ISRN KTH/EKT/FR-02/1-SE
Processing and Characterization of Silicon Carbide (6H- and
4H-SiC) Contacts for High Power and High Temperature
Device Applications
by
Sang-Kwon Lee
Ph.D Dissertation
KTH, Royal Institute of Technology
Department of Microelectronics and Information Technology
Device Technology Laboratory
Stockholm 2002
Processing and Characterization of Silicon Carbide (6H- and 4H-SiC) Contacts
for High Power and High Temperature Device Applications
A dissertation submitted to Kungliga Tekniska Högskolan, Stockholm, Sweden,
in partial fulfillment of the requirements for the degree of Teknisk Doktor (Ph.D.).
 2002 Sang-Kwon Lee
KTH (Kungliga Tekniska Högskolan)
Royal Institute of Technology
Department of Microelectronics and Information Technology
Electrum 229,
S-164 40, Kista-Stockholm
SWEDEN
ISRN KTH/EKT/FR-02/1-SE
ISSN 1650-8599
TRITA-EKT
Forskningsrapprt 2002:1
Printed in 250 copies by Kista Snabbtryck AB, Kista 2002
To Mejeong for her never-ending support
Lee, Sang-Kwon : Processing and Characterization of Silicon Carbide (6H- and 4HSiC) Contacts for High Power and High Temperature Device Applications, ISRN
KTH/EKT/FR-02/1-SE, KTH, Royal Institute of Technology, Department of
Microelectronics and Information Technology, Stockholm, 2002
Abstract
Silicon carbide is a promising wide bandgap semiconductor material for hightemperature, high-power, and high-frequency device applications. However, there are
still a number of factors that are limiting the device performance. Among them, one of
the most important and critical factors is the formation of low resistivity Ohmic
contacts and high-temperature stable Schottky diodes on silicon carbide.
In this thesis, different metals (TiW, Ti, TiC, Al, and Ni) and different deposition
techniques (sputtering and evaporation) were suggested and investigated for this
purpose. Both electrical and material characterizations were performed using various
techniques, such as I-V, C-V, RBS, XRD, XPS, LEED, SEM, AFM, and SIMS.
For the Schottky contacts to n- and p-type 4H-SiC, sputtered TiW Schottky contacts
had excellent rectifying behavior after annealing at 500 oC in vacuum with a thermally
stable ideality factor of 1.06 and 1.08 for n- and p-type, respectively. It was also
observed that the SBH for p-type SiC (φBp) strongly depends on the choice the metal
with a linear relationship φBp = 4.51 – 0.58φm, indicating no strong Fermi-level pinning.
Finally, the behavior of Schottky diodes was investigated by incorporation of sizeselected Au nano-particles in Ti Schottky contacts on silicon carbide. The reduction of
the SBH is explained by using a simple dipole layer approach, with enhanced electric
field at the interface due to the small size of the circular patch (Au nano-particles) and
large difference of the barrier height between two metals (Ti and Au) on both n- and pSiC.
For the Ohmic contacts, titanium carbide (TiC) was used as contacts to both n- and ptype 4H-SiC epilayers as well as on Al implanted layers. The TiC contacts were
epitaxially deposited using a co-evaporation method with an e-beam Ti source and a
Knudsen cell for C60, in a UHV system at low substrate temperature (500 oC). In
addition, we extensively investigated sputtered TiW (weight ratio 30:70) as well as
evaporated Ni Ohmic contacts on both n- and p-type epilayers of SiC. The best Ohmic
contacts to n-type SiC are annealed Ni (> 950oC) with the specific contact resistance of
≈ 8×10-6 Ωcm2 with doping concentration of 1.1 ×10-19 cm-3 while annealed TiW and
TiC contacts are the preferred contacts to p-type SiC. From long-term reliability tests
at high temperature (500 oC or 600 oC) in vacuum and oxidizing (20% O2/N2) ambient,
TiW contacts with a platinum capping layer (Pt/Ti/TiW) had stable specific contact
resistances for > 300 hours.
Keywords : silicon carbide, Ohmic and Schottky contacts, co-evaporation, currentvoltage, capacitance-voltage measurement, power devices, nano-particles, Schottky
barrier height lowering, and TLM structures.
Table of Contents
Appended Papers ......................................................................................................... iii
Summary of appended papers ................................................................................... vii
Acknowledgements ...................................................................................................... ix
Used Acronyms............................................................................................................ xii
1.
INTRODUCTION................................................................................................. 1
2.
PROPERTIES OF SIC......................................................................................... 5
2.1
SIC MATERIAL PROPERTIES .............................................................................. 5
2.1.1 Crystal structure ............................................................................................ 5
2.1.2 Polytypes of SiC ............................................................................................. 5
2.2
SIC ELECTRONIC PROPERTIES .......................................................................... 7
2.2.1 Density of States (DOS) and Energy bandgap (Eg) .................................... 7
2.2.2 Bandgap narrowing .................................................................................... 8
2.2.3 Incomplete ionization.................................................................................. 9
2.2.4 Carrier recombination .............................................................................. 10
2.2.5 Impact Ionization ...................................................................................... 10
2.2.6 Mobility ..................................................................................................... 11
3.
METAL-SEMICONDUCTOR JUNCTIONS .................................................. 15
3.1
CURRENT TRANSPORT MECHANISM ................................................................ 15
3.1.1 Schottky barrier formation........................................................................ 15
3.1.2 Current transport mechanism ................................................................... 16
3.2
OHMIC CONTACTS .......................................................................................... 18
3.3
SCHOTTKY DIODE PERFORMANCE .................................................................. 20
3.3.1 Specific on-resistance ............................................................................... 20
3.3.2 Forward voltage drop ............................................................................... 21
3.3.3 Breakdown voltage and reverse leakage current...................................... 21
3.3.4 Edge termination for high breakdown voltage ......................................... 23
3.3.5 Schottky barrier lowering ......................................................................... 25
3.4
OTHER RECTIFIERS ......................................................................................... 26
3.4.1 Junction barrier Schottky (JBS) diodes .................................................... 26
3.4.2 Merged P-i-N / Schottky (MPS) diodes..................................................... 27
3.4.3 DMT (Dual metal trench) diodes.............................................................. 28
3.4.4 TMBS (Trench MOS Barrier Schottky) diodes ......................................... 28
4.
FABRICATION PROCESS............................................................................... 29
4.1
PROCESS DESCRIPTION ................................................................................... 29
4.1.1 Wafer preparation and surface cleaning .................................................. 29
4.1.2 Etching process......................................................................................... 30
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4.1.3 Deposition Techniques.............................................................................. 32
4.1.4 Ion implantation........................................................................................ 35
4.1.5 Annealing .................................................................................................. 36
4.2
TEST STRUCTURES FOR OHMIC CONTACTS ..................................................... 37
4.2.1 Kuphal structure ....................................................................................... 37
4.2.2 Two-terminal contact resistance methods ................................................ 38
4.2.3 3-contacts, two-terminal methods............................................................. 39
4.2.4 Linear transmission line method (LTLM) ................................................. 40
4.2.5 Circular transmission line method (CTLM) ............................................. 42
4.2.6 Four-terminal contact resistance method................................................. 43
4.2.7 Six-terminal contact resistance method .................................................... 44
4.2.8 Comparison of each measurement technique ........................................... 45
5.
CHARACTERIZATION AND RESULTS....................................................... 47
5.1
MATERIAL CHARACTERIZATION ..................................................................... 47
5.1.1 X-ray Diffraction (XRD) ........................................................................... 48
5.1.2 Secondary Ion Mass Spectrometry (SIMS) ............................................... 52
5.1.3 Rutherford Backscattering Spectrometry (RBS) ....................................... 53
5.1.4 Transmission Electron Microscopy (TEM)............................................... 55
5.1.5 Atomic Force Microscopy (AFM) & Optical microscopy ........................ 55
5.2
ELECTRICAL CHARACTERIZATION OF SCHOTTKY CONTACTS ......................... 57
5.2.1 Measurement techniques........................................................................... 57
5.2.2 A review of the Schottky contacts (Paper I, IV, VIII)................................ 61
5.2.3 The relationship between metal work function and barrier height ..............
(Paper IV) ................................................................................................. 62
5.2.4 Reduction of the Schottky barrier height (Paper VIII) ............................. 65
5.3
SPECIFIC CONTACT RESISTANCE MEASUREMENTS ........................................... 70
5.3.1 TiC and Ti on n- and p-SiC (Paper II, III, V) ........................................... 71
5.3.2 Ni and TiW (30:70) contacts on n- and p-SiC (Paper V, VI, VII)............. 74
5.3.3 Microscopic mapping of specific contact resistance (Paper VI, VII)....... 76
5.4
LONG-TERM RELIABILITY TESTS AT HIGH TEMPERATURE ............................... 80
5.4.1 In vacuum.................................................................................................. 80
5.4.2 In oxidizing ambient.................................................................................. 81
6.
CONCLUSIONS AND FUTURE WORK ........................................................ 83
REFERENCES............................................................................................................ 85
- ii -
Appended Papers
I.
Schottky diode formation and characterization of titanium tungsten to nand p-type 4H silicon carbide.
S.-K. Lee, C.-M. Zetterling, M. Östling, J. Appl. Phys. 87(11), 8039 (2000).
II.
Low resistivity Ohmic titanium carbide contacts to n- and p-type 4H-Silicon
carbide
S.-K. Lee, C.-M. Zetterling, M. Östling, J.-P. Palmquist, H. Högberg, and U.
Jansson, Solid State Electronics 44(7), 1179 (2000).
III. Electrical characterization of TiC Ohmic contacts to aluminum implanted
4H-SiC
S.-K. Lee, C.-M. Zetterling, M. Östling, J.-O. Palmquist, H. Högberg, and U.
Jansson, Appl. Phys. Lett. 77(19), 1478 (2000).
IV. Schottky barrier height dependence on the metal work function for p-type
4H-SiC
S.-K. Lee, C.-M. Zetterling, and M. Östling, J. Electron. Mater. 30(3), 242
(2001).
V.
Low resistivity Ohmic contacts to silicon carbide for high temperature
device applications
S.-K. Lee, C.-M. Zetterling, M. Östling, J.-P. Palmquist, and U. Jansson,
Microelectronic Engineering, 60(1-2), 261 (2002).
VI. Ohmic contact formation on inductively coupled plasma (ICP) etched 4Hsilicon carbide using sputtered titanium tungsten
S.-K. Lee, S.-M. Koo, C.-M. Zetterling, and M. Östling, to be published in J.
Electron. Mater. (May 2002).
VII. The microscopic specific contact resistance mapping and long term
reliability on 4H-silicon carbide using sputtered titanium tungsten contacts
for high temperature device applications
S.-K. Lee, C.-M. Zetterling, and M. Östling, to be published in J. Appl. Phys.
(June 2002).
VIII. Reduction of the Schottky barrier height on silicon carbide using Au nanoparticles
S.-K. Lee, C.-M. Zetterling, M. Östling, I. Åberg, M. H. Magnusson, K. Deppert,
L.-E. Wernersson, L. Samuelson, and A. Litwin, to be published in Solid-State
Electron (June 2002).
- iii -
Related work not included in the thesis
Journal papers and proceedings
1.
Dry etching and metallization schemes in a GaN/SiC heterojuction device
process.
E. Danielsson, C.-M. Zetterling, S.-K. Lee, M. Östling, , K. Linthicum,
D.B. Thomson, O.-H. Nam, and R.F. Davis, Mater. Sci. Forum. 338-342, 1049
(2000).
2.
The formation and characterization of epitaxial titanium carbide contacts to
4H-SiC.
S.-K. Lee, E. Danielsson, C.-M. Zetterling, M. Östling, J.-P. Palmquist,
H. Högberg, and U. Jansson, Mat. Res. Soc. Symp. Proc. 622, T6.9 (2000).
3.
TiW (Titanium tungsten) for Ohmic and Schottky contacts to 4H-SiC
S.-K. Lee, C.-M. Zetterling, and M. Östling, Mat. Res. Soc. Symp. Proc. 640,
H7.2 (2000).
4.
Low damage Schottky diode formation on inductively coupled plasmas
etched 4H-silicon carbide
E. Danielsson, S.-K. Lee, C.-M. Zetterling, and M. Östling, J. Electron. Mater.
30(3), 247 (2001).
5.
Electrical characterization of titanium-based Ohmic contacts to 4H-SiC for
high-power and high-temperature operation
S.-K. Lee, C.-M. Zetterling, M. Östling, and B. M. Moon, to be published in J.
Korean Phys. Soc. (April 2002).
6.
Influence of the trenching effect on the characterization of buried-gate SiC
junction field-effect transistors
S.-M. Koo, S.-K. Lee, C.-M. Zetterling, M. Östling, U. Forsberg, and E. Janzen,
to be published in Mater. Sci. Forum (April 2002, presented at ICSCRM 2001).
7.
Reduction of the barrier height and enhancement of tunneling current of
titanium contacts using embedded Au nano-particles on 4H- and 6H-silicon
carbide
S.-K. Lee, C.-M. Zetterling, M. Östling, I. Åberg, M. H. Magnusson, K. Deppert,
L.-E. Wernersson, L. Samuelson, and A. Litwin, to be published in Mater. Sci.
Forum (April 2002, presented at ICSCRM 2001).
8.
Electrical characterization of metal-oxide-semiconductor capacitors on
plasma-etch damaged silicon carbide
S,-M. Koo, S.-K. Lee, C.-M. Zetterling, and M. Östling, to be published in SolidState Electron (June 2002).
- iv -
Conference presentation
(Oral, Poster, and invited talks)
1.
Schottky barrier height dependence on the metal work function for p-type
4H-SiC
S.-K. Lee, C.-M. Zetterling, M. Östling, presented at TMS Electron. Mater. Conf
(EMC), Denver, Colorado, U.S.A. June (2000).
2.
Low resistivity Ohmic contacts to silicon carbide for high temperature
device applications
S.-K. Lee, C.-M. Zetterling, and M. Östling, presented at European workshop on
Materials for Advanced Metallization (MAM), Sigtuna, Sweden, March (2001).
3.
Contact resistance on III-V (InP, InGaAs, and InGaAsP) semiconductors
H. Kosmaz, C. Zaring, S.-K. Lee, and M. Östling, presented at European
workshop on Materials for Advanced Metallization (MAM), Sigtuna, Sweden,
March (2001).
4.
Metallization schemes for combined unipolar / bipolar SiC process
U. Zimmermann, E. Danielsson, S.-K. Lee, C.-M. Zetterling, and A. Hallen,
presented at European workshop on Materials for Advanced Metallization
(MAM), Sigtuna, Sweden, March (2001).
5.
Ohmic contact formation on inductively coupled plasma (ICP) etched 4Hsilicon carbide using sputtered titanium tungsten
S.-K. Lee, S.-M. Koo, C.-M. Zetterling, and M. Östling, presented at Electron.
Mater. Conf. (EMC), Indiana, U.S.A. June (2001).
6.
Electrical characterization of Ohmic contacts to 4H-silicon carbide for high
power and high temperature operation
S.-K. Lee, C.-M. Zetterling, and M. Östling, presented at the 8th Korean Conf. on
Semiconductor (KCS), Seoul, Korea, Feb. (2001).
7.
Measurement on linear TLM structures with TiW/Ti/Pt contacts for
corrosive and high temperature applications
L. Uneus, S.-K. Lee, C.-M. Zetterling, L.-G. Ekedahl, I. Lundström, M. Östling,
and A. Lloyd Spetz, presented at Inter. Conf. on Silicon Carbide and Related
Mater. (ICSCRM), Tsukuba, Japan, Oct (2001).
8.
Recent advances and issues in SiC process and device technologies
M. Östling, S.-M. Koo, S.-K. Lee, E. Danielsson, and C.-M. Zetterling, presented
at the Solid State and Integrated Circuit Technology (ICSICT), Shanghai, China ,
Oct. (2001).
9.
Metal-oxide-semiconductor structures in inductively coupled plasma etch
damaged 6H- and 4H-SiC
S.-M. Koo, S.-K. Lee, C.-M. Zetterling, and M. Östling, presented at the
semiconductor interface specialists conference (SISC), Washington D.C. U.S.A,
Dec. (2001).
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10.
SiC device technology for high voltage and RF power applications
M. Östling, S.-M. Koo, S.-K. Lee, E. Danielsson, M. Domej, and C.-M.
Zetterling, presented at 23rd International Conference on Microelectronics, Nis,
Yugoslavia, May 12-15 (2002).
11.
Comparison study of Ohmic contacts in oxidizing ambient at high
temperature for gas sensor applications
S.-K. Lee, L. Uneus, S.-M. Koo, C.-M. Zetterling, L.-G. Ekedahl, I. Lundström,
A. Lloyd Spetz, and M. Östling, to be presented in TMS Electronic Material
Conference (EMC), Santa Barbara, U.S.A, June (2002).
12.
Influence of size-selected aerosol Au nano-particles in titanium Schottky
contacts on silicon carbide
S.-K. Lee, C.-M. Zetterling, M. Östling, I. Åberg, M.-H. Magnusson, K. Deppert,
L.-E. Wernersson, L. Samuelson, and A. Litwin, to be presented in 7th
International Conference on Nanometer-scale Science and Technology (Nano-7),
Malmö, Sweden, June (2002).
Not related work not included in the thesis
Part of MS thesis work
1.
Positron Annihilation study of phase transitions in Ethane physisorbed on
grafoil
P. C. Jain, S.-K. Lee, N. Hozhabri, and S. C. Sharma, Phys. Rev. B 49(4), 2821
(1994).
2.
Phase transition in Physisorbed Ethane investigated by positron annihilation
spectroscopy
P. C. Jain, S.-K. Lee, N. Hozhabri, and S. C. Sharma, Phys. Rev. B 60(3), 2057
(1999).
- vi -
Summary of appended papers
The appended papers are in chronological order and divided in two parts; Schottky and
Ohmic contacts to silicon carbide. In the beginning of the study the characterization of
metal-silicon carbide junctions and low resistivity Ohmic contacts with various
titanium-based metals (Ti, TiW, and TiC) was performed. Later on new concepts for
Schottky diodes and the performance of Ohmic contacts at high-temperature were
approached. Normally the first author is in charge of preparing manuscripts and
following up the publication in the appended papers.
Paper I. This paper presents the Schottky diode formation and characterization of
titanium tungsten (TiW) to silicon carbide: The main achievements are the first
characterization of TiW to both n- and p-type 4H-SiC with thermally stable ideality
factor and Schottky barrier height for complementary device applications. The author
performed all the processing, material and electrical characterizations, and all analysis
including manuscript writing and following up.
Paper II. This paper describes the results from a study of low resistivity titanium
carbide (TiC) Ohmic contacts to both highly doped n+- and p+-type epilayers of 4Hsilicon carbide using co-evaporation. The author performed all processing after the
deposition of TiC, most of the electrical characterization, and took part in the material
characterization, and most analysis including writing the manuscript.
Paper III. This paper investigates a novel combination of epitaxially grown TiC and
PECVD grown silicon nitride sacrificial layer to achieve lower specific contact
resistance on Al-implanted epilayers of 4H-SiC. The author performed all processing
after the deposition of TiC and ion implantation, most of the electrical characterization,
and took part in the material characterization, and most analysis including writing the
manuscript
Paper IV. This paper tried to answer the question of which metals to select on silicon
carbide using the relationship between Schottky barrier height and metal work
functions. The author performed all the processing, material and electrical
characterization, all analysis, and writing the manuscript.
Paper V. This paper presents the investigation of both low resistivity TiC and TiW
Ohmic contacts to silicon carbide. The long-term stability test of TiW is also
performed at high temperature. The author performed most of the processing, all
electrical characterization, and most of the analysis including writing the manuscript.
Paper VI. This paper describes the investigation of Ohmic contact formation on ICP
etch-damaged surface. For this study, sputtered TiW contacts were used as Ohmic
contacts. The author suggested the study, performed all the processing, performed the
analysis, and wrote the manuscript.
Paper VII. The investigation of the first microscopic mapping of specific contact
resistance is presented in this paper, where the long-term reliability tests on silicon
- vii -
carbide using sputtered TiW and a capping layer were also performed. The author
performed all processing, all of the electrical and material characterization, and all
analysis including writing the manuscript.
Paper VIII. This paper presents the new approach of SiC Schottky diodes using
aerosol deposited and embedded Au nano-particles, where the reduction of Schottky
barrier height is also discussed with a model of a dipole layer. The author performed
all the processing (except aerosol deposition of Au nano-particles), material and
electrical characterization, all analysis including computer simulation, and writing the
manuscript.
- viii -
Acknowledgements
At this moment, when I am finishing my Ph.D dissertation, I realize that I have to
thank a lot of people whom I knew and met in my life. I will borrow this space for
acknowledgement even though I feel it is not enough space for them! I also perceived
that this is not the end of study in my life. And, I know what kind of new life and
difficulties I have to solve and figure out are waiting for me in near future. I also
believe that I will do as before I did with some curiosity and confidence.
First of all, I would like to thank my academic supervisor Professor Mikael
Östling, who is head of our department, Microelectronics and Information Technology
(IMIT), without his guidance and continuous help this dissertation would never have
been finished. Prof. Östling accepted me as a Ph.D student in the former Device
Technology Lab. (EKT) at KTH (Royal Institute of Technology) in December of 1998.
Prof. Östling also introduced many opportunities to study the field of wide bandgap
semiconductor materials and collaborate with other Swedish research groups at
inorganic chemistry department in Uppsala University, at Solid State Physics in Lund
University, at S-SENCE in Linköping University, and with the ABB SiC research
groups. Prof. Östling encouraged me to attend and present my works at conferences. I
will not forget the 8th Korean Conference on Semiconductor (KCS) in Feb. 2001
because it was my first visit to Korea since I started my Ph.D in Sweden. It also gave
me many chances to meet Korean researchers in the field of silicon carbide and other
wide bandgap semiconductor materials.
Many thanks go to my supervisor, Dr. Carl-Mikael Zetterling, who is called
“Bellman” by people in EKT and IMIT, who is a leader of the SiC research groups in
EKT at KTH, and who gave me massive advise throughout this thesis. I have learned
many things from his courses and also Dr. Zetterling taught me how to write scientific
papers. Dr. Zetterling spent quite many hours to correct my manuscripts and my
research plan. Thank you so much, Bellman!
Thanks to my colleague, Dr. Erik Danielsson. Erik showed me processing and
electric characterization of SiC in the cleanroom as well as the measurement room.
Erik was happy to share his experiences with me throughout the SiC projects. Thank
you Erik! Good Luck! I also thank Uwe Zimmermann for his help in the cleanroom
and for high-voltage measurements in the FTE measurement room. I have to
acknowledge Sang-Mo Koo, who was my last office mate and Korean HUBAE at
KTH. Sang-Mo is the kindest man I have ever met. We spent many hours in the office,
the measurement room, and mainly the cleanroom together. We also spent quite many
times at PUB. I could not forget the time we had in last ICSCRM 2001 conference in
Japan, in Korea, and also in Sweden. I wish that you would do much better than I did
and good luck to you and your family!
I also thank all the member of SiC group, Dr. Martin Domej, Fanny Dahlqvist,
and Wei Liu as well as other members in EKT, Dr. Shi-li Zhang, Dr. Yong-Bin Wang,
Dr. Per-Erik Hellberg, Dr. Johan Pejnefors, Ann-Chatrin Lindgren, Erik Haralson,
Johan Seger, Erdal Suvar, Stefan Persson, Dongping Wu, and Christian Isheden.
- ix -
Special thanks go to Dr. Gunnar Malm for his help during the Ph.D. Gunnar will do his
defense at the end of April this year just before me. Congratulation, Gunnar!
Many thanks also go to Dr. Anders Hallen, Dr. Andrej Kuznetsov, Dr. Margareta
Linnarsson, Mikael Jargelius, Martin Janson, John Österman, Paulius Grivickas, and
Antonio Martinez in the Solid State Electronics Laboratory for their technological help,
measurements, and fruitful discussion. Special thanks to Leonardo Hillkirk, we had
quite many discussions not related to our own projects until last night. Leo, it was a
nice time. Please keep in touch.
I thank other Korean HUBAE-DUL at KTH, Jung-Hyuk Koh and new-comer
Jang-Yong Kim. Good luck!
During my Ph.D study I also met several special people, Dr. Henry Bleichner, Dr.
Mark Irwin, and Sofia Hatzikonstantinidou from ABB Corporate Research AB in Kista,
Sweden. They supported me overwhelmingly such as providing expensive wafers
($$$) and helping with the processes including ion implantation by Henry at ABB
cleanroom. I have learned many things like mass-production and industrial point of
view from the corporation with them. Specially I thank Sofia for her help, I wish that
you will have good luck in your own new business.
And the co-authors for the TiC papers, Dr. Ulf Jansson, Dr. Hans Högberg, and
Jens-Petter Palmquist in Uppsala are also acknowledged.
I would like to thank nano-particle (aerosol) and device people, Prof. Lars
Samuelson, Dr. Knut Deppert, Dr. Lars-Erik Wernersson, Dr. Martin H. Magnusson,
Dr. Andrej Litwin (also in Ericsson Microelectronics AB), and Ingvar Åberg in Solid
State Physics at Lund University. They introduced me to the world of nanometer-size
devices. Among them, I could not forget the endless discussion with Lars-Erik and
Ingvar (now in electrical engineering department at MIT) to improve our manuscript.
Thanks guys! Keep in touch as well.
Dr. Lars Uneus and Dr. Anita Lloyd Spetz at S-SENCE in Linköping University
are also acknowledged for their high-temperature measurements in oxygen ambient. Dr.
Spetz was also my opponent for my licentiate thesis defense on Oct. 23, 2000. Thanks
for the nice discussion during at that time.
I have to say “thank you” to Dr. Eun-Dong Kim and Dr. Nam-Kyun Kim at
Korea Electrontechnology Research Institute (KERI) in Korea for their warm
hospitality during my visit in Changwon in 2001.
I’d like to thank my former professors, Professor Suresh C. Sharma, who was my
first supervisor for my master period in the Physics department at the University of
Texas at Arlington in U.S.A, Professor Byung-Moo Moon at Korea University in
Korea, who introduced me to KTH in Sweden, and Professor K.V. Rao for support
during the first 6 months stay in Sweden. Thanks also to my past office mates, Hubert
and Linwei and our department secretary, Zandra Lundberg.
I’d like to thank all of the family in Korea. Very special thanks to my parents as
well as to my mother-in-law for their kind support in Seoul, Korea. I believe they will
be happy about my graduation, specially my father. I dedicate my Ph.D to my father. I
wish you, all my parents and mother-in-law, a long life without any health trouble.
Many many thanks to other my-side and my wife-side family members, specially my
sisters-in-law (called EONNEE-DUL) in Korea for their encouragements all the time.
Thanks EONNEE-DUL (usually I call them like my wife does).
Finally, I could not forget the great support of my wife, Me-Jeong during the stay
in Sweden. Thank you so much, Me-Jeong! Without your strong support (so called
-x-
NAE-JO in a good sense or in a wide meaning you said). Yes, I know, it was greatly
helpful to keep my study and finish my final Ph.D. work. In addition, everybody in our
family knows well you did a good support to me and GeeHee. Finally I thank my
daughter Geehee. I love you.
This work was supported by the Swedish Foundation for Strategic Research
(SSF) in one of the research projects in SiCEP (Silicon Carbide Electronics Program).
Thank you all.
_________________________
Sang-Kwon Lee
Stockholm
March 22, 2002
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Used Acronyms
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⋅
⋅
⋅
⋅
⋅
⋅
AFM
CTLM
C-V
DMT
ECR
FM
FMR
ICP
IMPATT
I-V
JBS
JFET
LEED
LOCOS
LPCVD
LTLM
MEMS
MESFET
MISiCFET
MOSFET
MPS
MS
PECVD
PVD
RBS
RESP
RIE
RTA
SBH
SEM
SiC
SIMS
TEOS
TEM
TFE
TiC
TiW
TLM
TMBS
UHV
XPS
XRD
: Atomic force microscopy
: Circular Transmission Line Method
: Capacitance-Voltage
: Dual Metal Trench
: Electron Cyclotron Resonance
: Figure of Merit
: Floating Metal Ring
: Inductively Coupled Plasma
: IMPact ionization Avalanche Transit Time diode
: Current-Voltage
: Junction Barrier Schottky
: Junction Field-Effect Transistor
: Low-Energy Electron Diffraction
: LOCal Oxidation of Silicon
: Low Pressure Chemical Vapor Deposition
: Linear Transmission Line Method
: MicroElectroMechanical Systems
: MEtal-Semiconductor Field-Effect Transistor
: Metal-Insulator Silicon Carbide Field Effect Transistor
: Metal-Oxide Semiconductor Field-Effect Transistor
: Merged P-i-N/Schottky
: Metal-Semiconductor
: Plasma Enhanced Chemical Vapor Deposition
: Physical Vapor Deposition
: Rutherford Backscattering Spectrometry
: REsistive Schottky barrier field Plate
: Reactive Ion Etching
: Rapid Thermal Annealing
: Schottky Barrier Height
: Scanning Electron Microscopy
: Silicon Carbide
: Secondary Ion Mass Spectrometry
: TetraEthylOrthoSilicate, Si(OC2H5)4
: Transmission Electron Microscopy
: Thermionic Field Emission
: Titanium Carbide
: Titanium tungsten
: Transmission Line Method
: Trench Metal-oxide-semiconductor (MOS) Barrier Schottky
: Ultra High Vacuum
: X-ray Photoelectron Spectroscopy
: X-Ray Diffraction
- xii -
1. Introduction
S
ilicon carbide (SiC) has received remarkable attention during the last decade as a
promising device material for high temperature, high frequency, and high power
device applications due to its high thermal conductivity and high critical field for
breakdown. It exhibits higher values of thermal conductivity (3-13 times), critical
electric field (4-20 times), and saturated carrier velocity (2-2.5 times) compared to the
conventional semiconductor materials such as silicon and gallium arsenide [1-3]. Table
1-1 shows the comparison of physical properties of SiC and other semiconductor
materials. These favorable properties of SiC are desirable for efficient high power
device operation[2, 3]. It is also an attractive material for high temperature operating
(> 650 oC) gas sensors as well as solid-state transducers such as pressure sensors and
accelerometers for automotive and space industry applications using
microelectromechanical systems (MEMS)[4, 5].
Table 1-1. Comparison of electrical / mechanical properties for various semiconductors.
Eg (eV)
Si
1.1
GaAs
1.4
3C-SiC
2.4
6H-SiC
3.0
4H-SiC
3.26
GaN
3.4
Ec(MV/cm)
0.3
0.4
1.2
2.5
2.2
3.3
vsat (107 cm/s)
1
2.0
2.0
2
2
2.5
µn(cm2/Vs)
1350
8500
900
370
720
1000
µp(cm2/Vs)
480
400
40
80
120
30
εr
11.8
12.8
9.7
10
10
8.9
λ (W/cmK)
1.5
0.5
5
5
5
1.3
5.65
4.36
5.3
3.2
a=3.08
c=15.12
3.2
a=3.08
c=10.08
3.2
a=3.19
c=5.19
6.1
lattice constant a=5.43
(Å)
2.3
ρ (g/cm3)
In order to qualify the advantages of semiconductors for various applications the
various FMs (figure of merit) have been used[6-9]. The analysis of these FM will help
in quantifying the benefits of using wide bandgap semiconductors for making unipolar
power devices. Chow and Tyagi[10] reported a critical evaluation of the performance
capabilities of various wide-bandgap semiconductors for high-power and highfrequency unipolar electron devices using seven different FMs such as JM, KM, BM,
BHFM, QF1,2, and 3. Johnson's[6] and Keyes'[7] figures of merit are basically used
for comparison purposes like high frequency and power evaluation. The other FMs are
more critical for power device performance. Table 1-2 summarizes the various figures
-1-
Chapter 1 Introduction
Sang-Kwon Lee
of merit for various semiconductors. For high-frequency device applications, SiC
devices can provide much higher speed compared to the Si devices due to the higher
saturated drift velocity vsat (influence the delay time τ = W/ vsat) and lower permittivity
εr (capacitance C ∝ εr). Apart from these advantages, there are two more benefits that
SiC-based electronics offer in the areas of high-temperature and high-power device
operation.
Table 1-2. various figures of merit for various semiconductors.
Material
JMa
(Ecvsat/π)2
KMb
λ(vsat/εr)1/2
QF1c
λσAg
QF2d
λσAEC
QF3(BM)e
σA = εrµEC3
BHFMf
µEC2
Si
GaAs
GaN
3C-SiC
4H-SiC
6H-SiC
1
7
756
278
278
215
1
0.5
1.6
5.2
5.1
5.1
1
36
644
117
594
448
1
48
7089
468
4357
3734
1
16
744
35
178
134
1
11
90
11
29
19
all values are normalized with respect to Si
a
Johnson's FM (JM) for the basic limit on the device performance (high power and frequency).
b
Keyes's FM is based on the switching speed of the transistor.
c
Quality factor 1 (thermal FM) for heat sink material and the active device area in power device
d
Quality factor 2 is based on perfect heat sinks.
e
Quality factor 3 is based on no assumptions about the sink materials or geometry.
f
Baliga FM for evaluation of high frequency application
g
σA is QF3 (Baliga FM)
Beyond on favorable properties of SiC, the full performance of SiC devices is limited
by the material quality itself and the fabrication of high temperature stable Schottky
contacts and low resistivity Ohmic contacts. Generally, contacts between metals and
semiconductors play a major role in all classes of devices. Contacts are used in
controlling certain device functions, as well as providing means for communication
between the active devices and the outside world. In view of the major role of these
device applications, the nature and essential parameters, which affects the properties of
contacts, should be addressed and studied in detail. Lowering the Schottky barrier
height (φB) or increasing the doping concentration can reduce the specific contact
resistance (ρC) since the most systematic way to compare different metallization
technologies with respect to the specific contact resistance is by measuring itself [11].
Indeed, unlike the actual contact resistance, this parameter (ρC) is unaffected by the
current crowding and geometry-independent characteristic of the metal-semiconductor
interface. However, it is hard to achieve Ohmic contacts with low (<10-6 Ωcm2)
specific contact resistance due to the relatively large Schottky barrier height (0.9∼1.8
eV and 1.3∼2.0 eV for n- and p-type, respectively, see chapter 2). Lower specific
contact resistance is obtained to n-type 4H-SiC and 6H-SiC (∼10-4 to 10-6 Ωcm2) than
to p-type 4H- and 6H-SiC (10-3 to 10-5 Ωcm2)[12-14].
According to calculation of the specific contact resistance ρC in case of a barrier height
of 0.3 eV, a doping level of 1 × 1019 cm-3, for n-type 6H-SiC using a general
thermionic field emission (TFE) theory[11], a specific ρC of 1 × 10-6 Ωcm2 can be
achieved (see chapter 2). The theoretical and experimental values are quite consistent
with each other except for p-type SiC. Currently the investigation on thermally stable
Schottky contacts and low resistivity Ohmic contacts is still required for high-power
-2-
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
and high-temperature device applications even though there are a plenty of
publications in this topic. In addition, a few publications and works are reported on
long-term reliability tests at elevated temperature (> 600oC) and on the issue of
distribution of the specific contact resistance on SiC wafer for stable performance of
devices at high temperature and high power operation. The answers for the questions:
what kinds of phases are created at the interface between metal and silicon carbide,
what kind of phase makes Ohmic and low resistivity contacts to SiC, what kind of
metal and metal-stacks can be applied for high-power as well as high-temperature
device applications, what kind of metal is compatible with the fabrication processes,
and what is the mechanism of metal-semiconductor junctions will be described in this
thesis.
This thesis is focused on the electrical characterization of Ohmic as well as Schottky
contacts to n- and p-type 4H- and 6H-silicon carbide. P-type Ohmic and Schottky
contacts are much more highlighted because of its importance and urgent need for
device applications. A background to silicon carbide is given in Chapter 2. Chapter 3
covers the basics of the current-transport mechanism and Schottky diode performance.
Chapter 4 describes the experimental technique and test structures for the measurement
of specific contact resistances. The characterization and results are presented in
Chapter 5. Chapter 6 contains concluding remarks and suggested future investigations.
-3-
2. Properties of SiC
A
s we realized in chapter 1, silicon carbide is a wide bandgap semiconductor,
which has many advantages compared to the conventional semiconductors. In
this chapter, the material properties of SiC is focused. Some of the important
parameters for device simulation and calculation are also presented with useful values,
which are based on the recent literature and commercially available simulators.
2.1 SiC material properties
2.1.1 Crystal structure
The common semiconductors occur in the diamond crystal structure (Si and Ge), the
zincblende crystal structure and the wurtzite crystal structure (for example, GaAs and
other III-V compound semiconductors) even though there is a large number of
different crystal structures possible in nature [15]. Silicon carbide has several stable
crystal structures.
2.1.2 Polytypes of SiC
SiC has equal parts silicon and carbon, both of which are group IV elements. The
distance between neighboring silicon (a) or carbon atom is approximately 3.08 Å for
all polytypes. The carbon atom is situated at the center of mass of the tetragonal
structure outlined by the four neighboring Si atoms (see Figure 2-1). The distance
between the C atom and each of the Si atoms is approximately 2.52 Å. The height of
the unit cell, called c, varies between the different polytypes. Therefore, the ratio of c/a
differs from polytype to polytype. This ratio is 1.641, 3.271, and 4.908 for the 2H, 4H,
and 6H-SiC polytypes, respectively (see also Table 1-1). The polytype is a variation of
crystalline material in which the stacking
order of planes in the unit cell is different.
Each SiC bilayer, while maintaining the
Si atom
tetrahedral binding scheme of the crystal,
a
can be situated in one of three possible
C atom
positions with respect to the lattice (A, B,
or C). The bonding between Si and C
atoms in adjacent bilayer planes is either of
a Zinc-blende (Cubic) or Wurtzite
a
(Hexagonal) nature depending on the
stacking order[16]. As shown in Figure 2-2, Figure 2-1. The tetragonal bonding of a
if the stacking is ABCABC...the cubic carbon atom with the four nearest
polytype commonly abbreviated as 3C-SiC, silicon.
is realized.
-5-
Chapter 2 Properties of SiC
Sang-Kwon Lee
Figure 2-2. The staking sequence of common 3C-, 2H-, 4H-, and 6H-silicon carbide
(after ref. [17]).
The purely Wurtzite ABAB... stacking sequence is called 2H-SiC. The 4H-SiC
(ABAC...) and 6H-SiC (ABCACB...) are also shown in Figure 2-2[17]. These two
types of SiC are the most common hexagonal polytypes [18]. 4H-SiC consists of equal
amounts cubic and hexagonal bonds, while 6H-SiC is two-thirds cubic. All the
polytypes of SiC are referred to in a hexagonal coordinate system consisting of three aplane coordinates a1, a2, and a3, and a c-axis coordinate. The c-axis is the direction of
the stacking of hexagonally close packed layers, and the three a-plane axes are all in
the plane perpendicular to c-axis (see Figure 2-3), with 120-degree angle between aplanes. Commercially available SiC bulk material is generally cut and polished 3∼8
degrees off-axis towards <11 2 0> for avoiding the growth of 3C inclusions in the
epitaxial layers of 4H, called step-controlled epitaxy by Matusunami et al. [19]. Two
different faces perpendicular to the c-axis (Si 0001 and C 000 1 ) exist in commonly
used SiC. SiC with the silicon face is commonly used for device applications since the
quality of epitaxial growth is better than that on the carbon face.
Figure 2-3. The Miller indices describing the
hexagonal structure.
-6-
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
2.2 SiC electronic properties
Physical models, which also include most important parameters, are presented based
on the recent literature and the corresponding parameters for the Silvaco ATLAS [20]
physical device simulator. The comparison of electrical and structural properties for
various semiconductors can be found from a Ph.D. thesis[21].
2.2.1 Density of States (DOS) and Energy bandgap (Eg)
The intrinsic carrier concentration ni in a semiconductor is a fundamental parameter
and high operating temperature limit. It has also been found that the formation of
current filaments (mesoplasmas) can be related thermal runway when the intrinsic
concentration becomes comparable to that of the background concentration. The
relationship between ni and temperature and energy bandgap is given by [22]
 Eg 

