1. Barrier Current Flow in Nitride Heterostructures
Peter Asbeck, S.S.Lau, Ed Yu
Lin Jia, Dongjiang Qiao, L.S.Yu
UCSD
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)
Depth B C E
AlGaN:
xAl=0.15
InGaN: xIn=0.10
-0.5
0.5 1
1.5
1000
2000
3000
Depth (Angstrom)
Energy (eV)
50A E B C
n-AlGaN/GaN
n-GaN
HET
Advantages
High mobility base
No Mg ionization problems
Potentially fast

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

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

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

12. C-V results of Ni/p-GaN Schottky contactCm C rs G Gm
Corrections for Rs and Gp
Needed to obtain C
B=2.68 eV 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 A
SiN
i-GaN A
n+ AlGaN 5e A Al:25%
n+GaN e A
i-AlGaN A Al:15%
n+AlGaN e A 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