Interface roughness scattering in ultra-thin GaN channels in N-polar enhancement-mode GaN MISFETs

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 *uttam@ece.ucsb.edu

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

3. E-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

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

5. Short 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

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 Et __ • 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

9. Mobility 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

10. Mobility 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

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

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 L ∆ Roughness parameter ∆ = 0.82 nm, L = 1.4 nm

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

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. Conclusionsand 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