Development of the Electrochemical Solution Growth (ESG) Technique for

Native GaN Substrates

DOE Energy Storage & Power Electronics Research Program30

September

2008

PI: Karen WaldripAdvanced Power Sources R&D, Dept 2546

PM: Stan Atcitty, John BoyesSandia National Laboratories, Albuquerque, NM, 87185

Sponsor: Gil Bindewald, DOE Power Electronics & Energy Storage Program

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Outline

•

Motivation•

Existing GaN Growth Technique–

Epitaxial Lateral Overgrowth–

Methods for Growing Bulk GaN

•

Development of the Electrochemical Solution Growth Technique

–

Electroplating GaN from Ga+3

and N-3

–

Electrochemical Solution Growth (ESG)–

Initial Results

Project Objective

To develop a novel, scalable, cost-effective growth technique for producing high quality, low dislocation density bulk gallium nitride for

substrates for GaN-based power electronics.

Project Start: 5/08Previous Funding: DOE’s Solid-State Lighting

Combined Figure of Merit

K (W/cm˚C) Ec (MV/cm) µ (cm2/Vs) vs

Combined Figure of

MeritSi 1.31 0.3 11.8 1350 1 x 107 1SiC 4.9 2 10 650 2 x 107 136GaN 1.3 3.3 9 1200 2.5 x 107 153

Energy gap -

lattice parameter diagram of III-nitrides

In-plane Lattice Constant a (Å)

Ener

gy G

ap (e

V)

Wav

elen

gth

(m

)

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

3.3 3.4 3.53.0 3.1 3.2

1.0

2.0

5.0

0.5

0.3

0.4

0.2AlN

InN

GaN

3.6

2.4% tension 11% compression

Heterostructure

Rectifiers Offer Improved Breakdown Voltages

9.7 kV for Al0.25

Ga0.75

N

Leakage current due to bulk defects

GaN is Grown Heteroepitaxially

on Sapphire (and Silicon Carbide) Substrates

•

As grown GaN nucleation layers contain disordered GaN with many stacking faults.

•

Once annealed, wurtzite

GaN forms on top of disordered GaN NL, forming nano-sized GaN nuclei from which further high temperature GaN growth occurs.

•

High temperature growth on the GaN nuclei produces GaN grains.

•

Growth conditions can be varied to enhance the pyramidal growth mode

or lateral coalescence. Dislocations are bent laterally on pyramidal facets.

•

Dislocations are concentrated in bunches located microns apart.

Figure from Lada et al., J. Crystal Growth 258, 89 (2003).

Methods for growing bulk GaN

“True”

Bulk Techniques

High Nitrogen Pressure Ammonothermal growth

Dislocation Filtering Techniques

Lateral Overgrowth

Sapphire Substrate

GaN layer

GaN layer

Liftoff process

GaN layer

Polishing

HVPE

15 mmP = 105atmT = 1500Ct = 100 hrh = 100 m

Seeds/Crystals

Nutrient BasketT1

T2

T1

< T2

Dislocation density = 102cm-2

4,000 –

5,000 atmT = 400 –

800oCG.R. = 50 m/dayMultiple seeds

KNH2

+ GaN + 2NH3 →

KGa(NH2

)4

GaN + 2NaNH2

+ 2NH3

→ Na2

Ga(NH2

)5

Desires/Requirements for a Bulk Growth Technique

•

Good crystalline quality (

≤

1x105cm-2)

•

High growth rate (~mm/hr): high throughput, high volume production

•

Low impurity content

•

Scalable

•

Controllable

•

Manufacturable

•

Reasonably inexpensive

•

Applicable to InN, GaN, AlN, and III-N alloys

N2

ON

N2

OFF

1/2N2 + 3e- N-3: The Reactive IntermediateT. Goto and Y. Ito, “Electrochemical reduction of nitrogen in a molten chloride

salt” Electrochimica Acta, Vol. 43, Nos 21-22, pp 3379-3384 (1998).

Advantages of using N2 gas:•

Clean•

Inexpensive•

Control over precursor conc.•

Continuous, controlled supply

Found that nitrogen was continuously

and nearly quantitatively

reduced to nitride ions

N2

gas OUT (RGA)

N2

gas INPower Supply

N3-

N3-

N3-

N3-

N3-

N3-

Molten LiCl-KCl + Li3

N450˚C

Report of nitride concentration in LiCl in literature: 12

mole %

Anodic ReactionN3-

= 1/2 N2

+ 3e-

Cathodic

Reaction1/2 N2

+ 3e-

= N3-

Initial Experimental Setup: Unseeded Growth of GaN in a Test Tube

Li3

N or (Li3

N + N2

)

