Matt Highland
‣
Synthesis away from equilibrium
‣
Metastable Materials
‣
Reactive Synthesis
Second Workshop on Photocathodes: 300nm-500nm
June 29-30 at the University of Chicago

‣
Typical thermodynamics gives us guide posts on synthesis near equilibrium
‣
Engineering materials with specific properties often requires synthesis away from equilibrium
‣
“Metastable” materials that demand non-equilibrium and kinetically controlled synthesis path ways
‣
Metastable synthesis requires additional stabilization during growth:
Strain Epitaxy Energetic ions Sputtering Chemical Activity Reactive Synthesis
SrRuO
3 and Co
3
O
4
2

‣
Reactive synthesis utilizes activity of chemical precursors to stabilize desired phases
‣
Practical example: (In,Ga)N solid solutions
‣
Band-gap tunable across solar spectrum by varying solid solution content
3

‣
The promise
• Solid-state lighting has the potential reduce U.S. energy consumption from 3.1 to 2.1 petawatt-hours/year *
• Roughly the output of 250 coal fired power plants
‣
The truth
• External quantum efficiency drops as InN content increases
• Driven by problems with crystal quality and the metastable nature of InN
High-power (>1 Watt input) visible-spectrum
LEDs
70%
60%
50%
In x
Ga
1-x
N
(Al x
Ga
1-x
)
0.52
In
0.48
P
V( l
)
V( l
)
40%
30%
20%
10%
0%
350
Tj =
25

C
450 550
Peak Wavelength (nm)
Peak wavelength, l p
(nm)
650
*“Energy Savings Potential of Solid State
Lighting in General Illumination Applications”, http://www.netl.doe.gov/ssl


‣
At desirable growth temperatures required nitrogen activity is equivalent to kilobars (~10 4 psi) of
N
2
‣
During MOCVD growth nitrogen activity provided by cracking ammonia
 
3

3


3
4
‣
Reaction we want to avoid:
InN

In

1 2 N
2

Ambacher et al., JVST B 14 , 3532 (1996)
5 5


‣
We know the overall reaction desired for growth
 
3

3


3
4
‣
However what are the intermediate chemical species that drive this growth
?
• All we know is the precursors crack somehow interact
 
3
3

?

?

X

?
2

6 6

‣
We’re employing multiply in-situ probes and computational techniques to understand the details of reactive synthesis
In-situ IR spectroscopy
In-situ X-ray Analysis
Theory & modeling
7 7

‣
Synchrotron x-rays are capable to penetrating the MOCVD environment and yield structural and elemental details in real time
• In-situ MOCVD reactor at sector 12ID-D of the
Advanced Photon Source
‣
Diffraction from GaN surfaces and InN crystals
‣
X-ray Fluorescence from deposited Indium
Scattering
Detector
Movie camera
‣
Measurements reveal a very complex growth behavior
Fluorescence
Detector
Visible illumination
Synchrotron x-rays
8
8 8

‣
By monitoring InN and In liquid formation we can map out an indium condensation phase diagram
‣
Upon increasing TMI flow
• At higher temp, elemental In liquid condenses
• At lower T, relaxed InN solid particles grow
In liquid droplets
Bare GaN surface pNH
3
= 27 Torr
InN crystals
F. Jiang, et al. PRL 101, 086102 (2008)

‣
Near phase boundaries system can spontaneously oscillate
• Inter-conversion between InN and liquid In
‣
AFM of quenched samples shows microstructure of distinct surface species
Epitaxial InN islands Elemental In droplets
F. Jiang, et al. PRL 101, 086102 (2008)
10

‣
Spatial variation between
InN and In can be resolved optically
• Dark regions: InN
• White regions: In liquid
‣
Waves of InN or In liquid
• Sweep across the sample
• Form concentric rings
• Spiral patterns
F. Jiang, et al. PRL 101, 086102 (2008)

‣
The key to this complex growth behavior is local nitrogen activity
‣
NH
3 impinges on the hot sample surface, cracks and forms some highly active chemical species (NH x
)
‣
These active species either interact with In and form InN or react to
‣ eventually form N
2 and leave the surface.
The efficiency with which NH
3 is cracked and the residence time of the intermediate species determines which material grows

NH
3 cracks on the GaN of InN surface and forms the intermediate species that allow InN to grow
Critical amount of liquid In metal condenses which accelerates conversion of NH
3 to N
2 and InN starts to decompose
Liquid In metal evaporates to expose
GaN surface and InN growth starts again

