High Doping Effects on in-situ
Ohmic Contacts to n-InAs
Ashish Baraskar, Vibhor Jain, Uttam Singisetti,
Brian Thibeault, Arthur Gossard and Mark J. W. Rodwell
ECE, University of California, Santa Barbara, CA,USA
Mark A. Wistey
ECE, University of Notre Dame, IN, USA
Yong J. Lee
Intel Corporation, Technology Manufacturing Group, Santa Clara, CA, USA
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Outline
• Motivation
– Low resistance contacts for high speed HBTs
– Approach
• Experimental details
– Contact formation
– Fabrication of Transmission Line Model structures
• Results
– InAs doping characteristics
– Effect of doping on contact resistivity
– Effect of annealing
• Conclusion
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Device Bandwidth Scaling Laws for HBT
We
To double device bandwidth*:
Tb
• Cut transit time 1:2
Wbc
Tc
• Cut RC delay 1:2
Scale contact resistivities by 1:4
1
  in  RC
2f
f
max

f
8    Rbb  Ccb eff
*M.J.W. Rodwell, IEEE Trans. Electron. Dev., 2001
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InP Bipolar Transistor Scaling Roadmap
Emitter: 512
16
256
8
128
4
64
2
32
1
width (nm)
access ρ, (m2)
Base:
300
20
175
10
120
5
60
2.5
30
1.25
contact width (nm)
contact ρ (m2)
f t:
fmax:
370
490
520
850
730
1300
1000
2000
1400
2800
GHz
GHz
- Contact resistivity serious barrier to THz technology
Less than 2 Ω-µm2 contact resistivity required for
simultaneous THz ft and fmax*
*M.J.W. Rodwell, CSICS 2008
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Emitter Ohmics-I
Metal contact to narrow band gap material
1. Fermi level pinned in the band-gap
InGaAs
Narrow band gap material:
InP
2. Fermi level pinned in the conduction band
• lower Schottky barrier height
• lower m* easier tunneling
across Schottky barrier
InAs
Better ohmic contacts with
narrow band gap materials1,2
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GaAs
1.Peng et. al., J. Appl. Phys., 64, 1, 429–431, (1988).
2.Shiraishi et. al., J. Appl. Phys., 76, 5099 (1994).
5
Emitter Ohmics-II
Choice of material:
• In0.53Ga0.47As: lattice matched to InP
- Ef pinned 0.2 eV below conduction band[1]
• Relaxed InAs on In0.53Ga0.47As
- Ef pinned 0.2 eV above conduction band[2]
Other considerations:
• Better surface preparation techniques
- For efficient removal of oxides/impurities
• Refractory metal for thermal stability
1. J. Tersoff, Phys. Rev. B 32, 6968 (1985)
2. S. Bhargava et. al., Appl. Phys. Lett., 70, 759 (1997)
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Thin Film Growth
Semiconductor layer growth by Solid Source Molecular Beam
Epitaxy (SS-MBE): n-InAs/InAlAs
- Semi insulating InP (100) substrate
- Un-doped InAlAs buffer
- Electron concentration determined by Hall measurements
100 nm InAs: Si (n-type)
150 nm In0.52Al0.48As: NID buffer
Semi-insulating InP Substrate
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In-situ Metal 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 Mo
100 nm InAs: Si (n-type)
150 nm In0.52Al0.48As: NID buffer
Semi-insulating InP Substrate
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TLM (Transmission Line Model) Fabrication
• E-beam deposition of Ti, Au and Ni layers
• Samples processed into TLM structures by photolithography
and liftoff
• Mo 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 Mo
100 nm InAs: Si (n-type)
150 nm In0.52Al0.48As: NID buffer
Semi-insulating InP Substrate
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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  C  RSh
2  RC 
W
1
0
0
0.5
1
1.5
2
2.5
3
3.5
Gap Spacing (m)
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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)
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6
Overlap Resistance
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5000
Tsub: 380
19
3000
oC
2000
1000
Mobility
18
10
18
10
10
19
0
20
10
2
4000
10
1.5 10
20
1 10
20
5 10
19
-3
Active carrier
concentration
20
10
Electron concentration, n (cm )
6000
Mobility (cm /Vs)
-3
Electron concentration, n (cm )
Results: Doping Characteristics
-3
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400
420
440
460
o
Si atom concentration (cm )
n saturates at high
dopant concentration