n i = N C N V exp −
2
kT


(2-1)
where NC and NV is the effective density of states in the conduction and valence band
states, respectively given by
3
2
3
 2m kT 
 T 2
 = 
N C (T ) = 2
 N C (300)
 300 
 h

*
e
2
3
(2-2)
3
 2m *h kT  2  T  2
 = 
N V (T ) = 2
 N v (300)
2
 300 
 h

(2-3)
where me* and mh* is 0.76 m0 and 1.20 m0, respectively[23]. Using equations 2-2 and 23, the NC and NV for 4H-SiC equal 1.66×1019 cm-3 and 3.19×1019 cm-3, respectively at
room temperature (300K). The temperature dependent energy bandgap is given by
 300 2
T2 
E g (T) = E g (300) + α ⋅ 
−

 300 + β T + β 
(2-4)
The effective density of states in the conduction and valence band as well as the energy
bandgap at room temperature (300K) for different semiconductors are summarized in
Table 2-1 [24].
Table 2-1. Calculated parameters for different semiconductors at 300K.
4H-SiC
Si
Ge
GaAs
NC (300) cm-3
NV (300) cm-3
Eg (300) eV
1.66 × 1019
3.19 × 1019
3.26
2.89 × 1019
1.04 × 1019
1.08
1.04 × 1019
6.00 × 1018
0.66
4.7 × 1017
7.0 × 1018
1.42
α
β
3.3 × 10-4
0
4.73 × 10-4
636
4.77 × 10-4
235
5.41 × 10-4
204
-7-
-3
Intrinsic carrier concentration ni (cm )
Chapter 2 Properties of SiC
Sang-Kwon Lee
20
10
300 K
18
10
16
10
Ge
14
10
Si
12
10
GaAs
10
10
4H-SiC
8
10
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
1000/T (1000/K)
Figure 2-4. Intrinsic concentrations for various semiconductors as a
function of the temperature.
From equations 2-1, 2-2, 2-3, and 2-4, the intrinsic carrier concentration as a function
of the temperature for different semiconductors having a different energy bandgap
based on the calculation (see Table 2-1) was plotted in Figure 2-4. For room
temperature (300K), ni is equal to be approximately 7 × 10-7 cm-3 for 4H-SiC. As
shown in Figure 2-4, the wider bandgap and thereby lower intrinsic carrier
concentration allows SiC to maintain semiconducting behavior at much higher
temperature than conventional Si and Ge semiconductors.
2.2.2 Bandgap narrowing
The bandgap narrowing can be induced both by doping and by carrier injection. For the
ATLAS[20] simulations of SiC devices, a model that is valid for Si is used since there
are few models available for the bandgap narrowing effect of SiC. The bandgap
narrowing (∆Eg) is given by
1


2
2


N
N 


−3 
∆E g = 9 ⋅10 ln
+  ln
+ 0.5 
17
17 
 1⋅10
 1 ⋅10 
 


(2-5)
However, Persson et al. [25, 26] theoretically investigated the energy bandgap
narrowing for 3C, 4H-, and 6H-SiC, where the valence band and conduction band
change are extracted as a function of ionized dopant concentration or carrier
concentration. This model [25, 26] can be described by equation 2-6
1
k
Ξ 
Ξ 
∆E x = A x   + B x  
 N0 
 N0 
-8-
1
l
(2-6)
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
where Ξ is the controlling property of the bandgap narrowing, and A, B, No, k, and l
are the fitting parameters where k and l are positive integers.
2.2.3 Incomplete ionization
One of the disadvantages with wide-bandgap semiconductors is that the dopant
ionization levels are quite deep. Hence, the dopants are not fully ionized even at
higher temperature. The carrier concentration N+D,A (i.e. the number of ionized donors
or acceptors) can be calculated with the following equations [3, 27],
N D+ , A
1



 N D,A 
 E D , A   2



1
1
4
exp
g
−
+
+



c


N
kT

 
,
C
V




= N D,A 
N
E 

2 g c D , A exp D , A 
N C ,V

 kT 


N C ,V
3
2
 T   mC
= 2.513 × 1019 
 
 300 K   mV









(2-7)
3
2


(2-8)
10
21
10
20
10
19
10
18
10
17
10
16
10
15
10
14
10
13
-3
Carrier concentration (cm )
where gC is the spin degeneracy (in this case 2 for donors and 4 for acceptors), NC(NV)
is the density of states given by equation 2-8 with the effective density of states masses
10
n-type
p-type
6H-SiC
ED=100meV
EA=200meV
800K
300K
13
10
14
10
15
10
16
10
17
10
18
10
19
10
20
10
21
-3
Doping NDor NA (cm )
Figure 2-5. Calculated ionization of aluminum and nitrogen in 6H-SiC at 300K and
800K using equations 2-7 and 2-8.
mC or mV (=1× m0) for electrons and holes, respectively, and ED and EA are the donors
and acceptors levels . Using equations 2-7 and 2-8, the calculated ionization aluminum
and nitrogen in 6H-SiC at 300K and 800K is shown in Figure 2-5. According to the
calculation, a doping concentration of 1020 cm-3 for aluminum at 300K would only
result in a carrier concentration of 5 × 1017 cm-3. However, at these doping
-9-
Chapter 2 Properties of SiC
Sang-Kwon Lee
concentrations 1% of the atoms have been replaced, and the bands are degenerate,
resulting in much higher free carrier concentration. Therefore, the equations 2-7 and 28 are not quite valid for higher doping concentration than about 1019 cm-3.
2.2.4 Carrier recombination
SRH Recombination
Schottky, Read, and Hall (SRH) described the recombination process with the phonon
transitions by the way of defects or traps. The SRH recombination rate is given by
R SRH
np − n ie2
=
E Trap − E i
E i − E Trap



kT



τ p n + n ie e
+ τ p + n ie e kT

 n







(2-9)
where ETrap-Ei is the distance between the trap level and the intrinsic level, and τn, and
τp are the lifetime of electrons and holes at low injection, respectively. Commonly used
values of carrier lifetimes are in the range of 0.1 ∼ 2 µs for n-type 4H-SiC [21] since
the reported lifetimes in the literature are varying with a big difference.
Auger Recombination
Auger recombination, which is an important model parameter for high-power device
design, occurs both at high doping level and in the high injection regime due to the
direct band-to-band recombination between an electron and a hole across the forbidden
gap, accompanied by the transfer of energy to other free electron or hole [28]. It is
given by equation 2-10, where Cn,p are the Auger coefficient for electrons and holes,
and their sum is extracted from the measurement in the high-level injection regime.
[
R Auger = [C p p + C n n ]⋅ np − nie2
]
(2-10)
The values we used for Cn and Cp is 5×10-31 and 2×10-31 cm-6s-1, respectively for n-type
4H-SiC with a doping concentration of 1×1018 cm-3 at room temperature [28].
2.2.5 Impact Ionization
The maximum electric field (critical field; EC) and the blocking voltage (VB) are
determined by the impact ionization rate αn and αp for electrons and holes respectively.
SiC shows lower impact ionization rate of electrons than that of holes, which is the
opposite compared to conventional Si. The ionization integral equation is given by[29,
30]
W

