View
33
Download
0
Category
Preview:
DESCRIPTION
Analysis of Al x Ga 1-x N 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 - PowerPoint PPT Presentation
Citation preview
UICPhysics
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
UICPhysics
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
UICPhysics
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
UICPhysics
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
UICPhysics
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
UICPhysics
STEM Images
HAADF Image ABF Image
UICPhysics
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
UICPhysics
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
UICPhysics
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
UICPhysics
Simulated Images - Pure GaN
GaN ABFGaN HAADF
UICPhysics
Simulated Images - Pure AlN
AlN HAADF AlN ABF
UICPhysics
Simulated Images - Single Graded
Graded HAADF
Graded ABF
GaN
AlN
Viewing Direction
GaN rich AlN rich
GaN rich AlN rich
UICPhysics
Simulated Images - Double Graded
GaN
AlN
Graded HAADF Graded ABF
GaN rich
AlN rich
GaN rich
AlN rich
UICPhysics
Comparison to STEM Images
AlN rich
GaN rich
UICPhysics
Comparison to EELS Spectra
GaN rich
AlN rich
UICPhysics
Comparison to EELS Spectra
V. J. Keast et al., Journal of Microscopy, Vol. 203, 170, 2001
UICPhysics
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
UICPhysics
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
UICPhysics
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
UICPhysics
FEFF9 Calculations
• Pure Crystals – GaN and AlN• Graded Crystal – change nearest neighbor sites
UICPhysics
FEFF9 Graphs
UICPhysics
FEFF9 Graphs
UICPhysics
Comparison to STEM Images
Recommended