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Slide # 1
Ohmic contacts
Common techniques to make ohmiccontacts Choose metal so that its work function
Fmetal is close to that of semiconductorsFsemi (thermal ionic)
Insert thin layer of narrow bandgapmaterial between metal andsemiconductor
Increase the doping level near thesemiconductor surface as high aspossible (tunneling assisted)
Ohmic contacts should be
Low contact resistance (< 10-6 cm2) Thermally stable (does not degrade at
elevated temperature or react withoxygen), which requires no phasechange or no phase change leading tohigh resistance
Smooth morphology
Compatibility with the whole deviceprocess
Both semiconductors and metalsources should be CLEAN!
Michaelson, IBM J. of R&D, 1978
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Slide # 2
Common Techniques for Ohmic contacts
Usually for compound semiconductorsthe ohmic contact by band alignment ishard to realize due to surface states and
Fermi pinning. For p-type, the problemis caused by unavailability of metalswith large enough work function
High n-type doping required for ohmiccontacts to n-type semiconductors,
which can also be realized by interfaciallayer reaction chemistry
M
Evac Evac
EF
M < +Ec-EF = SFor n-typesemiconductor.
Reverse for p-type
(ii) Ohmic contact by high doping
Electrons from
conduction band
can move veryeasily to the
metal and vice
versa by
tunneling
n+ doped
(i) Ohmic contact by band alignment
n doped
B = S -MEF
B = band bending
n-doped
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Slide # 3
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 tooxidize need a stable cap like Au
Al: formation of AlN (not intermetallic) and highN vacancies in GaN -> ok! Also easy tooxidize Au cap is necessary
In: most popularly used for quick contacts Ta: studied by Qiao et al. 5.7e10-7 cm2 on
AlGaN/GaN (2001); but others could notreproduce the results
Others: also studied but not as good as theone below
As deposited or alloy: generally alloyed unlessdoping 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/Alhas 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, othermetals were tried, Pd and Pt were worse)
State-of-art: 0.1-0.2 cm2 (~ 10-8 cm2 )
Lim et al, APL 78, 3797(2001)
Liu et al, Solid State Electronics, 42, 677(1998)
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Slide # 4
Schottky contacts
Schottky contacts areformed whenDoping in the
semiconductor is not veryhigh i.e. > ~5x1018 cm-3
The metal work function isgreater than the n- typesemiconductor workfunction
The metal work function islower than p-typesemiconductor work
functionVery high density of
surface states pinning theFermi level at the surface
w.r.t. the conduction band(Example: GaAs)
Schottky contact
n doped
Electrons from
conduction band
or in the metalfaces barrier to
free movement,
and tunneling is
also not easy
M
Evac Evac
EF
M > +Ec-EF = SFor n-type semiconductor andreverse for p-type
s
Bn =
M -
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Slide # 5
Conduction mechanisms in schottky contacts
Thermionic emissionElectrons emit over the barrier
Low probability of direct tunneling
Valid for low doping (ND < ~ 1017 cm-3)
Thermionic-field emissionElectrons 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); almostohmic
Leakage currentHigh probability of defect-assisted tunneling and
simple conduction
Occurs in poor material/interface quality; dislocations
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Slide # 6
Thermionic emission current: Schottky diode
I-V characteristics
Schottky diode I-V equation:J = J0 (e
qV / kT 1), where J0 is the
saturation current density given by
Forward bias Reverse biasTypical I-V
characteristics
= kT
qTAJ Bno
exp2* T = temperature, A* = effective Richardsons constant
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Slide # 7
Schottky on n-GaN
Experimentally shownvery weak surfacepinning
Surface cleanness hasbeen heavilyinvestigated, however External cleaning is
generally sufficient toachieve decentSchottky
Leakage is largely due
to dislocations Thermal stability is
IMPORTANT Ni does not react with
GaN below ~ 600 C
Pd reacts with GaN at ~
400-500 C W and Rd ~ 600 C Liu et al, Solid State Electronics, 42, 677(1998)
The higher the schottky barrier, the lowerthe leakage current
Using polarization in nitrides i.e.
GaN/AlGaN/GaN structure, the schottkybarrier can be made larger
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Slide # 8
Electrical properties of dislocations in MBE-grown n-GaN(Ed Yu --- UCSD)
1m
topography:
1m
current:
Pure screw dislocations can be highly conductive in MBE-grown n-GaN:
Screw: conducting, uncharged
Edge: nonconducting , e chargeMixed: nonconducting, e charge
Edge and mixed dislocations typically contain negative charge in dislocation core:
high current leakagein Schottky contacts
scattering, localcarrier depletion
[B.S. Simpkins, E.T. Yu, P. Waltereit, J.S. Speck, J. Appl. Phys. 94, 1448 (2003).]
