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Performance Enhancement of P-channel InGaAs Quantum-well FETs by
Superposition of Process-induced Uniaxial Strain and Epitaxially-grown Biaxial Strain
Ling Xia1, Vadim Tokranov2, Serge R. Oktyabrsky2, and Jesús A. del Alamo1
1 Microsystems Technology Laboratories, MIT, USA;2 College of Nanoscale Science and Engineering, SUNY-Albany, USA.
Dec. 06, 2011
Sponsors: Intel Corp. and FCRP-MSD Center. Fabrication: MTL at MIT.
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Outline
• Motivation
• Mechanical simulations
• Device technology
• Experimental results
• Conclusions
2
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• Interests in InGaAs CMOS – Fueled by excellent ve and µe• Key challenge for InGaAs CMOS
• Bridging performance gap between n- and p-FET.• Our approach – Introduce strain to InGaAs p-FET
• Uniaxial + biaxial compressive strain
Kuhn, IWJT, 2010
Logarithmic scale
Electron mobility
Hole mobility
Motivation
del Alamo, Nature, 20113
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• Sources for strain include:– Epitaxial lattice mismatch Biaxial strain– Fabrication process Uniaxial strain
Why biaxial strain + uniaxial strain?
4
• Enhancements of µh by biaxial and uniaxial strain add superlinearly• Similar effect found in Si simulations (Wang, TED, 2006)
-2.5 -2.0 -1.5 -1.0 -0.5 0.00
20
40
60
80
100
120
GaAs
In0.24Ga0.76As
Max
<11
0> p
iezo
resi
stan
ce c
oeffi
cien
t (1
012 c
m2 /d
yn)
Epitaxial compressive biaxial strain (%)
Ge
Gomez, EDL, 2010Weber, IEDM, 2007Xia, TED, 2011Xia, ISCS, 2011 Experiments
π = ‐Δµ/(µ0σ )
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InGaAs QW-FET with uniaxial + biaxial strain
• Induced stress scalable with LG (next slide)
5
• Biaxial strain in the channel as grown
• Carbon delta-doping
• Self-aligned stressor (compressively stressed)
• Self-aligned metal layer (Mo)
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Mechanical stress simulations• Parameters used in simulations: tSiN = 200 nm; SiN σint = -2 GPa
6
S DCut line
LG = 2 µm
• Desirable stress type can be obtained with the proposed stressor structure• Compressive longitudinal stress µh ↑• Tensile transverse stress µh ↑
Lside = 100 nm
tSiN=200 nm
x (µm)-1 0
Longitudinal stress distribution
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0%
30%
60%
90%
120%
150%
180%
10 100 1000 10000
Δµh/
µ h0
LG (nm)
FromFromTotal
Projected assuming Δµh/ µh = -π·σπ// = - 1.2x10-10 cm2/dynπ = 0.7x10-10 cm2/dyn(Xia, ISCS, 2011)
σσ//
-1,500
-1,200
-900
-600
-300
0
300
10 100 1,000 10,000
Stre
ss (M
Pa)
LG (nm)
LongitudinalTransverse
Slow increase when LG >> tSiN
Fast increase when LG ~ tSiN
Slow down due to a fixed Lside
LG scalability of induced stress at middle of gate
• LG ↓ Stress ↑ inside gate opening• Assume linear ∆µ with σ
>160% µh enhancement for LG < 50 nm7
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Device technology
8
Mesa isolation
Ohmic metalization
Molybdenum (Mo) deposition
PECVD SiN stressor and SiO2Anisotropic ECR RIE SiO2/SiN
Anisotropic ECR RIE Mo
Isotropic RIE Mo
GaAs cap recess by wet etching
Gate metalization
Key considerations:• Avoid Mo layer short to gate metal• Air gap as small as possible
ChannelBarrier
Buffer
Al0.42Ga0.58As
AlGaAsIn0.24Ga0.76As
DS
SiN
Mo
SiO2
Cap : 2x1019 cm-3 Carbon doped GaAs
Channel
Cap GaAs
Buffer AlGaAsIn0.24Ga0.76As
DS
SiNSiO2
MoCap : 2x1019 cm-3 Carbon doped GaAs
SiO2
Barrier Al0.42Ga0.58As
ChannelBarrier
Buffer
Al0.42Ga0.58As
AlGaAsIn0.24Ga0.76As
DS
SiN
Mo
SiO2
Cap : 2x1019 cm-3 Carbon doped GaAs
Channel
Cap
Buffer AlGaAsIn0.24Ga0.76As
DS
SiNSiO2
MoCap : 2x1019 cm-3 Carbon doped GaAs
