Title: Interface roughness scattering in ultra-thin GaN channels in N-polar enhancement-mode GaN MISFETs
1Interface 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
Departments University of California, Santa
Barbara, CA 2011 International Symposium on
Compound Semiconductors Berlin, Germany
uttam_at_ece.ucsb.edu
2Outline
- N-polar E-mode GaN HEMTs
- Mobility in the scaled channels
- Low-T mobility and roughness scattering
- Conclusion
3E-mode ultra-scaled N-polar GaN devices
No electron barrier
N-polar inverted HEMT
N-polar GaN
- No barrier to electron on top of 2-DEG? grading
to narrowgap InN ? - low resistance contacts (0.027 W-mm)1
- AlGaN back ? confinement of 2-DEG, control short
channel effects2 - E-mode devices
1. S.Dasgupta, APL 2010
2. S. Rajan, IEEE TED 2011
4E-mode device structure and design
Under gate
5Short channel effect, channel scaling
8 nm GaN channel
20 nm GaN
Vt roll-off with gate length
- 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
6Mobility in thin channel
- Need 5 nm thick GaN channel for sub-50 nm
devices - Mobility drops with decreasing GaN channel
thickness
7Mobility in ultra-scaled devices
Mobility under the sidewall access regions ? low
source access resistance Mobility under the gate
? Quasi-ballistic operation
8Device test structure
Et
__
- Design target 81012 to 101012 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
9Mobility dependence on Si doping
Si 5 e18 cm-3
Si 2 e 19 cm-3
- 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
10Mobility dependence on AlN etch
SiNx cap
8 nm channel graded back-barrier
mobility
AlN wet etch treated
- Selective AlN wet etching leads to reduction in
mobility - GaN etching negligible, surface roughening
feasible
11Low-temperature mobility
graded back-barrier 5e18 cm-3 Si
- Low temperature mobility ? remove phonon
contribution - Coulombic scattering dominant
12Mobility model with no roughness scattering
- Calculated mobility deviates significantly at
low temperature - Local Coulombic scattering
13Roughness scattering (I) Local field effect
- Roughness induced scattering depends on the
local field
Ferry and Goodnick
14Roughness scattering (II) Sub-band energy
- Ground state energy calculated from perturbation
theory
Sakaki, APL 1987
15Mobility model with roughness scattering
L
?
Roughness parameter ? 0.82 nm, L 1.4 nm
16N-face growth surface
5 nm GaN channel
8 nm GaN channel
- N-face surface rms roughness 1 nm
17Sub-band energy fluctuation with qw width
18Quantum well scattering in SOI
Riddet, IEEE TED 2010
- SOI body thickness variation due to roughness
leads to drop in mobility
19Conclusions 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
20Ga -polar