1. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Innovative Processing for GaN Power Devices Ilan Ben-Yaacov, Yan Gao, Sarah E. Monteith, S. DenBaars, U. Mishra, E.L. Hu University of California, Santa Barbara with thanks to Andrew Huntington, Stacia Keller, Andreas Stonas (UCSB)

2. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Outline Next steps for (Al)GaAs-GaN HBTs –Wafer fusion Current Aperture Vertical Electron Transistor (CAVET) –Through MOCVD regrowth –Through selective etching

3. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Structure AlGaAs-GaAs emitter-base –high mobility carriers –well-understood emitter-base interface –p contacts to GaAs base (rather than to p GaN) n-GaN collector –High-breakdown voltages possible n-GaN Collector n-AlGaAs Emitter p-GaAs Base fused interface In previous reviews demonstrated reliable fusion of GaAs-GaN : 500 -750 o C, 0.5-2 hours used SIMS, TEM, I-V measurements to characterize fused interface Carried out initial electrical characterization of (Al)GaAs-GaN HBT Formation of (Al)GaAs-GaN HBT

4. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 5 nm Fused at 750 o C for 15 minutes Ex situ fusion and the fused interface Spray etch to remove GaAs substrate Starting Materials GaN GaAs Fuse under 2MPa pressure at 500-750°C for 0.25-2hr GaAs A uniform, relatively smooth interface GaAs GaN Courtesy J. Jasinski 1 to 4 monolayers

5. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB >2 m uid-GaN (~5x10 16 Si) (001) sapphire substrate (1x10 19 C) 120 nm n-Al x Ga 1-x As (5x10 17 Si, x = 0.3) 30 nm Graded Al x Ga 1-x As (5x10 17 Si, x = 0 - 0.3) 30 nm Graded Al x Ga 1-x As (5x10 17 Si, x = 0.3 – 0) 100 nm n-GaAs (1x10 19 Si) 150 nm p-GaAs Au/Ge/Ni 415 o C Zn/Au Al/Au (Al)GaAs-GaN HBT Structure GaAs/GaN Interface fused at 750 o C for 1 hour Common Emitter Characteristic I B Step Size = 2mA 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 051015202530 V EC (Volts) Collector Current (mA) 20 micron x 52 micron emitter mesa

6. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB (Al)GaAs-GaN HBT IV Characteristics Common Emitter Characteristic I B Step Size = 2mA 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 051015202530 V EC (Volts) Collector Current (mA) Relatively small V CE offset (~1 V): can be improved with anneal p-GaAs contacts Reasonably good output conductance (~ mA; few hundred A/cm 2 ) Low current gain (< 1) –large base width (150 nm) –dopant profile after fusion? (AlGaAs-GaAs and GaAs- GaN) –recombination at the fused interface?

7. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Gummel Plot

8. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB (Al)GaAs-GaN HBT: Next Steps -5 0 5 10 15 20 25 -2-1.5-0.500.511.5 Applied Bias (Volts) Current Density (A/cm2) 6.5x10 -3 Wcm 2 Base-Emitter Junction (pGaAs-nAlGaAs) Set fused interface slightly into collector region - n-AlGaAs/p-GaAs/n-GaAs/n- GaN -Allows for uncertainties in GaAs-GaN band lineups -Previous experience indicates that n-AlGaAs/p-GaAs/n-GaAs structure will go through fusion process intact Reduce base layer thickness Carry out fusion at lower temperatures -Minimize dopant diffusion across fused interface Same characteristics before and after fusion EBC n-AlGaAs p-GaAs/ n-GaAs GaN

9. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB C urrent A perture V ertical E lectron T ransistor Current flows vertically from sources to drain Electron flow through aperture modulated by the gate High-field region below the gate instead of at the surface (as in a HEMT) Higher breakdown voltages When optimized, reduction in DC-RF dispersion 2DEG High-Field Region I DS Regrown Channel GaN CAVET

10. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 1.MOCVD growth of drain and insulating regions 2.Cl 2 RIE etch of aperture region 3.MOCVD maskless regrowth of aperture and source region, pattern device mesa, and deposit metal contacts CAVET Process Flow

11. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB CAVET: Theoretical Models Ideal device, pinch-off occurs between gate and aperture. 1. When aperture region is too insulating, pinch- off occurs across the aperture and current does not saturate due to DIBL. 2.

12. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB CAVET: DC Electrical Results For this device: W s = 2*W g = 200 m, L ap = 0.6 m, and L go = 2 m I DSS = 430 mA/mm, extrinsic g m = 100 mS/mm, V p = - 4 V Parasitic leakage current observed at pinch-off For all devices: I DSS and leakage current at pinch-off independent of L ap I DSS and pinch-off leakage current increase when L go is decreased

13. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Etched-Aperture GaN CAVET In x Ga 1-x N (600Å) x = 0.07 0.5 micron uid- GaN InGaN 1.7 microns n- GaN AlGaN Al 0.30 Ga 0.70 N (220Å) sapphire 1000 W Hg/Xe lamp (~20 W/cm 2 ) Au wire Sample KOH:H 2 O GaN filter Create an etched aperture Use an etch process that rapidly and selectively etches a sacrificial layer (InGaN) PhotoElectroChemical (PEC) Wet Etching Pt coil

14. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Bandgap-Selective PEC Etching Approximate absorption edge of InGaN h GaN InGaN GaN filter will select wavelengths that only excite carriers in InGaN GaN sapphire GaN InGaN SiO 2 Ni/Au After Etching 3 min, 1000 W, 2.2 M KOH I bias = 40 mA PROBLEM: roughness of undercut etch RESPONSE: Taguchi experiment to identify most critical etch parameters

15. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 0.55M 2.2M 8.8M 2.71V 1.24V 0.62V 2.71V 1.24V 2.71V0.62V 170W400W600W top view of undercut MRS PEC etching 50µm 0.62V Etched areaUnetched area Illumination power KOH concentration Systematically varied -KOH concentration - illumination power - bias applied to sample Evaluated - lateral etch rate - smoothness of etch front - roughness of etched surface Overwhelming dependence on KOH concentration: lower concentration produced smoother etched surface TOP-DOWN VIEWS OF ETCHED STRUCTURES Optimization of Etch Conditions

16. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Etched CAVET: future work Taguchi experiments helped to identify critical etch parameter: KOH concentration Samples etched at 0.001 M KOH, 1000W, no bias, showed smooth, well- controlled undercut Future work: fabricate full CAVET device, using optimized etch conditions

17. Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Summary Initial electrical characterization of first (Al)GaAs-GaN fused HBT –Try setback of fused interface, lower fusion temperature, thinner base region Initial CAVET device results for regrown structures –Optimize device structure and growth conditions Optimization of PEC etching for etched CAVET devices –Fabricate and characterize full CAVET device