Ohmic contacts
•
Common techniques to make ohmic
contacts
–
–
–
•
Ohmic contacts should be
–
–
–
–
•
Choose metal so that its work function
Fmetal is close to that of semiconductors
Fsemi (thermal ionic)
Insert thin layer of narrow bandgap
material between metal and
semiconductor
Increase the doping level near the
semiconductor surface as high as
possible (tunneling assisted)
Low contact resistance (< 10-6 Ωcm2)
Thermally stable (does not degrade at
elevated temperature or react with
oxygen), which requires no phase
change or no phase change leading to
high resistance
Smooth morphology
Compatibility with the whole device
process
Both semiconductors and metal
sources should be CLEAN!
Michaelson, IBM J. of R&D, 1978
Slide # 1
Common Techniques for Ohmic contacts
(i) Ohmic contact by band alignment
Evac
φM
Evac
EF
χ
EF
φM < χ+Ec-EF = φS
For n-type
semiconductor.
Reverse for p-type
φB = φS - φM
n doped
φB = band bending
(ii) Ohmic contact by high doping
• Usually for compound semiconductors
the ohmic contact by band alignment is
hard to realize due to surface states and
Fermi pinning. For p-type, the problem
is caused by unavailability of metals
with large enough work function
• High n-type doping required for ohmic
contacts to n-type semiconductors,
which can also be realized by interfacial
layer reaction chemistry
n-doped
n+ doped
Electrons from
conduction band
can move very
easily to the
metal and vice
versa by
tunneling
Slide # 2
Ohmic on n-GaN
•
Possible metals: Ag, Nb, Ti, Al, In, Ta, Cr
–
–
•
•
Ag: poor adhesion
Nb: extremely easy to oxidize thus difficult to
process
– Ti: formation of TiN (intermetallic) and high N
vacancies in GaN -> good! But easy to
oxidize… need a stable cap like Au
– Al: formation of AlN (not intermetallic) and high
N vacancies in GaN -> ok! Also easy to
oxidize… Au cap is necessary
– In: most popularly used for quick contacts
– Ta: studied by Qiao et al. 5.7e10-7 Ωcm2 on
AlGaN/GaN (2001); but others could not
reproduce the results
– Others: also studied but not as good as the
one below
As deposited or alloy: generally alloyed unless
doping near the surface is very high!
Popular schemes: Ti/Al/Ni/Au
–
–
–
–
Ti/Al bilayer: formation of N vacancies, TiN,
Al3Ti (thermally very stable ☺); but ratio of Ti/Al
has to be carefully controlled (~1/2.5) Add high conductive and protective layer of Au,
but Au diffuses easily
Add Ni as diffusion barrier (decent, other
metals were tried, Pd and Pt were worse)
State-of-art: 0.1-0.2 Ωcm2 (~ 10-8 Ωcm2 )
Lim et al, APL 78, 3797(2001)
Liu et al, Solid State Electronics, 42, 677(1998)
Slide # 3
Schottky contacts
• Schottky contacts are
formed when
Evac
Evac
φs χ
φM
Doping in the
EF
semiconductor is not very
high i.e. > ~5x1018 cm-3
The metal work function is
greater than the n- type
φM > χ+Ec-EF = φS
semiconductor work
For n-type semiconductor and
function
reverse for p-type
The metal work function is
lower than p-type
Schottky contact Electrons from
φBn =
semiconductor work
conduction band
φM -χ
function
or in the metal
faces barrier to
Very high density of
free movement,
surface states “pinning” the
n doped
Fermi level at the surface
and tunneling is
w.r.t. the conduction band
also not easy
(Example: GaAs)
Slide # 4
Conduction mechanisms in schottky contacts
• Thermionic emission
Electrons emit over the barrier
Low probability of direct tunneling
Valid for low doping (ND < ~ 1017 cm-3)
• Thermionic-field emission
Electrons use thermal energy to tunnel trough the
thin barrier in the upper end of the conduction
band
Valid for intermediate doping (~ 1017 cm-3 < ND <
~ 1018 cm-3)
• Field emission
Direct tunneling, as depletion region is very narrow
Valid for heavy doping (ND > ~ 1018 cm-3); almost
ohmic
• Leakage current
High probability of defect-assisted tunneling and
simple conduction
Occurs in poor material/interface quality; dislocations
Slide # 5
Thermionic emission current: Schottky diode
I-V characteristics
Typical I-V
characteristics
Forward bias
Reverse bias
Schottky diode I-V equation:
J = J0 (e qV / kT – 1), where J0 is the
saturation current density given by
 qφ 
J o = A*T 2 exp − Bn  T = temperature, A* = effective Richardson’s constant
 kT 
6
Slide #
Schottky on n-GaN
•
•
Experimentally shown
very weak surface
pinning
Surface cleanness has
been heavily
investigated, however
–
–
•
Thermal stability is
IMPORTANT
–
–
–
•
•
External cleaning is
generally sufficient to
achieve decent
Schottky
Leakage is largely due
to dislocations
Ni does not react with
GaN below ~ 600 C
Pd reacts with GaN at ~
400-500 C
W and Rd ~ 600 C
The higher the schottky barrier, the lower
the leakage current
Using polarization in nitrides i.e.
