barrier current flow in nitride heterostructures

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors

NC STATE UNIVERSITY UCSB

Barrier Current Flowin Nitride Heterostructures

Peter Asbeck, S.S.Lau, Ed Yu

Lin Jia, Dongjiang Qiao, L.S.Yu

UCSD

[email protected]

February 12, 2002



Outline

• Potential barriers in nitride devices– Structure and current flow

• Schottky barriers on p-GaN

• Status of HET fabrication



Hot Electron Transistor (HET)

Advantages

EB B

C

n-AlGaN/GaNn-GaN HET

High mobility baseNo Mg ionization problemsPotentially fast

DepthB CE AlGaN:

xAl=0.15

InGaN: xIn=0.10

-0.5

0

0.5

1

1.5

0 1000 2000 3000Depth (Angstrom)

En

erg

y (e

V)

50A

50A



GaN/AlGaN/GaN Barrier

n+ GaNBuffer layer

n SiC substrate

n- GaNAlGaNn-GaN

Schottky

Ohmic

Simulated conduction band energy

3000

-0.5

0

0.5

1

1.5

0 1000 2000Depth (A)

En

erg

y (

eV

)

AlGaN layer

0.E+00

5.E+17

1.E+18

1.5E+18

0 0.1 0.2 0.3

Depth (um)

Co

nce

ntr

atio

n (

cm-3

)

V VSample Expected MeasuredxAl 13 % d=100A 1.20eV 1.43eVxAl 13% d=50A 0.60eV 0.95eV

Materials by R. Davis Group



PM251-b4-m2-lowT (semi-log)

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

-1 -0.8 -0.6 -0.4 -0.2 0

Voltage (V)

Cur

rent

(A)

300K

275K

250K

225K

200K

175K

150K

125K

100K


1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

-1 -0.8 -0.6 -0.4 -0.2 0

Voltage (V)

Cu

rre

nt

(A)

300K

250K

225K

200K

175K

150K

125K

100K

GaN / AlGaN /GaN Barrier I-V Curves Vs Temperature

exp(V/Eoo)

100A AlGaN Barrier 50A AlGaN Barrier

Eoo ~ 48 meV (independent of temperature)

Eoo ~ 38 meV (independent of temperature)

Theoretical ~ 5meV




1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

0 1 2 3 4

Voltage (V)

Cu

rre

nt

(A)

300K

250K

225K

200K

175K

150K

125K

100K

GaN / AlGaN /GaN Barrier I-V Curves Vs Temperature

Reverse Characteristics100A AlGaN Barrier

ModifiedFowler-Nordheim Plot

Fit with Eoo=40meV Theoretical Eoo= 5 meV

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

0.2 0.4 0.6 0.8 1

1/sqrt(V+Vbarrier)

I/(V+

Vbar

rier)



1.E-12

1.E-10

1.E-08

1.E-06

1.E-04

1.E-02

0 0.2 0.4 0.6 0.8 1

1/sqrt(V+Vbarrier)

I/(V+

Vbar

rier)

GaN Schottky Barriers: Reverse Current

1.E-12

1.E-10

1.E-08

1.E-06

1.E-04

1.E-02

0 5 10 15 20 25

Vrev (V)

Irev

(A)

Ni on n GaN 3e17cm-3

VE

qVJ

boo

bb

2

)(exp)(

2/3

Fowler-Nordheim Tunneling Through Depletion Region

Expect Eoo=5 meVFit with Eoo= 50 meV

Schottky Barrier on n- GaNForward current

L.S.Yu, S.S.Lau et al, UCSD (1998)

T=220K

T=360K

•Log slope largely T invariant=> not thermionic emission

•Very good fit with tunneling formalismexcept Eoo= 19.5meV fitting

Eoo= 3.1meV theory

=> Defect assisted tunneling



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

Dislocation line charge > 0 Dislocation line charge < 0

n-GaN Easiertunneling



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 ~50Ato allow tunneling, need ~ 5e18cm-3

Dislocations can provide stateswithin gap correlated spatiallyfor convenient tunneling



Schottky barrier of Ni on p-GaN

Schottkycontact

ohmiccontact

0.0 0.5 1.0 1.51E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

140oC

82oC

61oC

T=29oC

Cu

rre

nt

(A)

Voltage (V)

Expect Eoo=16 meVFit with Eoo= 56 meV

Mg doped

MOCVD grown

Sapphire substrate

P~1e17cm-3



C-V results of Ni/p-GaN Schottky contact

CmC

rs

GGm

-4 -3 -2 -1 0 1 2 30.0

5.0x1020

1.0x1021

100KHz

10KHz

C-2

(F

ara

d)-2

Voltage (V)

B=2.68 eV - 2.87 eV

Corrections for Rs and Gp

Needed to obtain C



Profile of acceptor concentration

150 200 250 300 3501017

1018

1019

100 kHz 10 kHz

zero bias

Do

pin

g C

on

cen

tra

tion

NA

(C

m-3

)

Distance d (A)

•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)



HET - Fabrication Approach

C

BE

n+ GaNn AlGaN

n AlGaN

n+ GaN

SiC

n+ GaNn AlGaN

SiC

C

BE

n+ GaNn AlGaN

SiC

n+ GaNn AlGaN

SiC

Regrown emitter structureSi3N4

=> Base contacts can be formed after alloying of emitter and collector contacts=> No need to etch through GaN to reach base

n AlGaN n AlGaN

B



HET Fabrication Status

Initial growth NCSU

SiN deposition &patterning UCSD

Regrowth NCSU

Final processing - UCSD

i-GaN 940 A

n+GaN 8e18 500A

SiN n+ AlGaN 5e18 60A

Al:25%

SiC substrate JD634 AlGaN Barrier HET

n+AlGaN 5e18 6000A Al:15%

i-AlGaN 2000A Al:15% n+GaN 8e18 100A

i-GaN 60A

-0.5

0

0.5

1

1.5

200 600 1000 1400 1800 2200 2600 3000 3400 3800

Depth (A)

En

erg

y (e

V)



Plans

•Refine HET fabrication

•Continue barrier current investigation

•Continue p contact studies

barrier current flow in nitride heterostructures

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