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Interface roughness scattering in ultra-thin GaN channels in N-polar enhancement-mode GaN MISFETs
Uttam Singisetti*, Man Hoi Wong, Jim Speck, and Umesh Mishra
ECE and Materials DepartmentsUniversity of California, Santa Barbara, CA
2011 International Symposium on Compound SemiconductorsBerlin, Germany
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Outline
• N-polar E-mode GaN HEMTs
• Mobility in the scaled channels
• Low-T mobility and roughness scattering
• Conclusion
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− No barrier to electron on top of 2-DEG grading to narrowgap InN low resistance contacts (0.027 -mm)1
− AlGaN back confinement of 2-DEG, control short channel effects2
− E-mode devices
N-polar GaN
E-mode ultra-scaled N-polar GaN devices
1. S.Dasgupta, APL 2010
N-polar inverted HEMT
No electron barrier
2. S. Rajan, IEEE TED 2011
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Under gate
0 20 40 60 80 100
-4
-3
-2
-1
0
1
2
3
4
InN
n+ Graded InGaN(In: 0% to 65%)
GaN:SiAlN
GaN channel
EF
En
erg
y (e
V)
Depth (nm)
Under S/D contacts*
* S.Dasgupta, APL 2010
0 5 10 15 20 25 30 35 40-5
-4
-3
-2
-1
0
1
2
3
4
5
GaN:Si
SiNx
AlNAlN
GaNGaN
EF
EV
EC
En
erg
y (e
V)
Depth (nm)
Under sidewall
AlN removed under sidewall
20 40 60 80 100
-4
-3
-2
-1
0
1
2
3
4 n(x)
n (
x1019
cm
-3)
EF
EV
EC
AlN
GaN:SiSiN
x
GaN
GaN
En
erg
y (e
V)
Depth (nm)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
9.7×10128 nm
7.5×101210 nm
6.6×101212 nm
ns (cm-2)GaN channel
9.7×10128 nm
7.5×101210 nm
6.6×101212 nm
ns (cm-2)GaN channel
E-mode device structure and design
8, 10, 12 nm GaN channelTop AlN depletes 2-DEG under gate
Under gate
0 5 10 15 20 25 30 35 40-5
-4
-3
-2
-1
0
1
2
3
4
5
GaN:Si
SiNx
AlNAlN
GaNGaN
EF
EV
EC
En
erg
y (e
V)
Depth (nm)
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Short channel effect, channel scaling
• Vth roll off with gate length
• Vertical scaling needed to maintain E-mode at sub-50 nm gate lengths •Vertical scaling for high Rds at sub-50-nm gate lengths
8 nm GaN channel
Vt roll-off with gate length
20 nm GaN
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Mobility in thin channel
• Need 5 nm thick GaN channel for sub-50 nm devices
• Mobility drops with decreasing GaN channel thickness
0 5 10 15 20 25 30 35 40-5
-4
-3
-2
-1
0
1
2
3
4
5
GaN:Si
SiNx
AlNAlN
GaNGaN
EF
EV
EC
En
erg
y (e
V)
Depth (nm)
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Mobility in ultra-scaled devices
Mobility under the sidewall access regions low source access resistance
Mobility under the gate Quasi-ballistic operation
20 40 60 80 100
-4
-3
-2
-1
0
1
2
3
4 n(x)
n (
x1019
cm
-3)
EF
EV
EC
AlN
GaN:SiSiN
x
GaN
GaN
En
erg
y (e
V)
Depth (nm)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
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Device test structure
• Design target ~ 8×1012 to 10×1012 cm-2
• Modulation doping layer: GaN or AlGaN grade
• Si doping to keep Ef away from the trap level Et
• UV-Ozone, BHF treatment for process simulation
__Et
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Mobility dependence on Si doping
• High 3D Si doping to keep hole trap away from the Fermi level
• Similar 2-D Si density in the samples
• High Si density may lead to rougher interface
Si : 5 e18 cm-3
Si : 2 e 19 cm-3
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Mobility dependence on AlN etch
• Selective AlN wet etching leads to reduction in mobility
• GaN etching negligible, surface roughening feasible
mobility
AlN wet etchtreated
SiNx cap
8 nm channelgraded back-barrier
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Low-temperature mobility
• Low temperature mobility remove phonon contribution
• Coulombic scattering dominant
graded back-barrier5e18 cm-3 Si
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Mobility model with no roughness scattering
• Calculated mobility deviates significantly at low temperature
• Local Coulombic scattering
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Roughness scattering (I) : Local field effect
• Roughness induced scattering depends on the local field
1
0 22
4
2
,2
1)2
)((
where )2/(
1
22
ukq
uGu
euI
InNA
F
TF
duuLk
sSiD
F
*)()()(potential Scattering rzeFrV
* Ferry and Goodnick
ε
nNF
sD,Si
avg22
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Roughness scattering (II) : Sub-band energy
1
0 22
4
2323
1
1)2
)((
)42
(1
22
ukq
uGu
euI
Ikk
B
F
TF
duuLk
GaN
GaN
F
tt
*)(potential Scattering 1 rt
EeE
qw
• Ground state energy calculated from perturbation theory
* Sakaki, APL 1987
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Mobility model with roughness scattering
Roughness parameter ∆ = 0.82 nm, L = 1.4 nm
∆
L
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N-face growth surface
• N-face surface rms roughness ~ 1 nm
8 nm GaN channel5 nm GaN channel
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Sub-band energy fluctuation with qw width
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Quantum well scattering in SOI
Riddet, IEEE TED 2010
• SOI body thickness variation due to roughness leads to drop in mobility
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Conclusions and future work
• Study mobility drop in thin channels
• Effect of doping and process
• Low temperature mobility
• Roughness scattering included
• Remote surface roughness scattering
This work was supported by DARPA NEXT program
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Ga -polar