GaN/AlGaN/GaN structure, the schottky
barrier can be made larger
Liu et al, Solid State Electronics, 42, 677(1998)
Slide # 7
Electrical properties of dislocations in MBE-grown n-GaN
(Ed Yu --- UCSD)
• Pure screw dislocations can be highly conductive in MBE-grown n-GaN:
topography:
1µm
current:
1µm
[E. J. Miller, D. M. Schaadt, E. T. Yu, C. Poblenz, C. Elsass, J. Speck, J. Appl. Phys. 91, 9821 (2002).]
• Edge and mixed dislocations typically contain negative charge in dislocation core:
high current leakage
in Schottky contacts
Screw: conducting, uncharged
Edge: nonconducting , –e charge
Mixed: nonconducting, –e charge
scattering, local
carrier depletion
[B.S. Simpkins, E.T. Yu, P. Waltereit, J.S. Speck, J. Appl. Phys. 94, 1448 (2003).]
Slide # 8
Mitigation of dislocation-induced leakage
currents in MBE n-GaN (Ed Yu, UCSD)
AFM
AFM
2µm
2µm
current
current
AFM
1µm
current
~10µA
1µm
with electrochemical
process
• NaOH solution • V = 30V
• pH = 13.1
• I ~ 1-10mA
• T = 30°C
• t = 1000s
~100pA
φb = 0.80±0.02V
n = 1.74±0.01
φb = 0.86±0.02V
n = 1.13±0.02
AFM
unmodified
area = 1.23×10-4cm2
1µm
[E. J. Miller, D. M. Schaadt, E. T. Yu, P. Waltereit, C. Poblenz, and J. S. Speck, Appl. Phys. Lett. 82, 1293 (2003).]
Slide # 9
Ohmic to p-GaN
•
Similar techniques like
ohmic to n-GaN have
been tried, but:
–
•
P-GaN/Ni/Au annealed in
air (N2/O2) proved to be
one of the best:
–
•
rC~ 10-6 Ωcm2
Why?
–
–
•
rC~ 10-3 Ωcm2
After annealing, new
phases form: NiO, NiGa-O with Au particles,
GaN
NiO is p-semiconductor
with high Ni vacancies
Continuing challenges:
–
–
–
Transparency to visible
and UV
Ohmic to p-AlGaN
Tunneling junction
contacts
Ho et al. JAP 85, 4491 (1999)
Slide # 10
Another Contact Metal for p-GaN
• The absence of a metal with a sufficiently high work function. The band gap of
GaN is 3.4 eV, and the electron affinity is 4.1 eV, but metal work functions are
typically ~ 5 eV
• The relatively low hole concentrations in p-GaN due to the deep ionization level of
the Mg acceptor ~170 meV
• The tendency for the preferential loss of nitrogen from the GaN surface during
processing, which may produce surface conversion to n-type conductivity.
TEM
image
• Palladium gallide creates Ga vacancies that
reduce contact resistances
• Temperature and time of anneal also important
Slide # 11
Schottky to p-GaN
•
•
Schottky (Ni) on as grown GaN:Mg (MOCVD) --- quasi-ohmic (higher Mg near the surface?)