SiO2
Barrier Al0.42Ga0.58As
G
-
Device cross-section
• LG = 2 µm; channel along [-110]• Lside 400 nm
9
Artifact of imaging
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Experimental parameters for devices•Split experiments:
• SiN with -2.1 GPa stress vs SiN with 0 Pa stress• SiN film stress obtained from wafer curvature measurements
• LG = 2 µm to 8 µm• Four channel orientations:
10
[-110]
[110]
S D
GS
D
G[110][-110] [010][100]
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QW-FET electrical characteristics• Example: LG = 2 µm; channel along [-110]
• Significant drive current increase• Transconductance increase at all gate overdrives
11
-2.0 -1.5 -1.0 -0.5 0.00
2
4
6
8
10
-I D (m
A/m
m)
VDS (V)
-2.1 GPa SiN 0 GPa SiN
VGS - VT = -0.75 V
-0.55 V
-0.35 V
-0.15 V
-0.5 0 0.50
10
20
30
40
VGS - VT (V)|g
m| (
mS
/mm
)
0 GPa SiN-2 GPa SiN
VDS = -2 V
Intrinsic
Extrinsic
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Subthreshold characteristics and VT• Similar IG as chemically etched samples No RIE damage• VT shift between high- and low- stress samples
– Likely due to different anisotropic RIE overetch
12
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.01E-5
1E-4
1E-3
0.01
0.1
1
10
With -2 GPa SiN
|I| (m
A/m
m)
LG = 2 m
VDS = -2 V
ID
With 0 GPa SiN
VGS (V)
IG
Both S = 103 mV/dec
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LG dependence of gm
• Increasing enhancement observed with decreasing LG • Consistent with stress simulations + π measurements
>160% enhancement expected with LG < 50 nm13
2 4 6 80
5
10
15
20
25
30
|gm
-sat
| (m
S/m
m)
LG (m)
VDS = -2 VVGS – VT = -0.5 V
+36%
Channel along [-110]
Dots : dataLines: least-square fittings
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• Observed anisotropic ∆gmi and ∆RSD– gmi extracted using gmext, RS, RD and gD– RS, RD extracted using gate current injection method
• anisotropy consistent with measurements of piezoresistance coefficients– π[-110] : π [110] = 2.6 (Xia, ISCS, 2011)
Crystal direction dependence
14
gmi_sat
-
kx (2/a)
k y (2
/a)
-0.1 -0.05 0 0.05 0.1-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
0.1No uniaxial-500 MPa uniaxial
Theoretical discussions• Valence band change due to strain in InGaAs
– Used k.p methods (nextnano3)– Calculated subbands in In0.24Ga0.76As quantum well
15
In-plane iso-energy contours of hh1 with and without uniaxial strain
[100]
[010] [110]
• Compressive strain parallel to channel is desirable
-0.1 -0.05 0 0.05 0.1-0.16
-0.14
-0.12
-0.1
-0.08
-0.06
Ev
(eV
)
k//[-110]k[110]
[-110] uniaxial
No uniaxial
m*// ↓m*↑
hh1
lh1
hh2
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Effective mass model• Treat nonparabolic valence band using energy dependent
m* (De Michielis, TED, 2007)
From simulations:• ∆m* anisotropy induced by quantization change due to piezoelectric effect• ∆m* anisotropy consistent with gm measurements.
Gate
Pz(z)BarrierChannel
[-110] Strain
-0.1 -0.05 0 0.05 0.1-0.16
-0.14
-0.12
-0.1
-0.08
-0.06
Ev
(eV
)
k//k
[-110] uniaxial
[110] uniaxial
No uniaxial
m*// ↓m*↑
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Conclusions• Developed device architecture for InGaAs p-FETs that incorporates
uniaxial strain through self-aligned dielectric stressor
• Key results:
– Biaxial strain + uniaxial strain substantial enhancements in transport characteristics
– Up to +36% ∆gm observed in LG = 2 µm device
– Strong enhancement anisotropy due to piezoelectric effect
• For further enhancement:
– Scale down LG and bring S/D closer
– Project ∆gm >160% @ LG < 50 nm
• Study useful to other p-type III-V materials (e.g. InGaSb, InSb)
17
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