(
)
(
x
)
exp
(
x
)
−
(
x
)
dx
α
α
α

dx = 1
n
n
p
∫0
∫
x

W
- 10 -
(2-11)
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
 b 
α n ( x ) = a n exp  − n 
 E ( x) 
 bp 
α p ( x ) = a p exp  −

 E ( x) 
(2-12)
The critical field with an impurity concentration of 1015<ND<1018 can be calculated by
ECR, Ref
ECR =
1−
 N
1
log 10 D
4
 N Ref



(2-13)
It is known that, for 6H-SiC, the critical field parallel to the c-axis (E) is half the
critical field perpendicular to the c-axis (E⊥) and the temperature coefficient of the
breakdown voltage is negative for E, while it is positive for E⊥. There are no
measurement results on the anisotropic critical field in 4H-SiC. The total impact
ionization rate is expressed as
r
r
J
J
n
p
(2-14)
+αp
G II = α n
q
q
The specific impact ionization parameter for 4H-SiC can be found in the literature [21,
31].
2.2.6 Mobility
The Arora model [32] is used for the doping dependence of the mobility. Another
important model is the drift velocity saturation, which is implemented as a decrease in
the mobility at high electric fields. The low field mobility (Arora model) is given by
[23, 33, 34]
µ n,p
 T 
= µ n, p,min 

 300 
α n,p
+
µ n, p,max
 N + NA 

1+  D

 N
n,
p,
ref


γ n, p
 T 


 300 
α n,p
(2-15)
where ND and NA are the density of donors and acceptors respectively. The mobility is
more anisotropic for 6H-SiC than it is for 4H-SiC. Schaffer et al. [34] presented the
results from measurements of electron and hole majority carrier mobilities in both 4Hand 6H-SiC as functions of temperature, doping, and directions in epitaxilly grown
crystals. According to their measurements, the electron mobility perpendicular to the caxis ([11 2 0] for 4H- and [1 1 00] for 6H-SiC) is 0.83 times for 4H-SiC and 4.8 times
for 6H-SiC at above 200K than parallel to the c-axis ([0001])[23, 34]. For 4H-SiC, it is
independent of the temperature. The parameter values for different materials (Si, 6Hand 4H-SiC) are summarized in Table 2-2 below. Figure 2-6 and 2-7 shows the low
field mobilities of µn and µp as a function of the doping concentration at different
temperatures (300K, 450K, and 600K) for 4H-SiC, respectively. At high electric fields,
- 11 -
Chapter 2 Properties of SiC
Sang-Kwon Lee
the carrier drift velocity (v) saturates due to the increase of the optical phonon
scattering and reaches the saturation velocity (vsat). There is no model reported for this
effect in 4H-SiC yet. The high field mobility can be described as
µ n,p ( E ) =
µ n, p
0
1
  µ 0E β  β
1 +  n, p  
  vsat  


(2-16)
For SiC, the parameter for high field mobility i.e. β=2 and νsat=2×107 cm/s (see also
Table 1-1) is usually chosen.
Table 2-2. Mobility for different semiconductor materials (Ref. [23, 33, 34]).
Si
µn,min cm2/Vs
µn,max cm2/Vs
Nn,ref cm-3
γn
αn
µp,min cm2/Vs
µp,max cm2/Vs
Np,ref cm-3
γp
αp
92
1268
1.3×1017
0.91
-2.42
52
453
1.9×1017
0.63
-2.2
6H-SiC
to c-axis
⊥ to c-axis
µ[0001]
µ[1 1 00]
0
0
415
87
1.1×1018
0.59
-3
6.8
99
2.1×1019
0.31
-3
-
4H-SiC
⊥ to c-axis to c-axis
µ[0001]
µ[11 2 0]
0
0
947
1136
1.94×1017
0.61
-2.15
-2.40
15.9
124
19
1.76×10
0.34
-2.15
-
2
Electron Mobility (cm /Vs)
1200
1000
Parallel to c-axis
300 K
300 K
450 K
600 K
800
600
Perpendicular to c-axis
400
200
0
10
15
10
16
17
10
10
18
10
19
20
10
-3
ND (cm )
Figure 2-6. Low field electron mobility (Arora model) as a function of the doping
concentration at different temperatures (300 K, 450 K, 600 K) in 4H-SiC (after ref.
[23, 33, 34]).
- 12 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
2
Hole Mobility (cm /Vs)
140
300 K
450 K
600 K
120
100
80
60
40
20
0
15
10
10
16
10
17
10
18
10
19
10
20
-3
NA (cm )
Figure 2-7. Low field hole mobility (Arora model) as a function of the doping
concentration at different temperatures (300 K, 450 K, 600 K) in 4H-SiC (after ref.
[23, 33, 34]).
- 13 -
3. Metal-Semiconductor junctions
M
etal-semiconductor contacts have many applications such as the gate electrode
of metal semiconductor field-effect transistors (MESFET), the source and
drain contacts in metal oxide semiconductor field-effect transistors (MOSFET), and the
electrode for impact ionization avalanche transit time (IMPATT) oscillators[24]. In this
chapter, we will describe the basics of metal-semiconductor junctions, the key factors
for both Ohmic and Schottky contacts to semiconductor, and some examples of metalsemiconductor devices.
3.1 Current transport mechanism
3.1.1 Schottky barrier formation
When a metal and a semiconductor are brought in contact, the respective Fermi-levels
must coincide in thermal equilibrium as shown in Figure 3-1 (b). There are two
limiting cases such as the ideal case (referred to as Schottky-Mott limit[35]) and a
practical case (known as the Bardeen limit[36]) to describe the relationship between a
metal and a semiconductor. Figure 3-1 shows the energy band diagram for the ideal
case (Schottky-Mott limit) with the absence of surface states.
(a)
Vacuum level
qχ
qφs
EC
qφm
EF
Metal
EF
Semiconductor
n-type
EV
(b)
qVbi=qφm-qφs
EC
EF
qφB
EF
Metal
Semiconductor
EV
Figure 3-1. The formation of a barrier between the metal and the semiconductor when
(a) neutral and isolated and (b) in perfect contact without any oxide between them
(Schottky-Mott limit).
- 15 -
Chapter 3 Metal-Semiconductor junctions
Sang-Kwon Lee
In this case the barrier height for n-type semiconductor can simply be determined to be
the difference between the metal work function (φm) and electron affinity (χS) of the
semiconductor;
qφ Bn = q (φm − χ s )
(3-1)
For a given semiconductor and a metal, the sum of the barrier height on n- and p-type
semiconductor is expected to be equal to the energy bandgap
q (φ Bn + φ Bp ) = E g
(3-2)
This relationship for Schottky-Mott limit implies that the control of the barrier height is
achieved by the choice of the metal. The second limiting case is the Bardeen limit[36]
where a large density of states is present at the semiconductor to metal interface. In the
Bardeen limit the barrier height φB is completely independent of the metal work
function φm in contrast to the Schottky-Mott limit and the Fermi level is said to be
pinned by the high density of interface states.
3.1.2 Current transport mechanism
a
EC
EF
b
Metal
c
d
EV
Semiconductor
Figure 3-2. Current transport processes in a forward-biased Schottky barrier.
Figure 3-2 shows four basic transport processes for n-type semiconductors under
forward bias [11]. The four processes are a) emission of electrons from the
semiconductor over the top of the barrier into the metal, b) quantum mechanical
tunneling through the barrier, c) recombination in the space-charge region, and d)
recombination in the neutral region (called hole injection). For the lowly doped
semiconductor the current flows as a result of thermionic emission (TE)[37] as shown
(a) Low ND (TE)
(b) Intermediate ND (TFE) (c) High ND (FE)
Figure 3-3. Energy band diagram for (a) low doped, (b) intermediate doped, and (c)
high doped n-type semiconductor. The arrow indicates the electron flow.
- 16 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
in Figure 3-3 (a) with electrons thermally excited over the barrier. In the intermediate
doping range, thermionic field emission (TFE)[38] dominates as shown in Figure 3-3
(b). For high doping, the barrier is sufficiently narrow at or near the bottom of the
conduction band for the electrons to tunnel directly, known as field emission (FE). The
three regimes can be distinguished by considering the characteristic energy E00 defined
by[38]
E 00 =
[eV]
ND
qh
= 1.183 × 10 −11 N D
4p m *tun e s
(3-3)
where ND is the doping concentration, m*tun (≈ 0.25 m0 for 6H-SiC[10]) is the effective
tunneling mass , and εs (≈ 10 ε0) is the dielectric constant for SiC. Using equation 3-3,
a plot of characteristic energy E00 as a function of doping density is shown in Figure 34. A comparison of E00 to the thermal energy kT shows that when thermionic emission
dominates kT » E00, for thermionic-field emission kT ≈ E00, and for field emission kT «
E00. For simplicity, the range of each regime can be chosen by E00 ≤ 0.2 kT for TE, 0.2
kT < E00 < 5kT for TFE, and E00 ≥ 5 kT as shown in Figure 3-4. For 6H-SiC with a
tunneling effective mass of 0.25 m0, this corresponds approximately to TE for ND ≤
1.86 × 1017 cm-3, TFE for 1.86 × 1017 < ND < 1.15 × 1020, and FE for ND ≥ 1.15 × 1020.
0
10
*
6H-SiC (m tun~ 0.25m0)
20
-3
E00, kT (eV)
ND~1.1 x 10 cm (E00~ 0.127)
10
-1
TE
TFE
FE
kT
10
-2
17
-3
1.9 x 10 cm (E00~ 0.005)
E00
10
-3
16
10
17
10
18
10
19
10
20
10
10
21
-3
Doping density (cm )
Figure 3-4. E00 and thermal energy kT as a function of the doping density for 6H-SiC
with m*tun/m0=0.25, T=300K.
- 17 -
Chapter 3 Metal-Semiconductor junctions
Sang-Kwon Lee
3.2 Ohmic contacts
The specific contact resistance (ρC) is given by
 ∂V 
ρC =  
 ∂J  V→0
(3-4)
and, can be calculated by using the following equations for each current transport
process[10].
ρC =
ρC =
k
ϕ 
exp B  for TE
*
qA T
 kT 
(3-5)
E 
 E   ϕ + Uf Uf 
 for TFE (3-6)
cosh 00 coth 00 exp B
−
kT 
p (ϕ B + U f )
 kT 
 kT   E 0
k E 00
qA
*
 A * πqT 2
 − ϕ B  A *cq 2 T 2
 − ϕB

 − 2 2 exp
ρC = 
exp
− cU f 
c k q
 E 00 
 E 00

 kTsin(πckT )
−1
for FE
(3-7)
where A* is the modified Richardson’s constant, Uf is the Fermi level with respect to
the band edge, and E0 and c are given by
E 
E 0 = E 00 coth 00 ,
 kT 
c=
 4ϕ 
1
ln B 
2E 00  U f 
(3-8)
The theoretical specific contact resistance for different doping concentration can be
calculated using equation 3-5 to 3-7. For example, Schottky barrier height for a 0.3 eV
and surface concentration of 1 × 1019 cm-3, the specific contact resistance is 1.0 × 10-6
Ωcm2 using equations 3-3, 3-4 and 3-7. The plot of the calculated specific contact
resistance versus doping and barrier height is shown in Figure 3-5.
Since the first successful single transistor was built, device fabrication technology has
grown at a tremendous rate and device size is becoming smaller and smaller. Here, the
nanometer-size contacts are pointed out briefly due to its importance in future research.
Electronic properties of metallic nanometer-size contacts (or clusters or particles) on
semiconductors rely on the metal comprising the cluster, the size of cluster or contacts,
the semiconductor substrates, and the fabrication process and techniques. Until now,
metal/semiconductor nanostructures fall into three broad categories: (1) single electron
tunneling (SET) devices, (2) nanoscale Schottky barriers, and (3) Ohmic contacts.
When the size of a contact (particle or cluster) is reduced to a few nanometers, an
additional mechanism is required to understand and explain the new effects due to the
size. When we are working on the nanometer scale, the capacitance of the structure can
be low enough so that the single electron charge energy e2/2C can be large compared to
- 18 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
20
10
3
10
1
18
10
ΦB=1.0 eV
10
-1
10
-3
1x10
-5
10
-7
10
-9
10
17
ΦB=0.8 eV
ΦB=0.6 eV
2
ρC (Ωcm )
10 1019
ΦB=0.4 eV
ΦB=0.2 eV
0
1x10
-9
2x10
1/sqrt(ND) cm
-9
3x10
-9
3/2
Figure 3-5. calculated specific contact resistance (ρC) versus doping concentration for
barrier height from 0.2 to 1.2 eV. The calculation used Thermionic field emission (TFE)
for doping concentration from 1016 to 1018 cm-3 and field emission (FE) for doping
concentration from 1018 to 1021 cm-3.
the thermal energy KBT which is about 26 mV at 300K. For asymmetric structures, this
effect is observed as steps in an I-V curve and some oscillation in dI/dV and is known
as a Coulomb staircase. Also the tunneling probability is very small for voltages
smaller that e/2C, resulting in an energy gap in this voltage range. This phenomenon is
referred to as Coulomb blockage. Andres et al. [39] reported that a 1 nm diameter Au
cluster on Au (111) substrate showed the Coulomb blockade effects at room
temperature. But single electron tunneling effects at room temperature can not be
observed in a cluster larger than ∼ 3 nm in diameter [40]. Lee et al. [40] reported the
formation of characterization of nanometer size Ohmic contacts to n-type GaAs
substrates. They also determined the specific contact resistance (ρC=1×10-6 Ωcm2)
from I-V data of ultra high vacuum scanning tunneling microscopy (UHV STM) in the
Au cluster/xylyl dithiol/GaAs substrates. This nanometer size is required to further
research.
- 19 -
Chapter 3 Metal-Semiconductor junctions
Sang-Kwon Lee
3.3 Schottky diode performance
Schottky diodes are of interest for high-power devices because they are majority
carrier devices and consequently have very fast switching times and no reverse
recovery current. Here, some important properties of Schottky barrier diodes for power
rectifiers will be described.
3.3.1 Specific on-resistance
For high-power device application, the specific on-resistance should be as low as
possible. It is shown in equation 3-9, where W is the thickness of epilayer and
substrate, VB is the breakdown voltage.
R on −sp
4VB2
W
=
=
qµ n N D e µ n E 3C
 W 
 W 


+ 
= 
 qµ n N D  Epi −layer  qµ n N D  substrate
(3-9)
Using equation 3-9, the specific on-resistance versus the breakdown voltage for
different semiconductors such as Si, 4H-SiC, and GaN are shown in Figure 3-6.The
straight line (theoretical line) for Si, 4H-SiC, and GaN is calculated assuming that the
specific on-resistance of the substrate is less than ≈ 10-7 Ωcm2. The contribution of the
specific on-resistance for the substrate is also plotted in the same figure for the 4H-SiC.
As shown in Figure 3-6, SiC has large advantage for high power application in
comparison to Si. In order to design 1kV devices the specific on-resistance should
theoretically be lower than 10-4 Ωcm2.
2
Specific on-resistance Ron-sp(Ωcm )
4
10
2
10
with contribution of Substrate
0
10
4H-SiC
Si
10
-2
1x10
-4
10
-6
10
-8
10
-3
2
-5
2
Ron-sp=1x10 Ωcm
Ron-sp=1x10 Ωcm
GaN
Theoretical line (without contribution of Substrate)
-10
0
10
1
10
10
2
10
3
4
10
5
10
Breakdown voltage (V)
Figure 3-6. Specific on-resistance Ron-sp versus the breakdown voltage VB for Si,
4H-SiC and GaN.
- 20 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
3.3.2 Forward voltage drop
The forward voltage drop is given by
VF =
ηkT  J F 
ln * 2  + ηφ B + R on −sp J F
q
A T 
(
−5
= 8.61× 10 (T) ⋅ ln 0.685T
−2
)+ φ
B
(3-10)
+ R on −sp J F
where η is the ideality factor, k is the Boltzman's constant, JF is the forward current
density, and A* (≈ 146 Ak-2cm-2)[41] is the Richardson's constant. The forward voltage
drop (VF) is a function of the temperature, Schottky barrier height, and specific onresistance given by equation 3-9. From equation 3-10, if the specific on-resistance is
Forward Voltage Drop (V)
2.0
ΦBn=1.22 eV (for TiW) from Paper I
1.6
Ron ~ 4.3 mΩcm
2
1.2
ΦBn=1.22 eV
0.8
Theoretical line
ΦBn=1.0 eV
0.4
Ron negligible
0.0
0
50
100
150
200
250
300
o
Temperature ( C)
Figure 3-7. Forward voltage drop of a Schottky rectifier as a function of the
temperature and Schottky barrier height. The figure also includes the
experimental results of TiW Schottky diodes to 4H-SiC.
negligible and the logarithmic term is nearly constant and negative, the forward voltage
drop decreases linearly with increasing temperature as shown in Figure 3-7. The
results for a TiW Schottky diode (Paper I) with measured specific on-resistance (in
our case ≈ 4.3 mΩcm2) were plotted in the same Figure 3-7.
3.3.3 Breakdown voltage and reverse leakage current
The breakdown voltage depends on the critical field, epilayer doping and thickness,
and edge termination. The breakdown is given by[30]
- 21 -
Chapter 3 Metal-Semiconductor junctions
Sang-Kwon Lee
ε S E C2
VB =
2qN D
(3-11)
 E C2 

= 2.77 × 10 
N 


where critical electric field with doping dependence can be determined experimentally
for 4H-SiC[31]
2.49 × 10 6
EC =
(3-12)
1  ND 
1 − log 16 
4  10 
5
10
10
7
10
6
10
5
10
4
10
3
Breakdown Voltage (V)
4H-SiC
100µm
4
50µm
10
20µm
5µm
3
10
4H-SiC
2
10
1/8
Si for EC=(4010xND )
1
10
14
10
15
10
16
10
10
17
10
Maximum Electric field (V/cm)
6
18
-3
Background Doping (cm )
Figure 3-8. Breakdown voltage and maximum electric field versus epilayer doping
for 4H-SiC and Si for punch through and non-punch through epilayer thickness.
From Figure 3-8 shows that for a given epilayer thickness, a decrease in epilayer
doping does not necessarily increase the breakdown voltage since the decrease in
doping may correspondingly decrease the critical field. The reverse leakage current is
affected by Schottky barrier height, temperature, and image force barrier height
lowering. First of all, the reverse leakage current density (JL) without the contribution
of the image force lowering can be determined to be
 − qφ B 
J L = − A * T 2 exp
(3-13)

 kT 
Using equation 3-13, a plot of the leakage current of a Schottky rectifier as a function
of the temperature and Schottky barrier height is shown in Figure 3-9. As shown in
Figure 3-9, the leakage current density of the Schottky rectifier increases rapidly with
the temperature.
- 22 -
50
2
Leakage current density (mA/cm )
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
2
2
A* (=146 A/K cm )
40
30
20
ΦBn=0.8 eV
ΦBn=1.0 eV
10
ΦBn=1.2 eV
0
300
350
400
450
500
550
600
Temperature (K)
Figure 3-9. Reverse leakage current versus temperature and Schottky barrier
heights.
3.3.4 Edge termination for high breakdown voltage
For high voltage Schottky diodes to reach the ideal parallel plane breakdown voltage, it
is necessary to have an edge termination around the periphery of the diodes to reduce
the electric field crowding at the diode edge. Several techniques of edge terminations
have been shown to reduce the electric field crowding, resulting in higher breakdown
voltage. In this section, we will describe these techniques. Bhatnagar et al.[42] reported
a study on floating metal rings (FMR, see Figure 3-10) and resistive Schottky barrier
field plate (RESP, Figure 3-10) for 6H-SiC Schottky barrier diodes. For the FMR
termination, simulations indicate a breakdown voltage of 600 V for 3-ring termination
with a ring spacing of 0.8 µm. Experimentally it is determined to be >400 V compared
to a breakdown voltage of 220 V for un-terminated diodes with 10 µm n- epi-layer. For
the RESP termination, a breakdown voltage of 500 V was achieved with a length of 75
µm and a sheet resistance of 1 MΩ/£ for the TiOx. Ueno et al. [43] reported p-epi
guard ring (see Figure 3-11) formed by a local oxidation process (LOCOS). With this
ring-shaped p-n junction guard ring, they achieved a breakdown voltage of 600 V,
which is about 70 % of the ideal value of the p-n junction diodes. Alok and Baliga [44]
investigated an edge termination with a resistive layer created by high dose argon (Ar+)
ion implantation, resulting in close to the ideal parallel plane breakdown voltage (see
Figure 3.11). Other groups [45, 46] also investigated similar edge termination using
boron (B+) implantation (see Figure 3-12) for higher resistive regions at the edge to
improve the reverse breakdown voltage. Itoh et al. [46] reported the employment of B+
implantation (energy of 30 keV and a dose of 1.0 × 1015 cm-2 at room temperature) for
edge termination lead to an increase in the breakdown voltage close to the theoretical
voltage without increasing leakage current. With this structure the measured
breakdown voltage was up to 1750 V. Saxena et al. [47] also reported improved
breakdown voltage with oxide field ring termination.
- 23 -
Chapter 3 Metal-Semiconductor junctions
Floating metal rings
Schottky
Sang-Kwon Lee
Schottky
Al (1.5 µm)
Pt (100 nm)
Ti (5 – 8 nm)
TiOx RESP
N- epi (10 µm), 2×1016 cm-3
N- epi (10 µm), 2×1016 cm-3
N+ SiC (300 µm)
2×1018 cm-3
N+ SiC (300 µm)
2×1018 cm-3
Al-1% Si
Al-1% Si
Figure 3-10. Schematic Schottky diodes with FMR and RESP termination (after
ref. [42]).
Ar+ implantation
Al-Ti
Al (1µm)
p-epi
Ti (0.2 µm)
Schottky
N- epi (5 µm), 2×1016 cm-3
6H-SiC n+
300 µm
N+ SiC, 2×1018 cm-3
Ni
Figure 3-11. Schematic Schottky diodes with p-epi guard ring formed by LOCOS
process (after ref. [43]) and edge termination by Ar+ implantation (after ref.
[44]).
B+
(1.0×1015 cm-2, 30 keV)
Ti
Field oxide
(3000 Å)
Schottky
(Ni or Ti)
Schottky
10 µm
100 µm
4H-, 6H-SiC n+
N+ SiC, 200 µm
Ni
Ni
Figure 3-12. Schematic cross-section of Ti/4H-SiC Schottky diodes with B+ edge
termination (after ref. [46]) and field oxide termination (after ref. [47]).
- 24 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
3.3.5 Schottky barrier lowering
Under reverse bias voltage, there is a reduction of the Schottky barrier height due to
the image force lowering. When an electron approach a metal, the requirement that the
electric field must be perpendicular to the surface enables the electric field to be
calculated as if there is a positive charge of magnitude q located at the mirror-image of
the electron with respect to the surface of the metal. The barrier height reduction due
to the image force lowering is given by[11]
∆φ B =
qE m
,
4πε S
(3-14)
where Em is the maximum electric field given by
Em =
2qN D
(VR + Vbi )
εS
(3-15)
where VR is the reverse bias voltage and Vbi is the built-in voltage for SiC. Finally,
equation 3-13, the leakage current density including the image force barrier height
lowering can be given by
 φ 
 ∆φ 
J L = − A * T 2 exp − B  exp B 
 kT 
 kT 
(3-16)
Figure 3-13 shows the calculated Schottky barrier height reduction due to the image
force lowering effect as a function of the reverse bias voltage up to 10kV.
0.25
at 300 K
0.20
∆φΒ (eV)
0.15
0.10
0.05
16
-3
ND=1.0 x 10 cm for 4H-SiC
0.00
1
10
2
3
10
10
4
10
Reverse bias voltage (V)
Figure 3-13. The calculated Schottky barrier height reduction at room
temperature due to the image force lowering versus reverse bias voltage.
- 25 -
Chapter 3 Metal-Semiconductor junctions
Sang-Kwon Lee
3.4 Other rectifiers
In order to achieve better reverse blocking characteristics while maintaining Schottkylike forward conduction characteristics several modern rectifiers such as JBS, MPS,
DMT, and TMBS have been developed and presented in the literature. The most
common advantages and disadvantages of each device are presented in this section.[48,
49].
3.4.1 Junction barrier Schottky (JBS) diodes
The junction barrier Schottky diode is a device which has the advantage of a low
forward voltage drop while keeping a low leakage current at high blocking voltage. It
is normally a Schottky structure with a normally implanted P+ grid into its drift region
[30]. The schematic structure of a JBS diode is shown in Figure 3-14.
The unipolar current flows through the conductive channels under the Schottky metal
with a voltage drop that is normally determined by the Schottky barrier height like the
Schottky diode in forward mode. In reverse conduction mode the p+n junctions become
reverse biased and the depletion layers spread into the channel and pinch-off the
Schottky barrier. The spacing, width, and thickness of the p+ grid are important design
factors to optimize its performance. The total resistance which is a large contribution
of an increasing voltage drop in JBS is the sum of the resistance in grid (Rgrid), in drift
(Rdrift), and at backside Ohmic contacts (Rcathode) as shown in Figure 3-14. The
contribution of backside Ohmic contacts to the voltage drop might be small and can be
ignored. Because the contact resistance of annealed n-type Ni contacts is in the range
of below 10-5 Ωcm2, the voltage drop is in the range of ≈ 1mV at 100 A/cm2 [50].
Therefore, the total on-resistance (Ron) and the voltage drop (VF) in JBS diode are
given by equations 3-17 and 3-18, where s is the space between p+ grid, w is its width,
x is its depth, tepi is the thickness of the epi-layer, and d is the junction depletion width
from the p+ grid region [51].
Schottky metal (Anode)
P+
P+
P+
P+
P+
Rgrid
N- epi-layer
N+ Substrate
Rdrift
Rcathode
Ohmic contacts (Cathode)
Figure 3-14. Schematic JBS diode structure and its equivalent circuit.
- 26 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
R on

