Characterization of Northeastern University’s Carbon Nanotube Based Nanoswitch and a Study of Contact Resistance at Carbon Nanotube-Metal Interface
A Thesis Presented
by
Suchit Shreyas Shah
to
The Department of Electrical and Computer Engineering
in partial fulfillment of the requirement
for the degree of
Master of Science
in
Electrical Engineering
in the field of
Electronic Circuits and Semiconductor Devices
Northeastern University
Boston, Massachusetts
January, 2009
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NORTHEASTERN UNIVERSITY
Graduate School of Engineering
Thesis Title: Characterization of Northeastern University’s carbon nanotube based nano-switch
and study of contact resistance at carbon nanotube-metal interface
Author: Suchit Shreyas Shah
Department: Electrical and Computer Engineering Department
Approved for Thesis Requirement of the Master of Science Degree
______________________________________________________ ____________________
Thesis Adviser
Date
______________________________________________________ ____________________
Thesis Reader
Date
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Thesis Reader
Date
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Department Chair
Date
Graduate School Notified of Acceptance:
______________________________________________________ ____________________
DIRECTOR OF THE GRADUATE SCHOOL
DATE
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ABSTRACT
This thesis presents electrical test results of carbon nanotube based nanoswitches fabricated at Northeastern University. A study of contact resistances of
carbon nanotubes with various metals and a preliminary look at the reliability of the
contacts was also carried out. A test system was designed to make measurements of
low-level electrical signals. The test setup could measure currents down to the 100
femto-amps regime and voltages as low as 100V accurately. The noise margin for
the test setup was 78mdB (80 fA). Experiments include two-wire and four-wire
resistance measurements, switching experiments and anneals with temperatures
ranging from 100OC to 500OC.
A new template for nanoswitch was designed and fabricated to perform high
rate assembly. Following the assembly a study of contact resistance of carbon
nanotubes with different metals at different temperatures was made. The effects of
contact resistance of the annealed samples with time were also studied. This study
provides insight into the reliability of these switches and gives direction for
developing better contacts.
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ACKNOWLEDGEMENTS
I would like to thank everyone who has contributed to my research in the
preparation of this thesis. My advisor, Prof. Nicol McGruer, has been a tremendous
help during my time at Northeastern University. I am grateful for his knowledge and
support, as well as for giving me the opportunity to work under him in the
interesting and challenging field of Nanotechnology. Also, I would like to thank Dr.
Sivasubramnian Somu, for assisting me with all of the design aspects of nano-switch.
His knowledge in this area has been an invaluable contribution to my education. I
would also like to thank Prof. George Adams and Prof. Ahmed Busnaina for agreeing
to be on my thesis committee. In addition, I extend my deepest gratitude to the
graduate students who have helped me in various areas of the work for this thesis.
Specifically, Peter Ryan, Taehoon Kim and Anup Singh have helped me with ideas for
the test system and have contributed to the understanding of the CNT switches that
were used. Most of all, I would like to thank both my roommates and friends who
have provided me with unconditional motivation and support.
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TABLE OF CONTENTS
1. INTRODUCTION ................................................................................................................................ 11
1.1 SINGLE WALLED CARBON NANOTUBES (SWNT) ................................................................................... 11
1.1.2 Electronic Properties of CNT................................................................................................................12
1.1.3 Mechanical Properties of CNT .............................................................................................................14
1.1.4 Electrical Properties of CNT .................................................................................................................15
1.2 CONTACT RESISTANCE OF CNT................................................................................................................... 16
1.3 CARBON NANOTUBE BASED LOGIC DEVICES ............................................................................................. 17
1.4 CARBON NANOTUBE BASED MEMORY DEVICES ....................................................................................... 18
1.5 NEMS SWITCHES........................................................................................................................................... 19
2. DESIGN OF NANOSWITCH ............................................................................................................. 21
2.1 TEMPLATE DESIGN ........................................................................................................................................ 21
2.2 FABRICATION .................................................................................................................................................. 24
2.3 SWNT DIRECTED ASSEMBLY ...................................................................................................................... 26
2.4 ELECTRICAL CHARACTERIZATION ............................................................................................................... 31
2.5.1 E-beam Irradiation...................................................................................................................................34
2.5.2 Annealing ......................................................................................................................................................34
3. TEST-BENCH SYSTEM ..................................................................................................................... 36
3.1 TEST PROCEDURE .......................................................................................................................................... 39
3.2 TEST RESULTS ................................................................................................................................................ 40
3.2.1 Sweep Voltage Actuation .......................................................................................................................40
3.2.2 Step Voltage Actuation ...........................................................................................................................42
3.2.3 Applied Voltage versus time .................................................................................................................43
3.3 EVOLUTION OF THRESHOLD VOLTAGE....................................................................................................... 44
3.4 RESISTANCE MEASUREMENTS ..................................................................................................................... 45
3.4.1 Kelvin Measurement ................................................................................................................................46
3.4.2 Greek Cross Measurements ...................................................................................................................53
4. CONTACT RESISTANCE ................................................................................................................... 57
4.1 STUDY OF CONTACTS OF CARBON NANOTUBE WITH METALS ................................................................ 57
4.2 CONTACT RESISTANCE OF CARBON NANOTUBE WITH VARIOUS METALS ............................................ 61
4.3 TEST RESULTS FOR DIFFERENT METALS .................................................................................................... 67
Chromium-Gold (Cr-Au) ....................................................................................................................................67
Titanium-Gold (Ti-Au) .......................................................................................................................................70
Titanium-Palladium(Ti-Pd).............................................................................................................................73
Titanium-Tungsten (TiW) ................................................................................................................................75
Titanium(Ti) ...........................................................................................................................................................77
Ruthenium (Ru) .....................................................................................................................................................79
5. DISCUSSION ........................................................................................................................................ 82
6. CONCLUSION ...................................................................................................................................... 91
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7. BIBLOGRAPHY .................................................................................................................................. 92
8. FOOTNOTES ....................................................................................................................................... 97
LABVIEW SNAPSHOT FOR I-V MEASUREMENTS FOR NANO-SWITCH ....................................................... 97
NOISE MEASUREMENT FOR TEST SETUP .......................................................................................................... 98
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LIST OF FIGURES
Figure 1 - Structure of Graphene sheet which can be rolled up to realize a carbon
nanotube structure as explained by Dresselhaus [2] ................................................... 12
Figure 2: Arrangements of carbon atoms in a metallic and semiconducting CNTs
with band diagrams showing the cross-over of bands at Fermi level for metallic
and the existence of band gap for semiconducting nanotubes [3]......................... 13
Figure 3 - SEM Image of original design for nano-switch .................................................... 22
Figure 4 - Layout for new design ................................................................................................... 23
Figure 5- SEM Image of new design .............................................................................................. 24
Figure 6 - Schematic of fabrication process for nano-switch ............................................. 26
Figure 7 - Schematic of Dielectrophoresis assembly of CNT on nanoswitch ................ 27
Figure 8 - SEM Image of an assembled SWNT on a single trench design nanoswitch
........................................................................................................................................................... 28
Figure 9 - Schematic of a single trenched nanoswitch design ............................................ 28
Figure 10 - SEM Image of a single SWNT across a double trench nanoswitch ............ 29
Figure 11 - SEM Image of assembly of bundles of carbon nanotube on a single trench
nanoswitch .................................................................................................................................... 30
Figure 12 - SEM Image of assembly of bundles of carbon nanotubes on a double
trench nanoswitch ..................................................................................................................... 30
Figure 13 - SEM Image of a device due to ESD ......................................................................... 31
Figure 14 - SEM Image at high angle showing build-up of material after testing nanoswitch in air as seen by Peter Ryan[51] ............................................................................ 32
Figure 15 - Formation of high resistance Schottky contacts as well as ohmic contacts
can be observed after assembly for Cr-Au samples ...................................................... 33
Figure 16 - Reduction in resistance can be observed for same sample before and
after e-beam irradiation .......................................................................................................... 34
Figure 17 - A typical high contact resistance sample assembled using
dielectrophoresis before annealing .................................................................................... 35
Figure 18 - Schematic of a Guarded connection of SMU with a triaxial cable[54]...... 37
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Figure 19 - Schematic of a guard connection for voltage measurement using Keithley
2410[54] ........................................................................................................................................ 37
Figure 20 - Flowchart of testing procedure for nanoswitch actuation ........................... 39
Figure 21 - I-V Characteristics of nanoswitch between gate electrode and suspended
nanotube with possible indication of electro-mechanical switching due to large
hysteresis loop............................................................................................................................. 41
Figure 22 - I-V Characteristics of nanoswitch between gate electrode and suspended
nanotube with possible indication of charge trapping ................................................ 42
Figure 23 – Plot of resistance between gate electrode and suspended nanotube for
different read-out-voltages. The volatile behavior of nanoswitch is seen here
when the switch actuates at +12V and retreats back when measured again at
low bias read-out-voltage. ...................................................................................................... 43
Figure 24 Plot of current versus time for a constant voltage supply............................... 44
Figure 25- Switching cycles for a nanoswitch. ......................................................................... 45
Figure 26 - Schematic of 2-wire resistance measuremet electrical setup ..................... 47
