Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Effects of surface oxide on wafer bonding of GaN and SiC Jaeseob Lee, Robert F. Davis, and Robert J. Nemanich North Carolina State University Raleigh, NC USA February 12, 2002

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Outline Motivation - GaN/SiC HBT Introduction - Wafer Bonding of GaN/SiC Experiment Results - AES of GaN,SiC - AFM of GaN/SiC - I-V of GaN/SiC Discussion

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Advantage of GaN-SiC Device ; high temperature, high power, high frequency operation Larger bandgap emitter -restrict the diffusion of hole from base to emitter → high electron injection efficiency -heavily doped base → low base resistance Indirect bandgap base -longer carrier lifetime(longer diffusion length) → high base transport Short base width higher Emitter efficiency Larger current gain Al Emitter Contact n-GaN Emitter Al/Cr Base Contact p-SiC Base n-SiC Collector Al/Cr Collector Contact GaN/SiC HBT proposed by J. Pankove, S.S. Chang, H.C. Lee, R.J. Moustakas, B. Van Zeghbroeck (Int. Electron Devices Meet Tech. Dig. ’94) GaN/SiC HBT

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Direct Growth Nucleation Problem Defect due to large mismatch Buffer Layer(AlN) Growth Buffer Layer acts as a insulator Waferbonding Reduce defect formation at interface No insulating buffer layer between GaN and SiC SiC GaN SiC GaN AlN SiC GaN GaN/SiC WB

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Electronegativity values ElementElectronegativity N3.0 C2.5 Si1.8 Ga1.6 Bond and ionic character BondDifference in Electronegativity Ionic Character N-Ga1.439% N-Si1.230% C-Ga0.919% C-Si0.712% N-C0.57% Si-Ga0.21% GaN/SiC WB

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Dicing into 12.8×6.5 mm 2 pieces Degreasing, HF(SiC)/HCl(GaN) dip N 2 blow dry Processing Flow Chart Characterization (I-V) Ex situ Cleaning Ex situ Bonding In situ Annealing Characterization (AFM,AES) 600, 800, 1000 o C, 1hr

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB GaN SiC AFM of GaN/SiC RMS roughness 20 ± 5 Å in 20×20 µm 2 area of GaN and SiC

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Surface atomic concentration: Cl  0%, C  7  1%, N  33  8%, O  2  1%, Ga  57  9% AES of GaN

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Surface atomic concentration: Si  26  4%, C  20  4%, O  54  4% after HF(100:1) 1min dip Si  21  3%, C  73  13%, O  7  1% after HF( 10:1) 10min dip AES of (0001) Si SiC

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB HF( 10:1) 10min dipped SiC to GaN pair shows more ohmic behavior IV of GaN/SiC(n-type to n-type) Ti n-SiC

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Bonding process to 2H (0001) Ga GaN 6H SiC surface HFHF dip (min) Annealing Temp( o C) Annealing Time(min) Heating rate( o C/min) Bonding Results Trials (0001)Si100:111000,800240,60  10 No (1 partial bonding) ,800240,605No ,800605Yes2 10: ,800605Yes No1 (000-1)C100: ,305Yes5 10: ,605Yes Yes No1 Bonding result

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Surface Conc. of SiC Surface atomic concentration: SiC (0001) Si ; Si  27  4%, C  15  3%, O  57  5% after 1min dip Si  21  4%, C  76  14%, O  3  1% after 60min dip SiC (000-1) C ; Si  23  4%, C  73  13%, O  4  1% after 1min dip Si  21  4%, C  76  14%, O  3  1% after 60min dip HF(100:1) dip of (0001) Si /(000-1) C 6HSiC

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 6.5 mm 12.8 mm 406 µm pSiC 3.5E18 1 µm GaN <1E µm AlN Conductive 260 µm nSiC 4~6E18 Ti Pt or Ti 1 mm p-6H SiC(0001) Si ; Degreasing, HF(10:1) 10min dip 2H GaN(0001) Ga ; Degreasing, HCl 1min dip 600 o C, 800 o C, 1000 o C 1hr annealing for WB WB of GaN/SiC

