in-situ ohmic contacts to p-ingaas - ucsb...300 350 400 450 sb t (substrate temp. (oc) tendency to...
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In-situ Ohmic Contacts to p-InGaAs
Ashish Baraskar, Vibhor Jain, Evan Lobisser, Brian Thibeault,Ashish Baraskar, Vibhor Jain, Evan Lobisser, Brian Thibeault, Arthur Gossard and Mark Rodwell
ECE and Materials Departments, University of California, Santa Barbara CABarbara, CA
Mark WisteyfElectrical Engineering, University of Notre Dame, IN
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 1
Outline• MotivationMotivation
– Low resistance contacts for high speed HBTs– Approachpp
• 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 Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 2
Outline• MotivationMotivation
– Low resistance contacts for high speed HBTs– Approachpp
• 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 Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 3
Device Bandwidth Scaling Laws for HBT
To double device bandwidth:
• Cut transit time 2xeW
• Cut RC delay 2x
Scale contact resistivities by 4:1*
bcWcTbT
Scale contact resistivities by 4:1
HBT: Heterojunction Bipolar TransistorRC
f in 21
effcbbb CRff
8max
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 4
*M.J.W. Rodwell, CSICS 2008
InP Bipolar Transistor Scaling Roadmap256 128 64 32 nm width
Emitter256 128 64 32 nm width
8 4 2 1 Ω·µm2 access ρ
Base175 120 60 30 nm contact width
Base10 5 2.5 1.25 Ω·µm2 contact ρ
Collector106 75 53 37.5 nm thick
9 18 36 72 mA/µm2 currentµ4 3.3 2.75 2-2.5 V breakdown
fτ 520 730 1000 1400 GHzf 850 1300 2000 2800 GHzfmax 850 1300 2000 2800 GHz
Contact resistivity serious barrier to THz technologyy gy
Less than 2 Ω-µm2 contact resistivity required for simultaneous THz f and f *
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 5
simultaneous THz ft and fmax
Approach
To achieve low resistance, stable ohmic contacts• Higher number of active carriers
- Reduced depletion widthE h d t li t l- Enhanced tunneling across metal-semiconductor interface
B tt f ti t h i• Better surface preparation techniques- For efficient removal of oxides/impurities
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 6
• Scaled device thin base
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 Pd/Ti diff i
100 nm InGaAs grown in MBE
15 nm Pd/Ti diffusion
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 7
100 nm InGaAs grown in MBE
Outline• MotivationMotivation
– Low resistance contacts for high speed HBTs and FETs– Approachpp
• 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 Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 8
E il th b S lid S M l l B E it
Epilayer GrowthEpilayer growth by Solid Source Molecular Beam Epitaxy (SS-MBE)– p-InGaAs/InAlAs
- Semi insulating InP (100) substrateSemi insulating InP (100) substrate- Un-doped InAlAs buffer- CBr4 as carbon dopant source4
- Hole concentration determined by Hall measurements
Semi insulating InP Substrate
100 nm In0.52Al0.48As: NID buffer100 nm In0.53Ga0.47As: C (p-type)
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 9
Semi-insulating InP Substrate
In-situ contacts
In-situ molybdenum (Mo) deposition - E-beam chamber connected to MBE chamber- No air exposure after film growthNo air exposure after film growth
Why Mo? Refractory metal (melting point 2620 oC)- 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 In0.52Al0.48As: NID buffer100 nm In0.53Ga0.47As: C (p-type)
Semi insulating InP Substrate
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 10
Semi-insulating InP Substrate
TLM (Transmission Line Model) fabrication• E-beam deposition of Ti, Au and Ni layers
• Samples processed into TLM structures by photolithography d lift ffand liftoff
• Contact metal was dry etched in SF6/Ar with Ni as etch mask, isolated by wet etchisolated by wet etch
50 nm ex-situ Ni
20 nm in-situ Mo20 nm ex-situ Ti
500 nm ex-situ Au
100 nm In0.52Al0.48As: NID buffer100 nm In0.53Ga0.47As: C (p-type)
Semi insulating InP Substrate
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 11
Semi-insulating InP Substrate
• Resistance measured by Agilent 4155C semiconductor parameter
Resistance Measurementy g p
analyzer
• TLM pad spacing (Lgap) varied from 0.5-26 µm; verified from gap
scanning electron microscope (SEM)
• TLM Width ~ 25 µm
4
5
)
R22
3
Res
ista
nce
(W
RR ShC
C
22
0
1
0 0.5 1 1.5 2 2.5 3 3.5
R
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 12
Gap Spacing (m)
• Extrapolation errors:Error Analysis
• Processing errors:p– 4-point probe resistance
measurements on Agilent 4155C
g– Variable gap spacing along
width (W)– Overlap resistance4155C
– Resolution error in SEM3.5
Overlap resistance
2
2.5
3
ce (
)
R
d
Variable gap along width (W)1.10 µm 1 04 µm
0 5
1
1.5
2
Res
ista
nc d
Lgap
1.10 µm 1.04 µm
0
0.5
0 1 2 3 4 5 6Gap Spacing (m)
Rc W
Overlap Resistance
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 13
Outline• MotivationMotivation
– Low resistance contacts for high speed HBTs and FETs– Approachpp