Figure 27 - Schematic of 4-wire resistance measurement electrical setup .................. 47
Figure 28- Two-wire and Kelvin resistance measurements on Nanoswitch. Blue
readings are 2-Wire Measurements and Red readings are 4-Wire
Measurements ............................................................................................................................. 49
Figure 29 - Kelvin and two wire measurements on double trench nanoswitch. Blue
readings are 2-Wire resistance measurements and Red readings are 4-Wire
measurements. ............................................................................................................................ 50
Figure 30 - Two and Four wire measurements on nanoswitch. Blue are 2-wire
resistances and Red are 4-wire Resistances .................................................................... 51
Figure 31 - Resistance model for Contacts using transmission line effect .................... 52
Figure 32 - A resistance model considering transmission line effects for contacts
between electrode 1 & 3 and the nanotube. The Rcon is shown only for
electrodes 1 &3 but applies for electrodes 2&4 also. Current is sourced between
electrodes 2 & 4 while voltage is measured between electrode 1 & 3 .................. 53
Figure 33 - Schematic of electrical test setup for a Greek cross structure 4-wire
resistance measurement ......................................................................................................... 54
Figure 34 Schematic of a Greek Cross structure measurement ......................................... 55
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Figure 35 - Two wire and 4-wire resistance measurements on a single trench
nanoswitch. The contact resistance measured for this device was near 190kohms. ............................................................................................................................................... 56
Figure 36 - Schematic of bending of carbon nanotubes along the electrodes [62] .... 58
Figure 37 Heat treatment of two Pd-contacted samples. Pd-A samples had bundles
of SWNT while Pd-B is assumed to have a single SWNT[73]. ................................... 61
Figure 38 - (a)Schematic band diagram depicts the Schottky barrier height
differences in 3 CNTFET with different diameters (b) same diameter but
different metals (c) On current for Pd, Ti and Al contacts of CNTFET [75] ......... 63
Figure 39 Transmission coefficient v/s energy near Fermi level for different metals
(f) I-V curve for different metals (g) Contact resistivity per nm2 at metalgraphite interface[70] .............................................................................................................. 64
Figure 40 - Schottky barrier height and tunneling barrier height for different metal/
CNT combinations as a function of interfacial distance[76]. ..................................... 65
Figure 41 - I-V characteristics with respect to Cr1, Ni, Mo-Ni and Cr2 on same SWNT
from 30K to 295K at Vgs =0V[77] ........................................................................................ 66
Figure 42 I-V curves for Ti, Pd, Ta and W contacts with carbon nanotube[69] .......... 67
Figure 43 Plot of initial and after anneal resistances at different temperatures for
CNT on Cr-Au electrode samples. Square value is initial resistance while
triangle is after anneal.............................................................................................................. 69
Figure 44 Plot of change in resistance from initial resistance to after anneal and
after 15 days ................................................................................................................................. 70
Figure 45 Box & Whisker plot for resistance of Cr-Au samples after annealing ......... 70
Figure 46 Plot of initial resistance and after anneal resistance at different
temperatures for Ti-Au samples .......................................................................................... 71
Figure 47 Plot of change in resistance from initial value to after anneal and after 15
days for all Ti-Au samples ....................................................................................................... 72
Figure 48 Box & Whisker plot for final measured resistances after anneal for Ti-Au
samples........................................................................................................................................... 73
Figure 49 Plot of initial resistance and after anneal at different temperature for TiPd samples .................................................................................................................................... 73
Figure 50 Plot of change in resistance of CNT from initial assembly, after annealing
and after 15 days for Ti-Pd samples ................................................................................... 74
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Figure 51 Box & Whisker plot for distribution of measured resistance after
annealing Ti-Pd samples.......................................................................................................... 75
Figure 52 Plot of resistance of CNT before and after anneal at different temperatures
for TiW samples .......................................................................................................................... 75
Figure 53 Plot of change in resistance from initial assembled CNT to after anneal and
after 15 days ................................................................................................................................. 76
Figure 54 Probability and cummulative distribution plots of final resistances for
TiW samples after annealing. ................................................................................................ 77
Figure 55 Plot of initial resistance and after anneal resistance for CNT on Ti
electrode samples ...................................................................................................................... 78
Figure 56 Distribution plot of final resistances after anneal for Ti samples ................ 78
Figure 57 Plot of initial resistance and after anneal at different temperatures for
CNT on Ru electrodes ............................................................................................................... 79
Figure 58 Plot of change in resistance from initial assembly and after annealing and
after 15 days ................................................................................................................................. 80
Figure 59 Box & Whisker distribution plot for measured resistance of CNT on Ru
after anneal ................................................................................................................................... 81
Figure 60 An I-V characteristics of a typical Schottky behavior contact ........................ 83
Figure 61 Typical I-V characteristics of Ti-Pd samples showing ohmic contacts ....... 84
Figure 62 Typical I-V characteristics for CNT assembled on Ru metal samples ......... 85
Figure 63 Schematic explanation of energy band bending at metal-CNT junction
under forward and reverse bias ........................................................................................... 86
Figure 64 Probability distribution plot of log of resistances after anneal of samples
of CNT on electrode for different metals ........................................................................... 87
Figure 65 Average value of change in resistance after 15 days for different metals 88
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1. Introduction
This chapter outlines the properties of Single Walled Carbon Nanotubes (SWNT)
and their various applications, specifically for electronic devices. Therefore, various
types of logic, memory devices and NEMS switches are presented here.
1.1 Single Walled Carbon Nanotubes (SWNT)
Carbon Nanotubes (CNTs) are tubes made up of carbon atoms which can have a
length-to-diameter ratio greater than 1,000,000 and this makes them a very
interesting candidate for a variety of applications[1]. CNTs have quite unique and
surprising mechanical as well as electrical properties. Because of their symmetric
atomic lattice structure linked by covalent bonds in a cylindrical configuration, they
are extremely tough but flexible.
The main attributes of carbon nanotubes are:
 Number of Walls: Determines if the CNT is single-walled (SWNT), doublewalled (DWNT) or multi-walled (MWNT) nanotube.
 CNT Diameter: The diameter of a carbon nanotube has wide implications on
its physical properties such as mechanical, optical, electrical and magnetic
mainly due to band gap changes.
 Chirality: The chirality refers to the rolling angle of the graphene sheet, which
was used to create the carbon nanotube, and it affects on the band gap.
 CNT Wall thickness: This can be calculated by multiplying the number of
walls by the wall thickness of 0.34nm for MWNT.
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Figure 1 - Structure of Graphene sheet which can be rolled up to realize a carbon nanotube
structure as explained by Dresselhaus [2]
1.1.2 Electronic Properties of CNT
The carbon nanotube (CNT) can be visualized as a rolled up graphene sheet.
The particular roll orientation of a carbon nanotube is called the chirality[2].
Electronic properties of carbon nanotubes are determined by the exact arrangement
of the nanotube lattice. This determines whether the tube exhibits metallic or
semiconductor behavior. The CNT band structure can be derived from the band
structure of single sheets of graphene. The chiral angle  determines both the
structure and properties of CNT and is described by chiral vector Ch = na+mb,
where n and m are integers in a two-dimensional lattice space described by vectors
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a and b. There are three types of rolls: armchair, zigzag and chiral. Armchair are
formed when n=m thus making the angle between the chiral vectors 30o. These
nanotubes display metallic characteristics. Zigzag nanotubes are formed when
either n or m is equal to zero and chiral nanotubes are created from all other angles.
If n+m=3j, where j is an integer, the CNT forms a small band gap semiconductor, and
therefore displays semi-metallic behavior. Other angles including zigzag and chiral,
form semiconducting carbon nanotubes.
Figure 2: Arrangements of carbon atoms in a metallic and semiconducting CNTs with band
diagrams showing the cross-over of bands at Fermi level for metallic and the existence of
band gap for semiconducting nanotubes [3]
A metallic nanotube has two extended electron bands crossing at the Fermi
level; ideally it should act as an ideal two-channel ballistic conductor. Every electron
injected into it should pass through without scattering. Theoretically this should
result in a constant conductance of 4e2/h where e is the charge of electron and h is
Planck’s constant and is explained in detail in Section 1.2 of this thesis.
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The
structural symmetry of CNT limits the number of atomic standing wave vectors and
confines electrons to travel along the nanotube axis [4]. The number of possible
wave vectors for conduction depends directly on the diameter of the nanotube.
There is an increase in carbon atoms around the CNT perimeter for larger
diameters, and therefore more atomic wave vectors exist. The semiconducting band
gap energy is then lowered by the additional band states created by these extra
wave vectors. A nanotube with a lattice vector of (n,m) = (10,10) and a diameter of
1.4nm has a band gap of 0.5-0.65 eV [5].
There is also a great deal of interest in studying the electrical characteristics
of the metal-nanotube junction. Particularly in terms of what the mechanisms of
conduction are and why the junction characteristics can change even for metals that
have comparable work functions such as Au and Pd. Currently there are very few
known[6-8] ways to separate the highly entangled SWNT bundles into individual
tubes on the macro scale. Separation of carbon nanotubes is necessary for the
realization of many of their potential applications.
1.1.3 Mechanical Properties of CNT
The mechanical properties of carbon nanotubes are equally interesting.
Carbon nanotubes can be reversibly undergo high angle bending and exhibit tensile
strength which is unmatched by any known material. For example, the measured
tensile strength of SWNT is around five times greater than the tensile strength of
steel, but can be subjected to elastic deformation, which makes them a strong
candidate for switches. CNT’s Young modulus has been measured by thermal and
resonant vibration tests of cantilevered CNT, by direct stretching tests of CNT
samples and by bending/buckling tests using atomic force microscopy and is usually
around 1TPa and is dependent on the diameter of the nanotube.[9-11] Tensile
strengths of CNTs obtained from direct stretching tests have been found to vary
from 10 to 150 GPa.[12, 13] Khosravian et al [14]found the Poisson’s ratio of CNT to
vary from 0.14 to 0.34 according to simulation results. The bending strength of
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carbon nanotubes have been found around 8GPa using the strain at the initial
buckling point by Wong et al.[15]
1.1.4 Electrical Properties of CNT
It seems difficult to compare contact resistance of different metal/CNT
contacts because the properties of CNT vary significantly from tube to tube. Dekker
et al studied contacts between individual SWNT and metallic electrodes and found
three types of different behaviors. First they found that - (i) the I-V characteristics of
some single-walled carbon nanotube (SWNT) were non-linear at room temperature
with high zero bias resistance (>10Mohms) and resistance increases upon cooling.
These were categorized as large-gap semiconducting tubes. The second group of
SWNT had linear I-V curves at room temperature with lower resistances around 1M
and upon cooling the resistance fell to ~100K. These tubes were identified as
metallic of armchair variety.
A third type of behavior was observed where
nanotubes display similar room temperature characteristics as second group and
also a finite low-temperature density of states but the Coulomb charging signatures
were different than metallic tubes. These nanotubes were associated with low-bandgap semiconducting "quasimetallic" nanotubes as a strong dependence of
temperature on resistance was observed which was not seen in second group [16].
