Analysis of AlxGa1-xN Nanowires through
Simulated Methods of Scanning Transmission
Electron Microscopy and Electron Energy-Loss
Spectroscopy
Rajan Kumar
Northwestern University – Materials Science and Engineering
Dr. Robert Klie
Dr. Patrick Phillips
University of Illinois at Chicago – Nanoscale Physics Group
Acknowledgements:
UIC REU Program sponsored by
EEC-NSF Grant # 1062943
Dr. Christos Takoudis and Dr. Greg Jursich
UIC
Physics
Project Goals
• Simulate nanowire images to better
understand atomic composition of nanowires
-
Run Kirkland code, look for resolution changes in all
three elements
• Simulate EELS to better understand
electrical properties of nanowires
- Run multiple scattering code, look for energy peaks
and their shape relative to composition
• By characterizing the nanowire structure
qualitatively, we can improve its properties to
maximize efficiency for ultraviolet emission
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Physics
Graded AlxGa1-xN Nanowires
• New type of pn-junction not based on impurity doping,
but on grading composition from x = 0 to x = 1
• UV LED Applications
• Need for atomic-scale characterization
S.D. Carnevale et al., Nano Letters,
Vol. 10, 1-3, 2012
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Physics
Scanning Transmission Electron Microscopy (STEM)
• Incident electrons converge
on specimen
• Electrons scatter through
specimen, contribute to image
formation
• Two types of detection
-
-
High Angle Annular Dark Field
(HAADF) and Annular Bright
Field (ABF)
HAADF good for Z contrast
ABF good for low weight
elements
E.J. Kirkland: ‘Advanced computing
in electron microscopy’, 2nd edn, 11;
2010, New York, Springer
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Physics
Scanning Transmission Electron Microscopy (STEM)
• Incident electrons converge
on specimen
• Electrons scatter through
specimen, contribute to image
formation
• Two types of detection
-
-
High Angle Annular Dark Field
(HAADF) and Annular Bright
Field (ABF)
HAADF good for Z contrast
ABF good for low weight
elements
• JEOL JEM-ARM 200CF
-
Down to 65 pm spatial
resolution
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Physics
STEM Images
HAADF Image
ABF Image
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Physics
Imaging Simulation - Kirkland
• Predict what a STEM
image will look like for a
given structure and imaging
parameters
• Kirkland code takes a
continuous material and
sections it into discrete slices
- Each slice is one layer of
atoms separated by empty
space
E.J. Kirkland: ‘Advanced computing in
electron microscopy’, 2nd edn, 142-143;
2010, New York, Springer
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Physics
Electron Energy-Loss Spectroscopy (EELS)
• Analyzing energy
distribution of emitted
electrons after interacting
with a specimen
• Two types of interactions:
elastic and inelastic
• These interactions cause
the emitted electrons to
lose a characteristic
amount energy relative to
the specimen it travels
through
R.F. Egerton: ‘EELS
in the Electron
Microscope’, 3rd
edn, 6-21; 2011,
New York, Springer
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Physics
Spectra Simulation - FEFF9
Coherent superposition of
standing waves
J. J. Rehr & R.C. Albers, Review of
Modern Physics, Vol. 72, 624,
2000
Feff9 calculations vs experiment for O K-edge in SrTiO3
FEFF9 is a real space multiple scattering
code for electronic structure, x-ray spectra,
and EELS. Non-periodic structures (e.g.
nanowires, surfaces) can be modeled
efficiently.
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Simulated Images - Pure GaN
GaN HAADF
GaN ABF
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Physics
Simulated Images - Pure AlN
AlN HAADF
AlN ABF
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Simulated Images - Single Graded
Viewing Direction
GaN
GaN rich
AlN rich
Graded HAADF
AlN
GaN rich
AlN rich
Graded ABF
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Physics
Simulated Images - Double Graded
AlN
GaN
AlN rich
AlN rich
GaN rich
Graded HAADF
GaN rich
Graded ABF
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Comparison to STEM Images
AlN rich
GaN rich
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Comparison to EELS Spectra
GaN
rich
AlN
rich
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Physics
Comparison to EELS Spectra
V. J. Keast et al., Journal
of Microscopy, Vol. 203,
170, 2001
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Physics
Summary
• Simulated STEM (Kirkland code)
- Higher nitrogen contrast in aluminum rich
regions vs gallium rich regions
- Gallium sites have higher intensities
• Simulated EELS (FEFF9)
- Confirmed three dominant peaks in AlN
- More testing needed for GaN to determine why
there is a secondary peak
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Physics
Summary
• Future Work
- Graded FEFF9 calculations
- Experiment with FEFF9 parameters
- Collect more STEM images and EELS data to
compare with simulations
• Confirmed experimental image contrast
comes from chemical ordering
- This will help complete structural
characterization of nanowires to verify their
target growth
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Physics
Microscopy – UIC’s JEOL JEM-ARM 200CF
• Cold field emission (0.4 eV resolution)
• Probe spherical aberration corrector (less
than 78 pm spatial resolution)
• Gatan Enfina 1000 EELS system
• HAADF, LAADF, BF/ABF detectors
• Stages:
• DT, ST, tomography
• in situ: heating, cooling, liquid, STM
1
n m
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Physics
FEFF9 Calculations
• Pure Crystals – GaN and AlN
• Graded Crystal – change nearest neighbor sites
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FEFF9 Graphs
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FEFF9 Graphs
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Comparison to STEM Images
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