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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

*uttam@ece.ucsb.edu

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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