w
w

 x − 2
 t epi − x − 2 
=
 + 
q
N
µ
n
d
 qµ n N d



 drift 
VF =


 s+w
  s + w 
 ⋅ 

 ⋅ ln
  s + 2d   s − 2d 


 grid
ηkT  (s + w ) J F 
ln 
⋅ * 2  + ηφ Bn + R on J F
q
 (s − 2d ) A T 
(3-17)
(3-18)
Recently Dahlquist et al. [52] successfully demonstrated junction barrier Schottky
diodes with a blocking voltage of 2.8 kV and a forward drop of 1.8V at 100 A/cm2 and
low on-resistance of 8 mΩcm2 at room temperature. Asano et al. [53] fabricated hightemperature and high-voltage JBS diodes using 4H-SiC, which have high breakdown
voltage of 3.7 – 3.9 kV and the specific on-resistance of 31.4 – 40.2 mΩcm2. They also
reported a very fast recovery time of 9.7 ns, which is about 10 times faster than a Si
high-speed diodes.
3.4.2 Merged P-i-N / Schottky (MPS) diodes
The MPS [30] is an attractive approach to reducing the switching losses in high voltage
power rectification without increasing the on-state voltage drop. Figure 3-15 shows the
schematic view of the MPS structure. The MPS is a similar approach as JBS rectifier.
However, the operating mode of two rectifiers is different. In the MPS rectifier, the PN junction becomes forward biased in the on-state, unlike the case of the JBS, because
the drift region has a very high resistance due to its design for supporting high voltage
during the reverse blocking. Reverse leakage current and breakdown voltage can be
achieved by employing this MPS even though the performance of the reverse recovery
behavior of the diodes does not reduce.
Ti
Anode
P+
Ni
P+
4H-SiC (epi) n-
N- epi-layer
N+ Substrate
4H-SiC substrate (n+)
Cathode
Figure 3-15. Schematic structure of
MPS rectifier.
Ohmic contact
Figure 3-16. Device structure of Ti/Ni
dual metal-trench rectifier.
- 27 -
Chapter 3 Metal-Semiconductor junctions
Sang-Kwon Lee
3.4.3 DMT (Dual metal trench) diodes
Recently, Schoen et al.[54] reported the DMT diode utilizing metals with two different
barrier heights to achieve similar performance as conventional Schottky diode (see
Figure 3-16). The DMT rectifier has advantages such as a simple structure (selfaligned structure), simple process flow, and does not need an ion implantation process.
The DMT device had Ti deposited to achieve a low barrier height on top of the mesa
followed by conformal deposition of Ni over the entire device to achieve high barrier
height at the bottom of the trenches. The forward and reverse current reported for this
device was between Ti and Ni Schottky barrier diode. In particular, a forward voltage
drop close to that of Ti SBD but with leakage current two order of magnitude lower
than that of Ti SBD and about a factor of two higher than Ni SBD. The DMT diode
can be applied for applications such as lower voltage drop and lower leakage current.
3.4.4 TMBS (Trench MOS Barrier Schottky) diodes
Khemka et al. [55] reported that embedding a UMOS trench like grid instead of a pn
junction grid as in the JBS/MPS rectifier yields a structure known as the TMBS
rectifiers shown in Figure 3-17. The forward and reverse characteristics of a polysilicon planarized Ni-TMBS in 4H-SiC for two different Schottky areas (40% or 57%)
were compared to that of simultaneously fabricated Ni SBD and a pin diode. A forward
voltage drop of 1.75 V at 60 A/cm2 and the reverse leakage current density of 6 × 10-6
A/cm2 were obtained from this TMBS rectifiers.
Schottky diode metal
Oxide
N epi (n-)
N+ Substrate
Cathode
Figure 3-17. Schematic cross-section of a trench MOS barrier Schottky
(TMBS) rectifier.
- 28 -
4. Fabrication Process
S
ilicon carbide processing is similar to conventional Si and GaAs processing even
though it has quite different material properties. The largest difference is that there
are no wet etching methods for SiC at room temperature, and that much higher
temperatures are required to get thermal oxide and to activate implanted dopants in SiC
than those in conventional semiconductors. In this chapter, the most commonly used
processes are described. In addition, a summary of test structures for measuring the
specific contact resistances is included.
4.1 Process description
4.1.1 Wafer preparation and surface cleaning
Wafer Preparation
The starting wafers for the experiments were 4H- and 6H-SiC with Si-face orientation
(0001) from CREE Research Inc.[18]. Standard high doped substrates (1∼2 × 1018 cm-3)
were selected with a 4 µm-thick lowly doped epitaxial layer (1015 ∼ 1016 cm-3) for
Schottky contacts and around 1 µm-thick highly doped epitaxial layers (1019 ∼ 1020 cm3
) grown by chemical vapor deposition for Ohmic contacts. Some wafers from
Linköping university and Acreo AB were also used for the experiment. Normally both
nitrogen (n-type) and aluminum (p-type) are used for the dopants. The wafers we used
were 1 ½ inch and 2 inch in diameter. For experiments, they are cut into segments (1 ×
1 cm2) with a diamond cutting saw.
Table 4-1. Chemicals used for cleaning of SiC wafers and removing SiO2.
Chemical / Mixture
H2O:NH4OH:H2O2 (5:1:1)
H2O:HCl:H2O2 (5:1:1)
H2SO4:H2O2 (2.5:1)
H2O:HF:CH3CH(OH)CH3(100:3:1)
HCl:HNO3 (3:1)
HF:H2O(1:10)
HF:NH4F (1:7)
BHF+NH4OH
Temp./Time
75oC / 5min
75oC / 5min
100oC/5min
25oC/100s
50oC/5min
25oC
25oC
25oC
- 29 -
Comments
RCA SC1
RCA SC2
Seven-up
IMEC
aqua regia
Dilute HF
BHF
pH-modified BHF (pH 12)
Chapter 4 Fabrication process
Sang-Kwon Lee
Surface cleaning
The wafers were first directly degreased in acetone, propanol, and DI water for 2 min
each. Some standard chemical cleaning[3] is listed in Table 4-1. Two different chemical
cleaning recipes, so called "Seven-up" and "IMEC", were used prior to the oxidation
and metal deposition (See Table 4-1).
4.1.2 Etching process
Wet etching
Chemical etching has been a key technology in the fabrication of devices using
conventional silicon as well as silicon carbide material. Unfortunately, wet etching of
single crystal SiC by single acids at room temperature is impossible [56]. It can be wet
etched by molten salt fluxes at high temperature, for instance in solution of NaOH/KOH
at 480 oC [57], hot gases, electrochemical process or plasma etching. As a result, plasma
based etching is today the common way to etch SiC. However, wet etching of both
metals and dielectric materials has been used frequently for many of the device
processes in SiC. Table 4-2 shows various etchants for insulators and metals[22, 58].
Table 4-2. Etchants for insulators and metals.
Material
Etchant Composition
Etch rate
Si3N4
Buffered HF (HF:NH4F = 1:7)
H3PO4
H3PO4:CH3COOH:HNO3:H2O
(4:4:1:1)
NH3:H2O2 (1:5)
H2O2
NH3:H2O2 (1:5)
KI:I2:H2O (4g:1g:40ml)
HCl:HNO3:H2O (4:1:5)
H3PO4:CH3COOH:HNO3:H2O
(4:4:1:1)
HF(5%):H2O(1:2)
H3PO4:CH3COOH:HNO3:H2O
(5:4:2:150)
HNO3:HCl:H2O (1:7:8)
HCl:HNO3:CH3COOH(1:10:10)
5 Å/min
100 Å/min
350 Å/min at 35oC
in ultrasonic bath
600 ∼ 4000 Å/min
45 Å/min
300 Å/min
3500 Å/min
1500 Å/min (at 65oC)
500 Å/min at 35∼ 40oC
in ultrasonic bath
5000 Å/min
5000 Å/min
Al
TiW
TiC
Au
Ni
Ti
Mo
Pt
Pd
400∼500Å/min at 85oC
1000Å/min
Dry etching
Dry etching has been used to etch and pattern SiC since conventional wet chemical wet
etching is not straightforward due to the chemical inertness of SiC and due to the high
bond energies existing between silicon and carbon. Many publications on plasma based
etching of SiC in reactive ion etching (RIE)[5, 59], electron cyclotron resonance
(ECR)[60], and inductively coupled plasma (ICP) [61-64] were reported. Among these ,
ICP is preferable since RIE increases mask erosion at high ion energies and residual
lattice damage in the semiconductor [61] and these plasma chemistries cause micromasking problem when
- 30 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
Gas distribution (SF6, Ar, O2)
Chamber
2 MHz Power supply
Vacuum pump
Power electrode
13.56 MHz rf source
∼
Figure 4-1. Inductively coupled plasma etching equipment.
aluminum is used as an etch mask [59] [21]. However, ICP provides the potential of
achieving excellent anisotropy, low surface damage, smooth morphology, and even high
etch rates for SiC[65]. ICP etching was used on 4H-SiC using a fluorine-based
chemistry (SF6/Ar/O2) throughout the thesis. ICP etching is a high-density plasma
technique where the plasma is formed (see Figure 4-1) in a dielectric vessel encircled by
an inductive coil into which rf power is applied. Anisotropic profiles are obtained by
using low-pressure conditions to minimize ion scattering and lateral etching. Gas
switching between low O2 or Ar and high O2 percentage was tested to improve
selectivity to the Al mask. Generally in order to define the mesa structure for the TLM
(transmission line method) ICP was performed in a mixture of SF6 (21 sccm) and Ar (9
sccm) at 600 W rf forward power and 5.0 mTorr base pressure with 0.25 µm thick ebeam evaporated Al mask. The etch rate was around 0.16 µm/min with 30W of platen
power. We also reported that the performance of Schottky diodes (Ti/n-4H-SiC) have
been shown to change significantly due to ICP etch induced damage and we proved that
the etch damage can remove by high temperature passivation [64] since dry etching
damage is created in the near-surface region of the semiconductor by the bombardment
of energetic ions and it can have significant impact on the electrical performance of
fabricated devices. In paper VI, we proved that low power (30W) ICP etching process
did not affect the formation of Ohmic contacts and we did not observe any difference
between the un-etched and the 30W etched sample from TLM measurement, having the
same value of the ρC when medium platen power (60W) ICP etching showed significant
influence on the Ohmic contact formation. The trenching effect is known to occur for
most dry etching conditions [66], caused by the deflection of ions on the sidewall
inducing enhanced ion bombardment at the bottom. Our current results show that
junction field-effect transistors (JFET) fabricated with Ni metal mask show a trenching
profile (> 0.2 µm, see Figure 4-2) after dry etch, where 30W platen power, 600W coil
power, a mixture of SF6 and Ar using ICP were used, in the channel groove region and
also showed static induction transistor (SIT)-like characteristics in the sub-threshold
region of current-voltage curves [67].
- 31 -
Chapter 4 Fabrication process
Sang-Kwon Lee
(a)
(b)
θSiO2
θ
S = tan θ / tan θSiO2
Figure 4-2. Scanning electron microscopy image of (a) the trench on the bottom of
the sidewall and (b) trenching corner after using angled oxide mask which was wetetched by BHF (after ref. [67]).
4.1.3 Deposition Techniques
Several methods exist for metal deposition in device fabrication. In this section,
techniques we used throughout the thesis, such as sputtering and co-evaporation
including e-beam evaporation are described. In this thesis, most pure metals, such as
Au, Ti, Ni, Al etc. were deposited by e-beam evaporation shown in Figure 4-3 (a).
Alloy metal (TiW) was sputtered by DC magnetron sputtering system in our cleanroom.
It is known that sputtering can be performed for deposition of the compound and alloy
metals. We also used co-evaporation for the deposition of titanium carbide (TiC). TiC is
a promising material for low resistivity n- and p-type metallization on highly doped
silicon carbide using a co-evaporation method as shown in Paper II and III. This coevaporation set-up had a limited deposition area, normally we used 1×1cm2, slow
deposition rate, the substrate temperature, and lack of reproducibility.
+
-1kV
CHAMBER
Heater
Substrate
Substrate
Load lock system
e-
Chamber
B
Gas distribution
Source
10kV
Power supply
Target
Power supply
Vacuum
pumps
Vacuum
pumps
Plasma
(a)
(b)
Figure 4-3. (a) Typical e-beam source evaporation system and (b) cross-sectional view
of the DC sputtering system.
- 32 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
Sputtering
Sputtering is a physical vapor deposition (PVD) process involving the removal of
material from a solid cathode. This is accomplished by bombarding the cathode with
positive ions emitted from a rare gas discharge[58]. When ions with high kinetic energy
are incident on the cathode, the subsequent collisions knock loose, or sputter, atoms
from the material, which could not be deposited using the thermal evaporation
techniques of that time. A typical sputter system[22][69] is shown in Figure 4-3 (b).
The deposition of titanium tungsten (TiW, weight ratio 30:70) was performed in a DC
magnetron sputter under deposition condition of 3kW power, 120 ∼ 200 oC substrate
temperature, 50 ∼ 69 sccm of Ar gas flow, 5 × 10-7 Torr base pressure, and 5 × 10-3 Torr
deposition pressure. TiW has been used extensively as a diffusion barrier for Al
metallization in conventional Si process technology and fuse structures in
programmable log device. The addition of 5- 30 weight % titanium provides improved
adhesion between metals and semiconductor, corrosion, barrier, and contact resistance
properties. TiW has low resistivity, can be wet etched, and is very inert with respect to
reaction with SiC under high temperature due to its high melting point (1660 oC for Ti
and 3410 oC for W). The deposition rate of the TiW metal, the thickness, and the metal
resistivity were 500 ∼ 600 Å/min, 1500 ∼ 2500 Å, and 85 – 88 µΩcm. The sputtered
TiW can easily be wet-etched using a mixture of 25% NH3 : H2O2(1:5) with an etch rate
of 600 ∼ 4000 Å/min depending on the sample pre-treatment (Paper I, V, VI, VII). So
far we have considered two methods of metal deposition. These are the more widely
used metallization techniques. To conclude this section, we highlight the advantages
that the sputtering method offers. Because sputter deposition can be used to deposit
refractory materials, which are useful metals for long-term reliability, since such
materials are often difficult to evaporate, and hence sputtering may be the practical way
for their deposition on semiconductors.
Co-evaporation
For the growth of titanium carbide (Paper II and III) contacts, a co-evaporation
technique in a UHV chamber comprising an electron beam evaporator for Ti and a
Knudsen effusion cell for C60 was used for both Ohmic and Schottky contact studies.
The UHV system for TiC deposition consists of one deposition chamber connected to
two different analysis chambers via an in vacuo transfer system[69]. The commercial
metal evaporator works by directing an electron beam towards the tip of the metal rod
(5-6 mm in diameter) to be evaporated shown in Figure 4-4. The metal flux can be
regulated by control by the electron current (50 ~ 100mA) with a fixed bias voltage
(700V). The C60 from a Knudsen cell shown in Figure 4-4 was used as a carbon source
for making a TiC films. The carbon fluxes were controlled by tuning of the cell
temperature from 440 oC to 500 oC. In this thesis (Paper II, III) cell temperature
ranging from 450 oC to 460 oC were applied. The deposition rate was around 35Å/min
for TiC films. The substrate temperature for TiC and Ti deposition was 500 oC and 300
o
C, respectively. As mentioned in beginning of this section, the co-evaporation method
is still under development with a lack of applicability even though it was best deposition
method for TiC metals compared to the other methods.
PECVD & LPCVD
Low temperature (300 oC) plasma enhanced chemical vapor deposition (PECVD) was
used to deposit a 1400 Å Silicon nitride layer (Si3N4) on 4H-SiC. The Si3N4 was used as
- 33 -
Chapter 4 Fabrication process
Sang-Kwon Lee
5V/10A
Knudsen cell
440 oC – 500 oC
700 V/50-100 mA
C60
sample
e-beam
Ti metal rod
heater
Deposition chamber (UHV)
Vacuum pump
Figure 4-4. Schematic view of the co-evaporation system for TiC deposition.
a sacrificial layer prior to the ion implantation (See Paper III). Si3N4 layers can also be
deposited by a low pressure chemical vapor deposition (LPCVD) process with
intermediate temperature (750 oC). The difference between LPCVD and PECVD was
that the LPCVD films are of stoichiometric composition with high density (2.9 to 3.1
g/cm3) whereas the films deposited by PECVD are not stoichiometric and have a lower
density (2.4 to 2.8 g/cm3). In the PECVD process, silicon nitride is formed either by
reacting silane and ammonia in an argon plasma or by reacting silane in a nitrogen
discharge. The reactions are as follows :
SiH4 + NH3
2SiH4 + N2
SiNH + 3H2
2SiNH + 3H2
300 oC
(4-1)
(4-2)
Oxides also can be deposited by both PECVD and LPCVD at higher temperature. The
quality of these PECVD or LPCVD oxide can be improved by following up with high
temperature annealing (> 1000 oC for 1 hour)[3]. LPCVD deposited tetra ethyl
orthosilicate (TEOS) was also used as a dry etch mask for defining the mesa structures.
The deposition conditions for silicon nitride using PECVD are shown in Table 4-3.
Table 4-3. The deposition condition for Si3N4 using PECVD.
Gas Flow
SiH4(5%) + He (58 sccm), NH3 (1.4 sccm), N2 (268 sccm),
Ar (500 sccm)
Parameters
Pressure (600 mTorr), rf power (15W),
Temperature (300oC), Deposition rate (60Å/min)
- 34 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
4.1.4 Ion implantation
Ion implantation is the introduction of energetic, charged particles into a substrate. The
practical use of ion implantation in semiconductor technology has been mainly to
change the electrical properties of the substrate. Typical ion energies are usually
between 30 and 300 keV, and ion doses vary from 1011 to 1016 ions/cm2. Figure 4-5
shows a typical ion implantation system [22]. Implantation of dopants into SiC surface
has been recognized as crucial means of selective area doping since thermal diffusion is
not feasible due to slow diffusion rates of most dopants in SiC. Normally N (Nitrogen)
and Al (Aluminum) are used frequently for doping in SiC due to their low activation
energy in SiC (∼ 80 meV for N and 240 meV for Al in 6H-SiC) compared to other
impurities of the same type [70]. For n-type implantation, it has been demonstrated that
nitrogen gives sufficiently good n-type doping with low sheet resistance (less than
1kΩ/£) as well as reasonable n+-p diodes characteristics [71, 72]. On the contrary,
attempts at obtaining good p-type SiC by implantation, using either Al or B ions, have
encountered difficulties such as high sheet resistance and low activation efficiency. A
high temperature anneal is usually necessary to reduce the damage caused by highenergy ions during implantation [70, 73]. Normally post ion implantation annealing is
performed at temperatures around 1500 oC to 1800 oC for 10 ∼ 30 min, in a CVD
furnace with Ar ambient and with a small introduction of silane to avoid pitting of the
surface. Ion implantation can be used for making highly doped Ohmic contact regions.
Recently, Zhao et al.[74] showed the possibilities of achieving a low specific contact
resistance of 10-5 Ωcm2 for Al contacts on C-Al co-implanted 6H-SiC. Ion implantation
is an important technology for device applications and quite promising solutions with
many advantages for formation of low resistivity Ohmic contacts to SiC exist. In order
to determine the ion implantation condition TRIM[75] simulation was used. Figure
4-5 shows typical depth profile from TRIM simulation using a Si3N4 layer on the silicon
carbide surface with energy of 180keV and different doses, where the measured depth
profile of Al in SiC using SIMS is also included.
21
10
3
Concentration (atoms / cm )
14
2
Dose (3 x 10 ions / cm )
Energy (180 keV)
o
Annealed at 1700 C, 30min
20
10
19
10
18
10
14
SIMS profile (3 x 10 )
14
180 keV (3 x 10 )
15
180 keV (1 x 10 )
17
10
16
10
15
10
14
10
-0.1 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Depth (µm)
Figure 4-5. The depth profile generated by TRIM simulation and SIMS
using Si3N4 sacrificial layers for different doses with energy of 180 keV.
- 35 -
Chapter 4 Fabrication process
Sang-Kwon Lee
4.1.5 Annealing
In order to make the deposited metal contact Ohmic it is not enough to deposit them on
highly doped SiC epilayer because most of metals have a high Schottky barrier height.
Therefore, the best suggested way is to anneal and sinter the metal contacts at high
temperature in inert gas ambient. The required temperature to make the contacts Ohmic
for most of the metals was at least > 900 oC. In this thesis, both Ohmic and Schottky
contacts were annealed at 500 ∼ 980 oC in Ar or in 10% H2/Ar using rapid thermal
annealing (RTA). Some of the samples were annealed at the same temperature in a lowpressure vacuum chamber. Long-term reliability tests (Paper V and VII) were also
performed at 500 ∼600 oC in a vacuum chamber.
- 36 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
4.2 Test structures for Ohmic contacts
For an Ohmic contact, the parameter of interest is not the barrier height, but rather the
resistance of the linear pat of the current-voltage characteristics. This parameter is
important for metal-semiconductor metallization as well as scaling-down effects of
MOS transistors in modern VLSI circuits. For example, in the limit case, known as the
electrically long contact, the contact resistance RC ≈ρC/LTW increases as K (scaling
factor), when the lithographic dimensions of the integrated circuit are scaled down by a
factor of K[76]. This means that the contact resistance or specific contact resistance also
becomes an essential parameter in the design of modern integrated circuits as seen in
conventional Si world. Ohmic metal-semiconductor contacts are defined as a metalsemiconductor interface whose voltage drop is small, ideally zero, compared to the
active region of the devices. The parameter which characterizes an Ohmic contact is its
specific contact resistance or contact resistivity, ρc, and is commonly used to compare
the quality of each Ohmic contact, usually in unit of Ωcm2. In this section, we will
illustrate various measurement techniques and corresponding test structures.
4.2.1 Kuphal structure
A four-in-line circular structure was suggested by Kuphal [77]. It requires only one
mask process to fabricate and is the simplest test structure to measure the specific
contact resistance. Beyond these advantages, these simple test structures tend to
overestimate the specific contact resistance compared to the other test structures. A
schematic view of the test structures is shown in Figure 4-6. The total resistance
between two contacts consists of the contact resistance (2Rc), the spreading resistance
(2Rsp), and the sheet resistance of the epitaxial layer of the semiconductor. As shown in
Figure 4-6, a current Iad is applied between contact a and d, and the voltages Vab and Vbc
are measured, finally the specific contact resistance can be calculated by
I
b
a
c
d
Rc
Rc
Rsp
Rs
s
s
Rsp
epi-layer
s
Figure 4-6. Schematic view of Kuphal test structure (after ref. [77]).
- 37 -
Chapter 4 Fabrication process
Sang-Kwon Lee