+ Ga, 450˚C, current sweep, 2 hours

Produced numerous wurtzite

GaN crystals; This crystal was ~1.25mm long x 0.8mm wide

Potentiostat

N-3

Quartz tube

LiCl-KClLi3

N, 450˚C

Ni cathode Pt/Ga or Mo/Gaanode

Furnace coils

Ga(l)

Ga Ga+3

+ 3e-

Ga+3

+ N-3

GaN3Li+

+ 3e-

3 Li

120o

GaN

ESG Produces Photoluminescent

GaN

Crystallites

0

2500

5000

7500

Inte

nsity

(Cou

nts)

01-074-0243> GaN - Gallium Nitride

20 30 40 50 60 70 80Two-Theta (deg)

[KW-G15.raw] KW-G15

0

2500

5000

7500

Inte

nsity

(Cou

nts)

01-074-0243> GaN - Gallium Nitride

20 30 40 50 60 70 80Two-Theta (deg)

[KW-G15.raw] KW-G15

TG

= 450oCPG

= 1 atm

GNOEM Systems, Inc. Boulder Creek, CA

300 350 400 450 500 550 600 650

0.1

1

10

100

1000

PL In

tens

ity (a

rb. u

nits

)

Wavelength (nm)

DNZ00652C GaN Epilayer on SapphireG15 GaN sample

266 nm 1 mW pulsed pump

366.8 nm

362.4 nm

1

10

100

1000325 nm 5 mW CW pump

PL In

tens

ity (a

rb. u

nits

)

DNZ00652C GaN Epilayer on SapphireG15 GaN sample

372.3 nm

362.2 nm

Mary Crawford, SNL

New Growth Technique: Electrochemical Solution Growth (ESG)

Use salt flow to deliver precursors

Increase growth rate through flux of reactants (increase spin rate)

Precursors can be replenished as they are consumedAdvantage: Continuous, isothermal or steady-state growth

•

Half-reaction 2: –

Ga Ga+3

+ 3e-

–

Ga+3

equilibrium concentrations ~1 mole %

U.S. Patent filed April 11, 2005

N-3

Ga+3

N2

GaN boule

Potentiostat

Spinning Motor

•

Half-reaction 1:–

1/2N2

+ 3e- N-3

–

N-3

concentrations ~12 mole %

Example of Nitrogen Gas Reduction Cyclic Voltammograms

1/2N2

+ 3e-

N-3

200mV/sec

SEM of RD-ESG Growth Run #1MOCVD-grown GaN

Ga, Al-containing layer

Film surface

0.38” = 480nm

•

SIMS revealed the layer to be a graphitic carbon layer, with Ga,

N, and GaN clusters–

GaN content was about 10%–

Profile was consistent with an increasing concentration•

Problem with salt purity from supplier–

Working it out with supplier–

Developing in-house purification technique for reagent grade salt

Industrial Partner (GNOEM) Hardware Development

First GNOEM RD-ESG Experiment

•

Hardware failure–

susceptor sheared, not sure when•

Black line on sample surface delineated a higher, specular

region and lower, roughened area •

Defect selective etching observed (several microns/hr)•

Highly encouraging for crystal quality–

Must identify the conditions under which this takes place

•

Polished cross sections of control and experiment sample consistently measure about 1m thicker for experiment

Control

ESG-227.011m

5.907 m

epoxy

Growth Rate vs. Rotation Speed and Concentration

0.01

0.1

1

10

100

0 2000 4000 6000 8000 10000 12000

Rotation Rate (rpm)

Gro

wth

Rat

e (m

m/h

r)

1.00E+19

1.00E+20

1.00E+21

1 mole %

10 mole %

0.1 mole %

Growth Rate vs. Rotation Speed and Concentration

Summary: Path For Development

•

Demonstrate that chemistry is viable–

Kinetics and thermodynamics are favorable in this setup

•

Check for dissolution and precipitation approach

•

Develop N2

electrochemical reduction methods

•

Develop initial fluid dynamics schemes

•

Deposit GaN on a seed crystal

•

Improve crystal quality

•

Optimize growth rate

Acknowledgments

•

Jeff Tsao•

Tom Kerley•

Frank Delnick•

David Ingersoll•

Bill Averill•

Bob Biefeld•

Mike Coltrin•

Ryan Egidi•

National Energy Technology Laboratory/Energy Efficiency and Renewable Energy Office of Solid-State Lighting

•

Paul Butler•

Tom Wunsch•

Dan Doughty•

Randy Creighton•

Christine White•

Dan Koleske•

Dave F. Smith•

George Antypas•

Mary Crawford•

Bertha Montoya