• 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 Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 14
Doping Characteristics-IHole concentration Vs CBr4 flux
1020
70
80
3 )
4
60
70hole concentration
ty (c
m2 -V
s)
ntra
tion
(cm
-3
Tsub = 460 oC
40
50mobility
Mob
ilit
Hol
e C
once
n
1019
40
0 10 20 30 40 50 60CBr
4 foreline pressure (mtorr)
– Hole concentration saturates at high CBr fluxes– Number of di-carbon defects as CBr flux
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 15
Number of di carbon defects as CBr flux
Doping Characteristics-IIHole concentration Vs V/III flux
10
9 ) cm
-3
Hole concentration Vs V/III flux
6
8
ratio
n (1
019CBr = 60 mtorr
4
e C
once
ntr
210 20 30 40 50 60
Group V / Group III
Hol
As V/III ratio hole concentration
h pothesis As deficient s rface dri es C onto gro p V sites
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 16
hypothesis: As-deficient surface drives C onto group-V sites
Doping Characteristics-IIIH l t ti V b t t t t
2 1020
)
Hole concentration Vs substrate temperature
1.2 1020
1.6 1020
tratio
n (c
m-3
)
19
8 1019
Hol
e C
once
nt
CBr = 60 mtorr4 1019
300 350 400 450
H
S b T (oC)Substrate Temp. (oC)
Tendency to form di-carbon defects as Tsub *
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 17
*Tan et. al. Phys. Rev. B 67 (2003) 035208
y
Doping Characteristics-IIIH l t ti V b t t t t
2 1020
)
Hole concentration Vs substrate temperature
)
1.2 1020
1.6 1020
tratio
n (c
m-3
)
1020
ratio
n (c
m-3
)
19
8 1019
Hol
e C
once
nt
CBr = 60 mtorr1019
Tsub = 460 oCTsub = 350 oC
Hol
e C
once
ntr
4 1019
300 350 400 450
H
S b T (oC)
10
0 20 40 60 80 100CBr
4 foreline pressure (mtorr)
HSubstrate Temp. (oC)
Tendency to form di-carbon defects as Tsub *
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 18
*Tan et. al. Phys. Rev. B 67 (2003) 035208
y
Results: Contact Resistivity - I
Metal Contact ρc (Ω-µm2) ρh (Ω-µm)In-situ Mo 2.2 ± 0.8 15.4 ± 2.6
20
• Hole concentration, p = 1.6 x 1020 cm-3
10
15
tanc
e (
)• Mobility, µ = 36 cm2/Vs• Sheet resistance, Rsh = 105 ohm/ (100 nm thick film)
ρ lower than the best reported contacts to
5Res
ist
WR
R ShCC
22
(100 nm thick film)
ρc lower than the best reported contacts to pInGaAs (ρc = 4 Ω-µm2)[1,2] 0
0 1 2 3 4Gap Spacing (m)
1 G iffith t l I di Ph hid d R l t d M t i l 2005
W
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 19
1. Griffith et al, Indium Phosphide and Related Materials, 2005.2. Jain et al, IEEE Device Research Conference, 2010.
Results: Contact Resistivity - II
10
(-
m2 )
*
1exp
pCTunneling
sist
ivity
, c
pTunneling
ThermionicE i i ~ constant*
onta
ct R
es
Data suggests tunneling
Emission c ~ constant
15 10 15
Co
[p]-1/2 (x10-11 cm-3/2)
gg g
High active carrier concentration is the key to low resistance contacts
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 20
* Physics of Semiconductor Devices, S M Sze
Mo contacts annealed under N2 flow for 60 mins at 250 oC
Thermal Stability - IMo contacts annealed under N2 flow for 60 mins. at 250 C
Before annealing After annealing
Molybdenum C substrate
ρc (Ω-µm2) 2.2 ± 0.8 2.8 ± 0.9
• ρc increases on annealing
• Mo reacts with residual
interfacial carbon?* Kiniger et. al.*
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 21
*Kiniger et. al., Surf. Interface Anal. 2008, 40, 786–789
Mo contacts annealed under N2 flow for 60 mins at 250 oC
Thermal Stability - IIMo contacts annealed under N2 flow for 60 mins. at 250 C
TEM of Mo-pInGaAs interfacep
- Suggests sharp interface
- Minimal/No intermixing
Au
- Minimal/No intermixing Ti
InGaAsMo
200 nm InAlAs
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 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)( ρc µ )
Contacts s itable for TH transistors Contacts suitable for THz transistors
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 23
Thank You !
Questions?
AcknowledgementsONR, DARPA-TFAST, DARPA-FLARE
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 24
Extra SlidesExtra Slides
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 25
Correction for Metal Resistance in 4-Point Test Structure
WL /R W/)( 2/1 WLsheet /metalR Wcontactsheet /)( 2/1
WI
V
L
W V
V
I
xRWLW t lh tt th t ///)( 2/1
Error term (R /x) from metal resistance
xRWLW metalsheetcontactsheet ///)(
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 26
Error term (Rmetal/x) from metal resistance
Random and Offset Error in 4155C
0.6646
0.6647• Random Error in
resistance
0.6645
0.6646
nce
() measurement ~ 0.5
m• Offset Error < 5 m*
0.6644
Res
ista
n • Offset Error < 5 m
0.6642
0.6643
0 5 10 15 20 25Current (mA)
*4155C datasheet
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 27
Accuracy Limits
• Error Calculations• Error Calculations– dR = 50 mΩ (Safe estimate)
dW 1– dW = 1 µm– dGap = 20 nm
2• Error in ρc ~ 40% at 1.1 Ω-µm2
2010 Electronic Materials Conference Ashish BaraskarJune 23-25, 2010 – Notre Dame, IN 28
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