 3s 1  
ln −  

A
d 2

Vab − R sp − Vbc 
ρc =
I ad 
2 ln 2 




(4-3)
where s is the spacing between the contacts. Rsp can be neglected if the ratio between
the contact area and the epilayer thickness is small.
4.2.2 Two-terminal contact resistance methods
This method is the simplest, earliest, and also with questionable accuracy if not properly
executed [78]. These subdivide into two-element structure and multi-element structures
that are widely known as contact strings or contact chains. The contact string
technologies are considered to be a coarse measurement method that is not very useful
for detailed evaluations of contact resistance. It is used, however, as a process monitor.
Figure 4-7 shows 2-terminal test structure (a) and contact string (b).
(a)
(b)
Figure 4-7. A lateral two-terminal contact resistance structure (a) in cross section
and top view and (b) contact string (after ref. [84]).
To confine the current flow, the region on which the contact is located must be isolated
from the remainder of the substrate. This is done by either confining the implanted or
diffused region by planar techniques or by etching the region surrounding the island,
leaving in as a mesa. The contact resistance RC is
ρd


RT − S + Rd + RW 
W

RC = 
2
(4-4)
where RT is the total resistance, ρS is the sheet resistance of the n-layer, Rd is the
resistance due to current crowding under the current, and RW is a contact width
correction if Z<W.
- 38 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
4.2.3 3-contacts, two-terminal methods
From Figure 4-8 (a), the total resistance and contact resistance are given by
ρSd1
+ 2R C
W
ρ d
= S 2 + 2R C
W
R T1 =
R T2
finally,
(R d − R T1d 2 )
R C = T2 1
2(d1 − d 2 )
(4-5)
(a)
(b)
Figure 4-8. (a) Test structure and (b) the equivalent circuit of the metalsemiconductor with the current choosing the path of least resistance (after
ref. [84]).
As shown in equation 4-5 this structure does not have the ambiguities of the simpler
two-terminal structure, because neither the bulk resistance nor the layer sheet resistance
need be known. This structure only allows the contact resistance to be determined. The
specific contact resistance, which is quite useful and practical parameter, cannot be
directly extracted from the two resistance measurements. Murrmann and Widmann [79]
used a simple transmission line model (TLM) considering both the semiconductor sheet
resistance and the contact resistance to take current crowding into account and to be
able to extract the specific contact resistance ρC. Berger [80] extended this method.
When current flows from the semiconductor to metal, it sees the resistance ρC and ρs,
shown in Figure 4-8 (b). The potential distribution under the contact is determined by
both ρC and ρs according to [80]
V( x ) =
I ρ s ρ C cosh[(L − x ) / L T ]
 L
Z sinh 
 LT



(4-6)
where L is the contact length, Z is the contact width, and I is the current flowing into the
contact. It is obvious that the voltage is the highest near the contact edge x=0 and drops
nearly exponentially with distance x as shown in Figure 4-9 (a). The 1/e distance of the
voltage curve is defined as the transfer length LT (= ρ C ρ S ). Figure 4-9 (b) also
shows transfer length as a function of the specific contact resistance. The transfer
length is on the order of 1µm or less for such contacts. Using equation 4-6, the contact
resistance can be written by
- 39 -
Chapter 4 Fabrication process
RC =
Sang-Kwon Lee
ρSρ C
 L
 L  ρC
V
 =
=
coth
coth
I
Z
 LT
 LT  LT Z



(4-7)
There are two limiting cases to simplifications of equation 4-7, that is, for L≤ 0.5LT,
coth(L/LT) ≈ LT/L and simply, RC ≈ ρC/LZ. For L ≥ 1.5LT. coth(L/LT) ≈ 1 and finally,
-2
1.0
10
0.8
ρs=10 Ω/square
-3
10
-4
1000
300
LT(cm)
0.6
V(x)
100
2
ρC=10 Ωcm
-4
1x10
0.4
-5
2
10 Ωcm
-5
1x10
0.2
-7
2
10 Ωcm
-6
2
10 Ωcm
0.0
-6
0
2
4
6
8
10
X (µm)
10
-8
10
-7
10
-6
10
-5
-4
1x10 1x10
-3
10
-2
10
2
ρC(Ωcm )
(a)
(b)
Figure 4-9. (a) Normalized potential under a contact versus x as a function of ρC
(where L & Z =100µm, ρs = 100 Ω/£ using equation 4-6) and (b) shows the transfer
length versus specific contact resistance and sheet resistance.
RC =
ρC
LT Z
(4-8)
4.2.4 Linear transmission line method (LTLM)
The transmission line method (TLM) was originally proposed by Shockley [81]. The
TLM structure is very much like 3-contacts (two-terminal methods) shown in Figure 43, but consists of more than three contacts. In this thesis, only this linear TLM method
was used to extract the specific contact resistance (Paper II, III, V, VI, and VII). From
equation 4-5 through 4-8 for contacts with L ≥ 1.5 LT the total resistance between any
two contacts can be given by
RT =
ρS d
ρ
+ 2R C ≈ S (d + 2L T )
Z
Z
(4-9)
The total resistance is plotted as a function of spacing d as shown in Figure 4-10 (a).
From Figure 4-10 three parameters including sheet resistance (ρS) under the contacts
- 40 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
(from the slope), contact resistance (RC), and the transfer length (LT) (from the intercept)
can be extracted. Finally, from equation 4-8 the specific contact resistance (ρC) is
determined. The TLM method has its own problems even if it is used a great deal in the
practical study. The x-axis from Figure 4-10 (at RT=0) giving LT is sometimes not very
distinct, leading to incorrect ρC values. This problem is due to the uncertainty of the
sheet resistance under the contacts. The sheet resistance from TLM methods includes
the sheet resistance both under the contacts and between contacts. Reeves and Harrison
[82] suggested that the sheet resistance under the contacts might differ from that of
between contacts due to the alloying effects of contact formation. They modified
equations 4-6 and 4-7, that is

 and

ρ
ρd
ρ 
R T = S + 2R C ≈ C d + 2 SC
Z
Z 
 ρS
R Cf =
 L
ρC
coth
L Tc Z
 L Tc
 
L Tc 
 
(4-10)
where ρSC is the sheet resistance under the contact and LTc=(ρC /ρSC )1/2. The slope still
gives the sheet resistance (ρS/Z) and the intercept at d=0 gives 2RC. However, the
intercept at RT=0 now gives 2LTc(ρSC /ρS) instead of 2LT in the simple TLM method and
the parameter ρC can not be extracted from this method because the parameter ρSC is
RT
(a)
2R c
slope = Rs/Z
0
d
2L T
(b)
5 µm
10 µm
15 µm
20 µm
25 µm
I (current)
Metal
n+ or p+ epilayer
Substrate (p+ or n+ 4H- or 6H-SiC)
Figure 4-10. (a) A plot of total resistance versus spacing d (from 5 µm to 25 µm) and
(b) schematic view of TLM structure.
- 41 -
Chapter 4 Fabrication process
Sang-Kwon Lee
still unknown. Therefore, they combined the TLM method (determining Rc) and the end
resistance method (determining Rce shown in equation 4-11). From equations 4-10, 411, and 4-12, the parameters, LTc and ρC, are extracted. This modified TLM method can
determine the contact resistance and the specific contact resistance in addition to the
sheet resistance between and under the contacts as well.
ρSC ρ C
R ce =
 L 

Z sinh 
L
TC


R ce
1
=
Rc
 L 

cosh
 L TC 
=
ρC
 L
ZL TC sinh 
 L TC



(4-11)
(4-12)
In our case, we did not see any large difference between LTLM and modified methods
because there is not too much alloying effect (reaction between SiC and metals).
Therefore, we have used LTLM for our Ohmic contact measurements through this
thesis.
4.2.5 Circular transmission line method (CTLM)
The circular transmission line method (CTLM) was proposed by Reeves [83]. It
eliminates the necessity of mesa isolation of the contact pattern, thus simplifying test
structure fabrication. It can also avoid the problem of linear TLM, inconsistency of the
width of pads and mesa structures due to the etching process. Circular contact pattern
r2
r1
r0
C1
C2
Figure 4-11. Schematic view of the circular
patterns
for
specific
contact
resistance
measurement (after ref. [83]).
C0
and cross-sectional view of circular TLM are shown in Figure 4-11 and 4-12,
respectively. Since the contacts are circularly symmetrical, the pattern of current flow
between two contacts should be circularly symmetrical when the contacts are equipotential surfaces. The resistance between the inner two contacts and the outer two
contacts, are called R1 and R2, respectively. From the Figure 4-11, the resistances
measured between the inner two contacts, R1 and between outer two contacts, R2 are
given by
(
R 1 = R A + R C 0 + R 'C1
)
R 2 = R B + (R C1 + R C 2 )
- 42 -
(4-13)
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
Using two measurements between the inner and outer pads and the circular transmission
line model, the specific contact resistance (contact resistivity, ρc) can be determined by
the directly following equations.

 r'
ρ c = log e  2
 r1


 r' 

 ⋅ R 1 − log e  1  ⋅ R 2  ⋅ (r0 )2 ⋅ ∆

 r0 

(4-14)
2π
(αr ) ⋅ φ
∆=
 A (r , r ) ⋅ B(r , r )

+ D(r , r )

C(r , r )