J.E. Fisher[17] and C.L. Kane [18] found that the resistivity of single walled
carbon nanotubes increase linearly with rise in temperature upto 600K due to
twiston scattering. The twiston is a new kind of particle which possesses mass,
charge, spin and magnetic moment in quantum mechanics[19]. The dc and ac
electrical resistivity measurements on mats and ropes of SWNT show that at high
temperature, SWNT are metallic and have a positive change in resistivity with
respect to temperature while they exhibit a non-metallic behavior (negative d/dT)
at low temperature due to 1D localization. The increase in resistivity is measured
around d/dT ~ 0.1ohm-cm/K to 0.1mohm-cm/K. Electrical transport in the lowbias region of the I-V is limited by the finite resistance of the contacts and by
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electron scattering with acoustic phonons at room temperature.[20] Joule selfheating in this region is not significant but suspended SWNT under high bias
(Vsupply>0.3V) show an electrical resistance an order of magnitude higher and strong
electron scattering with high-energy optical phonons. Pop et al [20]modeled the
resistance of SWNT as a function of supply voltage (V) and temperature (T) as
,where Rc is the contact resistance, and eff = (1/AC + 1/OP(ems) + 1/OP(abs))-1 is the
effective electron mean free path which includes elastic scattering with AC phonons,
and inelastic OP emission and absorption, h is Plank’s constant, q is charge of
electron, L is the length of tube and  is mean free path. The mean free path is
dependent on temperature and bias.
Very few articles on the dynamic conductance of carbon nanotubes have
been published in the literature [21-25]. AC conductance in a material is
complicated by the presence of time-dependent fields that can take the system out
of equilibrium. Under an applied AC field, electrons can absorb photons and thus
causes photon-assisted tunneling.[26] The dynamic response of carbon nanotubes
to AC has been investigated in the wide-band limit[21]. The dynamic effects are
quite small for low AC frequencies while there is a general reduction in the
conductance for larger frequencies. The induced displacement currents act to
reduce the normal conduction current while at even higher frequencies, conduction
is increased due to photon-assisted tunneling.
1.2 Contact Resistance of CNT
Imagine a perfect, extremely thin, straight wire in which electrons are
allowed to move only along the axis of the wire. This wire should have no defects,
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kinks or obstacles other than a connection at each end to allow current to pass
through an external circuit. If the length of the wire is shorter than the mean
collision distance between the electrons the conduction of electricity is considered
ballistic (4e2/h) and the wire should exhibit zero resistance. Carbon nanotubes are
considered potential candidates for such a perfect wire. The resistance of such a
wire can be separated into two parts: an "intrinsic resistance" due to the scattering
of electrons by imperfections in the wire and a "contact resistance" associated with
the connections to the external probes or circuit.
The two terminal resistance of such a ballistic wire is found to be ~13k ohms.
In this case it was found that this resistance is just the sum of the contact
resistances[27]. Carbon nanotubes are expected to behave like quasi onedimensional systems with quantized electrical resistance. As an example, metallic
armchair nanotubes should have resistance of h/4e2 at low bias[28]. However,
typically the electrical resistance of metal-nanotube contacts is too high for practical
application. Currently, the resistance varies widely among experiments. This reflects
a combination of inherent contact resistance and extraneous factors such as
contamination. A topic of continued experimental and theoretical investigation
remains in the study of nature of the electrical connection between the SWNT s and
macroscopic electrodes. For SWNT's transport measurements have observed
conductance G=4Te2/h with transmission probability up to T~0.5-0.6 [29]. Low
resistance ohmic contacts to carbon nanotubes are critical for exploiting their
intrinsic electrical properties and ultimately building functional electronic devices
with useful characteristics.
1.3 Carbon Nanotube based Logic Devices
Bachtold et al. [30, 31]demonstrated logic devices using field effect
transistors based on single carbon nanotubes. They built inverters, ring oscillators,
NOR gates and demonstrated SRAM capabilities in nanotube based devices. They
employed p-type cnt as well as n-type by doping it to make n-type cnt. Derycke
17
et al. [32]also transformed p-type carbon nanotubes into n-type by annealing in
vacuum. They demonstrated the technique by creating an inverter using this
approach of making p-type and n-type CNT-FETs. Honjie Dai et al. [33] also
demonstrated multistage NOR, OR, NAND and AND logic gates and ring oscillators
(frequency ~220Hz) with arrays of p-type and field manipulated n-type nanotube
field effect transistors. Martel et al[32] studied carbon nanotube field effect
transistors that have been made either by modifying the Schottky barrier or by
doping the bulk of the nanotube. They investigated the device performance and
their general characteristics and concluded that CNT based logic devices can
compete with silicon MOSFETs in terms of performance and integration capabilities
and thus present all the desirable properties needed for future electronic
applications.
1.4 Carbon Nanotube based Memory Devices
Thomas Rueckes et al. [34]developed a novel concept of molecular
computing by displaying the potential of CNT to be used as reading and writing
information. They proposed the devices could be electrostatically switched between
ON/OFF states and these reversible, bistable device elements could be used to
construct non-volatile memory elements at an integration level approaching 1012
elements per square centimeter and operation frequency in excess of 100GHz. Won
Bong Choi et al. [35] fabricated a SWNT based non volatile memory device using
oxide-nitride-oxide layer as a storage node which is typically observed in SONOS
memory. J.B. Cui et al [36]developed memory devices of high charge stability by
sweeping gate voltages in range of 3V, associated with a storage stability of more
than 12 days at room temperature. Johnson et al. [37]fabricated non-volatile
memory elements based on ambipolar nanotube FET. Leonid Maslov [38]studied
about a concept of non-volatile memory based on multi-walled carbon nanotubes
where two stable Van der Waals states exist between the inner core and vertically
top mounted electrode. Udayan Ganguly et al. [39]developed carbon nanotube
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based memory with charge storage in metal nanocrystals. Thus with the growing
potential for non-volatile memory and the scaling of flash memories towards higher
bit density operations, carbon nanotube based memories can serve an ideal
candidate with integration capabilities of 1013 elements per sq cm and operating
frequency of 100GHz theoretically.
1.5 NEMS Switches
NEMS have been a rapidly growing area of research with considerable
potential for future applications. The basic idea underlying NEMS is the strong
electromechanical coupling in devices on a nanometer scale in which the Coulomb
forces associated with device operation are comparable to the chemical binding
forces. Some prototype CNT-based NEMS have already been demonstrated, such as
gigahertz oscillators[40], nanotweezers[41, 42], random access memories[34],
sensors and nanorelays[43, 44]. Lee et al[44, 45] fabricated CNT nanorelay devices,
and Nantero Inc. presented an electromechanical memory array using nanotube
ribbons. Dequesnes et al[46] theoretically investigated the NEM switch based on a
nanotube bridge. Sapmaz et al[47] investigated theoretically the interplay between
the electrical and mechanical properties of the NEM switch based on a nanotube
bridge. Sazonova et al[48] fabricated a tunable NEM CNT oscillator. CNT bridges
were suspended over a trench between the two metal electrodes. Lieber worked on
making a SWCNT nanotweezer, which again demonstrates a prototype for a
potential NEMS switch.
NEMS have mechanical resonance frequencies in the range of 100MHz to
5GHz [49]. NEMS have very low masses and very high mechanical quality factors
attaining 1000-10000. Nano-relays with switching time of a few nanoseconds and
tunable resonators based on NEMS are recently reported. The NEMS switches have
widespread applications in the RF frequency range due to their lower losses and
higher isolation than switches based on semiconductor devices.
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1.6 Background of my thesis
This thesis is aimed at understanding of contact resistance of carbon
nanotubes with different metals and experimental results for resistance of
“nanotube-on-metal” contacts with various metals. The Center for High Rate
Nanomanufacturing at Northeastern University has designed an electromechanical
switch using a CNT as the actuating element. It is their hope that this may replace
the current semiconductor technology for logic and memory applications. The
switch template fabrication process was devised by Dr. Sivasubramanian Somu. In
this design, CNT was placed between the source and drain electrodes using a
directed assembly process developed by Taehoon Kim. The parameters for this
assembly process were later modified by Peter Ryan and myself for our
applications. High contact resistance was found during the actuation of the switch.
The interface between the carbon nanotubes with the Au electrodes played a major
role in test results as CNT formed ohmic as well as Schottky contacts with the metal.
A systematic study of contact resistance of CNT and different metals was carried out.
Moreover, ways to reduce the contact resistance between CNT and metal were also
studied using annealing techniques at different temperatures. Reliability studies
were carried out by looking at the contact resistances as a function of time. Thus an
understanding of contact resistances of dielectrophoretically assembled CNTs and
recommendations for choices of metals to be used for development of Northeastern
University’s nano-switch was made in this thesis.
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2. Design of Nanoswitch
In 2006, The Center for High Rate Nanomanfacturing proposed a new
electromechanical switch design consisting of a conducting CNT bridge placed on a
trenched substrate and connected to fixed source and drain electrodes. The gate
electrode for actuation is at the bottom of the trench between the source and drain
electrodes. The resulting capacitive force generated by applying voltage on the gate
electrode causes a bend of CNT towards the gate thus making a contact with the
trench. This two-trench design would allow a single nanotube to be actuated
electrostatically into one trench and then released from the trench by actuating the
other trench electrode. The switch displays a huge potential for design of future flipflops, memories and other logic devices.
2.1 Template Design
The template for the originally proposed nanoswitch design appears in the
figure 3. The source/drain electrodes were spaced apart from each other by a
distance of 2um and different widths of gate electrodes were used with single and
double trench designs.
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Figure 3 - SEM Image of original design for nano-switch
A new template for high rate assembly of these nanoswitches was designed
by me that involved multiple electrodes connected to a single large pad for
increasing the number of devices results from a single assembly process. The
connecting lines are then scratched off using sharp probe tips so as to have 7
separate devices assembled on a single chip by a single assembly process. The
design also serves a good example of application of the switch as LUT (Look up
Table) for a potential hybrid FPGA-NEMS approach.
