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Analysis of AlxGa1-xN Nanowires through Simulated Methods of Scanning Transmission Electron

Microscopy and Electron Energy-Loss Spectroscopy

Rajan KumarNorthwestern University – Materials Science and Engineering

Dr. Robert KlieDr. Patrick Phillips

University of Illinois at Chicago – Nanoscale Physics Group

Acknowledgements:UIC REU Program sponsored by

EEC-NSF Grant # 1062943Dr. Christos Takoudis and Dr. Greg Jursich

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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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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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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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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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STEM Images

HAADF Image ABF Image

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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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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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Spectra Simulation - FEFF9

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.

J. J. Rehr & R.C. Albers, Review of Modern Physics, Vol. 72, 624, 2000

Coherent superposition of standing waves

Feff9 calculations vs experiment for O K-edge in SrTiO3

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Simulated Images - Pure GaN

GaN ABFGaN HAADF

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Simulated Images - Pure AlN

AlN HAADF AlN ABF

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Simulated Images - Single Graded

Graded HAADF

Graded ABF

GaN

AlN

Viewing Direction

GaN rich AlN rich

GaN rich AlN rich

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Simulated Images - Double Graded

GaN

AlN

Graded HAADF Graded ABF

GaN rich

AlN rich

GaN rich

AlN rich

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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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Comparison to EELS Spectra

V. J. Keast et al., Journal of Microscopy, Vol. 203, 170, 2001

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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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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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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 nm1 nm

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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