[E. J. Miller, D. M. Schaadt, E. T. Yu, C. Poblenz, C. Elsass, J. Speck, J. Appl. Phys. 91, 9821 (2002).]
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Slide # 9
Mitigation of dislocation-induced leakagecurrents in MBE n-GaN (Ed Yu, UCSD)
1m
AFM
1m
AFM
1m
current
NaOH solution
pH = 13.1 T= 30C
AFM
2m
current
~10A
AFM
2m
current
~100pA
b= 0.800.02V
n= 1.740.01
b= 0.860.02V
n= 1.130.02
unmodified
with electrochemicalprocess
area = 1.2310-4cm2
V = 30V
I ~ 1-10mA t= 1000s
[E. J. Miller, D. M. Schaadt, E. T. Yu, P. Waltereit, C. Poblenz, and J. S. Speck, Appl. Phys. Lett. 82, 1293 (2003).]
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Slide # 10
Ohmic to p-GaN
Similar techniques likeohmic to n-GaN havebeen tried, but:
rC~ 10-3 cm2
P-GaN/Ni/Au annealed inair (N2/O2) proved to beone of the best:
rC~ 10-6 cm2
Why?
After annealing, newphases form: NiO, Ni-Ga-O with Au particles,GaN
NiO is p-semiconductorwith high Ni vacancies
Continuing challenges: Transparency to visible
and UV
Ohmic to p-AlGaN
Tunneling junctioncontacts
Ho et al. JAP 85, 4491 (1999)
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Slide # 11
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.
Palladium gallide creates Ga vacancies that
reduce contact resistances
Temperature and time of anneal also important
TEM
image
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Slide # 12
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)
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Slide # 13
Schottky contact characterization
Current-Voltage (IV) measurements
Capacitance-Voltage (CV) measurements
So the intercept of 1/C2 vs. V gives the barrier height
Photoelectric measurements (by photon incidenton the schottky contact; this is very accurate)
Photocurrent R is related to the barrier height as
( )VqNW BnDs
= 2
( )V
Nq
CBn
Ds
=
2 ( )VC Bn 21
BnqhvR ~ So the intercept gives the barrier height
kT
q Bn
eTAJ
= 2*0
=
0
2*
ln
J
TA
q
kTBn. VF vs. J intercept gives J0 and
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Slide # 14
Evaporation systems
Contact Metallization (Ti, Al, Ni, Au etc)
Metal Electron-Beam Evaporation System
Rapid Thermal Annealing System
from 20 oC to 1000 oC in seconds
Target Metal Sourcewith e-beam
Sample
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Slide # 15
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 C 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, N-
vacancies are created, which can dope thecontact region and create ohmic contact
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Slide # 16
Specific contact resistivity and sheet resistance
Product of contact resistance Rc and area
A is called specific contact resistivityc:
0
1
=
=
V
c
V
J ( . cm2)
Semiconductor layer resistivity:
ne
1= ( . cm)
tZ
ddx
A
R
d
s
==
0
1()
d
Z
t
Sometimes semiconductor resistance is
expressed in terms of sheet resistance sh
For any semiconductor device there are two main resistances: Contact resistance
Semiconductor resistance
( ) tnetsh ==
1(Can also be expressed in terms of . mm)
The total semiconductor resistance is thengiven by
(/)
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Slide # 17
Ohmic contact characterization:
Transmission line method (TLM)
( )TT LxLx BeAexI // +=
c
ZxV
dx
dI
)(=
Z
I
tZ
I
dx
dV shs ==sh
cTL
=,)(
22
2
TLxI
dxId =
shx/Z
c/(Zx)
I
I(x)
V(x)
I(x+x)
V(x +x)
where
The solution forI(x) is given as:
Now putting the boundary conditionI(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:
L
( ) ( )00 === xIxVRC
is called the transfer length.
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Slide # 18
Transmission line method (TLM) II
T
CC
ZLR
=
Z
dRRRR
shcscTot
+=+= 22
sh = /t
d
Z
t
When the following conditions are further
satisfied, d LT , we have,
=
TT
CC
LL
ZLR coth
,ZLR Tcc=Putting Rtot = 0, and using the relation
.2)0( TT LRd ==
we have,
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.
OhmicsL
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Slide # 19
Measurement technique
-2 -1 0 1 2-100
-50
0
50
100
B1205 UV LED
n-TLM
Curr
ent,mA
Voltage, V
4um
6um
8um10um
12um
14um
16um
0 5 10 15 200
10
20
30
40
B1205 UV LED
n-TLM
Y =14.51607+1.13839 XR
c=7.258
LT=6.373um
c=6.93*10-5-cm2
Rsh
=170.7/sq
Resistanc
e,
Ohm
gap, um
Typical measurement set up
Slope =
sh/Z
What is wrong in
this measurement?
Plot of total resistance
vs. distance