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB IV of metal/SiC Ohmic behavior of Metal/SiC 800 o C,20min Ti n-SiC Ti n-SiC Pt p-SiC 800 o C 20min Metal SiC Metal/polished side Metal/unpolished side  Ohmic contact

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB (0001) C SiC/(0001) Ga GaN pair shows low resistance than (000-1) Si SiC/(0001) Ga GaN pair IV of GaN/SiC(n-type to n-type) (0001) Si, (000-1) C SiC ; HF(10:1) 10min dip (0001) Ga GAN ; HCl 1min dip Ti n-SiC

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB IV of GaN/SiC(n-type to p-type) (0001) C SiC/(0001) Ga GaN pair and (000-1) Si SiC/(0001) Ga GaN pair show rectifying behavior 1000ºC 1hr in-situ annealing Ti Pt n-SiC p-SiC Ti n-SiC p-SiC

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB IV of GaN/SiC(n-type to p-type) (0001) C SiC/(0001) Ga GaN pair shows rectifying behavior But (000-1) Si SiC/(0001) Ga GaN pair shows ohmic behavior 800ºC 1hr in-situ annealing Ti Pt n-SiC p-SiC Ti n-SiC p-SiC

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB IV of GaN/SiC(n-type to p-type) (0001) C SiC/(0001) Ga GaN pair shows rectifying behavior But (000-1) Si SiC/(0001) Ga GaN pair shows ohmic behavior 800ºC 1hr in-situ annealing Ti Pt n-SiC p-SiC Ti n-SiC p-SiC

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB (0001) C SiC/(0001) Ga GaN pair and (000-1) Si SiC/(0001) Ga GaN pair do not bond at 600ºC 1hr in-situ annealing 600ºC 1hr in-situ annealing 5mm (a) (0001) Si SiC/(0001) Ga GaN (b) (000-1) C SiC/(0001) Ga GaN 5mm IR image of GaN/SiC

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Conclusion Polarity is important factor in WB of SiC/GaN (000-1) C n-SiC/(0001) Ga n-GaN pair has the low resistance (nearly Ohmic) (000-1) C p-SiC/(0001) Ga n-GaN pair keep good rectifying behavior at lower bonding temperature(800 o C) No bonding happened at 600 o C with wafer surface having RMS roughness 20Å in 20×20 µm 2.

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Future Research Characterize the bonded interface with FiB-TEM Bond patterned GaN structures appropriate for HBT Explore improved polishing of SiC and GaN surface to get low T bonding

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Fabrication of HBT Substrate ; 300µm nSiC 4~6E18 Epi 1 ; 12.0µm nSiC 6.9E15 Epi 2 ; 0.2 µm pSiC 3.5E18 Substrate ; 260µm nSiC 4~6E18 0.1µm AlN cunductive 1 µm nGaN 4~6E18 From Dr. Davis group From Cree Research Inc. 2in

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB After bonding & Polishing 6.5 mm 0.2 μm pSiC 3.5E18 12 μm nSiC 6.9E15 substrate nSiC 4~6E18 1 μm nGaN 4~6E μm AlN Conductive 1 μm nSiC 4~6E18 2 μm 300 μm Polishing SiC substrate ; 260µm to 50 µm Bonding Polishing SiC substrate : 50 µm to 2 µm by Diamond lapping film GaN on Si wafer is easy for layer transfer

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB 0.2 μm pSiC 3.5E18 12 μm nSiC 6.9E15 substrate nSiC 4~6E μm nGaN 4~6E18 Al Al/Cr RIE/metallizing Plan SF 6 500Å/min SiC 20min for 1μm Cl 2 /Ar 4000Å/min GaN 2.5min for 1μm Al deposition Al/Cr deposition RIE By Parallel-plate RIE