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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