‣
The local intermediate chemical species dictate growth behavior
‣
Different surfaces catalytically crack NH residence time of intermediate species
3 differently and possibly change
‣
If we can understand which intermediate species enable InN growth, then we can better stabilize and encourage its formation
‣
What are the intermediate nitrogen species?
• First principle calculations
• Additional in-situ probes

‣
We can calculate the lowest energy configurations of NH
3 a GaN and InN surface
, NH
2
, NH, N, and H on
‣
We can then create a phase diagram predicting the equilibrium coverage species for given conditions
• “We” = Peter Zapol, Weronika Walkosz, and Xin
Tan
(2x2) surface unit cell
- 4 H3 “hollow” sites
- 4 T1 “on top” Ga sites
- 4 T4 “on top” N sites
- 12 br “bridge” site fixed

‣
Lowest energy surface species differ depending temperature and nitrogen activity
‣
One of these configuration maybe be what enable InN growth
‣
Can we find these phases experimentally ?
Predicted structures on GaN surface
N-rich Ga-rich
W. Walkosz, et al. PRB 85, 033308 (2012)

‣
An abrupt crystalline surface in realspace creates an extended rod of scattering in reciprocal space
‣
Scattering that occurs along this Crystal
Truncation Rod (CTR) away from the
Bragg peaks is very sensitive to surface changes

‣
First Principle can be used to predict
CTRs for each phase
‣
Can we see these changes with in-situ x-rays ?
Predicted structures on GaN surface
W. Walkosz, et al. PRB 85, 033308 (2012) N-rich Ga-rich

‣
With different amounts of NH
3
, N
2
, and H
2 see large changes at anti-Bragg conditions in the sample environment we
‣
Modeling shows that CTR changes are consistent with a number of predicted surface structures
• Uniqueness problem: Modeling generates a number of structures that fit equally well.
Surface studies of GaN at 450 ° C as a function of chemical environment
20L Rod

‣
How can we get information about the intermediate chemical species on the surface ?
• X-rays are great at looking at the In phases (the heavy stuff), but how about highly reactive surface species (the light stuff)?
‣
Photons of a different length: in-situ Reflection-Absorption IR Spectroscopy
(RAIRS)
• Can distinguish between NH
3
, NH
2
, and NH
• Can penetrate MOCVD environment

‣
Heater is IR Source
• Solution: Bandpass filtering to mask black body radiation
‣
Surface vs. Gas species
• Solution: Polarize emitted spectrum
• Gas species are isotropic
• Surface species show polarization dependence
‣
Metallic Surface:
• Solution: ZrN
• 10% lattice mismatch to InN
• 0.6% lattice mismatch to GaN
• Stable in MOCVD
Environment
01L rod of ZrN

‣
By combining Reflection-Absorption IR Spectroscopy with grazing incidence surface x-ray scattering we correlate InN structure, surface chemical species, and theoretical surface structure predictions we will understand what are the intermediate chemical process the allow InN to form and grow
In-situ IR spectroscopy
In-situ X-ray Analysis
Theory & modeling
‣
We hope to use this knowledge to design new synthesis pathways and improve the quality of InN and InGaN alloys

‣
Synthesis of Metastable Materials requires we exploit kinetically limited and non-equilibrium pathways.
‣
We’ve shown that the synthesis of InN with highly reactive chemical species is a complex interplay of surface chemistry and structure
‣
Through a fundamental understanding of these metastable path ways we may be able to push the boundaries of the materials we synthesis and properties we can engineer

Edith Perret, Materials Science Division, Argonne National Laboratory
Weronika Walkosz, Chemical Sciences and Engineering Division, Argonne National Laboratory
Xin Tan, Chemical Sciences and Engineering Division, Argonne National Laboratory
Kedar Manandhar , Department of Physics, University of Illinois at Chicago
Paul Fuoss, Materials Science Division, Argonne National Laboratory
Carol Thompson, Department of Physics, Northern Illinois University
Peter Zapol, Chemical Sciences and Engineering Division, Argonne National Laboratory
Stephen Streiffer, Physical Sciences & Engineering, Argonne National Laboratory
Mike Trenary , Department of Physics, University of Illinois at Chicago
Brian Stephenson, Advanced Photon Source, Argonne National Laboratory
Work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy
Sciences, under contract No. DE-AC02-06CH11357