22
Figure 4 - Layout for new design
23
Figure 5- SEM Image of new design
2.2 Fabrication
The fabrication process was originally developed by Dr. Sivasubramanian
Somu. According to his process, a 300~500nm thick thermal SiO2 layer is grown on
a wafer followed by Tungsten metal deposition by sputtering. 150nm of PMMA is
spun on the wafer for electron-beam lithography to define the pattern for the
metallization. The W mask is then etched along with oxide layer using dry ICP etch.
The plasma etch consisted of CHF3 flow at base pressure of 1x10 -4 torr for 3
minutes to etch 50nm deep into the SiO2 layer. A 50:1 HF: DI H2O solution is used to
isotropically undercut the oxide trench sidewalls. The metal of interest is then
deposited on wafer using E-beam evaporation followed by a lift off process for
PMMA and Tungsten removal.
24
25
Figure 6 - Schematic of fabrication process for nano-switch
2.3 SWNT Directed Assembly
After the fabrication of template, the wafer is diced into chips for individual
assembly of carbon nanotubes on it. The wafer is then cleaned using Acetone and
Piranha etch to remove any organic residues and to make the surface more
hydrophilic. A dielectrophoresis technique originally developed by Taehoon Kim et
al is used for directed assembly and alignment of single carbon nanotube on the
chip. Dielectrophoresis is a method where a dielectric particle exerts an external
force due to formation of a dipole moment when subjected to an AC field. In an AC
field,
the
time
averaged
force
on
the
particle
is
given
by
where,  is a factor depending on geometry, εm is the real part
of the permittivity of the suspending medium and E is the electric field. The factor Kf
depends on the complex permittivity of both the particle and the medium.[50]
26
Figure 7 - Schematic of Dielectrophoresis assembly of CNT on nanoswitch
A solution of 1:10,000 (CNT Solution:DI H2O) is prepared and sonnicated for
10 minutes. CNT solution provided by Brewer Science and Nantero Inc. was used for
characterization of nano-switch. 1uL of drop of this solution is then placed on the
device and a 1MHz sine wave with 2.5Vpp is applied for 60 seconds across the
electrodes for assembly of CNT. Phase shift of 180 degrees is maintained between
27
the voltages of the two opposite electrodes and ground so that assembly is uniform
and nanotube crosses over both the electrodes.
Figure 8 - SEM Image of an assembled SWNT on a single trench design nanoswitch
Figure 9 - Schematic of a single trenched nanoswitch design
28
Figure 10 - SEM Image of a single SWNT across a double trench nanoswitch
An example of a single bundle SWNT crossing over two electrodes is shown
in figure 10. Though the yield for finding such a device is merely 10% through
current assembly methods. More often, few bundles of CNT were obtained through
directed assembly that serves as the major devices for the results presented in the
thesis.
29
Figure 11 - SEM Image of assembly of bundles of carbon nanotube on a single trench
nanoswitch
Figure 12 - SEM Image of assembly of bundles of carbon nanotubes on a double trench
nanoswitch
30
2.4 Electrical Characterization
With such low yield, collecting enough data for statistical analysis was not
feasible for electrical characterization of nanoswitch devices. Moreover, the single
SWNT devices were highly susceptible to external parameters and ESD issues. An
example of blown up of device is shown in figure 13. Also, since the structures were
very small in the nanometer regimes, there have been fabrication and ESD issues
which could be seen where there is an assembly on a device. So building up the test
setup was one of the major tasks for testing these devices wherein knowledge of
potential of every part on the tester was necessary and grounding as much as
possible. Also, the CNT devices seemed to be susceptible to air and showed evidence
of collection of particles while under test as seen in figure 14. So later on all the
testing was done under dry nitrogen all the times.
Figure 13 - SEM Image of a device due to ESD
31
Figure 14 - SEM Image at high angle showing build-up of material after testing nano-switch in
air as seen by Peter Ryan[51]
2.5 Contact Resistance Reduction Methods
The dielectrophoretically assembled carbon nanotubes on the switch had
large contact resistances. The CNT formed Schottky contacts as well as ohmic
contacts with metals as seen in figure 15. Various techniques have been followed in
the literature for contact resistance reduction between CNT and metals. [45, 52]Two
methods were followed for reduction of contact resistances in the nano-switch.
32
8.00E-12
6.00E-12
4.00E-12
Current (A)
2.00E-12
-0.5
-0.4
-0.3
-0.2
0.00E+00
-0.1
0
0.1
0.2
0.3
0.4
0.5
-2.00E-12
-4.00E-12
-6.00E-12
-8.00E-12
Voltage (V)
Figure 15 - Formation of high resistance Schottky contacts as well as ohmic contacts can be
observed after assembly for Cr-Au samples
33
2.5.1 E-beam Irradiation
The electron irradiation creates displacements of the carbon atoms in
nanotubes, leading to very reactive broken chemical bonds, which improves its
electrical conductivity. [53] So the electrodes were bombarded with the electron
beam in a Scanning Electron Microscope with reduced raster view wherever the
CNT overlaps the metal electrodes. This injection of electrons into the CNT is
assumed to create displacements of carbon atoms in the CNT Metal interface, which
helps in reduction of resistance between them due to formation of a reactive broken
chemical bond. A reduction of 3 orders of magnitude in resistance was observed
with e-beam irradiation as seen in figure 16.
Figure 16 - Reduction in resistance can be observed for same sample before and after e-beam
irradiation
2.5.2 Annealing
Another explored approach was annealing of the chip in nitrogen ambient at
high temperatures. Very high resistances of CNT with metals were observed after
34
dielectrophoresis assembly process as seen in figure 17. So in the effort to reduce
this resistance, the nanoswitch was placed in a Tempress Mini Brute furnace MB-71
for annealing. The furnace is pre-filled with nitrogen at temperatures around 350C.
The reduction in contact resistance of the switch was 3-4 orders of magnitude with
this technique. Moreover, an extensive study of contact resistance with different
metals at various temperatures is being carried out using the same annealing
technique.
Figure 17 - A typical high contact resistance sample assembled using dielectrophoresis before
annealing
35
3. Test-bench System
A low-level measurement test system was developed for measuring currents
accurately at 100 femto-ampere levels and 100uV voltages Agilent/HP 4155
Semiconductor Parametric Analyzer was used for the current measurements. The
instrument was remotely configured using LabVIEW for data acquisition and
control. Four wire measurements were made using a Keithley 2410 Source Meter
unit and a Keithley 487 Picoammeter/Voltage source. A switch box for changing
connections between different instruments without lifting the probes from the
device was made. This avoids errors in measurements due to re-probing and
minimizes the risk of ESD. The complete test setup was built in a Faraday cage to
prevent external EMI.
Low-level signal measurements are very susceptible to noise pickup and
even minor disturbances can affect the measurements. Short triax cables were used
for current measurements with the outer conductor grounded. The nanoswitch was
highly sensitive to electrostatic discharges and grounding all floating potentials was
a major requirement in setting up the test system. The test station, switch box,
chuck, faraday cage, all measuring instruments and cables as well as the operator
were grounded to common earth to avoid ground loops and prevent build up of any
static charges. All the tests were carried out in the dark and special care was taken
to prevent blowing up of device due to switching on/off the lights and ESD.
Triax cables provided the electrical connections for current measurements
and a guard was used on the inner shielding. The guard works by driving the
shielding conductor at the same potential as the center conductor, which carries the
signal. This reduces the external noise interferences on the signal of interest and
minimizes cable leakage. Similarly, four-wire measurements were driven with the
guarded connections using a Keithley 2410 as shown in figure 18 & 19.
36
Figure 18 - Schematic of a Guarded connection of SMU with a triaxial cable[54]
Figure 19 - Schematic of a guard connection for voltage measurement using Keithley 2410[54]
In the guarded circuit shown in Figure 18, the inside shield conductor of the
triax cable is connected to the guard terminal of the HP 4155 SMU. This guard
terminal is driven by a unity-gain, low impedance pre-amplifier. So a nearly 0V
potential difference is built up between the guard terminal and center-most signal
conductor. This helps to reduce the cable leakage current between signal and
outermost ground return as well as helps to reduce the external noise interferences
affecting the signal. Similarly, as shown in figure 19, the shield conductor of triax
cable during voltage measurements is connected to the guard terminal on Keithley
2400 Source Meter Unit. The guard terminal is driven by a preamplifier which helps
37
to reduce the effect of cable leakage and making the response speed of the
measurements faster by reducing capacitive effects of the cable.
Another important task in developing the test-system was configuring the
data acquisition system. Proper configuration of the data acquisition system with
use of a settling delay for source and line cycle integration of the measured signal in
the instrument was carried out to prevent any errors due to the capacitive effects in
the cabling and to minimize random noise. With this knowledge about the complete
test setup, testing was performed on the nanoswitch.
38
3.1 Test Procedure
The test procedures for testing nanoswitch devices followed this flow-chart.
Figure 20 - Flowchart of testing procedure for nanoswitch actuation
39
Continuity testing was performed by using a voltage ramp with a peak value
of 100mV and step value of 1mV. For these measurements the current compliance
was set to 200 nA. The results for different devices are shown below which displays
ohmic as well as Schottky barrier contacts with Au pads. The switch actuation was
carried with current compliance set to 200nA and sourcing voltage as high as 10V.
3.2 Test Results
Results demonstrate volatile switching behavior from the nanotube to the
trench electrode. Some devices showed a clear indication of electro-mechanical
switching as seen in Figure 21 while other devices (Figure 22) showed signs of
different behaviors. The other behaviors include indication of charge trapping in the
silicon dioxide that lead to a hysteresis loop while sweeping voltages and tunneling
current between nanotube and gate electrode. It is believed that the carbon
nanotube can trap charge inside them as well as in the silicon dioxide [35, 55, 56].
3.2.1 Sweep Voltage Actuation
The results of voltage sweep between gate electrode and nanotube can be
seen in figure 21 and 22. Pink readings are for ramping up the voltage while blue
readings are for ramping down the voltage source. The effect of charge trapping and
a possible electro-mechanical switching could be observed from the test results.
Though one can not confidently claim from the test results about the observation as
whether it is due to switching behavior, tunneling current, charge trapping
mechanisms or some other phenomena but an increase in current with hysteresis
loop could be seen in the results.
