1. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Measurements of the E-field Breakdown and Band Offsets of SiO 2 on GaN Ted Cook, Ed Hurt, Kieran Tracy, R.F. Davis, G. Lucovsky, and R.J. Nemanich North Carolina State University Raleigh, NC 27695-8202 USA February 12, 2002

2. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Introduction The band offsets have a direct effect on the properties of the interface, and therefore affect the reliability and stability of the device Investigation of the band offsets, as well as their effects, such as E-field breakdown, is important for device fabrication, and will be explored in this study

3. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Goal and Motivation Goal: Measure the Valence Band Offsets between atomically clean GaN and SiO 2 using photoemission techniques Motivation: Passivation of high voltage devices Wafer Bonding – fusion bonded interfaces Gate insulator applications

4. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Experimental Procedure 1.Achieve atomically clean GaN surface using CVC anneal in NH 3 atmosphere @ 860°C for 15 minutes 2.XPS and UPS performed after each of the following steps to track the evolution of peak shifts 1.Deposition of 2Å Si via MBE 2.Deposition of a second 2Å Si layer 3.Oxidation of Si layer using a Remote O 2 plasma 4.Deposition of 2Å, and oxidation of the Si layer 5.650°C anneal for 15 minutes to densify the oxide 6.Deposition of 3Å, and oxidation of the Silicon layer 7.Final anneal at 650°C for densification

5. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Integrated UHV Surface and Interface Processing System CVD Diamond High Voltage Testing Field Emission XPSGSMBE AES / LEED e-beam evap Si-Ge MBE ARUPS Hydrogen/Oxygen Plasma LoadLock ECR N 2 Plasma Wafer Bonding

6. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Proposed Valence Band Line-up between n-GaN and SiO 2 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Ef Ec Ev Evac Eg = 3.40eVEf-Ev = 3.32eV  = 2.9eV Eg = 9.0eV Ef-Ev = 5.30eV Ec-Ef =0.08eV Ec-Ef = 3.70eV  = 1.1eV  Ec = 3.6eV  Ev = 2.0eV GaN SiO 2 Clean Ed = 0.3eV

7. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Gallium 3d XPS Evolution of n-type GaN

8. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Energy Band Diagram Clean GaN SurfaceFinal Surface EfEf EfEf CB VB

9. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB UPS Spectra of VB turn-on 0.3 eV shift due to band bending

10. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Valence Band Offset between GaN and SiO 2 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Ef Ec Ev Evac Eg = 3.40eVEf-Ev = 3.32eV  = 2.9eV Eg = 9.0eV Ef-Ev = 5.30eV Ec-Ef =0.08eV Ec-Ef = 3.70eV  = 1.1eV  Ec = 3.6eV  Ev = 2.0eV GaN SiO 2 Clean Ed = 0.3eV

11. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Oxygen 1s XPS Spectra showing Oxide Formation

12. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB UPS Spectra of Spectral Width W=11.1 eV E g SiO2 =9.0 eV E g GaN =3.4 eV W=14.9 eV  =21.2-11.1-9.0 = 1.1 eV  =21.2-14.9-3.4 = 2.9 eV  =h -W-E g

13. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Electron Affinity difference between GaN and SiO 2 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Ef Ec Ev Evac Eg = 3.40eVEf-Ev = 3.32eV  = 2.9eV Eg = 9.0eV Ef-Ev = 5.30eV Ec-Ef =0.08eV Ec-Ef = 3.70eV  = 1.1eV  Ec = 3.6eV  Ev = 2.0eV GaN SiO 2 Clean Ed = 0.3eV

14. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Models of Heterojunctions Electron Affinity Model Valid if small or no change in the interface Dipole Interface Dipole Model Change in the Electric field at interface Deviation from the electron affinity of the heterojunction alignment represents the interface dipole EAM IDM

15. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Interface Dipole in Band Line-up of GaN and SiO 2  =1.9 eV -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Ef Ec Ev Evac Eg = 3.40eVEf-Ev = 3.32eV  = 2.9eV Eg = 9.0eV Ef-Ev = 5.30eV Ec-Ef =0.08eV Ec-Ef = 3.70eV  = 1.1eV  Ec = 3.6eV  Ev = 2.0eV GaN SiO 2 Clean Ed = 0.3eV

16. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Conclusions for n-type experiment Special care has been taken to prevent oxidation of gallium Band bending of ~0.3 eV for CVC GaN surface Electron Affinity ~2.9 eV for CVC GaN surface Flat Bands at the GaN-SiO 2 Interface Valence Band Offset of ~2 eV for GaN-SiO 2 for 1x10 17 cm -3 n-type GaN Conduction Band offset of ~3.6 eV for GaN-SiO 2 assuming E gSiO2 =9.0 eV Deviation from the Electron Affinity model due to Interface dipole of ~1.9 eV

17. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Gallium 3d XPS Evolution of p-type GaN

18. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Proposed Valence Band Line-up between p-GaN and SiO 2 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Ef Ec Ev Evac E g = 3.40eV Ef-Ev = 2.2eV Eg = 9.0eV Ef-Ev = 4.5eV Ec-Ef =3.1eV Ec-Ef = 4.5eV Ec = 3.3eV Ev = 2.3eV GaNSiO 2 E B =0.3eV Ef-Ev=1.7eV Clean

19. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Conclusions for p-type experiment Initial Band Bending of 1.4 eV for CVC GaN surface Additional downward Band Bending of ~0.5 eV as the surface was formed Valence Band offset of ~2.3 eV for GaN-SiO 2 for 2x10 18 cm -3 p-type GaN Conduction Band Offset of 3.3 eV for GaN-SiO 2 assuming E g Sio2 =9.0 eV

20. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB E-field Breakdown Procedure 1.Achieve atomically clean GaN surface using CVC anneal in NH 3 atmosphere @ 860°C for 15 minutes 2.Deposit 4Å Si via MBE on surface to help prevent oxidation of the Gallium 3.Deposit 300Å SiO 2 via Remote O 2 Plasma 4.Anneal to 650°C to densify the oxide 5.Form 0.068 mm 2 Al contacts on the surface using photolithographic techniques 6.Electrical characterization of sample

21. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Device Structure for Electrical Testing SiC SiO2 GaN AlN W Contact Al Contacts V CRT I v

22. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Breakdown Frequency 300Å SiO 2 unintentionally doped, n-type <1.0x10 17 cm -3

23. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Conclusions These high values of breakdown voltage lends to the notion that the GaN layer is having an insulating effect A full understanding of this effect requires more study Film deposition rate of 2.5 Å/min established; verified by ellipsometry and C-V measurements

24. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Future Work Reduce Band Bending for p-type GaN/SiO 2 -Improve cleaning methods -Oxide integrity Effect of different passivation layers –Si 3 N 4 –High k dielectrics