2
0
1
'
1
1
1
and
α=
'
1
1
'
1
(4-15)
'
1
R SK
ρc
(4-16)
where RSK is the series resistance element under the contacts.
x=0
r0
RC0
R'C1
r2
r1
RC1
RA
RC2
RB
Figure 4-12. Cross-section view of the circular transmission line
method (after ref. [83]).
4.2.6 Four-terminal contact resistance method, Kelvin structures
As discussed in previous section of TLM, the contact resistance RC is often a small
fraction of the total measured resistance RT. This problem can be solved by using the
test structure as illustrated in Figure 4-13. The four-terminal contact resistance method
is also known as the Kelvin test structure or the cross-bridge Kelvin structure [84]. In
principle, this method also allows the specific contact resistance to be measured without
being affected by the underlying semiconductor or the contacting metal parasitic
contribution. Figure 4-13 (a) shows the equivalent circuit of the Kelvin structure and
(b) the principle of the Kelvin structure method, respectively. Since the internal
resistance of the voltmeter is much larger than the resistance of the device under test, RX,
or the connecting wire, the current I and the voltage drop in the resistor R1 and R2 can be
- 43 -
Chapter 4 Fabrication process
Sang-Kwon Lee
neglected. Hence the voltage V34 (=V3-V4) measured by the voltmeter coincides with
the voltage drop across the resistance RX=V34/I. RX can present to RC given by
RC =
V34
,
I
ρC = R C ⋅ A C
(4-17)
R1
V34
Rx
I
R2
(a)
(b)
Figure 4-13. (a) equivalent circuit of Kelvin resistance test structure and (b) schematic
view of four-terminal contact resistance test structure (after ref. [84]).
4.2.7 Six-terminal contact resistance method
In this case two more contacts provide additional measurement options and additional
information which is not available with the conventional 4-terminal Kelvin structure.
This method is shown in Figure 4-14. For the conventional Kelvin structure contact
resistance measurement, the current is forced between contacts 1 and 3 as shown in
Figure 4-14, and the voltage is measured between contacts 2 and 4. (RC=V24/I and
6
1
Contact
Metal
Diffusion
5
2
3
4
Figure 4-14. Six-terminal Kelvin test structure (after ref. [84]).
- 44 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
ρC=RCAC). This analysis for conventional Kelvin structure is one-dimensional case.
However, modified six-terminal Kelvin structure as shown in Figure 4-14 can provide
the contact resistance, the specific contact resistance, the contact end resistance, the
contact front resistance, and the sheet resistance under the contact to be determined. In
order to measure the contact end resistance (Rce=V54/I) the current is forced between
contacts 1 and 3, and the voltage is sensed across contacts 4 and 5. The sheet resistance
of the epilayer under the contact can be determined from the end resistance using
equation 4-11. Then the front resistance can be calculated using equation 4-10.
4.2.8 Comparison of each measurement technique
In this thesis, only the linear TLM (LTLM) method was used to extract the specific
contact resistance (Paper II, III, V, VI, and VII). Table 4-4 shows a comparison
among the measurement techniques. We can measure the specific contact resistance of
>10-6 Ωcm2, which is low enough to apply to the high-power device applications
(≈voltage drop of 0.1 mV at 100 A/cm2). But the Kuphal structure can be useful, it can
extract the specific resistance of >10-4 Ωcm2 and is easy to fabricate (1 mask step) even
though it overestimate the specific contact resistance.
Table 4-4. A comparison of each measurement technique.
Kuphal method
TLM
Kelvin method
Indirect method
Simple structure
1 mask
(Contact)
Measurement limit
≈ 10-4 Ωcm2
(overestimates ρC)
Indirect method
Simple structure
2 mask steps
(Contact, Mesa)
Measurement limit
≈ 10-6 Ωcm2
Direct method
Complex
3 mask steps
(Contact, Metal, Diffusion)
Measurement limit
≈ 10-8 Ωcm2
- 45 -
5. Characterization and results
B
oth Schottky and Ohmic contacts should be evaluated by material and electrical
characterizations when they have been manufactured. Blanket pieces (samples)
are prepared simultaneously for additional material characterization. Various material
techniques such as XRD, RBS, LEED, SIMS, AFM, and TEM are introduced to
understand the contact properties. For material characterization, we mostly stress the
solid-state reaction on Ti/, Ni/, TiC/, and TiW/4H-SiC. We will try to answer the
question : what kinds of phases are created at the interface and what kind of phase
makes Ohmic and low resistivity contacts. These are also related to the answers for the
electrical characterization for both Schottky and Ohmic contacts to silicon carbide. The
main results for Schottky diodes, specific contact resistance, and microscopic mapping
of specific contact resistance are included with short introduction of the measurement
techniques. Finally, the results and discussion for the long-term reliability tests at high
temperature and different ambients (in vacuum and oxidizing ambient) are described at
the end of chapter.
5.1 Material characterization
The samples, as-deposited and annealed at high-temperature, were characterized by 2.4
MeV 4He+ Rutherford backscattering spectrometry (RBS) for depth profiles of
composition and solid-state reaction after high-temperature annealing, X-ray diffraction
(XRD) for phase identification, and atomic force microscopy (AFM) for the surface
morphology. The ion implanted Al depth profiles were also analyzed by secondary ion
mass spectrometry (SIMS). For both good and reproducible Schottky and Ohmic
contacts, it is essential to study the solid-state reaction between the metal and SiC.
There are materials that do not react (Ag, Au etc.), materials that form the silicide but
Si
Si
(a)
T=1000 oC
o
T=850C
TiSi2
NiSi2
NiSi
Ni2Si
Ni5Si2
Ni3Si
Ni
TiSi
Ti5Si4
SiC
Ti5Si3
Ti3Si
SiC
T2
T1
Ti
C
C
TiCx
Figure 5-1. Ternary phase diagrams of Ni/SiC (a) and Ti/SiC (b), where T1 and T2
denote Ti3SiC2 and Ti0.6Si0.34C0.05, respectively.
- 47 -
Chapter 5 Characterization and results
Sang-Kwon Lee
not the carbide (Co, Ni, Pd, Pt etc.), materials that form both silicide and carbide (Cr, Fe,
Mn, W etc.), and finally materials that forms silicides, carbides, and a ternary phase
(Mo, Ta, Ti, Zr etc.)[85]. In this section, we will focus on the phase formation, related
to the specific contact resistance and Schottky barrier height. The ternary phase
diagrams of the two different systems (Ni/SiC and Ti/SiC) are shown in Figure 5-1 (a)
and (b)[85]. It can be known that in the case of Ni reaction on SiC, the only stable
silicide phase is the Ni2Si (See sub-section below). The carbon present in the consumed
silicon carbide layer should precipitate. The Ti/SiC system has a ternary phase (Ti-Si-C),
carbide phase (TiC), and silicide phases (Ti2Si, TiSi2 etc.) after high-temperature
annealing.
5.1.1 X-ray Diffraction (XRD)
XRD measurements were performed in the θ-2θ geometry using a Siemens D 5000
powder diffractometer (CuKα1 radiation) to identify the phases formed on the SiC
substrate before and after high-temperature annealing at > 950oC. For the measurements,
the samples were tilted 3.5 ∼ 8-degree off-axis to find the exact phase of metals on SiC
(0001) and finally the oblique tilt (± 0.5 degree) to reduce the single crystal SiC
substrate signal.
TiC(111)
o
3
5
Ti (002)
SiC (004)
5
3
5
3
Intensity (A.U)
Ti Si (030)
Ti Si (121) + TiC (002)
T i/S iC ( 9 5 0 C )
Ti Si (012)
5
3
Ti Si (002)
Titanium (Ti)
The x-ray spectrum of a Ti film deposited on 4H-SiC for as-deposited and annealed at
950oC are shown in Figure 5-2 (Paper II, IV,VIII). The spectra for as-deposited Ti/SiC
indicate that Ti films are highly textured on silicon carbide with {001}-type reflection
from Figure 5-2. After high-temperature annealing at 950 oC, the phase at the interface
was changed into two different phases (see also Figure 5-2), such as Ti5Si3 and TiC,
which is in good agreement with previous results from Porter et al. [86]. These two
phases (titanium silicide and titanium carbide) have a much lower resistivity than that of
Ti only.
A s - d e p o s ite d
30
35
40
45
50
2Θ (deg)
Figure 5-2. x-ray spectra of Ti/SiC(4H-SiC) for as-deposited and
annealed at 950 oC.
- 48 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
Nickel (Ni)
For Ni, the reaction at the interface at high-temperature annealing only proceeds to the
formation of the Ni2Si phase (see Figure 5-3) as seen from our X-ray diffraction
measurements. For as-deposited Ni, the x-ray spectra show that it is highly oriented
with the (111) orientation on silicon carbide (Paper IV and VII).
50
Ni (111) Ni2Si
30
20
Ni2Si
Intensity (A.U)
40
Ni2Si
Ni/SiC annealed
Ni/SiC as-dep.
10
0
30
35
40
45
50
2Θ(Deg)
Figure 5-3. X-ray spectra of Ni metal deposited on 4H-silicon carbide (0001)
for as deposited and after anneal at 950 oC.
Au (111)
Intensity (A.U)
SiC (004)
200
100
30
35
40
45
2Θ(Deg)
Figure 5-4. X-ray diffraction spectra of as-deposited Au on 4H-SiC.
- 49 -
Chapter 5 Characterization and results
Sang-Kwon Lee
Gold (Au) and Ti/Al/TiW
The XRD spectra show that Au metals are textured on silicon carbide with {111}-type
reflection (see Figure 5-4) . Figure 5-5 shows the x-ray spectra for as-deposited and 950
o
C annealed multiple-stack contacts (Ti/Al/TiW). It indicates that as-deposited films
have strong Ti and Al peaks and the new compound phases (aluminum titanium, such as
AlTi or AlTi3) are created after annealing at 950 oC (Paper IV and VII).
Al (122) (003)
Al (112)
20
10
0
30
40
50
60
70
Al (013)
As-dep
Annealed
Al (111)
Intensity (A.U)
30
SiC (004)
Ti (002)
40
80
2Θ(Deg)
Figure 5-5. X-ray diffraction spectra for as-deposited Ti/Al/TiW contacts,
indicating no strong peak for tungsten (W).
Titanium carbide (TiC)
X-ray diffraction spectra of typical TiC films are shown in Figure 5-6. The
diffractogram shows only reflections of the {111}-type from the films suggesting a
Figure 5-6. X-ray diffractogram of a 900Å thick blanket TiC0.7 film
deposited on 4H-SiC with clear hexagonal LEED pattern.
- 50 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
highly textured or an epitaxial growth with the relationship TiC(111)//4H-SiC(0001).
This was also confirmed by low-energy electron diffraction (LEED) that showed a clear
hexagonal pattern indicating epitaxial growth of the film (see inset of Figure 5-6). TiC
can be formed with two stacking sequences, ABCABC and ACBACB, which are
equivalent to an 180o rotation about the [111] direction in the TiC film. These domains
will give rise to identical and overlapping LEED patterns (Paper II, III, V).
Intensity (A.U)
TiW(@RT)
o
TiW(@120 C)
Ti (002)
1500
SIC(004)
2000
1000
500
0
30
35
40
45
50
2θ
Figure 5-7. (a) X-ray diffraction spectra of a 1000 Å-thick blanket sample of asdeposited TiW on 4H-silicon carbide.
SIC(004)
100
7.7 deg.
7.9 deg.
8.1 deg.
(Ti,W)C1-x (002)
Intensity (A.U)
150
(Ti,W)Si2 (111). (003)
200
50
30
35
40
45
50
2θ
Figure 5-7. (b) X-ray diffraction spectra of a 1000 Å thick blanket sample of 950
o
C annealed TiW on 4H-silicon carbide.
- 51 -
Chapter 5 Characterization and results
Sang-Kwon Lee
Titanium tungsten (TiW)
Ti30W70 was deposited by sputtering from a compound TiW target (nominally 30:70
weight percent ratio). The x-ray diffraction spectra of as-deposited TiW films with
different substrate temperatures are shown in Figure 5-7 (a). X-ray results show that the
Ti-W alloys of all compositions are β-Ti rich with a weak bcc solid solution of W,
indicating that the substrate heating transforms the β-Ti phase to the α-Ti phase.
Babcock et al.[87] observed similar behavior with Ti8W2 after anneal at 500oC using
XRD measurements. Figure 5-7 (b) shows the XRD spectra for the TiW film after
annealing at 950oC. This result indicates that polycrystalline (Ti,W)Si2 and (Ti,W)C1-x
phases form due to high temperature annealing. We suggest that these silicide and
carbide phases with (Ti,W) make the TiW films exhibit low resistivity at the interface ,
which is finally changed to Ohmic behavior from the previously rectifying with high
resistance (Paper I, V, Vi, VII).
5.1.2 Secondary Ion Mass Spectrometry (SIMS)
SIMS is an analytical technique based on the fact that under particle bombardment of a
target, atoms and molecules are ejected[88]. SIMS data can be recorded as mass spectra,
depth profiles and ion images. Depth profiling is the primary mode of detection in
semiconductor analysis where the secondary ion intensity is recorded as a function of
sputtering time. Commonly used primary sputtering ions are O-, O2+, Ne+, Ar+, Kr+, Xe+
and Cs+ with typical impact energies in the range of 1∼20 keV[89]. Figure 5-8 shows
the depth profile of SIMS of Al implanted 4H-SiC after post-implantation annealing at
1700 oC for 30 min compared with results of the TRIM simulation (Paper III). Both
TRIM simulation and SIMS depth profile show that the surface Al peak concentration is
around 2 × 1019 cm-3. From SIMS it was found that 60% of the dose remains close to
the surface (≈ 1.8 × 1014 cm-2). This Al dose corresponds to a theoretical sheet
20
3
Concentration (atoms / cm )
10
19
10
18
10
SIMS Profile (Al Implanted)
TRIM simulation results
17
10
16
10
15
10
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Depth (µm)
Figure 5-8. The depth profile after Al implantation using SIMS and TRIM. The
SIMS results show the depth profile after high temperature annealing (1700 oC,
30 min in a CVD furnace. The TiC was removed prior to the SIMS
measurements.
- 52 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
resistance of about 0.5 kΩ/£ if completely activated, using a hole-mobility of 70
cm2/Vs for the calculation[23, 90].
5.1.3 Rutherford Backscattering Spectrometry (RBS)
The RBS technique normally uses 1 to 3 MeV ions (usually 4He ions) to analyze the
surface and the outer 0.5 to 3.0 µm of semiconductors and other materials[91]. In this
thesis RBS measurements (2.4 MeV 4He+ ions) were performed to investigate the TiW
(Paper I, V, VI) and TiC (Paper II, III, V) film thickness, composition ratio, and
interface reaction between metals and the silicon carbide substrates. For the TiW
Schottky contacts shown in Figure 5-9, the atomic composition ratio (Ti:W) and
thickness was determined to be Ti:W (0.58:0.42) and 1250Å, respectively. In addition,
16000
W
14000
As-deposited
o
Annealed at 500 C, 30 min
Yield (arb. units)
12000
10000
8000
6000
4000
C
Ti
Si
2000
0
0.5
1.0
1.5
2.0
2.5
Backscattering Energy (MeV)
Figure 5-9. RBS spectra for as-deposited and 500 oC annealed TiW Schottky
contacts.
we obtained no detectable reaction between TiW and SiC substrate after annealing at
500oC in a vacuum chamber (Paper I). In order to investigate the TiW film thickness,
the composition ratio, and the interface reaction between TiW and SiC, Rutherford
backscattering measurements were carried out with a 1000 Å-thick blanket sample,
which was deposited simultaneously with the other samples. The RBS spectra for
samples both the as-deposited and annealed sample at 950 oC are shown in Figure 5-10
(Paper VI, VII). After annealing, the intensity of the Ti and W peaks decreases. This
implies that there is a reaction between W and Ti and SiC at the interface. These results
are in good agreement with our previous XRD results after annealing at 950 oC (see
Figure 5-7 b). The RUMP simulator [92] shows the atomic composition ratio of asdeposited TiW was 0.61 and 0.39 for Ti and W, respectively. This corresponds to the
nominal weight ratio of the TiW target (30:70 in weight and 62:38 in atomic ratio).
From the Figure 5-11 for TiC Ohmic contacts, RBS spectra indicated around 10%
oxygen at the surface after 950 oC 180 s RTA. The presence of oxygen after RTA was
also supported by XPS depth profiles, which shows the region of this film, contained
around 15% oxygen and then decreased to around 1% at the interface. RBS and XPS
- 53 -
Chapter 5 Characterization and results
Sang-Kwon Lee
20
Ti(0.61)W(0.39)
Ti
Yield (A.U)
15
As-deposited
o
950 C annealed
10
5
W
Si
0
0.5
1.0
1.5
2.0
2.5
Energy (MeV)
Figure 5-10. RBS spectra for as-deposited and 950 oC annealed TiW Schottky contacts.
5000
o
Annealed (950 C RTA)
RBS raw data
RUMP simulation
Yield (arb. units)
4000
RUMP simulation
Composition : Ti0.51C0.39O0.10
3000 Thickness : 900 Å
Ti
C
2000
O
Si
1000
0
0.4
0.8
1.2
1.6
2.0
2.4
Backscattering Energy (MeV)
Figure 5-11. RBS spectra for 950 oC annealed TiC Ohmic contacts to 4H-SiC. The solid
line indicates the simulation results using the RUMP simulator.
analysis showed that high temperature annealing caused the oxidation of the TiC
contacts and finally resulted in an increase of the specific contact resistance for n-type
TiC Ohmic contacts on epilayer and implanted 4H-SiC (Paper II, III, V).
- 54 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
5.1.4 Transmission Electron Microscopy (TEM)
TEM is used to examine cross-sections of the films to determine the phases and
sharpness of the interface. As mentioned in section 5.1.1, Ti/SiC creates new phases,
such as titanium silicide (Ti5Si3) and titanium carbide (TiC) at the interface after hightemperature annealing. Porter et al. [86] have shown using high-resolution transmission
electron microscopy that around 10-15 nm of cubic TiC1-x and an orthorhombic Ti5Si3
layer is formed at the interface after annealing Ti/6H-SiC at 700 oC for 20 – 60 min. For
the system of Ni/SiC the TEM view of Ni/SiC after annealing at 950oC shows that the
δ-Ni2Si phase is formed by high temperature annealing[85], which is in good agreement
with the observation by X-ray diffraction.
TiC
50 nm
4H-SiC
Figure 5-12. The cross-section view of TiC/4H-SiC using TEM
(Paper II).
TEM analysis of an as-deposited TiC film on 4H-SiC is shown in Figure 5-12. The TiC
exhibits a columnar microstructure, where the individual columns have an average
width of about 200 ∼ 2500Å. The step repetition length on 4H-SiC (0001) substrate with
an 8 degree cut-off angle can be estimated to be about 70Å. The fact that the column
width is significantly larger than the repetition length suggests that the simple model of
domain formation by nucleation at surface steps may be incorrect (Paper II).
5.1.5 Atomic Force Microscopy (AFM) & Optical microscopy
The purpose of the AFM measurement is to monitor the surface morphology. In Paper
VI, we investigated Ohmic contact formation on ICP etched 4H-silicon carbide using
AFM. The roughness of samples such as un-etched, 30W etched, 60 W etched with
sacrificial oxidation, and 60 W etched samples, was 14∼16Å, 24∼27Å, 23∼28Å, and
44∼80Å, respectively. Figure 5-13 shows typical AFM images of SiC surface of unetched sample and 60-W of platen power etched samples. From the roughness
measurements, a sacrificial oxidation (1250 oC, 1hr) seems to improve and recover the
roughness to the same extent as that of a 30W etched sample (Paper VI). In addition,
- 55 -
Chapter 5 Characterization and results
Sang-Kwon Lee
optical microscopy view of nickel (Ni) and titanium tungsten (TiW) contacts after hightemperature annealing (950 oC) in a vacuum chamber (see also Section 4.1.5) is shown
in Figure 5-14(a) and (b), respectively. As shown in Figure 5-14 (a), the surface of Ni is
quite rough even after low-temperature annealing (650 oC) due to the creation of a new
phase (δ-Ni2Si, nickel silicide) after high-temperature annealing (see also section
5.1.1)[93]. In view of wire bonding and packaging, and long-term reliability, Ni
contacts have disadvantages, causing ultimate device failure via contact degradation and
wire bond failure under the exposure of high-power and high-temperature operation
even though they initially have extremely low specific contact resistance. In order to
keep away from this kind of problems, we introduced sputtered TiW contacts as shown
in Figure 5-14 (b), indicating no surface changes even after high-temperature annealing
(950 oC). In Paper VII, we suggested this TiW with a capping layer to be the best
candidate for long-term reliability tests.
(a)
(b)
Figure 5-13. AFM images of the silicon carbide surface of (a) un-etched sample and
(b) 60-W etched sample.
(a)
(b)
Figure 5-14. Optical microscope view of (a) Ni contacts and (b) TiW contacts after
950oC annealing in a vacuum chamber for 30 min.
- 56 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
5.2 Electrical characterization of Schottky contacts
5.2.1 Measurement techniques
The Schottky barrier height (φB) and ideality factor (η) are essential paramters to
characterize Schottky diodes. In general, there are four methods such as the currentvoltage (I-V) measurement, the capacitance-voltage (C-V) measurement, photoelectric
measurement, and activation energy measurement for evaluating the barrier height of
metal-semiconductor contacts.[11] In this section, we will describe briefly how they
work.
Capacitance-voltage measurement
When a small ac voltage (normally Vp-p ≈ 30 ∼ 50 mV) is superimposed upon a dc bias,
charges of one sign are induced on the metal surface and charges of opposite sign in the
semiconductor. If the Schottky diode is nearly ideal and the semiconductor has a
uniform donor concentration the differential capacitance C under reverse bias Vr is
given for non-generate semiconductor by
1
 qN ε  2
C = A D s 
 2 

kT 
⋅ φ b − ξ + Vr −
q 

−
1
2
(5-1)
where A is the area of the contact and ξ is the difference in energy between the Fermi
level and the bottom of the conduction band in the bulk semiconductor (n-type).
Equation 5-1 can be written in the following form
21
5x10
p-type 4H-SiC
φBp=1.56 eV (Ni)
21
4x10
VI=1.36
φBp=2.07 eV (Ti)
VI=1.87
2
-2
1/C (F )
21
3x10
21
2x10
Intercept
φBp=1.49 eV (Au)
21
1x10
VI=1.30
0
-2
-1
0
1
2
3
Reverse Voltage VR (V)
Figure 5-15. 1/C2 versus reverse bias voltage for Ti, Ni, and Au Schottky
contacts to p-type 4H-SiC in the frequency of 100 kHz at room
temperature. The area of the diodes is 1.26 × 10-3 cm2.
- 57 -
Chapter 5 Characterization and results
Sang-Kwon Lee

 
kT 
2
⋅ φ b − ξ + Vr − 
C −2 =  2

q 
 A qN Dε s  
(5-2)
If the barrier height (φb) is independent of Vr, a plot of 1/C2 versus Vr should give a
straight line with an intercept –VI on the horizontal axis equal to –(φb-ζ-kT/q). Hence,
the barrier height is given by
φ bn = VI + ξ +
kT
− ∆φ
q
(5-3)
where VI is the voltage intercept and ∆φ, given in equation 3-16, is the image force
lowering of the Schottky barrier. From the slope of this plot, we can also extract the
doping concentration, given in equation 5-4, of the epilayer. Figure 5-15 shows typical
1/C2 versus reverse voltage for Ti, Au, and Ni Schottky contacts to p-type 4H-SiC.
ND =
2
 d (1 C 2 ) 2
qε s  −
⋅A
dVr 

(5-4)
Current (A)
Current-voltage measurement
According to the thermionic-emission theory, the current density (J)-forward voltage
(V) characteristics is given by
1x10
2
1x10
0
1x10
-2
1x10
-4
1x10
-6
1x10
-8
1x10
-10
10
-12
0.0
n-type 4H-SiC
Ti Schottky
Au Schottky
300
1.03
1.05
1.06
1.06
Au/4H-SiC
Temp ΦB
25
1.75
100 1.84
200 1.77
300 1.78
1.30
1.20
1.10
1.05
η
200
100
0.5
Ti/4H-SiC
Temp ΦB
25
1.12
100 1.11
200 1.12
300 1.14
1.0
o
25 C
1.5
2.0
2.5
η
3.0
Forward Voltage (VF)
Figure 5-16. The current (log I) versus forward bias voltage (VF) for Ti
(titanium) and Au (gold) Schottky diodes to n-type 4H-SiC.
- 58 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
  qV  
J = J S exp
 − 1
  ηkT   ,
where
 qφ 
J S = A *T 2 exp − e 
 kT 
(5-5)
where JS is the saturation current density, η is the ideality factor, A* is effective
Richardson's constant (146 Acm-2K-2), and φe is the effective barrier height (=φb-∆φ).
The barrier height and ideality factor are obtained from the following equation 5-6
φB =
kT  A * T 2 

ln
q  J S  ,
η=
q  ∂V 
kT  ∂ (ln J )
(5-6)
The ideality factor (η) and the saturation current density (Js) can be extracted from the
experimentally obtained forward current density-voltage (ln J-V) characteristics.
Typical current (log I) versus forward bias voltage (VF) for Ti and Au Schottky contacts
to n-type 4H-SiC is shown in Figure 5-16. From Figure 5-16, the Schottky barrier
height for titanium metal contacts to n-type 4H-SiC was 1.11∼ 1.14 eV in the range of
25 to 300 oC with stable ideality factor of 1.03 ∼1.06.
Photoelectric measurement
When a monochromatic light is incident upon a metal surface (see Figure 5-17), photocurrent (Y) is generated. The photo-current per absorbed photon Y as a function of the
photon energy, hν, is given by
Y≈
 x 2 π 2  − x e −2 x

