Transcript PPT - University of Chicago

Solid State Solutions, Phase Diagrams, and
Phase Transitions
Matt Highland
‣ Synthesis away from
‣
‣
equilibrium
Metastable Materials
Reactive Synthesis
Second Workshop on Photocathodes: 300nm-500nm
June 29-30 at the University of Chicago
Synthesis away from Equilibrium
‣ 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
SrRuO3 and Co3O4
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Reactive Synthesis of Metastable Materials
‣ 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
LEDs for solid state lighting
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InGaN: The promise and truth
• 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
*“Energy Savings Potential of Solid State
Lighting in General Illumination Applications”,
http://www.netl.doe.gov/ssl
quantum efficiency, hext
External
External Q uant um Eff iciency
‣ The promise
High-power (>1 Watt input) visible-spectrum
LEDs
70%
(AlxGa1-x)0.52In0.48P
InxGa1-xN
60%
V(
V(l)
l)
50%
40%
30%
20%
10%
0%
Tj =
25C
350
450
550
650
Peak wavelength, lp (nm)
Peak Wavelength (nm)
The Fundamental Problem
‣ At desirable growth temperatures
required nitrogen activity is
equivalent to kilobars (~104 psi) of
N2
‣ During MOCVD growth nitrogen
activity provided by cracking
ammonia
InCH3 3  NH3  InN 3CH4
‣ Reaction we want to avoid:
InN  In 1 2 N 2
Ambacher et al., JVST B 14, 3532 (1996)
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Intermediate Chemical Species
‣ We know the overall reaction desired
for growth
InCH3 3  NH3  InN 3CH4
‣ However what are the intermediate
chemical species that drive this growth
?
• All we know is the precursors crack
somehow interact
InCH 3 3  In? ?
NH 3  NH X  ?H2
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Attacking a Problem on Multiply Fronts
‣ 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
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Probing the Growth Environment
‣ 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
Fluorescence
Detector
Movie
camera
Visible illumination
Synchrotron x-rays
Scattering
Detector
‣ Measurements reveal a very
complex growth behavior
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In-InN Phase Boundaries
‣ 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
•
In liquid
droplets
liquid condenses
At lower T, relaxed InN solid
particles grow
Bare GaN
surface
pNH3= 27 Torr
InN crystals
F. Jiang, et al. PRL 101, 086102 (2008)
Oscillatory Growth and Decomposition
‣ 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)
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Chemical Wave Patterns
‣ 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)
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What Drives the Oscillatory Behavior ?
‣ The key to this complex growth behavior is local nitrogen activity
‣ NH3 impinges on the hot sample surface, cracks and forms some highly active
‣
‣
chemical species (NHx)
These active species either interact with In and form InN or react to
eventually form N2 and leave the surface.
The efficiency with which NH3 is cracked and the residence time of the
intermediate species determines which material grows
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Oscillatory Growth Mechanism
NH3 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 NH3to N2 and InN
starts to decompose
Liquid In metal
evaporates to expose
GaN surface and InN
growth starts again
Intermediate Chemical Species
‣ The local intermediate chemical species dictate growth behavior
‣ Different surfaces catalytically crack NH3 differently and possibly change
residence time of intermediate species
‣ 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
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First-principle Calculations
‣ We can calculate the lowest energy
configurations of NH3, NH2, NH, N, and H on
a GaN and InN surface
‣ We can then create a phase diagram
predicting the equilibrium coverage species
for given conditions
• “We” = Peter Zapol, Weronika Walkosz, and Xin
Tan
fixed
(2x2) surface unit cell
- 4 H3 “hollow” sites
- 4 T1 “on top” Ga sites
- 4 T4 “on top” N sites
- 12 br “bridge” site
Predicted Phase Diagram
Predicted structures on GaN surface
‣ 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 ?
N-rich
Ga-rich
W. Walkosz, et al. PRB 85, 033308 (2012)
Surface and Crystal Truncation Rods
‣ An abrupt crystalline surface in real‣
space 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
(10L) CTR
(00L) CTR
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Predicted CTRs
Predicted structures on GaN surface
‣ First Principle can
be used to predict
CTRs for each
phase
‣ Can we see these
changes with in-situ
x-rays ?
W. Walkosz, et al. PRB 85, 033308 (2012)
N-rich
Ga-rich
Experimentally measured CTRs
‣ With different amounts of NH3, N2, and H2 in the sample environment we
‣
see large changes at anti-Bragg conditions
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
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In-situ Surface Chemistry
‣ 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 NH3, NH2, and NH
• Can penetrate MOCVD environment
RAIRS: Challenges to Overcome
‣ 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
01L rod of ZrN
‣ Metallic Surface:
•
•
•
•
Solution: ZrN
10% lattice mismatch to InN
0.6% lattice mismatch to GaN
Stable in MOCVD
Environment
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Future Plans
‣ 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
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Summary
‣ 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
Acknowledgment
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
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