40
2.30E-10
1.80E-10
Current (A)
1.30E-10
8.00E-11
3.00E-11
0
0.5
1
1.5
2
2.5
3
3.5
-2.00E-11
Gate Voltage (V)
Figure 21 - I-V Characteristics of nanoswitch between gate electrode and suspended nanotube
with possible indication of electro-mechanical switching due to large hysteresis loop
41
2.00E-08
1.80E-08
1.60E-08
1.40E-08
Current (A)
1.20E-08
1.00E-08
8.00E-09
6.00E-09
4.00E-09
2.00E-09
0.00E+00
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Gate Voltage (V)
Figure 22 - I-V Characteristics of nanoswitch between gate electrode and suspended nanotube
with possible indication of charge trapping
3.2.2 Step Voltage Actuation
Nanoswitch testing also included step voltage actuation that showed
potential volatile switch characteristics. A low read-out-voltage of 10mV was
applied between the gate electrode and CNT and current was measured. Then high
actuation voltage of 12V was applied and current was measured. There was a
change in 6 orders of magnitude in current between low voltage and high voltage
read-outs. The carbon nanotube was able to switch between ON and OFF states for
two cycles. The behavior appeared volatile because when the read out voltage was
reduced to 10mV it returned to a state of very little current flow. The ON/OFF ratio
was more than 5 orders of magnitude and there was a constant current flow
through the tube in ON state at read-out-voltage of 12V. Moreover, no significant
ON-current was flowing between the gate electrode and CNT for lower actuation
42
voltages (5V and 10V) as seen in figure 23. This rules out a possibility of “other
behaviors” as we observed in previous test results and suggests an indication of
actual electro-mechanical switching for two cycles. The device burnt-out after two
cycles.
Figure 23 – Plot of resistance between gate electrode and suspended nanotube for different
read-out-voltages. The volatile behavior of nanoswitch is seen here when the switch actuates
at +12V and retreats back when measured again at low bias read-out-voltage.
3.2.3 Applied Voltage versus time
Nano-switch testing with constant voltage supply as a function of time was
also being carried out. A constant voltage was applied between the trench and the
nanotube and current was measured as a function of time and showed some
conduction from the trench to the tube. As seen in figure 24, a constant gate voltage
was applied between the trench electrode and CNT for 400 seconds and current was
measured as a function of time. One can see an increase in current as a function of
43
time and voltage for 4V and 5V. This increase in current as voltage increased could
be potentially because of bending of nanotube towards the trench and measured
current as a function of time could be the tunneling current between CNT and gate
electrode. But the effect of charge trapping or some other unknown effect cannot be
ruled out by looking at this result alone and further characterization needs to be
performed to differentiate between charge trapping behavior and switching
characteristics. The switch could not withstand the large amount of current flowing
through the electrodes and eventually burnt out due to I2R heating at 5V.
Figure 24 Plot of current versus time for a constant voltage supply.
3.3 Evolution of Threshold Voltage
Repeated cycling of the same device yielded different threshold voltage. The
reduction in threshold voltage could possibly be attributed to the slip of nanotube
along the side electrodes. Due to lack of extensive test results conclusions about slip
44
can not be made for this gradual reduction in the threshold voltage. Moreover,
phenomena like tunneling through the nanotube and charge trapping in the
nanoswitch complicate the interpretation of test data further. A theoretical study
for the calculation of threshold voltages and more systematic methods for studying
the evolution of threshold voltage need to be made.
Switching Cycles
1.80E-10
1.60E-10
1.40E-10
Current (A)
1.20E-10
1.00E-10
8.00E-11
6.00E-11
4.00E-11
2.00E-11
0.00E+00
0
0.5
1
1.5
2
2.5
3
Gate Voltage (V)
Figure 25- Switching cycles for a nanoswitch.
3.4 Resistance Measurements
The contact resistance measurements were made by combination of a 2-wire
and 4-wire measurements. A four wire measurement is different from a two wire
resistance measurement in terms of it eliminates the resistance of cables and
contacts. This in turn gives a true voltage drop across the device-under-test (DUT).
Various configurations were used for making these 4-wire measurements. A Kelvin
measurement gives the resistance of only DUT thus eliminating resistances of wire
leads and contacts at probes & electrodes. Contact resistance between the CNT and
45
3.5
metal electrode can me calculated by measuring the two wire and four wire
measurements. A Greek cross structure measures the voltage drop across the
contacts of metal and CNT thus giving the true contact resistance at the interface.
Both of these configurations are described and how measurements with nanoswitch
structures were made in detail in following sections.
3.4.1 Kelvin Measurement
The following figure illustrates the basic test setup for a 4-wire
measurement. In a typical 2-wire measurement, the source current flows through
the wire to the DUT and voltage is measured across it. Such a setup faces a
disadvantage, since the voltage drop across the leads of wire, across the contacts
between the DUT and test pin as well as drop across the resistance of DUT is
measured. Thus the effective resistance measured by V/I = R2W = RDUT + 2(RCON +
RLEAD). In a Kelvin measurement a known current flows from the two outside
terminals and a voltage drop is measured between the inner electrodes. The
resistance calculated using this technique gives the true resistance of the DUT alone
since the current flowing through the leads of voltmeter is negligible owing to high
input impedance of voltmeter compared with RDUT. Thus, the resistance measured
by V/I = R4W = RDUT. + 2RLEAD . Contact resistance (RCON) can then be calculated by
following formula: RCON = (R2w-R4W)/2.
46
RCON
RDUT
RCON
RLEAD
RLEAD
V
Figure 26 - Schematic of 2-wire resistance measuremet electrical setup
RCON
RDUT
RLEAD
RCON
RLEAD
V
RLEAD
RLEAD
Figure 27 - Schematic of 4-wire resistance measurement electrical setup
47
The above-explained 4-wire measurement assumes that the drop across the
voltage measurement lead is uniform along the width. Thus if the voltage drop along
the width of the voltage measuring electrode (1 & 3) is not negligible, the reading
V/I is not the true measured contact resistance. However a transmission line model
can be applied for the calculation of the voltage drop along the contacts between the
metal electrodes.
For the measurements done in figure 27-29, a two wire resistance
measurement (blue readings) is carried out by sweeping a voltage source between
two electrodes and measuring the current flowing through it. For a four-wire
resistance measurement (red readings), a known current is swept between two
electrodes and voltage measurement is taken between the electrodes of interest. For
example, to read a four-wire resistance between terminal 1 & 3, current is flowed
through electrodes 2 to electrode 4 and voltage measurement is taken directly
between electrode 1 & 3. Thus, the measured voltage drop divided by the known
source current, gives us the 4-wire resistance measurement. Similarly, for a 4-wire
terminal resistance between 1 & 4, current is sourced between electrodes 2 & 3
while voltage is measured between 1 & 4. In the same fashion, all the measurements
are performed for rest of the electrodes.
48
1
3
2
4
Figure 28- Two-wire and Kelvin resistance measurements on Nanoswitch. Blue readings are
2-Wire Measurements and Red readings are 4-Wire Measurements
49
1
2
3
4
Figure 29 - Kelvin and two wire measurements on double trench nanoswitch. Blue readings
are 2-Wire resistance measurements and Red readings are 4-Wire measurements.
50
1
3
2
4
Figure 30 - Two and Four wire measurements on nanoswitch. Blue are 2-wire resistances and
Red are 4-wire Resistances
From the above measurements, the CNT resistance measured across the two
gate electrodes is always around ~160kThe 4-wire measurements suffer from
transmission line problem since there is a voltage drop in the carbon nanotube
along the width of the trench electrode. The four-wire resistance measured while
sourcing current through the nanotube was always higher than sourcing current
from trench to the nanotube. As you can observe from above figures, resistance
across terminal 1&3 is always higher than between terminal 1&4 or 2&3. The
explanation for this observation is, there is a voltage drop across the width of
electrode 1 or electrode 3 when current flows through it. So the effective voltage
measured is not ideally the voltage drop across the CNT between gate electrodes
1&3 but also includes a voltage drop along the width of gate electrodes. Ideally, if
51
the measurements did not suffer from transmission line problem, then the gate
electrode should have no resistance (RLEAD ~ 0). A resistance model considering the
transmission effects for the contacts between the electrode and nanotube can be
viewed as shown in figure 30. This model, assumes a voltage drop across the
contacts (RCON) as well as a shunt resistance (RSH). Thus when current flows through
the CNT, the measured voltage drop is across RCNT + 2GSH. The resistance calculated
includes a shunt resistance a shunt resistance drop. So in case when current is
sourced between electrodes 1 & 4, the measured voltage drop is RCNT + Gsh. This
model is a simplified -model for a transmission line problem, but includes a shunt
resistance which solves the problem of resistance being lower for measured
between two gate electrodes (1&3) than between gate and source electrode (1&4 or
2&3).
Gs
Rcnt
I
Rcon
Rcnt
Gs
Rcnt
h
h
Rcon
Rcon
Rcon
Figure 31 - Resistance model for Contacts using transmission line effect
52
1
3
2
4
Gsh
Rcnt
Rcnt
Rcnt
Gsh
I
Rcon
Rcon
Rcon
Rcon
V
Figure 32 - A resistance model considering transmission line effects for contacts between
electrode 1 & 3 and the nanotube. The Rcon is shown only for electrodes 1 &3 but applies for
electrodes 2&4 also. Current is sourced between electrodes 2 & 4 while voltage is measured
between electrode 1 & 3
3.4.2 Greek Cross Measurements
A Greek cross structure is used for measuring the contact between the CNT
and metal directly using a four-wire technique. In this structure the current is
sourced from the metal to the CNT from one of the electrodes to the trench and a
voltage drop is measured across the other end of CNT and metal as illustrated in
figure 32. This four wire measurement yields the contact resistance of CNT and
metal across the width of the metal and CNT which is in contact.
53
Figure 33 - Schematic of electrical test setup for a Greek cross structure 4-wire resistance
measurement
In this setup, a direct contact resistance is measured as explained in figure
33. When a current is sourced between CNT and trench, voltage is measured across
opposite electrodes and resistance is calculated. Thus as seen in figure 33, when
current flows from CNT to metal electrode and voltage is measured directly
between other end of CNT and metal electrode, measures the RCON.