380
Substrate Temperature ( C)
- Enhanced n for colder growths
- hypothesis: As-rich surface
drives Si onto group-III sites
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Results: Contact Resistivity - I
Metal Contact
ρc (Ω-µm2)
ρh (Ω-µm)
In-situ Mo
0.6 ± 0.4
2.0 ± 1.5
5
Resistance ()
4
3
2
2  RC 
1
0
0
0.5
1
1.5
2
2  C  RSh
W
2.5
3
• Electron concentration, n = 1×1020 cm-3
• Mobility, µ = 484 cm2/Vs
• Sheet resistance, Rsh = 11 ohm/
(100 nm thick film)
3.5
Gap Spacing (m)
Lowest ρc reported to date for n-type InAs
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4
2
3
c
• ρc measured at various n
- ρc decreases with increase in n
Contact Resistivity,  (m )
Results: Contact Resistivity - II
• Shiraishi et. al.[1] reported
ρc = 2 Ω-µm2 for ex-situ
Ti/Au/Ni contacts to n-InAs
• Singisetti et. al.[2] reported
ρc = 1.4 Ω-µm2 for in-situ
Mo/n-InAs/n-InGaAs
2
1
Shiraishi et. al.[1]
(ρc = 2 Ω-µm2)
Singisetti et. al.[2]
(ρc = 1.4 Ω-µm2)
Present work
0
0
2
4
6
8
10
12
19
-3
Electron Concentration, n (x 10 cm )
Extreme Si doping improves contact resistance
1Shiraishi
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et. al., J. Appl. Phys., 76, 5099 (1994). 2Singisetti et. al., Appl. Phys. Lett., 93, 183502 (2008).
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Results: Contact Resistivity - III
Thermal Stability
• Contacts annealed under N2 flow at 250 oC for 60 minutes
(replicating the thermal cycle experienced by a transistor during
fabrication)
• Observed variation in ρc less than the margin of error
Contacts are thermally stable
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Application in transistors !
• Optimize n-InAs/n-InGaAs interface resistance
• Mo contacts to n-InGaAs*: ρc = 1.1±0.6 Ω-µm2
n-InAs
n-InGaAs
*Baraskar et. al., J. Vac. Sci. Tech. B, 27, 2036 ( 2009).
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Conclusions:
• Extreme Si doping improves contact resistance
• ρc= 0.6±0.4 Ω-µm2 for in-situ Mo contacts to n-InAs with 1×1020 cm-3
electron concentration
• Need to optimize n-InAs/n-InGaAs interface resistance for transistor
application
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Thank You !
Questions?
Acknowledgements
ONR, DARPA-TFAST, DARPA-FLARE
ashish.baraskar@ece.ucsb.edu
University of California, Santa Barbara, CA, USA
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Extra Slides
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Results
-3
Active carrier concentration, n (cm )
1. Doping Characteristics
10
20
9 10
19
8 10
19
7 10
19
6 10
19
5
10
15
20
25
30
-7
As flux (x10 torr)
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c
2
Contact Resistivity,  (m )
10
1
0.1
0
5 10
n
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-1/2
-10
(cm
1 10
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Results: Contact Resistivity - III
– : simulation[1]
x : experimental data[2]
n-InAs
p-InAs
Possible reasons for decrease in contact resistivity with increase in electron
concentration
• Band gap narrowing
• Strain due to heavy doping
• Variation of effective mass with doping
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Accuracy Limits
• Error Calculations
– dR = 50 m (Safe estimate)
– dW = 0.2 m
– dGap = 20 nm
• Error in c ~ 50% at 1 -m2
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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
Current (mA)
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*4155C datasheet
24
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
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W
Variable gap along width (W)
1.10 µm
1.04 µm
Overlap Resistance
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•
•
•
•
•
•
•
Variation of effective mass with doping
Non-parabolicity
Thickness dependence
SXPS (x-ray photoemission spectroscopy)
BEEM (ballistic electron emission microscopy)
Band gap narrowing
Strain due to heavy doping
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Emitter Ohmics-I
Metal contact to narrow band gap material
For the same number of active carriers
(in the degenerate regime)
Ef - Ec > E f - Ec
(narrow gap, Eg1) (wide gap, Eg2)
m*e(Eg1)< m*e(Eg2)
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