Schottky (Ni) on etched GaN:Mg --- rectifying (tunneling and defect-assisted tunneling still
significant thus it is difficult to extract barrier height and Richardson constant from I-V)
Slide # 12
Schottky contact characterization
• Current-Voltage (IV) measurements
*
2
J0 = A T e
−
qφ Bn
kT
. VF vs. J intercept gives J0 and φ Bn
kT  A*T 2 

ln
=
q  J 0 
• Capacitance-Voltage (CV) measurements
1
qε s N D
2ε s
⇒ 2 ∝ (φBn − V )
(φBn − V ) ⇒ C =
W=
C
2(φ Bn − V )
qN D
– So the intercept of 1/C2 vs. V gives the barrier height
• Photoelectric measurements (by photon incident
on the schottky contact; this is very accurate)
– Photocurrent R is related to the barrier height as
R ~ hv − qφBn So the intercept gives the barrier height
Slide # 13
Evaporation systems
Contact Metallization (Ti, Al, Ni, Au etc)
Metal Electron-Beam Evaporation System
Sample
Target Metal Source
with e-beam
Rapid Thermal Annealing System
from 20 oC to 1000 oC in seconds
Slide # 14
Ohmic contacts: n-type or undoped nitride
• Standard recipe for ohmic contact:
– Ti/Al/Ti/Au or Ti/Al/Ni/Au deposition. Ti/Al thickness ratio is important
– Annealing at 800 – 900 ºC for about 1 min for alloying. Alloying temperature
and alloying time are important factors controlling contact resistance.
Ti/Al/Ni/Au
Since TiN and AlN are formed by reaction
between the nitride layers and Ti or Al, Nvacancies are created, which can dope the
contact region and create ohmic contact
Slide # 15
Specific contact resistivity and sheet resistance
For any semiconductor device there are two main resistances:
• Contact resistance
• Semiconductor resistance
d
Z
t
Product of contact resistance Rc and area
A is called specific contact resistivity ρc:
ρc =
1
 ∂J 
 ∂V 
V =0
(Ω . cm2)
(Can also be expressed in terms of Ω . mm)
Semiconductor layer resistivity ρ:
1
ρ=
enµ
(Ω
. cm)
Sometimes semiconductor resistance is
expressed in terms of sheet resistance ρsh
1
ρ
=
ρ sh =
t (e n µ ) t
(Ω/ )
The total semiconductor resistance is then
given by
d
1
ρd
(Ω)
Rs = ∫ ρ dx =
A0
Zt
Slide # 16
Ohmic contact characterization:
Transmission line method (TLM)
I(x)
L
I(x+∆x)
ρsh∆x/Z
V(x)
ρc/(Z∆x)
V(x +∆x)
∆I
dI
V ( x) Z
=−
dx
ρc
Iρ
Iρ
dV
= − s = − sh
dx
Zt
Z
d 2 I I ( x)
ρc
⇒ 2 = 2 , where LT =
ρ sh
dx
LT
The solution for I(x) is given as:
is called the transfer length.
I ( x ) = Ae x / LT + Be − x / LT
Now putting the boundary condition I(x = L) = 0, and finding the
solution for V(x), we can find the contact resistance as the ratio of the
input voltage and input current as: RC = V (x = 0 ) I ( x = 0)
Slide # 17
Transmission line method (TLM) II
 L
coth 
The contact resistance Rc is then given by: RC =
ZLT
 LT 
ρ
For L >>LT , we have, RC = C
Ohmics
ZLT
ρC
Z
When the following conditions are further
d
satisfied, d << Z and t << LT (to avoid
t
current spreading in the sides or into the film),
ρsh = ρ /t
ρ sh d
Then, RTot = 2 Rc + Rs = 2 Rc +
Z
Putting Rtot = 0, and using the relation ρ c = Rc LT Z , we have,
d ( RT = 0) = − 2 LT . So, the transfer length can be found from the
intercept of the total resistance on the x-axis.
Note that the contact resistivity is not given by the product of the
contact resistance and the total contact area, but by the product of
contact resistance, width Z, and transfer length LT.
Slide # 18
L
Measurement technique
100
Typical measurement set up
B1205 UV LED
n-TLM
Current, mA
50
4um
6um
8um
10um
12um
14um
16um
0
-50
-100
-2
-1
0
1
2
Voltage, V
40
Slope =
ρsh/Z
30
What is wrong in
this measurement?
Resistance, Ohm
Plot of total resistance
vs. distance
B1205 UV LED
n-TLM
20
Y =14.51607+1.13839 X
Rc=7.258Ω
LT=6.373um
10
ρc=6.93*10-5Ω-cm2
Rsh=170.7Ω/sq
0
0
5
10
15
gap, um
Slide # 19
20