54
RCNT
RCNT
RCON
V
RMETAL
RMETAL
Figure 34 Schematic of a Greek Cross structure measurement
55
Figure 35 - Two wire and 4-wire resistance measurements on a single trench nanoswitch. The
contact resistance measured for this device was near 190k-ohms.
The CNT resistance calculated from the extracted contact resistance and two
wire measurements comes out to be around 20kohmsRcnt = R2W – (2 x R4W) =
53.33k – (2 x 19.5k) = 15.2k in this case.
56
4. Contact Resistance
4.1 Study of Contacts of Carbon nanotube with metals
The experimentally measured contact resistances between various metals
and CNT have been found to be way too high to achieve the quantum conductance
value of CNT calculated theoretically. Sometimes resistances with metals like Au are
greater than 108Ω[52]. This observation leads to various theoretical explanations
for such a high measured contact resistances between CNT and metals. San Hang Ke
et al suggested in his paper[57] that on the carbon side, the tube-metal connection
is made by either straight σ-bonds from the tube end or π-bonds from the side of the
tube, while the metal is modeled by a thin nanowire or small cluster. The electronic
states of the Au and Pd electrodes are very different: Au has active s states while Pd
has only d states. The Pd d-states have stronger interaction with Carbon P-states
than Au s-d states thus Pd is predicted to be a better electrode material than Au. In
general, the ability of transition metals to bond with carbon atoms increases with
the number of unfilled d orbital. Metals with few d vacancies such as Ni, Fe and Co
exhibit poor interaction with CNTs compared to 3d and 4d metals with many d
vacancies like Ti and Nb which forms strong chemical bonds with carbon and thus
form highly stable carbide compounds in certain temperature ranges. The strong NiSWNT interaction is attributed to the curvature induced rehybridization of carbon
sp2 orbital with the Ni d-orbital [58]the Ti-SWNT interaction should be stronger
than that from Ni-SWNT and involves covalent bonding between the two. [59].
Moreover in case of SWNTs the coulomb blockade effect has been commonly
observed. The location of the tunnel barrier depends on whether the nanotube is
deposited on the metal electrode or vice versa. Various configurations including
"metal-on-tube", "tube-on-metal", "buried nanotube" and “point contact” have been
studied[60]. It is believed that the difference in contact resistances in different
configurations is because the tunnel barrier for conduction is formed not only
between the SWNT and an electrode but also inside the SWNT due to its bending in
57
cases of “tube-on-metal” configurations. When NT's are laid on the top of the metal
pads, the NT's tend to bend around the pads which drastically alters the conduction
in armchair carbon nanotubes.[28, 61]
Figure 36 - Schematic of bending of carbon nanotubes along the electrodes [62]
Li, Tongcang et al [63]suggested that in a side contacted carbon nanotube
often dangling ends contribute to the high resistances that are observed typically.
Classically these hanging ends have little effect on the conductance but in a CNT,
electron transport behavior occurs in wave fashion. Electrons will leak in a CNT
with an infinite dangling end but they will be reflected by a CNT with finite dangling
end. The conductance of a CNT with an infinite suspending end is independent of
energy around Fermi level while the conductance of a CNT with a finite suspended
end oscillates as a function of energy through the averaged local density of states
(LDOS) of a carbon atom in a CNT. Hence, the average conductance in a CNT system
with infinite dangling end will be smaller than that in a CNT system with a finite
dangling end.
However in most experimental situations, metal atoms surround a tube. As
the contact structure and quality changes from case to case, experimental results are
scattered. Contact quality will depend experimentally on the number of good
carbon-metal connections, which is essentially determined by whether the metal
wets the CNT surface[59].
58
Despite extensive experimental efforts to improve the contact transparency
and reveal the relevant factors behind it like metal material, contact structure, and
type of tube a clear picture is still not available for minimum contact resistance.
Various theoretical studies of the contacts between CNTs and metals have been
carried out with different tubes (3,3), (4,4), (5,5) and different metals like Al [64], Ni
[65], Co [66], Pd [67], Fe [68] Ti, Au, Cu, Ta [69]. The quantum mechanical
calculations predict that the cohesive strength of metal-carbon interface follows the
sequence Ti>Pd>Pt>Cu>Au and contact resistance follows the reverse sequence[70].
Experiments have been performed on study of contact resistance of SWNT
with different metals in various configurations. Maiti et al [59]studied metalnanotube contact properties and their interactions with different metals by electrowetting CNT with various metals. Based on the observation they suggested
Eb(Ti)>Eb(Ni)>Eb(Pd)>Eb(Fe)>Eb(Al)>Eb(Au) where Eb denotes the binding
energy of the metal atom to SWNT. According to their experiments, Ti coating on
nanotubes appear uniform which points to a high nucleation density [71],
suggesting strong metal-substrate interaction and Ti-SWNT binding. On the
contrary, the Au coating appeared highly discontinuous with very low nucleation
density suggesting weak Au-SWNT interaction leading to a high contact resistances
[68]. However, the observed results were in disagreement with the computed
interaction energies of a single atom of metal on SWNT[72]. The computed results
fail to explain why Ti inspite of its good wetting properties on SWNT surface, yields
ohmic contacts rarely while Pd gives reliable ohmic contacts compared to Ti and Pt
as seen by D. Mann et al [67]. Maiti suggested through simulations that isolated
atoms performed exactly as theory suggested, however, for films the metal-metal
binding within the metal-film was found to be much stronger than binding between
the film and graphite surface.
It was reported by J.O.Lee[45]that annealing could reduce the metal/CNT
contact resistance because of the formation of metal carbide at the interface.
Zhengchun Liu et al in[69] measured the contact resistance between CNT and
different metals before and after annealing. They found the side contact resistances
59
for Ta and W reduces after rapid thermal annealing at 400oC. Moreover, there was
an increase of Ti/CNT and Pd/CNT side contacts after anneal. For their experiments
they grew vertical CNT strips on Fe/Al catalyst film, then immersed them in an
organic solvent isopropanol to form dense solid-like strips due to capillary
coalescence and deposited metal on top of these strips using e-beam evaporation.
Similar temperature based contact resistance reduction with different metals
was observed by Alexander Kane et al in[73]. They grew SWNT by CVD and
deposited 50nm Ti and Ti/Pd on top of these grown CNTs using e-beam deposition.
They found a mild increase in resistance for their Pd samples with bundles of SWNT
and it followed an irreversible change on first heating cycle where the resistance
increased upon cooling than previous value during heating. For Ti samples they
found huge reduction in resistances (from 100MΩ to 350kΩ) when subjected to
temperature cycles upto 1000K and a uniform contact resistance for their 12
devices could be obtained after heat treatment. They suggested that variations in
the temperature coefficients between similar devices contacted by the same metal
could be an explanation to the unavoidable physical and chemical variations at the
SWNT-metal interface. The temperature dependence remains dominated by
thermalization of carriers of contacting metals and possibly the bulk α values of the
metal electrodes. Their results for Pd samples could be seen in figure 36.
60
Figure 37 Heat treatment of two Pd-contacted samples. Pd-A samples had bundles of SWNT
while Pd-B is assumed to have a single SWNT[73].
4.2 Contact Resistance of Carbon Nanotube with various metals
Interconnect requirements for the near and long term needs innovative
solutions to local, intermediate and global wiring. The challenges in interconnect
technology arise from both material requirements and difficulties in processing. The
susceptibility of common interconnect metals to electromigration at high current
densities is a major problem for scaling of metals. Moreover, the electrical resistivity
of Cu increases with a decrease in dimensions due to scattering effects. On the
processing side, maintaining high aspect ratios for trenches and vias would be an
extremely difficult task. Innovative material and process solutions are critical to
61
sustain the growth curve according to ITRS. In this regard, the use of CNT as an
interconnect material holds a great hope. The extraordinary electrical, mechanical
and thermal properties of CNTs may provide near term solutions for problems in
interconnect, chip cooling etc. in silicon IC technology. Kreupl et al [74]showed CNT
are better than Au nanowires with higher current density applications.
Chen Z. et al [75]studied CNT-FET with different metals and found that oncurrent of field effect transistor can be related to a tunneling barrier whose height is
determined by diameter of the nanotube and type of metal as seen in figure 37. They
found the large variations in current from device to device was mainly due to
diameter of carbon nanotube and concluded that for CNTs having diameter above
1.4nm on Pd contacts are a good choice for high on-state performance of p-type
CNT-FETs.
62
Figure 38 - (a)Schematic band diagram depicts the Schottky barrier height differences in 3
CNTFET with different diameters (b) same diameter but different metals (c) On current for Pd,
Ti and Al contacts of CNTFET [75]
Based on first-principles quantum mechanical density function and matrix Green’s
function methods Matsuda et al [70] finds the lowest contact resistance is obtained with
Ti and followed by Pd, Pt, Cu and Au. The I-V models in figure 38 were constructed
theoretically from the optimized geometries of the metals deposited on stacking positions
of a graphene sheet.
63
Figure 39 Transmission coefficient v/s energy near Fermi level for different metals (f) I-V
curve for different metals (g) Contact resistivity per nm2 at metal-graphite interface[70]
An ab initio study of Schottky barrier height and tunneling barrier of a
semiconducting carbon nanotube (8,0) junction with Au, Pd and Pt metal contacts
was done by Bin Shan and Kyeongjae Cho [76]. They concluded that the type of
metal and surface orientation of metal had influence on the Schottky barrier height.
Pd was calculated to have the lowest barrier height according to their calculations.
The tunneling barrier height and Schottky barrier height calculated for Au, Pd and Pt
metals and CNT interface is shown in figure 39.
64
Figure 40 - Schottky barrier height and tunneling barrier height for different metal/ CNT
combinations as a function of interfacial distance[76].
Schottky barriers at the nanotube-metal electrode was observed by Perello et
al [77] for Cr, Mo-Ni, Ni metals for different temperatures between 30K and
295K.The Cr device displayed Schottky behavior with a barrier height of 38meV
indicating that the thermionic emission and thermally assisted tunneling
dominating the total current of the device. For Ni devices they found the Schottky
barrier height to be around 40.5meV and for Mo-Ni devices the height was
estimated at 41meV at the Mo-nanotube electrode junction. Figure 40 shows the I-V
characteristics of their device for different metals at 30K and 295K.
