TLM Lecture
-
date post
24-Mar-2015 -
Category
Documents
-
view
233 -
download
0
Transcript of TLM Lecture
ELECT 871 03/31/03
Schottky contacts• Schottky contacts are
formed whenDoping in the
semiconductor is not very high i.e. > ~5x1018 cm-3
The metal work function is greater than the n- type semiconductor work function
The metal work function is lower than p-type semiconductor work function
Very high density of surface states “pinning” the Fermi level at the surface w.r.t. the conduction band (Example: GaAs)
Schottky contact
n doped
Electrons from conduction band or in the metal faces barrier to free movement, and tunneling is also
not easy
M
Evac Evac
EF
M > +Ec-EF = S
For n-type semiconductor and reverse for p-type
s
Bn =
M -
ELECT 871 03/31/03
Ohmic contacts
• 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
M
Evac Evac
EF
M < +Ec-EF = S
For n-type semiconductor.
Reverse for p-type
Ohmic contact by high doping
Electrons from conduction band can move very easily to the metal and
vice versa by tunneling
n+ doped
Ohmic contact by band alignment
n doped
B = M - EF
ELECT 871 03/31/03
Conduction mechanisms in schottky contacts
• Thermionic emissionElectrons emit over the barrier Low probability of direct tunnelingValid 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 bandValid for intermediate doping (~ 1017 cm-3 < ND < ~ 1018 cm-3)
• Field emissionDirect tunneling, as depletion region is very narrowValid for heavy doping (ND > ~ 1018 cm-3); almost ohmic
• Leakage currentHigh probability of defect-assisted tunneling and simple conductionOccurs in poor material/interface quality; dislocations
ELECT 871 03/31/03
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 Bn
o
exp2* T = temperature, A* = efective Richardson’s constant
ELECT 871 03/31/03
Schottky contact for nitride devices
• Schottky contacts are usually made in nitride HFET devices by depositing Ni/Au layer on n-type GaN
• 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
Variation of the schottky barrier height for different metal deposited on GaN
ELECT 871 03/31/03
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 incident on the schottky contact; this is very accurate)– Photocurrent R is related to the barrier height as
VqN
W BnD
s 2 V
NqC
Bn
Ds
2 V
C Bn 2
1
BnqhvR ~ So the intercept gives the barrier height
kT
q Bn
eTAJ
2*
0
0
2*
lnJ
TA
q
kTBn. VF vs. J intercept gives J0 and
ELECT 871 03/31/03
Evaporation systemsContact Metallization (Ti, Al, Ni, Au etc)Metal Electron-Beam Evaporation System
Rapid Thermal Annealing Systemfrom 20 oC to 1000 oC in seconds
Target Metal Sourcewith e-beam
Sample
ELECT 871 03/31/03
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, N-vacancies are created, which can dope the contact region and create ohmic contact
ELECT 871 03/31/03
Contacts for p-type doped nitrides
• 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
ELECT 871 03/31/03
Specific contact resistivity and sheet resistance
Product of contact resistance Rc and area A is called specific contact resistivity c:
0
1
V
c
VJ
. cm2)
Semiconductor layer resistivity :
ne
1 ( . cm)
tZ
ddx
AR
d
s
0
1(
dZ
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 then given by
(/)�
ELECT 871 03/31/03
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
TL
xI
dx
Id
shx/Z
c/(Zx)
I
I(x)
V(x)
I(x+x)
V(x +x)
where
The solution for I(x) is given as:
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:
L
00 xIxVRC
is called the transfer length.
ELECT 871 03/31/03
Transmission line method (TLM) II
T
CC ZL
R
Z
dRRRR sh
cscTot
22
sh = /t
d
Z
t
When the following conditions are further satisfied, d << Z and t << LT (to avoid current spreading in the sides or into the film), Then,
The contact resistance Rc is then given by:
For L >>LT , we have,
TT
CC L
L
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.
Ohmics L
ELECT 871 03/31/03
Measurement techniqueTypical measurement set up
Slope = sh/Z
What is wrong in this measurement?
Plot of total resistance vs. distance