1 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 Departments University of California, Santa Barbara, CA 2011 International Symposium on Compound Semiconductors Berlin, Germany *

2 Outline N-polar E-mode GaN HEMTs Mobility in the scaled channels Low-T mobility and roughness scattering Conclusion

3 − 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 effects 2 − 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

4 Under gate Under S/D contacts* * S.Dasgupta, APL 2010 Under sidewall AlN removed under sidewall E-mode device structure and design 8, 10, 12 nm GaN channel Top AlN depletes 2-DEG under gate Under gate

5 Short channel effect, channel scaling V th roll off with gate length Vertical scaling needed to maintain E-mode at sub-50 nm gate lengths Vertical scaling for high R ds at sub-50-nm gate lengths 8 nm GaN channel V t roll-off with gate length 20 nm GaN

6 Mobility in thin channel Need 5 nm thick GaN channel for sub-50 nm devices Mobility drops with decreasing GaN channel thickness

7 Mobility in ultra-scaled devices Mobility under the sidewall access regions  low source access resistance Mobility under the gate  Quasi-ballistic operation

8 Device test structure Design target ~ 8×10 12 to 10×10 12 cm -2 Modulation doping layer: GaN or AlGaN grade Si doping to keep E f away from the trap level E t UV-Ozone, BHF treatment for process simulation __ EtEt

9 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

10 Mobility dependence on AlN etch Selective AlN wet etching leads to reduction in mobility GaN etching negligible, surface roughening feasible mobility AlN wet etch treated SiN x cap 8 nm channel graded back-barrier

11 Low-temperature mobility Low temperature mobility  remove phonon contribution Coulombic scattering dominant graded back-barrier 5e18 cm -3 Si

12 Mobility model with no roughness scattering Calculated mobility deviates significantly at low temperature Local Coulombic scattering

13 Roughness scattering (I) : Local field effect Roughness induced scattering depends on the local field * Ferry and Goodnick

14 Roughness scattering (II) : Sub-band energy Ground state energy calculated from perturbation theory * Sakaki, APL 1987

15 Mobility model with roughness scattering Roughness parameter ∆ = 0.82 nm, L = 1.4 nm ∆ L

16 N-face growth surface N-face surface rms roughness ~ 1 nm 8 nm GaN channel 5 nm GaN channel

17 Sub-band energy fluctuation with qw width

18 Quantum well scattering in SOI Riddet, IEEE TED 2010 SOI body thickness variation due to roughness leads to drop in mobility

19 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

20 Ga -polar