T2
−  e −
+ ⋅ ⋅ ⋅ 
 +
6 
4
E S − hν  2

x≥0
for
(5-7)
where hν0 is the barrier height qφBn, Es is the sum of hν0 and x is defined as h(ν-ν0)/kT.
equation 5-7 can be rewritten to
10
Metal
8
(Arbitrary Unit)
Semiconductor
hν
(Back illumination)
hν
(Front illumination)
Intercept (=qφBn)
4
(Y)
1/2
Ohmic contact
6
2
A
0
0
1
2
3
Photon energy hυ (ev)
Figure 5-17. Schematic set-up for
photoelectric measurement (see
ref. [24]).
Figure 5-18. A plot at square of photo
current vs photon energy. The
extrapolated
values
are
the
corresponding barrier heights (see ref.
[24).
- 59 -
Chapter 5 Characterization and results
Sang-Kwon Lee
Y ≈ (hν − qφ Bn )
2
for hν-qφBn > 3kT
(5-8)
When the square root of the photo response (Y) is plotted as a function of the photo
energy (see Figure 5-18), a straight line should be obtained. The extrapolated value on
the photo energy axis directly yields the barrier height (qφB).
Activation energy measurement
The advantage of this method is that no assumption of electrically active area is
required. Using active area (Ae), we can obtain
I 
ln F2  = ln(A e A * ) − q (φ B − VF ) kT
T 
(5-9)
where q(φB-VF) is the activation energy. Thus for a given forward bias voltage VF, the
slope of a plot of ln(IF/T2) versus 1/T yields the barrier height and the intercept at 1/T=0
yields the product of the active area Ae and the effective Richardson constant A*.
Summary
I-V and C-V measurements have been the main methods to characterize the Schottky
diodes to silicon carbide. These two methods are convenient for evaluating the
Schottky barrier height of metal-semiconductor contacts, which is an essential factor to
characterize Schottky diodes. C-V measurements were mainly used for extracting the
doping concentration of the epilayer as shown in equation 5-4. The Schottky barrier
heights from C-V measurements are slightly higher than those from I-V measurements
as shown in Paper I. Therefore, in order to characterize the Schottky diodes the best
electrical characterization is I-V measurements among various measurements as
described above.
- 60 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
5.2.2 A review of the Schottky contacts (Paper I, IV, VIII)
During recent years, the interest in 4H-SiC for device applications has increased
because of its higher electron mobility and wider band gap than the 6H polytype. There
are a few reports on the metal/4H-SiC systems compared to those of on 3C-SiC and 6HSiC system. In this section, some of the earlier works related to the Schottky contacts to
only 4H-SiC are summarized in Table 5-1.
Table 5-1. Schottky barrier height of metals using I-V, C-V and other methods on n- and
p-type 4H-SiC.
n
or
p
Metals
n
Ti
Ni
n
Au
Ni
Ti
Au
Ni
Ti
n
TiW
p
p
n
Ni
Au
Ti
Ti
n
Ni
n
Ni
Pt
Barrier
Height (eV)
I-V
0.80
C-V
-
0.85
1.30
1.40
1.50
1.73
1.85
1.80
2.10
1.62
1.75
1.60
1.90
0.95
1.17
1.16
1.30
1.81 (IPE)
2.07 (IPE)
1.69 (IPE)
1.87 (IPE)
1.09 (IPE)
1.25 (IPE)
1.22
1.23
1.18
1.19
1.41
2.11
1.91
1.66
1.31
1.56
1.35
1.49
1.94
2.07
0.91
∼0.94 0.99
∼1.04
1.6
1.9
1.59
1.39
-
η
face
Comments
Refs.
1.15
Si-
as-deposited
(20 oC)
122 oC
20 oC
122 oC
255 oC
[45]
1.10
1.21
1.12
1.12
1.02
∼
1.20
1.05
1.10
3.11
1.08
1.29
1.08
1.07
1.17
∼1.22
1.03
∼1.04
2
1.1
1.05
1.01
SiCSiCSiCSiCSiCSiCSi-
[41]
IPE
(Internal
Photoemission
Spectroscopy)
as-deposited
500 oC, 30 min
as-deposited
500 oC, 30 min
Si-
Si-
Paper I
Paper IV
as-deposited
[94]
500 oC annealed
SiSi-
- 61 -
as-deposited
500 oC annealed
[95]
[47]
Chapter 5 Characterization and results
Sang-Kwon Lee
5.2.3 The relationship between metal work function and barrier height
(Paper IV)
Itoh et al.[41] reported Schottky barrier height of several metals (Ti, Ni, and Au) to ntype 4H-SiC using I-V, C-V and IPE measurements (also see Table 5-1). They also
proved that the barrier height depends on the metal work function with evidence of no
strong Fermi-level, and with a linear relationship with slope (S≡φBn/φm), referred to the
index of interface behavior, of 0.7 for Si-face of n-type 4H-SiC between the barrier
height (φBn) for n-type 4H-SiC and metal work function. This value agrees with works
of Waldrop et al.[96] for n-type 6H-SiC. In view of a basic understanding of metalsemiconductor interfaces, Schottky barriers on p-type 4H-SiC are also of interest.
Unfortunately, there are only few publications on the subject of Schottky barrier on ptype 4H-SiC. In this section, we will describe our investigation of Schottky diodes of
several metals to p-type 4H-SiC (Si-face) using I-V and C-V characteristics. We also
suggest that there be a strong relationship between barrier height and metal work
function and that it will reach the Schottky-Mott limit with these results (Paper IV).
Here, highly doped p-type 4H-SiC (0001) substrate (1018 cm-3) with 4 µm thick lightly
doped p-type epilayer (1.3×1016 cm-3) was used. The Schottky diodes (1500 Å thick)
were fabricated by e-beam evaporation of metals (Ti, Au, and Ni). The metal work
function (φm) of a metal is defined by the amount of energy required to raise an electron
from the Fermi level to a state of rest out side the surface of metals (called vacuum
level), and it consists of two parts; the volume contribution and the surface contribution,
which indicates that any of surface modification in the surface electron charge
distribution (adsorption of gas on surface and different crystallographic faces of the
same crystals) will lead to a change in work function [11]. Some results also show that
there is 0.78 eV difference between the work function of the tungsten (110) and (111)
faces due the difference of the surface contribution. For example, there is 0.81 eV
difference between the metal work function of the (111) plane and the (331) plane of
platinum. In order to get the exact barrier height on a metal-semiconductor, the exact
crystallographic faces are important. The XRD spectra (see also Section 5.1.1) show
that the metals are highly textured on SiC with {111},{001}, and {111}-type reflections
for Au, Ti, and Ni, respectively. From the XRD results, we have the crystallographic
face dependent work function of each metal; Au (5.31 eV) and Ni (5.35 eV) for {111}.
For Ti (4.33 eV) the work function for the polycrystalline phase was used instead of
{001} because of no available reference data [97]. The Schottky barrier height was
determined by C-V and I-V measurements. Figure 5-15 shows the plot of the square of
the inverse of the capacitance per unit area as a function of reverse voltage for Ni, Ti,
and Au on p-type 4H-SiC at the frequency of 100 kHz at room temperature. The barrier
height was 1.56, 1.49, and 2.07 eV for Ni, Au, and Ti, respectively. The current-voltage
characteristics of Au on p-type 4H-SiC in the range of 24 oC to 300 oC are shown in
Figure 5-19. All of the contacts show good rectifying behavior with stable ideality
factor of 1.07, 1.23, and 1.06 for Ti, Ni, and Au, respectively, in the range of 24 oC to
300 oC. The barrier heights from C-V characteristics were 0.13 ∼ 0.25 eV higher than
those from C-V measurements. It could be explained by additional capacitance at the
interface due to a thin oxide during the sample preparation. The barrier heights of each
metal for n-type and p-type 4H-SiC using C-V and I-V measurements as a function of
the metal work function at room temperature are plotted in Figure 5-20 and Figure 5-21,
respectively. From the both Figure 5-20 and 5-21, the slope S (S≡dφBn,p/dφm) for n-type
and p-type was 0.64 – 0.73 (C-V, I-V) and 0.54 – 0.61 (C-V, I-V), respectively.
- 62 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
2
10
2
Current Density J (A/cm )
Au (p-type 4H-SiC)
0
10
-2
10
-4
1x10
-6
0
300 C (φBn:1.51 eV, η:1.05)
10
0
200 C (φBn:1.42 eV, η:1.06)
0
100 C (φBn:1.40 eV, η:1.05)
-8
10
0
24 C (φBn:1.35 eV, η:1.08)
-10
10
0
1
2
3
4
5
Forward Voltage (VF)
Figure 5-19. Current density (log J) versus forward voltage (VF) for Au
Schottky diodes to p-type 4H-SiC with measurement temperature from
24 to 300 oC.
2.4
2.5
Barrier Height ΦBn (eV)
I-V
C-V
Fitting line
Schottky Barrier Height φBp (eV)
4H-SiC (n-type)
2.0
ΦB-n=0.64 Φm-0.58
1.5
1.0
0.5
4.0
ΦB-n=0.73 ΦM-2.21
AuNi
Ti
4.5
5.0
5.5
6.0
P-type 4H-SiC
C-V measurement
I-V measurement
2.2
2.0
φBp=4.42 - 0.54 φm
1.8
1.6
φBp=4.58 - 0.61 φm
1.4
1.2 At room temperature
Ti
1.0
4.0
AuNi
4.5
5.0
5.5
6.0
Metal work function φm (eV)
Metal work function Φm (eV)
Figure 5-20. Schottky barrier height of
Ni, Ti, and Au to n-type 4H-SiC using IV and C-V characteristics as a function
of each metal work function (after
ref.[41]).
Figure 5-21. Schottky barrier height of
Ni, Ti, and Au to p-type 4H-SiC using
I-V and C-V characteristics as a
function of each metal work function.
These results indicate that the Schottky barrier height strongly depends on the metal
work function even though there is partial Fermi level pinning. Figure 5-22 summarizes
the barrier height from C-V and I-V for n- and p-type 4H-SiC as a function of the metal
work function. From the Schottky-Mott limit [11], the sum of the Schottky barrier
- 63 -
Chapter 5 Characterization and results
Sang-Kwon Lee
height for n- and p-type semiconductor should be given by (φBn+φBp ≈ Eg-4H-SiC). From
Figure 5-22, the sum of the SBH is very close to the energy band gap of 4H-SiC,
indicating that our results satisfy the Schottky-Mott model without strong Fermi-level
pinning. It also indicates that n-type Schottky barrier diodes have weaker Fermi-level
pinning compared to p-type Schottky diodes. In addition, we plotted the various metals
as a function of the metal work function in Figure 5-23. It shows there is a great deal of
scatter in the experimental data for a given metal compared to our data on 4H-SiC. As
pointed out above the surface contribution caused by different sample preparation and
surface quality are important factors to have good Schottky barrier diodes without
strong Fermi level pinning.
3.5
2.8
o
Egat 24 C (4H-SiC)
(Eg=φBp-φBn)
3.0
o
2.5
φBp=4.51 - 0.58 φm (p-type) at 24 C
2.0
1.5
1.0
0.5
φBn=0.67φm - 1.85 (n-type) at room temp.
2.0
1.6
1.2
4.5
5.0
5.5
Φ B-n=0.39 Φ m-0.60
0.8
0.4
AuNi
Ti
0.0
4.0
4H-SiC (n-type)
I-V
C-V
IPE, BEEM
Average
Fitting line
2.4
Barrier Height ΦBn (eV)
Schottky Barrier Height φB (eV)
4.0
6.0
0.0
4.0
AuNi Pd
Ti
4.5
5.0
5.5
Pt
6.0
6.5
Metal work function Φm (eV)
Metal work function φm (eV)
Figure 5-22. Schottky barrier heights
of Ni, Ti, and Au to both n- and p-type
type 4H-SiC as a function of the metal
work function (see ref. Paper IV and
[41]).
Figure 5-23. Schottky barrier heights
of various metals on n-type 4H-SiC as
a function of the metal work function.
Data were taken from Table 5-1.
- 64 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
5.2.4 Reduction of the Schottky barrier height (Paper VIII)
A lower barrier height of the contacts was observed by the incorporation of sizeselected Au nano-particles in Ti Schottky contacts on silicon carbide (Paper VIII). For
this study, the contacts were formed by first depositing Au aerosol nano-particles with a
diameter of 20 nm and with a density of 90 ∼ 100 µm-2 on the SiC surface (see Figure 524) [98]. After deposition of the Au aerosol, the samples were loaded into the
evaporation chamber to deposit additional Ti (200 nm) on the Au nano-particles (see
Figure 5-25). The reason we selected Au and Ti for our study as Schottky metals is that
they have a large barrier height difference. Finally, the Schottky diodes (50 to 1000 µm
diameter) were patterned using a standard photo-lithographic process and wet-etched
using solutions of H2O : HF 5% (2:1) and KI : I2 : H2O (4:1:10) for Ti and Au,
respectively. Then large area Al backside Ohmic contacts (250 nm thick) were
evaporated onto the backside of the SiC with a photo-resist mask in order to prevent the
contamination of the front side metals. Diodes were finally annealed with a 10% H2/Ar
ambient at 350 ∼ 550oC for 60 s using rapid thermal annealing (RTA). I-V as well as CV measurements were performed on different diodes for different temperatures up to
300oC The SBHs were extracted from both I-V and C-V measurements for comparison
at different temperature ranges
Figure 5-24. A picture of the aerosol machine and schematic view of aerosol apparatus
(See after ref. [98])
200-400 µm
Ti
Au nano-particles
0.2 µm
n- or p- epilayer
4 µm
n- or p- epilayer
4H- or 6H-SiC (Substrate)
Backside Ohmic
Figure 5-25. Schematic view of Ti Schottky contacts with embedded Au nano-particles on
SiC. The thickness of the epilayer is 4 µm.
- 65 -
Chapter 5 Characterization and results
Sang-Kwon Lee
1
1
10
1x10
(a)
10
-3
10
-7
4H-SiC (n-type)
(b)
-3
1x10
Current (A)
Current (A)
6H-SiC (n-type)
Au-embedded Schottky
Ti Schottky
-7
1x10
Au-embedded Schottky
Ti Schottky
o
Temperature (25,50,75,100,and 125 C)
0
10
0.0
0.5
1.0
0
0
0
Temperature 25 C, 100 C, 200 C, 300 C
-11
-11
10
1.5
0.0
Forward Voltage (VF)
0.5
1.0
1.5
Forward Voltage (VF)
Figure 5-26. The current (log I)-forward voltage (VF) characteristics of particle free
control Ti Schottky contacts and Ti Schottky contacts with embedded Au nanoparticles to (a) n-type 6H-SiC at different measurement temperature 25 oC, 50 oC, 75
o
C, 100 oC, and 125 oC and (b) n-type 4H-SiC at different measurement temperature
25 oC, 100 oC, 200 oC, and 300 oC.
Table 5-2. Summary of the Schottky barrier height and ideality factor as a function of the
measurement temperature for particle-free control Ti Schottky contacts and Ti Schottky
contacts with embedded Au nano-particles to n- and p-type 4H- and 6H-SiC using
current-voltage measurements.
Measurement Temperature (oC)
Samples
25
1.66
1.41
1.71
1.33
0.93
1.04
1.12
1.03
1.60
1.38
1.70
1.24
0.59
1.05
0.76
1.07
50
x
x
x
x
0.93
1.03
1.11
1.03
x
x
x
x
0.59
1.07
0.76
1.07
75
1.71
1.37
1.76
1.31
x
x
x
x
1.61
1.44
1.73
1.24
0.59
1.13
0.76
1.10
100
x
x
x
x
0.93
1.03
1.11
1.05
x
x
x
x
0.59
1.27
0.76
1.12
125
1.77
1.33
1.75
1.35
x
x
x
x
1.66
1.41
1.79
1.20
0.61
1.36
0.77
1.13
φBp
η
φBp
η
a
n Nano
φBn
η
b
Ti
φBn
η
6 p Nanoa φBp
H
η
b
Ti
φBp
η
n Nanoa φBn
η
b
Ti
φBn
η
x : not performed
a
Ti Schottky Contacts with embedded Au nano-particles
b
Particle-free control Ti Schottky contacts
a
4 p Nano
H
Tib
- 66 -
150
x
x
x
x
0.93
1.04
1.11
1.05
x
x
x
x
0.63
1.32
0.78
1.13
175
x
x
x
x
x
x
x
x
x
x
x
x
0.65
1.35
0.78
1.20
200
x
x
x
x
0.94
1.03
1.12
1.06
x
x
x
x
0.68
1.30
0.78
1.30
250
x
x
x
x
0.95
1.02
1.13
1.14
x
x
x
x
x
x
x
x
300
x
x
x
x
0.95
1.11
1.14
1.06
x
x
x
x
x
x
x
x
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
Figure 5-26 shows typical I-V characteristics of 350 oC annealed control Ti Schottky
contact and Ti Schottky contacts with embedded Au nano-particles to n-type 6H- and
4H-SiC. The results are also summarized in Table 5-2. The results from the I-V
measurements clearly indicated that the SBH for Ti Schottky contacts with embedded
Au nano-particles on n-type SiC was 0.19 eV (4H-SiC) and 0.15 eV (6H-SiC) lower
than those of the control Ti Schottky contacts. The SBHs for p-type Ti Schottky
contacts with embedded Au nano-particles on SiC were also 0.02 ∼0.05 eV (4H-SiC)
and 0.1∼0.13 eV (6H-SiC) lower than those for the control Ti Schottky contacts in the
temperature range 25 ∼ 125 oC. In order to understand this reduction of the SBH for Ti
Schottky contacts with embedded Au nano-particles to 4H- and 6H-SiC both from I-V
and C-V measurements, it has been proposed that SBH lowering is caused by an
enhanced electric field at the depletion region close to the surface of the semiconductor
due to the small size of the Au nano-particles and the large SBH difference. According
to Tung's dipole-layer approach of the potential and the electronic transport at metalsemiconductor (MS) interface, the potential distribution for circular patch geometry at
MS interface is given by [99]


2
z
z



V(0,0, z ) = Vbi 1 −  + Va + Vn − ∆φ Ti − Au 1 −
1

 w
2
2 2 
 (z + R 0 ) 


1
z2
 2 2z 

E(0,0, z ) = Vbi  − 2  − ∆φ Ti − Au 
−
2
2 3 
 z 2 + R 20
w w 
(z + R 0 ) 

