Barrier Current Flow in Nitride Heterostructures

1. Barrier Current Flowin Nitride Heterostructures Peter Asbeck, S.S.Lau, Ed Yu Lin Jia, Dongjiang Qiao, L.S.Yu UCSD asbeck@ece.ucsd.edu February 12, 2002

2. Outline • Potential barriers in nitride devices • Structure and current flow • Schottky barriers on p-GaN • Status of HET fabrication

3. Hot Electron Transistor (HET) 50A B AlGaN: xAl=0.15 C E Depth InGaN: xIn=0.10 50A 1.5 E 1 B B Energy (eV) HET 0.5 n-GaN n-AlGaN/GaN 0 -0.5 0 1000 2000 3000 C Depth (Angstrom) Advantages High mobility base No Mg ionization problems Potentially fast

4. 1.5E+18 1.5 Schottky 1 n-GaN 1.E+18 Ohmic AlGaN 0.5 n- GaN Energy (eV) Concentration (cm-3) 5.E+17 0 n+ GaN Buffer layer -0.5 0.E+00 3000 n SiC substrate 0 1000 2000 0 0.1 0.2 0.3 Depth (A) AlGaN layer Depth (um) GaN/AlGaN/GaN Barrier Materials by R. Davis Group Simulated conduction band energy DV DV Sample Expected Measured xAl 13 % d=100A 1.20eV 1.43eV xAl 13% d=50A 0.60eV 0.95eV

5. GaN / AlGaN /GaN Barrier I-V Curves Vs Temperature 100A AlGaN Barrier 50A AlGaN Barrier exp(V/Eoo) Eoo ~ 38 meV (independent of temperature) Theoretical ~ 5meV Eoo ~ 48 meV (independent of temperature)

6. GaN / AlGaN /GaN Barrier I-V Curves Vs Temperature Reverse Characteristics 100A AlGaN Barrier Modified Fowler-Nordheim Plot Fit with Eoo=40meV Theoretical Eoo= 5 meV

7. GaN Schottky Barriers: Reverse Current Ni on n GaN 3e17cm-3 Fowler-Nordheim Tunneling Through Depletion Region Expect Eoo=5 meV Fit with Eoo= 50 meV

8. Schottky Barrier on n- GaN Forward current L.S.Yu, S.S.Lau et al, UCSD (1998) T=360K T=220K • Log slope largely T invariant • => not thermionic emission • Very good fit with tunneling formalism • except Eoo= 19.5meV fitting • Eoo= 3.1meV theory => Defect assisted tunneling

9. Dislocation Effects Line Charge effect: Electrostatic effects reduce potential barrier, allowing tunneling to occur more readily For reduction of barrier for electrons, require positively charged dislocation n-GaN Easier tunneling Dislocation line charge > 0 Dislocation line charge < 0

10. Trap-Assisted Tunneling • Explains SILC (stress-induced leakage current) • in Si nonvolatile memory • Explains leakage currents in LT or IT GaAs For point defects separated by ~50A to allow tunneling, need ~ 5e18cm-3 Dislocations can provide states within gap correlated spatially for convenient tunneling

11. ohmic contact Schottky contact Schottky barrier of Ni on p-GaN Mg doped MOCVD grown Sapphire substrate P~1e17cm-3 Expect Eoo=16 meV Fit with Eoo= 56 meV

12. Cm C G Gm rs C-V results of Ni/p-GaN Schottky contact Corrections for Rs and Gp Needed to obtain C B=2.68 eV - 2.87 eV

13. Profile of acceptor concentration • Na ~1019/cm3 within 200Å from the sample surface • tapers off to ~ 1018/cm3 • 10 to 100 times higher than p ~1017/cm3 (determined from Hall measurement)

14. HET - Fabrication Approach Regrown emitter structure Si3N4 n AlGaN n AlGaN n+ GaN n+ GaN n AlGaN n AlGaN SiC SiC E E B B B n+ GaN n AlGaN C C n+ GaN n+ GaN n AlGaN n AlGaN SiC SiC => Base contacts can be formed after alloying of emitter and collector contacts => No need to etch through GaN to reach base

15. HET Fabrication Status n+GaN 8e18 500A i-GaN 940 A SiN i-GaN 60A n+ AlGaN 5e18 60A Al:25% n+GaN 8e18 100A i-AlGaN 2000A Al:15% n+AlGaN 5e18 6000A Al:15% SiC substrate JD634 AlGaN Barrier HET Initial growth NCSU SiN deposition &patterning UCSD Regrowth NCSU Final processing - UCSD

16. Plans • Refine HET fabrication • Continue barrier current investigation • Continue p contact studies