Transcript In-situ Ohmic Contacts to p-InGaAs Ashish Baraskar, Vibhor Jain, Evan Lobisser, Brian Thibeault, Arthur Gossard and Mark Rodwell ECE and Materials Departments, University.

In-situ Ohmic Contacts to p-InGaAs
Ashish Baraskar, Vibhor Jain, Evan Lobisser, Brian Thibeault,
Arthur Gossard and Mark Rodwell
ECE and Materials Departments, University of California, Santa
Barbara, CA
Mark Wistey
Electrical Engineering, University of Notre Dame, IN
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
1
Outline
• Motivation
– Low resistance contacts for high speed HBTs
– Approach
• Experimental details
– Contact formation
– Fabrication of Transmission Line Model structures
• Results
– Doping characteristics
– Effect of doping on contact resistivity
– Effect of annealing
• Conclusion
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
2
Outline
• Motivation
– Low resistance contacts for high speed HBTs
– Approach
• Experimental details
– Contact formation
– Fabrication of Transmission Line Model structures
• Results
– Doping characteristics
– Effect of doping on contact resistivity
– Effect of annealing
• Conclusion
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
3
Device Bandwidth Scaling Laws for HBT
To double device bandwidth:
We
• Cut transit time 2x
Tb
• Cut RC delay 2x
Wbc
Tc
Scale contact resistivities by 4:1*
1
  in  RC
2f
f
max

HBT: Heterojunction Bipolar Transistor
f
8    Rbb  Ccbeff
*M.J.W. Rodwell, CSICS 2008
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
4
InP Bipolar Transistor Scaling Roadmap
256
128
64
32
nm width
8
4
2
1
Ω·µm2 access ρ
175
120
60
30
nm contact width
10
5
2.5
1.25
Ω·µm2 contact ρ
106
75
53
37.5
nm thick
9
18
36
72
4
3.3
2.75
2-2.5
V breakdown
fτ
520
730
1000
1400
GHz
fmax
850
1300
2000
2800
GHz
Emitter
Base
Collector
mA/µm2 current
Contact resistivity serious barrier to THz technology
Less than 2 Ω-µm2 contact resistivity required for
simultaneous THz ft and fmax*
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
5
Approach
To achieve low resistance, stable ohmic contacts
• Higher number of active carriers
- Reduced depletion width
- Enhanced tunneling across metal-
semiconductor interface
• Better surface preparation techniques
- For efficient removal of oxides/impurities
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
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Approach (contd.)
• Scaled device
thin base
(For 80 nm device: tbase < 25 nm)
• Non-refractory contacts may diffuse at higher temperatures through
base and short the collector
• Pd/Ti/Pd/Au contacts diffuse about 15 nm in InGaAs on annealing
Need a refractory metal for thermal stability
15 nm Pd/Ti diffusion
100 nm InGaAs grown in MBE
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
7
Outline
• Motivation
– Low resistance contacts for high speed HBTs and FETs
– Approach
• Experimental details
– Contact formation
– Fabrication of Transmission Line Model structures
• Results
– Doping characteristics
– Effect of doping on contact resistivity
– Effect of annealing
• Conclusion
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
8
Epilayer Growth
Epilayer growth by Solid Source Molecular Beam Epitaxy
(SS-MBE)– p-InGaAs/InAlAs
- Semi insulating InP (100) substrate
- Un-doped InAlAs buffer
- CBr4 as carbon dopant source
- Hole concentration determined by Hall measurements
100 nm In0.53Ga0.47As: C (p-type)
100 nm In0.52Al0.48As: NID buffer
Semi-insulating InP Substrate
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
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9
In-situ contacts
In-situ molybdenum (Mo) deposition
- E-beam chamber connected to MBE chamber
- No air exposure after film growth
Why Mo?
- Refractory metal (melting point ~ 2620 oC)
- Easy to deposit by e-beam technique
- Easy to process and integrate in HBT process flow
20 nm in-situ Mo
100 nm In0.53Ga0.47As: C (p-type)
100 nm In0.52Al0.48As: NID buffer
Semi-insulating InP Substrate
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
10
TLM (Transmission Line Model) fabrication
• E-beam deposition of Ti, Au and Ni layers
• Samples processed into TLM structures by photolithography
and liftoff
• Contact metal was dry etched in SF6/Ar with Ni as etch mask,
isolated by wet etch
50 nm ex-situ Ni
500 nm ex-situ Au
20 nm ex-situ Ti
20 nm in-situ Mo
100 nm In0.53Ga0.47As: C (p-type)
100 nm In0.52Al0.48As: NID buffer
Semi-insulating InP Substrate
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
11
Resistance Measurement
• Resistance measured by Agilent 4155C semiconductor parameter
analyzer
• TLM pad spacing (Lgap) varied from 0.5-26 µm; verified from
scanning electron microscope (SEM)
• TLM Width ~ 25 µm
5
Resistance ()
4
3
2
2  RC 
1
0
0
0.5
1
1.5
2
2  C  RSh
W
2.5
3
3.5
Gap Spacing (m)
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
12
Error Analysis
• Extrapolation errors:
• Processing errors:
– 4-point probe resistance
measurements on Agilent
4155C
– Resolution error in SEM
– Variable gap spacing along
width (W)
– Overlap resistance
3.5
Resistance ()
3
2.5
Variable gap along width (W)
1.10 µm
1.04 µm
dR
2
dd
1.5
Lgap
1
0.5
0
dRc
0
1
W
2
3
4
Gap Spacing (m)
2010 Electronic Materials
Conference
5
6
Overlap Resistance
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
13
Outline
• Motivation
– Low resistance contacts for high speed HBTs and FETs
– Approach
• Experimental details
– Contact formation
– Fabrication of Transmission Line Model structures
• Results
– Doping characteristics
– Effect of doping on contact resistivity
– Effect of annealing
• Conclusion
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
14
Doping Characteristics-I
Hole concentration Vs CBr4 flux
80
20
70
Tsub = 460 oC
60
2
hole concentration
Mobility (cm -Vs)
-3
Hole Concentration (cm )
10
mobility
50
40
19
10
0
10
20
30
40
50
CBr foreline pressure (mtorr)
60
4
– Hole concentration saturates at high CBr fluxes
– Number of di-carbon defects as CBr flux
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
15
Doping Characteristics-II
10
19
Hole Concentration (10 ) cm
-3
Hole concentration Vs V/III flux
8
CBr = 60 mtorr
6
4
2
10
20
30
40
50
60
Group V / Group III
As V/III ratio
hole concentration
hypothesis: As-deficient surface drives C onto group-V sites
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
16
Doping Characteristics-III
Hole concentration Vs substrate temperature
20
Hole Concentration (cm -3)
2 10
20
1.6 10
20
1.2 10
19
8 10
CBr = 60 mtorr
19
4 10
300
350
400
450
o
Substrate Temp. ( C)
Tendency to form di-carbon defects
as Tsub *
*Tan et. al. Phys. Rev. B 67 (2003) 035208
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
17
Doping Characteristics-III
Hole concentration Vs substrate temperature
20
-3
Hole Concentration (cm )
Hole Concentration (cm -3)
2 10
20
1.6 10
20
1.2 10
19
8 10
CBr = 60 mtorr
19
4 10
300
350
400
20
10
o
Tsub = 350 C
o
Tsub = 460 C
19
10
0
450
4
o
Substrate Temp. ( C)
Tendency to form di-carbon defects
100
80
60
40
20
CBr foreline pressure (mtorr)
as Tsub *
*Tan et. al. Phys. Rev. B 67 (2003) 035208
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
18
Results: Contact Resistivity - I
Metal Contact
ρc (Ω-µm2)
ρh (Ω-µm)
In-situ Mo
2.2 ± 0.8
15.4 ± 2.6
20
15
Resistance ()
• Hole concentration, p = 1.6 x 1020 cm-3
• Mobility, µ = 36 cm2/Vs
• Sheet resistance, Rsh = 105 ohm/
(100 nm thick film)
10
5
2  RC 
ρc lower than the best reported contacts to
pInGaAs (ρc = 4 Ω-µm2)[1,2]
0
0
1
2  C  RSh
W
2
3
4
Gap Spacing (m)
1. Griffith et al, Indium Phosphide and Related Materials, 2005.
2. Jain et al, IEEE Device Research Conference, 2010.
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
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19
2
Contact Resistivity, c (-m )
Results: Contact Resistivity - II
10
Tunneling 
 1 