(5-10)
(5-11)
where z is the distance from the surface of semiconductor, w is the depletion width, R0
is the radius of the circular patch, and ∆φTi-Au is the difference of the barrier height
between Ti and Au metals. Figure 5-27 (a) and (b) shows the calculated conduction
band potential and electrical field as a function of the distance from the surface and the
radius of the circular patch using equation 5-10 and 5-11, respectively. Equation 5-10
also suggested that the potential at the metal-semiconductor interface be highly
dependent on the radius of the circular patch (R0) and SBH difference (∆φTi-Au) between
Ti and Au contacts. The magnitude of electric field for n- and p-type SiC was calculated
to be 0.07 ×107 V/cm (n-4H- and 6H-SiC), 0.06 ×107 V/cm (p-4H-SiC), and 0.04 ×107
V/cm (p-6H-SiC), respectively. In order to calculate the value of Schottky barrier
lowering, the calculated electrical field for each n- and p-type was plugged into
following well-known equation 3-16. In general we could see that there is no image
force lowering at the forward bias in Schottky diode due to low electric field at the
forward bias. However, from our calculation results, there is enough electric field to
reduce the barrier height due to the small size of metal particles and large difference of
the barrier height between two metals. The ∆φ for our experimental results is in
reasonable agreement with the theoretical calculation using a dipole layer approach with
the circular patch [99] even though the ∆φ from the theoretical calculation is a factor of
2 lower than what we obtained from our measurements. The reason for this could be a
much higher electric field than we expected and calculated at the metal-semiconductor
interface. Detailed further studies are required for a more solid explanation. In order to
evaluate the predominant current of lower barrier height Schottky contacts and confirm
an enhanced high electric field, a 2-D simulation was performed with the device
simulator ATLAS [20]. Figure 5-28 (a) and (b) shows ATLAS simulation set-up and
- 67 -
Chapter 5 Characterization and results
Sang-Kwon Lee
0.14
2.0
(a)
200nm
Z
φB-∆ (Au)
φB (Ti)
1.4
100nm
1.2
Semi
Metal
20nm 50nm
1.0
0.8
10nm (in this work)
0.6
R0=5nm (Au nano-particles)
0.4
0
50
100
150
0.10
0.08
10 nm (in this work)
0.06
0.04
20nm
0.02
50nm
100nm
200nm
n-type 4H-SiC
0.00
-10 0
200
10 20 30 40 50 60 70 80 90 100
Depth from the surface Z (nm)
Depth z (nm)
Figure 5-27. (a) calculated conduction band potential distributions and (b) electric field
distribution for n-type 4H-SiC as a function of the radius of the circular patch and the depth
from the surface (z) The insert shows a schematic diagram of a high barrier circular patch
(Au) surrounded by low barrier metals (Ti) on SiC.
10
φBn=φm-χs
1
-1
10
M1(Au)
M2 (Ti)
+
η=1.02, φB=1.11 eV
-3
10
-5
VDC
-
1nm
4µm thick epi (4∼7×1015 cm-3)
SiC (∼ 1018 cm-3) (0001)
10
Current (A)
Potential (V)
R0
(b)
7
1.6
R0=5nm (Au nano-particles)
0.12
Electric field x 10 (V/cm)
n-type 4H-SiC
1.8
-7
10
-9
10
10
-11
10
-13
10
-15
Backside ohmic
GND
0.0
η=1.01, φB=1.13 eV
η=1.02, φB=1.64 eV
Ti/Au (Dual metals)
Ti only
Au only
0.5
1.0
1.5
2.0
Forward voltage (V)
Figure 5-28. (a) ATLAS simulation set-up and (b) the results of forward I-V
characteristics for Ti, Au, and dual metals on silicon carbide at 300 K.
the simulation results of the forward I-V characteristics for Ti, Au and dual metal (Ti
and Au) on SiC. The simple double barrier devices consisted of a silicon carbide with
two metal electrodes (called M1 and M2, with 1nm spacing between them) of different
work functions and barrier heights on the surface. The simulation results confirmed that
the small barrier height Ti Schottky contact conducts current dominantly indicating the
SBH values form simulation only depends on the lower SBH diodes as shown in Figure
5-28 (b). In addition, we did not observe the SBH difference between Ti Schottky and
dual metal (Ti/Au) Schottky contacts from our simulation. However, ATLAS
simulation showed the electric field increase at the triple interface (two metals and SiC
- 68 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
substrate) as compared to the control Ti/SiC interface as shown in Figure 5-29. The
electric field at the triple interface was around 3 × 106 V/cm.
Figure 5-29. The lateral electric field distribution for dual metal on SiC.
- 69 -
Chapter 5 Characterization and results
Sang-Kwon Lee
5.3 Specific contact resistance measurements
Linear transmission line method (LTLM) with mesa structures was used to characterize
the Ohmic contacts to 4H- and 6H-SiC (mainly 4H-SiC).
Table 5-3. A review of the Ohmic contacts to SiC
n
or
p
4H (n)
Metals
Doping
(cm-3)
ρc
(Ωcm2)
Face
Annealing
Ref.
Ni-Cr
4.8×1017
1.0×10-4
∼1.6×10-5
1.2×10-5
∼10-3
∼10-4
Si-
1100oC, 3
min
[101]
Si-
[102]
4H(p)
Si/Pt
Al/Ti
4H(n)
Ni
Cr
W
TiC
4H(n)
4H(p)
6H(n)
4H(p)
4H(n)a
4H(p)b
4H(n)
Ti
Ti
TiC
Ni
6H(n)
4H(n)
4H(p)
TiW
(30:70)c
6H(n)
3C(n)
TiW
(10:90)c
6H(n)
1.3×1019
1×1019
1017∼1018
10-4∼10-6
Si-
1.3×1019
>1020
>1020
>1020
2×1019
4×10-5
6×10-5
8×10-4
2×10-5
1×10-4
Si
30oC ∼400oC
1100oC, 3
min
1000oC∼
1050oC, 5
min
950oC
Si
Si
950oC
850oC
[104]
Paper III
1 × 1019
1 × 1021
1 × 1019
1.1× 1019
6.0 × 10-6
1.5 × 10-4
1.5 × 10-5
7.5× 10-6
Si
[105]
5 × 1019
7.8 × 1018
3.2 × 1017
2 ∼5 × 1018
1.3 × 1019
6 × 1018
∼ >1020
7 × 1018
1.7 × 1020
1 × 10-6
4∼9 × 10-6
3 × 10-6
8∼9 × 10-5
2∼6 × 10-5
1.2 × 10-4
∼ 4 × 10-6
1 × 10-4
7.8 × 10-5
1050oC
10 min
1000oC, 5min
950oC,
30
min
1000oC, 5min
950oC, 2min
1200oC, 1min
1000oC
950oC, 30min
6 × 1018
3.4 × 10-4
C
Si
[103]
Paper II
[106]
Paper VII
[107]
[50]
[108]
Paper V,
VI, VII
750oC, 5min [109]
900oC, 15min [110]
a
Nitrogen was implanted.
Aluminum and carbon were co-implanted.
c
Weight ratio (Ti:W)
b
In this thesis, various metals (Ti, TiC, TiW, and Ni) were used as a metal for Ohmic
contact studies. According to previous works, the specific contact resistance to n- and ptype of 4H-SiC and 6H-SiC is in the range of 10-4 ∼ 10-6 Ωcm2 and 10-3 ∼ 10-5 Ωcm2,
- 70 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
respectively, which seems to be highly depending on the surface doping concentration,
the choice of the metals, the post heat treatment, the sample preparation (deposition,
cleaning and other processes), and also quality of the silicon carbide materials [100].
The review results of most promising Ohmic contacts to n- and p-type silicon carbide
are summarized in Table 5-3 (also see ref. [100] in detail).
5.3.1 TiC and Ti on n- and p-SiC (Paper II, III, V)
TiC/n- and p-SiC (epi)
A summary of Ohmic specific contact resistance as a function of measurement
temperature and annealing temperature using Ti and TiC to n- and p-type 4H-SiC is
shown in Table 5-4. The results of the specific contact resistance for p- type (a) and ntype (b) TiC is also shown in Figure 5-30 (a) and (b), respectively. The lowest specific
contact resistance for as-deposited p-type TiC was 1.1 × 10-4 Ωcm2 at 25oC. After
annealing at 700oC, the specific ρC did not improve. The specific ρC reached its lowest
value of 1.9 × 10-5 Ωcm2 at 300oC after annealing at 950 oC. These results can be
explained by a shorter transfer length and lower contact resistance caused by a smoother
interface and no interface reaction. As shown in Figure 5-30 (b) n-type TiC have
different behaviors. In order to explain the exact mechanism at the interface between the
metals and epilayer, further investigation is needed. From the study of TiC contacts
deposited by co-evaporation in a UHV system have promising advantages to make the
lower contact formation on 4H-SiC. One reason for this is that no post-annealing is
required.
Ti/p-SiC (epi)
As-deposited Ti contacts showed a good Ohmic characteristic with the lowest ρC of 2.5
× 10-4 Ωcm2 at 200oC and broadly uniform distribution of the specific contact resistance.
After sequential annealing at 700oC and 950oC using RTA in 10%H2 /Ar the specific ρC
increased by a factor of 2. A possible reason for this increase is due to the creation of
-2
2
Specific contact resistance (Ω cm )
2
Specific contact resistance (Ω cm )
-1
10
(a)
-2
10
-3
10
1x10
-4
1x10
-5
(b)
-3
10
-4
1x10
-5
1x10
As-deposited
o
700 C, RTA in 10% H2/Ar
-6
10
o
950 C, RTA in 10% H2/Ar
-7
10
10
0
50
100
150
200
250
300
350
o
Temperature ( C)
-6
10
-7
o
10
950 C,60S, RTA in 10% H2/Ar
As-deposited
-8
10
0
50
100
150
200
250
300
350
o
Temperature ( C)
Figure 5-30. The results of the specific contact resistance of TiC Ohmic contacts to ptype (a) and n-type (b) to 4H-SiC as a function of the measurement and annealing
temperature.
the new phase such as Ti5Si3 and TiC1-x which have a higher contact resistance (RC) than
- 71 -
Chapter 5 Characterization and results
Sang-Kwon Lee
that of as-deposited contacts (see also section 5.1)[86]. The results from the LTLM
showed and confirmed the above indication because the contact resistance (RC) for asdeposited contact at 25oC was about 40Ω, which is 40% lower than that of the 950oC
annealed contacts measured at the same temperature.
Table 5-4. The summary of Ti and TiC Ohmic contacts to n- and p-type 4H-SiC for
different measurement and annealing temperature (Paper II).
Measurement
Temperature
(oC)
Annealing
Temperature
(oC)
25
100
200
300
25
100
200
300
25
100
200
300
As-deposited
700 oC, 180s
RTA in 10%H2/Ar
950 oC, 180s
RTA in 10%H2/Ar
Average specific contact resistance (Ω cm2)
Titanium Carbide (TiC)
Titanium (Ti)
n-type
p-type
p-type
-6
-4
9.28 × 10
1.08 × 10
3.44 × 10-4
3.07 × 10-6
1.31 × 10-4
2.91 × 10-4
7.38 × 10-7
1.12 × 10-4
2.50 × 10-4
-6
-4
5.26 × 10
2.32 × 10
2.93 × 10-4
X
1.75 × 10-4
4.70 × 10-4
X
3.28 × 10-4
3.90 × 10-4
-4
X
1.96 × 10
3.18 × 10-4
X
2.61 × 10-4
2.75 × 10-4
7.70 × 10-4
5.62 × 10-5
4.01 × 10-5
6.19 × 10-4
4.18 × 10-5
2.77 × 10-5
-5
-5
5.04 × 10-4
3.16 × 10
4.59 × 10
4.31 × 10-4
1.87 × 10-5
8.72 × 10-5
X : not performed
TiC on implanted layers
The results of the specific contact resistance as a function of measurement temperature
and annealing temperature are shown in Figure 5-31 and Table 5-5 The best result was
achieved after 500 oC annealing. For as-deposited contact (TLM1 structure, see Table
5-5) the sheet resistance (ρs) of the epilayer increased from 0.6 kΩ/£ to 1.4kΩ/£ when
-3
2
Contact resistivity ρC (Ω cm )
10
1x10
-4
1x10
-5
As-deposited
o
500 C RTA
o
700 C RTA
o
850 C RTA
-6
10
0
50
100 150
200
250
300 350
o
Temperature ( C)
Figure 5-31. The specific contact resistance of TiC Ohmic contact (TLM1) on
implanted 4H-SiC versus measurement temperature and annealing temperature.
- 72 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
the temperature was increased from 25oC to 300oC. Considering the theoretical sheet
resistance of 0.5kΩ/£, this would correspond to over 80% activation of Al acceptors.
We also observed that the TLM structures had different values of ρs varying from 0.6
kΩ/£ to 6.3kΩ/£ for as-deposited contacts. This non-uniformity of the sheet resistance
might be due to the different activation of Al in the epilayer or not completely
recrystallized SiC after 1700oC annealing. This behavior affected the variation of the
specific contact resistance as shown in Table 5-5. The specific contact resistance from
TLM1 was as low as 2 × 10-5 Ωcm2 after 500oC. After sequential annealing at 700oC
and 850oC, no further improvement of ρC was observed. As mentioned in the material
characterization part (see also section 5.1), RBS and XPS depth profiles indicated that
there were around 10 to 15% oxygen on the surface after 950oC RTA, but the oxygen
content decreased to 1 at% close to the metal/substrate interface. The increase in the
specific contact resistance after high temperature annealing (> 500oC RTA in
10%H2/Ar) was correlated with the oxygen detected from XPS and RBS analyses. The
oxygen incorporation results in substantial degradation (increasing specific contact
resistance) of the contacts, TiC/SiC.
Table 5-5. The summary of TiC Ohmic contacts on Al implanted 4H-SiC for 3 different
sets (TLM1, TLM2, and TLM3) (Paper III).
Annealing
Temperature
(°C)
As-deposited
500°C RTA
700°C RTA
850°C RTA
Meas.
Temp.
(°C)
25
100
200
300
25
100
200
300
25
100
200
300
25
100
200
300
TiC TLM 1
TiC TLM 2
TiC TLM 3
ρC
×10-4
Ωcm2
0.87
1.07
1.12
0.48
0.19
0.32
0.37
0.37
0.48
0.66
0.74
0.37
1.00
0.95
0.61
0.99
ρC
×10-4
Ωcm2
1.66
1.65
2.08
5.83
8.63
7.62
7.70
8.34
7.78
7.05
6.94
8.37
X
X
X
X
ρC
×10-4
Ωcm2
1.32
1.22
1.34
2.87
4.46
4.27
4.20
4.48
4.17
3.92
3.78
4.16
X
X
X
X
ρS
Ω/£
601
629
743
1400
1947
1895
1936
2096
1922
1780
1762
2054
3350
3033
3088
3314
X denotes no measurement
- 73 -
ρS
Ω/£
2492
2614
3021
6273
8642
8168
8159
8732
8265
7733
7692
8758
X
X
X
X
ρS
Ω/£
1810
1865
1993
3305
4757
4611
4656
5118
4737
4506
4491
4756
X
X
X
X
Chapter 5 Characterization and results
Sang-Kwon Lee
5.3.2 Ni and TiW (30:70) contacts on n- and p-SiC (Paper V, VI, VII)
Ni/n-SiC
Nickel contacts are widely used as Ohmic contacts to n-type both 4H- and 6H-SiC due
to its lower specific contact resistance and the reproducibility after high-temperature
annealing (see also Table 5-3). Normally high-temperature annealing (> 650 oC) creates
a new polycrystalline δ Ni2Si phase at the interface as mentioned in section 5.1 (See
also Figure 5-3, 5-12, and 5-14). The specific contact resistance for Ni is summarized in
Table 5-3. Figure 5-32 (a) shows the SEM view of Ni contacts after 950oC annealing,
indicating the new phase on the surface. Figure 5-32 (b) and 5-33 show the microscopic
mapping of the specific contact resistances for annealed Ni Ohmic contacts on highly
doped (1.1 × 1019 cm-3) n-type 4H-SiC (Paper VII).
7
1E-4
6
6,3E-5
6,3E-6
4E-6
4E-5
5
Y- Position
Ni2Si
2,5E-5
6,3E-6
1E-5
1,6E-5
4
1E-5
6,3E-6
6,3E-6
6,3E-6
4E-6
3
2,5E-6
1,6E-6
2
1E-6
M7
1
1
2
3
4
5
6
7
X- Position
o
2
Specific Contact resistance ρC(Ωcm )
Figure 5-32. (a) SEM of Ni contacts after 950 C annealing and (b) the contour
mapping of specific contact resistance for Ni contacts. The solid line indicates the
contour line of each measurement.
-4
1x10
19
-3
Ni on n+ 1.1x10 cm
A
A'
M7
A
-5
1x10
A'
Top view
-6
10
1
2
3
4
5
6
TLM Position (A-A')
Figure 5-33. The bar-graph mapping of he specific contact resistance for Ni
contacts. The right circle shows the exact position of the sample on the wafer.
- 74 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
TiW/n- and p-SiC
As mentioned in the section on material characterization, polycrystalline (Ti,W)Si2 and
(Ti,W)C1-x phases were created at the interface and then finally changed it to Ohmic
behavior due to the high-temperature annealing (above 950oC). We found that it is
reproducible, has low specific contact resistance, and compatible to the end process
including bonding process. This makes TiW the best candidate for both n- and p-type
Ohmic contacts to SiC [93]. As shown in Figure 5-34 (a) and (b), both p-and n-type
TiW Ohmic contacts have a good uniformity with the specific contact resistance of 1.2
×10-4 Ωcm2 and 3.3 × 10-5 Ωcm2, respectively. We will describe more detail about TiW
in the next section with some important factors to make low resistivity Ohmic contacts.
10
-3
1x10
-4
1x10
-5
10
-6
-3
10
19
-3
2
Contact resistivity ρc(Ωcm )
N-type (TiW) 1.3x10 cm
-4
o
After cooling down to 25 C
2
ρc(Ωcm )
1x10
-5
1x10
-6
10
0
50
100
150
200
250
300
Average ρc
0
o
Temperature ( C)
5
10
15
20
25
Data number
Figure 5-34. (a) The temperature dependence of the specific contact resistance for n-type
TiW Ohmic contacts and (b) the distribution of the p-type TiW Ohmic contacts. The
average specific contact resistance was 1.2 × 10-4 Ωcm2.
- 75 -
Chapter 5 Characterization and results
Sang-Kwon Lee
5.3.3 Microscopic mapping of specific contact resistance (Paper VI, VII)
For this study, the works on the microscopic mapping of the specific contact resistances
using sputtered TiW contacts are described.
Whole wafer mapping (n+-SiC)
The detailed sample preparation and differences among the samples are summarized in
Table 5-6. The inset denotes the exact sample position on the 35 mm wafer. Figure 5-35
shows the contour mapping of the specific contact resistance for each sample (M1 to
M6). In order to investigate the gradient behavior of the ρc for each adjacent sample the
bar-graph mapping of the ρc with the standard deviation is also presented in Figure 536(a) to (d). Comparing sample M4 and M6, which was treated under exact same
sample preparation, we found the specific contact resistance had gradient behaviors
decreasing from the center to edge as shown in Figure 5-36(a). It indicates that the
center region has a lower doping concentration than that of the edge region. Figure 536 (b) also shows similar behavior even though these samples are differently prepared.
Table 5-6. Summary of the process and specific contact resistances for all samples
Metals
Pretreatment
ICP
etchinge
(W/min)
S/Oh
Specific contact
resistance
(Ωcm2)
Average
STDEV
1250oC
(1 hr)
2.34×10-4
4.15×10-4
1.27×10-3
1.40×10-3
200
30W(3)→
60W(3)
30W(3)→
60W(3)
30W(2)
3.26×10-5
4.96×10-6
Pt/Ti/TiW
200
Xa
6.47×10-5
8.17×10-6
Au/Ti/TiW
None
Xa
2.40×10-5
1.76×10-5
3.95×10-5
5.79×10-6
1.84×10-5
1.47×10-6
3.77×10-5
6.31×10-6
7.47×10-6
2.47×10-6
Sub.
Tem
p
(oC)b
Samplesj
Mapping
Long term
(Capping
layersg)
M1
TiW
Xa
200
M2a
TiW
Xa
200
M3
TiW
Xa
M4
TiW
M5i
M5-1 TiW
None
None
c
M6i
M7d
M5-2
TiW
M6-1 TiW
Au/Ti/TiW
M6-2
TiW
Ni
Xa
200
None
Xa
Xa
c
a
Not performed.
Substrate temperature for the TiW sputtering.
c
Without substrate heating.
d
M7 was prepared by e-beam evaporator for reference.
e
Platen power (30W or 60W) under the same coil power (600W).
g
Capping layer was performed by e-beam evaporator on annealed TiW layers and it is not
annealed by deposition.
h
S/O denotes sacrificial oxidation.
i
M5 and M6 samples were cut into two called M5-1, M5-2, M6-1, and M-6-2
M5-1 and M6-1 samples were capped with sequentially evaporated Ti and Au.
j
All samples are annealed at 950oC in a vacuum chamber for 30 min and measured at room
temperature.
b
- 76 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
5
1
1E-3
2
6.3E-4
4
1.6E-5
4E-4
Y position
2.5E-4
3
1.6E-4
3
4E-4
1E-3
1E-4
6.3E-5
4
4E-5
2.5E-5
2
5
1.6E-5
2.5E-4
1E-5
M1
1
1
2
3
4
5
M2
6
6
1
2
3
X Position
4
5
6
7
X Position
1
1
6,3E-5
1E-3
6,3E-4
Y Position
2
2
4E-4
2,5E-4
1,6E-4
3
3
1E-4
6,3E-5
4E-5
4
4
2,5E-5
4E-5
1,6E-5
1E-5
5
6,3E-5
M3
5
1
2
3
4
5
6
7
M4
6
8
X Position
1
2
3
4
5
6
XPosition
1
1
1E-3
6,3E-4
2
2
4E-5
Y Position
4E-4
2,5E-4
3
3
1,6E-4
1E-4
4
6,3E-5
4
4E-5
4E-5
5
2,5E-5
5
1,6E-5
4E-5
6
1
2
3
4
5
1E-5
6
M6
M5
7
6
X Position
1
2
3
4
5
6
7
XPosition
Figure 5-35. The contour mapping of the specific contact resistance of sputtered TiW
Ohmic contacts for sample M1, M2, M3, M4, M5, and M6. The measurement was
performed at room temperature.
- 77 -
-3
2
10
Sang-Kwon Lee
Specific contact resistance ρC(Ωcm )
2
Specific contact resistance ρC(Ωcm )
Chapter 5 Characterization and results
(a)
A
Top view
M4
M6
A
M4
10
-4
10
-5
A'
A'
M6
10
-3
A
M3 M4 M5
M4
M3
10
-4
10
-5
(b)
A' Top view
M5
A
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
A'
5
10
15
20
TLM Position (A-A')
-1
A
10
A'
(c)
M5
M2
-2
Specific contact resistance ρC(Ωcm2)
10
2
Specific contact resistance ρC(Ωcm )
TLM Position (A-A')
A
M2
10
-3
1x10
-4
M5
1x10
-5
10
-6
A'
Top view
0
1
2
3
4
5
6
7
8
9 10 11 12 13
TLM Position (A-A')
-2
10
TiW contact on n+ 4H-SiC
(d)
A
A'
M3
M1
-3
10
A
M1
M3
-4
10
A'
Top view
-5
10
0 1 2 3 4 5 6 7 8 9 10 11 12
TLM Position (A-A')
Figure 5-36. The bar-graph mapping of the specific contact resistances of different
regions (a) M4→M5, (b) M3→M4→M5, (c) M1→M3, and (d) M2→M5. The inset
indicates the position of each sample and scan directions on the wafer.
The influence of the surface pre-treatment (Paper VI)
We will discuss how the surface roughness induced by ICP etch affects the formation of
the Ohmic contacts. For this study, we prepared three different samples (M1, M2, and
M3) on the same wafer (see Table 5-6). The comparison of the I-V curves for three
different samples (M1, M2, and M3) after annealing at 950oC shows that the samples
etched with 60W (M1 and M2) have higher resistances compared to the 30W-etched
sample M3. Figure 5-36 (c) and (d) also show big differences of the ρC and step
behaviors compared to sample M3 and M5. The specific contact resistances for the unetched sample (M6) and 30W etched, M3 (≈ 0.22 µm etched), was 3.8 × 10-5 Ωcm2 and
3.3×10-5 Ωcm2, respectively with good uniformity of the specific contact resistances.
The specific contact resistances of these samples are almost identical even though
sample M3 has slightly lower specific contact resistance, indicating sample M3 (30W
etched sample) does not contain any critical damage, which is enough to degrade the
specific contact resistances. However, the ρc for sample M2 increased by a factor of 40
- 78 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
2
Specific contact resistance ρc (Ωcm )
compared to the un-etched sample and 30W etched sample, having a much higher
specific contact resistance of 1.3 × 10-3 ± 1.4×10-3 Ωcm2. We also observed that 5 out of
35 TLM structures did not work due to the severe damages induced by the ICP etch. To
evaluate the passivation of a surface damaged sample, M1 was oxidized (1250oC, 1 hr,
≈ 58nm) after an ICP etch with 60W of platen power and the oxide was etched prior to
metal deposition (sacrificial oxidation). From Figure 5-36 (c), we observed the specific
contact resistance decreased by a factor of 6 with good uniformity on different TLM
structures even though it did not reach the ρc for un-etched and 30W etched sample.
This result indicates that the deep etching by ICP with medium power (60W) can cause
severe surface damage, which is corresponding to our earlier study on Schottky
contacts[64], and it directly affects the Ohmic contact formation. High temperature
oxidation can remove some of the surface damage but not all. The oxide thickness was
58 nm, which means that we removed damage to a depth of about 30 nm. Figure 5-37
shows the comparison of the specific contact resistances depending on the roughness for
sample M1, M2, M3 and M6. From Figure 5-37 we found that the Ohmic contacts
formed on a much rougher surface had much higher specific contact resistance.
10
-2
M2(60W etched)
10
-3
10
-4
M1 (60W, S/O)
M6 (un-etched)
M3 (30W etched)
10
-5
0
10
20
30
40
50
60
70
80
90
Roughness (Å)
Figure 5-37. Comparison of the specific contact resistance with roughness for sample
M1, M2, M3, and M6.
- 79 -
Chapter 5 Characterization and results
Sang-Kwon Lee
5.4 Long-term reliability tests at high temperature
5.4.1 In vacuum
2
Specific contact resistance ρC(Ωcm )
Long-term operating Ohmic contacts are also required for stable performance of device
at high temperature and high power operation. For the long-term study, we tested 3
different samples such as Au/Ti/TiW and Pt/Ti/TiW with reference contacts without
capping layer. The results of long-term reliability tests at 500 and 600 oC in a vacuum
furnace for 308 hours are shown in Figure 5-38. The initial ρc was 2×10-5, 6×10-5, and
4×10-5 Ωcm2 for Au/Ti/TiW, Pt/Ti/TiW and TiW, respectively. The difference of initial
values between contacts was due to on the wafer (see also section 5.3.3). The reason to
use capping layers (in our case, Pt and Au) is to protect degradation, e.g. oxidation and
to be compatible to the wire bonding. As shown in Figure 5-38, all of the samples with a
capping layer show excellent properties of the ρc without any Ohmic contact
degradation. It also shows that the specific contact resistance of a sample without a
capping layer increased slightly due to the oxidation at the interface or the surface. We
also found that after 308 hours test the sample with Au capping layer had much surface
degradation and was damaged by the long-term reliability tests (see Figure 5-39).
However, the sample with a Pt capping layer did not show any surface damage or a
specific contact resistance degradation.
#M5-1 (Au/Ti/TiW @ Alice)
#M5-2 (TiW @ Alice)
#M6-1 (Au/Ti/TiW @ ET sputter)
#M6-2 (TiW @ ET sputter)
#M4 (Pt/Ti/TiW @ ET sputter)
-4
10
o
o
500 C
600 C
-5
10
0
50
100
150
200
250
300
350
Annealing Time (hrs)
Figure 5-38. The specific contact resistance for different capping layers as a function
of annealing time at 500 oC or 600 oC in a vacuum furnace.
- 80 -
Processing and characterization of silicon carbide contacts for high power and high temperature device applications
Figure 5-39. (a) SEM view of
Pt/Ti/TiW Ohmic contacts after a
long-term reliability test for 308
hours.
Figure 5-39. (b) SEM view of
Au/Ti/TiW Ohmic contacts after a
long-term reliability test for 308
hours.
5.4.2 In oxidizing ambient
High-temperature operation chemical sensors with fast gas responses are of
considerable interest, for instance, for the control of combustion processes in
automotive industry. Time constants of less that about 20 ms enable the monitoring of
individual cylinders in a normal automobile engine. In order to satisfy this automotive
specification more stable sensors during the high-temperature operation are required
[111]. In this section, we tested TiW with capping layer in oxidizing atmosphere as well
as at high-temperature (500 oC or 600 oC)[112]. As shown in Figure 5-40, TLM
structures were glued on heaters. A Pt-100 element is also glued on the heater for the
temperature control. The heater is mounted on a 16-pin socket, which is put in an Alblock with a gas flow channel. The gas flow over the sensors was around 80 ml/min of
20% O2 in N2 and the annealing temperature was 500 or 600 oC.
Figure 5-40. Measurement setup including gas-handling system. A gold plated 16pin holder with a heater welded to the pins. On the heater, TLM structures, a
MISiCFET, and Pt-100 element are mounted. The metal contacts of the sensor and
TLM structures are gold bonded to the pins (after ref. [112]).
- 81 -
Chapter 5 Characterization and results
Sang-Kwon Lee
0
o
20% O2in N2 at 600 C
o
20% O2in N2 at 500 C
2
Specific contact resistance (Ωcm )
10
-2
10
-4
1x10
-6
10
0
200
400
600
Annealing time (hours)
Figure 5-41. Specific contact resistance for Pt/Ti/TiW/n-SiC measured at 500 oC and
600 oC in oxidizing ambient (20 %O2/N2) up to 500 hours.
As-deposited samples had a specific contact resistance of ≈ 1×10-5 Ωcm2. The results of
the measurement of two LTLM structures annealed in an oxidizing ambient at 500 oC
and 600 oC are shown in Figure 5-41. The ρC increases over time, but the contacts were
still working after more than 500 hours at 500oC with a 10-20 times higher values of ρC.
At 600 oC, the ρC increases faster than for 500 oC and the measurements were stopped
after 200 hours due to the failure of the contacts. Other work [113] with different
metallization schemes such as Ni/TaSix/Pt and TaSix/Pt for high temperature gas sensor
applications also confirms that the TiW/Ti/Pt shows the best performance while both
Ni/TaSix/Pt and TaSix/Pt show a rather poor performance at high temperature (500-600
o
C) in oxidizing ambient. Both metal stacks (Ni and TaSix based), which is commonly
used for MISiCFET [111], did not work at all after less than 100 hours at 600oC in same
oxidizing ambient [113].
- 82 -
6. Conclusions and future work
B
oth Schottky and Ohmic contacts to silicon carbide have been characterized by
electrical and material measurements. Several promising metallization schemes
such as sputtered TiW, co-evaporated TiC, and evaporated Ti and Ni for both Ohmic
and Schottky contacts to n- and p-type silicon carbide (4H- and 6H-SiC) were presented
and characterized in this thesis. The main achievements, divided in two parts (Schottky
and Ohmic parts), of each appended paper are summarized.
Improvement of Schottky diode performance
Sputtered TiW (weight ration 30:70) Schottky contacts have been fabricated and
investigated on n- and p-type 4H-SiC. The thermally stable ideality factors for n- and ptype after annealing at 500 oC in a low-pressure vacuum furnace were 1.06 and 1.08,
respectively. Our results from I-V and C-V measurements show that there is no Fermilevel pinning in sputtered TiW Schottky contacts on SiC after low-temperature
annealing. This can reduce the barrier height inhomogeneities of Schottky diodes and
improve the backside contacts.
Observation of the relationship between SBH and metal work function (ptype contacts to silicon carbide)
Schottky barrier diodes of several metals (Ti, Ni and Au), having different metal work
function to p-type 4H-SiC (Si-face) were characterized using I-V and C-V
measurements. From our measurements, the SBH and metal work function show a
linear relationship of φBp=4.58-0.61φm and φBp=4.42-0.54φm for I-V and C-V
characteristics at room temperature, respectively. It means that the SBH strongly
depends on the metal work function even though the Fermi level is partially pinned.
A new approach for Schottky contacts using Au nano-particles
By the incorporation of size-selected gold nano-particles (normally ≈ 20 nm in
diameter) in Ti Schottky contacts on silicon carbide, we observed considerably lower
barrier height of the contacts. The reduction of the Schottky barrier height is explained
and simulated using a model with enhanced electric field at the triple point (Ti-Au-SiC)
due to the small size of the Au nano-particles and the large difference of the barrier
height between Ti and Au on SiC.
Low-resistivity titanium carbide (TiC) contacts
Co-evaporated TiC on highly doped n- and p-type grown epilayers and aluminumimplanted layers was investigated. TiC was formed by UHV co-evaporation with Ti and
C60 at low substrate temperature. The lowest specific contact resistances were 5×10-6,
2×10-5, and 2×10-5 Ωcm2 for TiC contacts on n+ epilayers, on p+ epilayers, and on Al
implanted layers, respectively, even though there was a lack of homogeneity of the
specific contact resistances.
- 83 -
Chapter 6 Conclusions and future work
Sang-Kwon Lee
Ohmic contact formation on an ICP etched surface of SiC
For this study, we used and tested sputtered TiW Ohmic contacts to n-type SiC. High
temperature oxidation recovered some of the etch damage caused by ICP even though it
did not fully recover etch damage for the sample etched medium power (60W). We also
found that the specific contact resistance is highly related to the surface roughness and
quality of the metals. A lower specific contact resistance was obtained due to the
smoother interface.
Microscopic mapping of specific contact resistance
The mapping of specific contact resistance was performed on a whole wafer (35 mm
wafer from CREE) using sputtered TiW in order to see the distribution on the wafer.
The results show that the specific contact resistance had a decreasing distribution from
the center to the edge region on the wafer.
Long-term reliability tests
Long-term reliability tests of contacts were performed at 500 – 600 oC in vacuum as
well as in oxidizing ambient for high-temperature operation gas-sensor applications.
Titanium tungsten (Pt/Ti/TiW) contacts with Pt capping layer shows the best results
without surface degradation and have stable specific contact resistance at 500-600 oC in
a vacuum chamber for 308 hours..
General conclusions
Based on our studies, the best Ohmic contacts found so far to n-type SiC was annealed
Ni, which had a lowest specific contact resistance of ≈ 8 × 10-6 Ωcm2 and a good
reproducibility. However, for the device application point of view such as long-term
reliability and complete metallization schemes (that is a contact layer, a diffusion barrier
layer, and a wire-bond compatible conducting layer), Ni show a lack of reliability and
manufacturability due to its rough surface after high-temperature annealing, while
sputtered titanium tungsten (TiW) has much more flexibility instead. TiW can be
applied for n-type as well as p-type contacts to SiC with a low specific contact
resistance (≈ 10-4 – 10-5 Ωcm2), is compatible to the wire-bonding, and is very inert with
respect to reaction with SiC under high-temperature due to its high melting point. TiC is
also a promising material for low resistivity n- and p-type metallization on highly doped
silicon carbide using a co-evaporation method even though it is still under development
due to the lack of reproducibility, the limit of sample size, and slow deposition rate.
Future work
Finally, a more detailed study of Ohmic contact formation on ion implanted layers of
SiC is needed since ion implantation can be used to make selective doping and it has
many advantages even though its process is not fully optimized in SiC technology. The
Ohmic contact formation correlated with Al and B implanted layers should be
investigated more. In addition, as we considered three different methods of metal
deposition, the sputter deposition should be highlighted because it offers many
advantages as we discussed previously. One main reason is that sputter deposition can
be used to deposit refractory materials, which is useful for long-term reliability, since
such materials are often difficult to evaporate, and hence sputtering may be the best
practical way.
- 84 -
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