65
Figure 41 - I-V characteristics with respect to Cr1, Ni, Mo-Ni and Cr2 on same SWNT from 30K
to 295K at Vgs =0V[77]
Zhengchun Liu et al [69] studied about side contact resistances with strips of
CNT and Ti, Ta, Pd and W metals. They found rapid thermal annealing at 400oC for 5
min caused increase in resistances for Ti and Pd samples while reduction in
resistance for W and Ta. The I-V curves for resistance of CNT strips with different
metals are shown in figure 41.
66
Figure 42 I-V curves for Ti, Pd, Ta and W contacts with carbon nanotube[69]
4.3 Test results for different metals
A study of contact resistance of carbon nanotubes with different metals, the
effect of annealing to reduce the initial contact resistance and a preliminary look at
the contact aging is presented here. The carbon nanotubes were assembled on top of
the metal contacts using dielectrophoresis as explained before and sputtering or ebeam evaporation was used for metal deposition. Thus the results are for “tube-onmetal” contacts for small bundles of SWNT crossing across the two electrodes.
Chromium-Gold (Cr-Au)
The initial resistance for Cr-Au samples varied over a wide range between 100k to
10G using dielectrophoresis assembly between the two electrodes. The sample
67
was prepared with 2nm of Cr as adhesion layer and 60nm of Au using e-beam
evaporation and CNT was assembled on top of this metal. The two-wire resistance
of the sample after assembly and after annealing at different temperature is plotted
in figure 42. The squares indicate initial resistance while triangles attribute to
resistance after annealing. The colors indicate different samples that were
measured. One can observe the fall in resistance to around 10k to 1M from the
initial high resistance. The variation in the final resistances may be partially due to
to the varied number of carbon nanotubes bridging the two electrodes. The longterm effect of these samples when stored in laboratory air at room temperature is
also studied. The initial resistance right after assembly, the resistance after
annealing and the resistance measured after 15 days left open in ambient conditions
is plotted in figure 43. The variation in final resistance is plotted in figure 44. One
can observe from the box-whisker plot that the median for the final resistance after
anneal is around 78kΩ. Moreover it can be seen that almost all the samples would
have resistance less than 30M ohms after anneal irrespective of their initial
resistances.
(Triangles attribute to values after anneal and Squares are the initial measured
values for all the graphs henceforth).
68
Cr-Au
1.00E+11
2W-Resistance (ohm)
1.00E+10
1.00E+09
1.00E+08
1.00E+07
1.00E+06
1.00E+05
1.00E+04
T100
T200
T250
T300
T325
T350
T400
T450
T475
T500
Anneal Temperature (C)
Figure 43 Plot of initial and after anneal resistances at different temperatures for CNT on CrAu electrode samples. Square value is initial resistance while triangle is after anneal
Cr-Au
1.00E+10
Resistance(ohm)
1.00E+09
1.00E+08
1.00E+07
1.00E+06
1.00E+05
1.00E+04
R0
RT
Event
69
R15
Figure 44 Plot of change in resistance from initial resistance to after anneal and after 15 days
25th
Min
Median
Max
Mean
75th
Resistance(ohm)
1.00E+07
1.00E+06
1.00E+05
1.00E+04
Cr-Au
Figure 45 Box & Whisker plot for resistance of Cr-Au samples after annealing
Titanium-Gold (Ti-Au)
The initial resistances for Ti-Au samples also were along the whole range between
k to G. The samples are fabricated with Ti for adhesion layer of 2nm and Au as
the main metal with 60nm thickness. The two-wire resistance of the samples at
different temperatures is shown in the figure 45. One can observe the drop in
resistances from the initial high intrinsic resistance to less than 44M. When
compared with Cr-Au samples, the median value of final resistance for Ti-Au
samples is just 64k higher. The long-term resistances for the Ti-Au sample remain
unchanged as seen from figure 46. For few samples the long-term resistance
measured after 15 days was reduced even to the values below the initial resistance.
Figure 47 shows the distribution plot of resistances after annealing for Ti-Au
70
samples. One can observe from the plot that the range of measured resistances after
annealing is between 30kΩ and 500MΩ.
Ti-Au
1.00E+09
2-W Resistance (ohm)
1.00E+08
1.00E+07
1.00E+06
1.00E+05
1.00E+04
T100
T200
T250
T300
T325
T350
T400
Anneal Tem perature (C)
Figure 46 Plot of initial resistance and after anneal resistance at different temperatures for
Ti-Au samples
71
Ti-Au
1.00E+09
Resistance (ohm)
1.00E+08
1.00E+07
1.00E+06
1.00E+05
1.00E+04
R0
RT
R15
Event
Figure 47 Plot of change in resistance from initial value to after anneal and after 15 days for
all Ti-Au samples
25th
Min
Median
Resistance (ohm)
1.00E+07
1.00E+06
1.00E+05
1.00E+04
Ti-Au
72
Max
Mean
75th
Figure 48 Box & Whisker plot for final measured resistances after anneal for Ti-Au samples
Titanium-Palladium(Ti-Pd)
The Ti-Pd samples were prepared by e-beam deposition of 2nm Ti and 60nm
of Pd followed by CNT assembly on top of this. The resistance before annealing and
after annealing at different temperatures is shown in figure 46. One can observe that
annealing the samples reduced the resistances in some case while there was an
increase in resistances for some samples also. It has been reported in literature[73],
that Pd contacts with CNT sometimes yielded higher resistances than initial when
subjected to heat treatment. The change in resistances from the annealed values
after 15 days when left in laboratory air at room temperature can be observed in
figure 49 where we see that resistances reduced from their final annealed values.
Ti-Pd
1.00E+11
1.00E+10
2-W Resistance
1.00E+09
1.00E+08
1.00E+07
1.00E+06
1.00E+05
1.00E+04
T250
T300
T350
T400
T450
Anneal Temperature
Figure 49 Plot of initial resistance and after anneal at different temperature for Ti-Pd samples
73
Ti-Pd
1.0E+11
Resistance (ohm)
1.0E+10
1.0E+09
1.0E+08
1.0E+07
1.0E+06
1.0E+05
1.0E+04
R0
RT
R-15
Event
Figure 50 Plot of change in resistance of CNT from initial assembly, after annealing and after
15 days for Ti-Pd samples
25th
Min
Median
1.00E+11
1.00E+10
Resistance (ohm)
1.00E+09
1.00E+08
1.00E+07
1.00E+06
1.00E+05
1.00E+04
Ti-Pd
74
Max
Mean
75th
Figure 51 Box & Whisker plot for distribution of measured resistance after annealing Ti-Pd
samples
Titanium-Tungsten (TiW)
The TiW samples were prepared by sputtering the mixture of TitaniumTungsten and CNT was assembled using dielectrophoresis on them. The initial
resistance of TiW samples had a wide range between 100K to 10G ohms. After
annealing them at different temperatures there was a reduction in resistance for all
the samples but the spread of the final resistances was all over the range between
10k ohms to 1G ohms. The plot for TiW samples after dielectrophoresis and after
annealing at different temperatures is shown in figure 51. One can see the spread of
final resistances after annealing in the distribution plot in figure 53. The change in
resistance from the measured values after annealing and after 15 days is shown in
figure 52.
TiW
2W Resistance (ohm)
1.00E+12
1.00E+11
1.00E+10
1.00E+09
1.00E+08
1.00E+07
1.00E+06
1.00E+05
1.00E+04
T200
T250
T300
T350
T400
T450
T500
Anneal Temperature (C)
Figure 52 Plot of resistance of CNT before and after anneal at different temperatures for TiW
samples
75
TiW
1.00E+12
1.00E+11
Resistance (ohm)
1.00E+10
1.00E+09
1.00E+08
1.00E+07
1.00E+06
1.00E+05
R0
RT
Event
R15
Figure 53 Plot of change in resistance from initial assembled CNT to after anneal and after 15
days
76
25th
Min
Median
Max
Mean
75th
1.00E+12
1.00E+11
Resistance (ohms)
1.00E+10
1.00E+09
1.00E+08
1.00E+07
1.00E+06
1.00E+05
TiW
Figure 54 Probability and cummulative distribution plots of final resistances for TiW samples
after annealing.
Titanium(Ti)
The Ti-samples were prepared by sputtering the metal and assembling
carbon nanotubes on top of it using dielectrophoresis. All the Ti samples showed a
Schottky behavior at the contacts and had resistances higher than 1G ohms before
annealing. The plot for resistances of samples before and after annealing at different
temperatures is shown in figure 54. The I-V characteristic indicating a back-to-back
diode type behavior at contacts of CNT and metal for Ti-samples is shown in figure
59.
77
Ti
2-W Resistance (ohm)
1.00E+12
1.00E+11
1.00E+10
1.00E+09
T200
T250
T300
T350
T400
T450
Anneal Temperature (C)
Figure 55 Plot of initial resistance and after anneal resistance for CNT on Ti electrode samples
25th
Min
Median
Max
Mean
75th
Resistance (ohm)
1.00E+12
1.00E+11
1.00E+10
1.00E+09
Ti
Figure 56 Distribution plot of final resistances after anneal for Ti samples
78
Ruthenium (Ru)
Ruthenium was sputtered and CNT was assembled on top of the metal using
dielectrophoresis. The plot for initial resistance after assembly and after annealing
at different temperatures under nitrogen ambience is shown in figure 56. One can
observe that there is a significant reduction in resistances for the sample but unlike
Au samples the distribution of the final resistances if spread on wide range between
100kohms to 1Gohms. The change in resistance when the samples were left in
ambient conditions for 15 days could be observed from plot in figure 57. We
observe that there is no significant change in resistances for CNT on Ru samples
with time. The plot for spread of measured resistances after annealing for all
samples is shown in figure 58. The median value for Ru samples is around 28M.
