Measurements of the E-field Breakdown and Band Offsets of SiO 2 on GaN

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

NC STATE UNIVERSITY UCSB

Measurements of the E-field Breakdown and Band Offsets of

SiO2 on GaN

Ted Cook, Ed Hurt, Kieran Tracy, R.F. Davis, G. Lucovsky,

and R.J. Nemanich

North Carolina State University

Raleigh, NC 27695-8202 USA

February 12, 2002



Introduction

• The band offsets have a direct effect on the properties of the interface, and therefore affect the reliability and stability of the device

• Investigation of the band offsets, as well as their effects, such as E-field breakdown, is important for device fabrication, and will be explored in this study



Goal and Motivation

Goal:• Measure the Valence Band Offsets between

atomically clean GaN and SiO2 using photoemission techniques

Motivation:• Passivation of high voltage devices• Wafer Bonding – fusion bonded interfaces• Gate insulator applications



Experimental Procedure

1. Achieve atomically clean GaN surface using CVC anneal in NH3 atmosphere @ 860°C for 15 minutes

2. XPS and UPS performed after each of the following steps to track the evolution of peak shifts

1. Deposition of 2Å Si via MBE

2. Deposition of a second 2Å Si layer

3. Oxidation of Si layer using a Remote O2 plasma

4. Deposition of 2Å, and oxidation of the Si layer

5. 650°C anneal for 15 minutes to densify the oxide

6. Deposition of 3Å, and oxidation of the Silicon layer

7. Final anneal at 650°C for densification



Integrated UHV Surface and Interface Processing System

CVD Diamond

High Voltage Testing

Field Emission XPS GSMBE

AES / LEEDe-beam evap

Si-GeMBE

ARUPS

Hydrogen/OxygenPlasma

LoadLock

ECR N2

PlasmaWafer

Bonding



Proposed Valence Band Line-up between n-GaN and SiO2

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0 Ef

Ec

Ev

Evac

Eg = 3.40eV Ef-Ev = 3.32eV

= 2.9eV

Eg = 9.0eV

Ef-Ev = 5.30eV

Ec-Ef =0.08eV

Ec-Ef = 3.70eV

= 1.1eV

Ec = 3.6eV

Ev = 2.0eV

GaN SiO2Clean

Ed = 0.3eV



Gallium 3d XPS Evolution of n-type GaN

1012141618202224262830

Binding Energy (eV)

Inte

nsi

ty (

au)

CVC

Final Anneal 650°C

2Å Si Total

4Å Si Total

O Plasma

6Å Si Total & O Plasma

Anneal 650°C

9Å Si Total & O Plasma



Energy Band Diagram

Clean GaN Surface Final Surface

Ef Ef

CB CB

VBVB



UPS Spectra of VB turn-on

-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0Fermi Level Referenced Energy (eV)

Co

un

ts (

a.u

.)

CVC GaN

650°C Final Anneal

0.3 eV shift due to band bending



Valence Band Offset between GaN and SiO2

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0 Ef

Ec

Ev

Evac

Eg = 3.40eV Ef-Ev = 3.32eV

= 2.9eV

Eg = 9.0eV

Ef-Ev = 5.30eV

Ec-Ef =0.08eV

Ec-Ef = 3.70eV

= 1.1eV

Ec = 3.6eV

Ev = 2.0eV

GaN SiO2Clean

Ed = 0.3eV



Oxygen 1s XPS Spectra showing Oxide Formation

520522524526528530532534536538540Binding Energy (eV)

Inte

ns

ity

(a

u)

CVC

Final Anneal 650°C

2Å Si Total

4Å Si Total

O Plasma

6Å Si & O Plasma

Anneal 650°C

9Å Si & O Plasma



UPS Spectra of Spectral Width

-21 -20 -19 -18 -17 -16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0

Fermi Level Referenced Energy (eV)

Co

un

ts (

a.u

.)