 C  exp 
 p 


*
Thermionic
*

~
constant
c
Emission 
Data suggests tunneling
1
5
10
15
[p]-1/2 (x10-11 cm-3/2)
High active carrier concentration is the key to low resistance contacts
* Physics of Semiconductor Devices, S M Sze
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
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20
Thermal Stability - I
Mo contacts annealed under N2 flow for 60 mins. at 250 oC
Before annealing
After annealing
2.2 ± 0.8
2.8 ± 0.9
ρc (Ω-µm2)
Molybdenum
C substrate
• ρc increases on annealing
• Mo reacts with residual
interfacial carbon?*
Kiniger et. al.*
*Kiniger et. al., Surf. Interface Anal. 2008, 40, 786–789
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
21
Thermal Stability - II
Mo contacts annealed under N2 flow for 60 mins. at 250 oC
TEM of Mo-pInGaAs interface
- Suggests sharp interface
- Minimal/No intermixing
Au
Ti
Mo
InGaAs
200 nm
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
InAlAs
Ashish Baraskar
22
Summary
• Maximum hole concentration obtained = 1.6 x1020 cm-3 at a substrate
temperature of 350 oC
• Low contact resistivity with in-situ metal contacts
(lowest ρc=2.2 ± 0.8 Ω-µm2)
 Contacts suitable for THz transistors
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
23
Thank You !
Questions?
Acknowledgements
ONR, DARPA-TFAST, DARPA-FLARE
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
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Extra Slides
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
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Correction for Metal Resistance in 4-Point Test Structure
Rmetal
I
W
L
( sheet contact )1/ 2 / W
 sheet L / W
V
V
I
(sheet contact )1/ 2 / W  sheet L / W  Rmetal / x
Error term (Rmetal/x) from metal resistance
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
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26
Random and Offset Error in 4155C
0.6647
• Random Error in
resistance
measurement ~ 0.5
m
• Offset Error < 5 m*
Resistance ()
0.6646
0.6645
0.6644
0.6643
0.6642
0
5
10
15
20
25
Current (mA)
*4155C datasheet
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
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27
Accuracy Limits
• Error Calculations
– dR = 50 mΩ (Safe estimate)
– dW = 1 µm
– dGap = 20 nm
• Error in ρc ~ 40% at 1.1 Ω-µm2
2010 Electronic Materials
Conference
June 23-25, 2010 – Notre Dame, IN
Ashish Baraskar
28