Ru
1.00E+11
2-W Resistance (ohm)
1.00E+10
1.00E+09
1.00E+08
1.00E+07
1.00E+06
1.00E+05
T100
T200
T250
T300
T350
T400
T450
T500
T550
Anneal Temperature (C)
Figure 57 Plot of initial resistance and after anneal at different temperatures for CNT on Ru
electrodes
79
Ru
1.00E+12
1.00E+11
Resistance (ohm)
1.00E+10
1.00E+09
1.00E+08
1.00E+07
1.00E+06
1.00E+05
1.00E+04
R0
RT
R15
Event
Figure 58 Plot of change in resistance from initial assembly and after annealing and after 15
days
80
25th
Min
Median
Max
Mean
75th
1.00E+11
Resistance (ohm)
1.00E+10
1.00E+09
1.00E+08
1.00E+07
1.00E+06
1.00E+05
Ru
Figure 59 Box & Whisker distribution plot for measured resistance of CNT on Ru after anneal
81
5. DISCUSSION
We found that contact resistances with “tube-on-metal” configuration are
higher than “metal-on-tube” devices. One possible explanation as given by Kong et al
[62] is nanotube bending on the metal electrodes changes the conduction behavior
due to formation of tunnel barrier between the metal and CNT as well as inside the
SWNT. CNT assembled using dielectrophoresis give Schottky behavior as well as
ohmic contacts with metals and usually the initial resistance is pretty high. This
behavior is observed in literature also with CNT assembled dielectrophoretically on
Au electrodes[78, 79]. The nanotubes that are assembled by dielectrophoresis are
held by Van der Waals forces only so a strong bonding between metal and CNT is
not formed. In case of “metal-on-tube” structures, nanotube is laid down first and
then metal is deposited on top of this tube using conventional sputtering or e-beam
deposition. This causes a strong sticking of metal atoms onto the graphite surface
depending upon its wetting properties as explained before and also forms better
bonds by physically rupturing the surface to some extent during sputtering where
the metal atoms are bombarded on SWNT. Also, the contact area of CNT with metal
increases in case of “metal-on-tube” configurations as SWNT can form side-contacts
as well as end-contacts. So effects of dangling ends as predicted by Li et al [63] are
eliminated in this case.
We found that CNT assembled using dielectrophoresis could give Schottky
contacts as well as ohmic contacts depending upon the metal. For example, Ru and
Ti-Pd always gave an ohmic contact for all our samples while Ti had Schottky
contacts only for all the measured samples. Schottky contacts formed between CNT
and Ti has been observed in literature and Lu et al fabricated a Schottky diode using
Ti electrodes[64]. Au and TiW yielded both, either Schottky or ohmic contacts in our
samples. Figure 59 shows typical I-V characteristics of CNT on Ti electrodes with
sweepback voltage. One can observe that the CNT has a higher Schottky barrier
height at one electrode than the other electrode in this particular device. Figure 60
82
& 61 shows typical I-V characteristics for Ti-Pd and Ru samples showing ohmic
contacts.
Figure 60 An I-V characteristics of a typical Schottky behavior contact
83
Figure 61 Typical I-V characteristics of Ti-Pd samples showing ohmic contacts
84
Figure 62 Typical I-V characteristics for CNT assembled on Ru metal samples
The explanation for this kind of behavior is explained below in figure 60. A
potential barrier is created between metal-cnt junction due to the difference in work
function of metal and band gap of CNT. Formation of oxide not only changes the
work function of metal but also serves as a tunneling barrier. Thus one electrode
acts more has a lesser Schottky barrier height compared to other electrode, which
causes an I-V behavior as seen in figure 62. The conduction in one direction of
current flow (forward bias) is higher than conduction under reverse bias.
85
Figure 63 Schematic explanation of energy band bending at metal-CNT junction under
forward and reverse bias
Annealing the samples in nitrogen does helps in reducing the initial high
contact resistances with many metals. Moreover, change in resistance when these
annealed samples were left in laboratory air at room temperature was studied and
was found that there is an increase in resistance compared to initial and measured
values after anneal. Oxygen absorption by the metals to form oxides or a thin
insulating layer formation during assembly either by contaminants or organics
could be one of the possible reasons for high contact resistances observed when
CNT is assembled on the metals. A comparison of measured resistances of CNT on
different metals is made in figure 63. A probability distribution plot for logarithmic
value of the measured resistance after anneal for all the samples of different metals
is calculated. The probability distribution function for samples is given by
86
f(x) =
,where probability density function f(x) is defined as x is the log value of
measured resistances,  is the mean of samples,  is the standard deviation of x
from mean and  is mathematical constant.
The plot is fitted on a Gaussian curve and plotted against a log R scale here
and gives us a general idea how the spread of final resistances varied for different
metals.
0.7
Probability Distribution Function
0.6
0.5
0.4
0.3
0.2
0.1
0
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
1.E+09
1.E+10
1.E+11
1.E+12
Resistance (LOG) (ohms)
TiW
Ti-Au
Ru
Cr-Au
Ti
Ti-Pd
Figure 64 Probability distribution plot of log of resistances after anneal of samples of CNT on
electrode for different metals
We can observe from the above plot that resistance for Au samples seemed
to have a lesser spread than Ru, Pd and TiW samples. Moreover, Ti also has a spread
87
between 1G ohms to 100G ohms. One of the possible explanations for this behavior
could be a possible oxidation of the metal after or during assembly process, which
forms a thin layer of metal-oxide on top of it. The argument could possibly
supported by observing the change in resistance of the samples of different metals
from their final annealed values to resistance values measured after 15 days when
stored in laboratory air. A higher change in resistance value would suggest, a
formation of insulating oxide layer as well as prone to more contaminants
developing on the metal surface which causes the resistance to increase from
annealed values. One can observe in figure 64, a plot for average value of change in
resistance in 15 days from their annealed measured resistances for different metals.
AVERAGE VALUE OF RATIO OF RESISTANCE MEASURED AFTER 15 DAYS AND
FINAL RESISTANCE AFTER ANNEAL FOR ALL SAMPLES
1000
R15days / Rfinal
100
10
1
0.1
Cr-Au
TiW
Ti-Pd
Ti-Au
Ru
Ti
Figure 65 Average value of change in resistance after 15 days for different metals
Among the metals we studied so far, Gold seems to be a good candidate for
building devices with CNT on metal configurations, as Au does not oxidizes easily.
88
We can observe from Figure 62 that changes in resistances for Ti-Au samples were
almost zero. Moreover, resistance for Cr-Au samples changed only marginally by an
order of magnitude. Chromium can form chrome-oxide layer easily which is
insulating and the metal film becomes more prone to oxidation as well as other
contaminants as the morphology changes while Cr diffuses into Au metal layer. The
resistances of the annealed sample increased with time which may be due to oxygen
adsorption effect. The Cr diffuses into Au particle[80] which might form a chromeoxide layer. This could be one of the possible arguments for the gradual increase in
the contact resistance between the nanotube and Cr-Au film with time but more
detailed study is required. Titanium can be used as adhesion layer for depositing Au
as it is observed that the contact resistance with CNT does not change with time for
Ti-Au samples. Ru samples also did not have a significant change in their resistance
values after 15 days. RuOx is a good conductor of electricity and so the oxide
formation on Ru layer does not varies the resistance value a lot with time. Though
the large spread of resistance values observed after annealing on Ru samples inspite
of forming ohmic contacts might be due to contaminants forming on the Ru metal
layer. Ti sample also showed a very slight increase in its change in resistance values
but Ti samples were already measuring in > 109 ohms. TiW samples had the highest
change in resistance values and so the observation does coincide with the argument
of insulating oxide layer formation on its top. One interesting result is reduction in
resistance for Ti-Pd samples. It is established that surface work function of Pd can
be modified upon exposure to molecular hydrogen[81, 82]. Hydrogen molecules
dissociate at the surface of Pd and thus lower its work function. The reduction in
resistance with time for Ti-Pd samples suggests hydrogen absorption at the metal
surface from the atmospheric humidity. Such a study of contact resistance due to
oxygen absorption in metal is carried out by Tsui et al [83] where they spun SWNT
on SiO2 substrate and deposit different metals (Al, Ti, Cu, Ni, Cr, Ta, W, Au & Pt) on
selective SWNT.
Thus absorption of oxygen and other gases degrade the contacts of CNT with
metal with time is observed from the results. Pati et al and Tang et al studied effects
of H2O adsorption on electron transport in a CNT theoretically and concluded that
89
water molecules absorbed on the NT surface reduce the electronic conduction in the
tube [84-86]. Though, Zhao et al studied theoretically various gas molecules in air
on SWNT and suggested that air exposure effect should be dominated by O2 and
effects of most gas molecules in air such as N2, NO2, CO2 and H2O are relatively
weak[87]. A preliminary understanding of contacts of “tube-on-metal” using
dielectrophoresis assembly is made here.
90
6. CONCLUSION
Thus, this thesis presents electrical test results of single walled carbon
nanotube based nano-switch fabricated at Northeastern University. Moreover, a
preliminary study of various types of configurations for study of contact resistance
of CNT with metal and the results are presented here. Also, methods to reduce the
initial as-deposited CNT contact resistance using rapid thermal annealing under
nitrogen for 5 min is developed and explained here. An understanding of electron
transport through CNT-metal interface for “tube-on-metal” structures is explained
in detail throughout the thesis. Formation of oxide layer and effects of absorption of
gases by the CNT-metal interface when left open in air at room temperature is
observed from the test results presented in this thesis.
Further recommendations for use of Ti-Au as a metal for future nanoswitch
development would be made from the observed test results. Moreover, storing the
samples in a clean dry nitrogen box is highly advisable to prevent the metal surface
from contaminants and other adsorption of gases which potentially degrades the
contacts between CNT and metal and thus hampers the realization of true intended
applications for the fabricated sample. Also, trying to encapsulate the top metal
surface with either electroplating or other methods can be tried to lower down the
resistance and increase the reliability of contacts in long term.
Future work from the presented test results would be to understand the
surface of the metal and try to do a surface analysis of my samples to determine the
exact proportion of oxide formation and signatures of other gas absorption by the
metals at contact.
91
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8. FOOTNOTES
LabVIEW Snapshot for I-V Measurements for Nano-Switch
97
Noise Measurement for Test Setup
98