CVC GaN

650°C Final Anneal

W=11.1 eVEg SiO2 =9.0 eV

Eg GaN =3.4 eVW=14.9 eV

=21.2-11.1-9.0 = 1.1 eV

=21.2-14.9-3.4 = 2.9 eV

=h-W-Eg



Electron Affinity difference between GaN and SiO2

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0 Ef

Ec

Ev

Evac

Eg = 3.40eV Ef-Ev = 3.32eV

= 2.9eV

Eg = 9.0eV

Ef-Ev = 5.30eV

Ec-Ef =0.08eV

Ec-Ef = 3.70eV

= 1.1eV

Ec = 3.6eV

Ev = 2.0eV

GaN SiO2Clean

Ed = 0.3eV



Models of Heterojunctions

• Electron Affinity Model

Valid if small or no change in the interface Dipole• Interface Dipole Model

Change in the Electric field at interface• Deviation from the electron affinity of the heterojunction

alignment represents the interface dipole

EAM IDM



Interface Dipole in Band Line-up of GaN and SiO2

=1.9 eV

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0 Ef

Ec

Ev

Evac

Eg = 3.40eV Ef-Ev = 3.32eV

= 2.9eV

Eg = 9.0eV

Ef-Ev = 5.30eV

Ec-Ef =0.08eV

Ec-Ef = 3.70eV

= 1.1eV

Ec = 3.6eV

Ev = 2.0eV

GaN SiO2Clean

Ed = 0.3eV



Conclusions for n-type experiment

• Special care has been taken to prevent oxidation of gallium

• Band bending of ~0.3 eV for CVC GaN surface• Electron Affinity ~2.9 eV for CVC GaN surface

• Flat Bands at the GaN-SiO2 Interface

• Valence Band Offset of ~2 eV for GaN-SiO2 for 1x1017

cm-3 n-type GaN

• Conduction Band offset of ~3.6 eV for GaN-SiO2 assuming EgSiO2 =9.0 eV

• Deviation from the Electron Affinity model due to Interface dipole of ~1.9 eV



Gallium 3d XPS Evolution of p-type GaN

1012141618202224262830

Binding Energy (eV)

Inte

ns

ity

(a.u

.)

Clean GaN

2Å Si

4Å Si

4Å Si and O2 Plasma


650 C anneal


Final anneal



Proposed Valence Band Line-up between p-GaN and SiO2

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0 Ef

Ec

Ev

Evac

Eg

= 3.40eV

Ef-Ev = 2.2eV

Eg = 9.0eVEf-Ev = 4.5eV

Ec-Ef =3.1eV

Ec-Ef = 4.5eV

Ec = 3.3eV

Ev = 2.3eV

GaN SiO2

EB=0.3eV

Ef-Ev=1.7eV

Clean



Conclusions for p-type experiment

• Initial Band Bending of 1.4 eV for CVC GaN surface• Additional downward Band Bending of ~0.5 eV as

the surface was formed

• Valence Band offset of ~2.3 eV for GaN-SiO2 for 2x1018 cm-3 p-type GaN

• Conduction Band Offset of 3.3 eV for GaN-SiO2 assuming Eg Sio2 =9.0 eV



E-field Breakdown Procedure

1. Achieve atomically clean GaN surface using CVC anneal in NH3 atmosphere @ 860°C for 15 minutes

2. Deposit 4Å Si via MBE on surface to help prevent oxidation of the Gallium

3. Deposit 300Å SiO2 via Remote O2 Plasma

4. Anneal to 650°C to densify the oxide

5. Form 0.068 mm2 Al contacts on the surface using photolithographic techniques

6. Electrical characterization of sample



Device Structure for Electrical Testing

SiC

SiO2GaN AlN

W Contact

Al Contacts V

CRT

I

v



Breakdown Frequency

0

2

4

6

8

10

12

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96 101 106

Voltage

Co

un

ts

300Å SiO2

unintentionally doped, n-type <1.0x1017 cm-3



Conclusions

• These high values of breakdown voltage lends to the notion that the GaN layer is having an insulating effect

• A full understanding of this effect requires more study

• Film deposition rate of 2.5 Å/min established; verified by ellipsometry and C-V measurements



Future Work

• Reduce Band Bending for p-type GaN/SiO2

-Improve cleaning methods

-Oxide integrity

• Effect of different passivation layers

– Si3N4

– High k dielectrics

Measurements of the E-field Breakdown and Band Offsets of SiO 2 on GaN

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