UIC Physics 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 University of Illinois at Chicago – Nanoscale Physics Group Acknowledgements: UIC REU Program sponsored by EEC-NSF Grant # 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

UIC Physics Graded Al x Ga 1-x N 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

UIC 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’, 2 nd edn, 11; 2010, New York, Springer

UIC 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

UIC Physics STEM Images HAADF ImageABF Image

UIC 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’, 2 nd edn, ; 2010, New York, Springer

UIC 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’, 3 rd edn, 6-21; 2011, New York, Springer

UIC Physics 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 SrTiO 3

UIC Physics Simulated Images - Pure GaN GaN ABFGaN HAADF

UIC Physics Simulated Images - Pure AlN AlN HAADFAlN ABF

UIC Physics Simulated Images - Single Graded Graded HAADF Graded ABF GaN AlN Viewing Direction GaN rich AlN rich GaN richAlN rich

UIC Physics Simulated Images - Double Graded GaN AlN Graded HAADFGraded ABF GaN rich AlN rich GaN rich AlN rich

UIC Physics Comparison to STEM Images AlN rich GaN rich

UIC Physics Comparison to EELS Spectra GaN rich AlN rich

UIC Physics Comparison to EELS Spectra V. J. Keast et al., Journal of Microscopy, Vol. 203, 170, 2001

UIC 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

UIC 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

UIC 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

UIC Physics FEFF9 Calculations Pure Crystals – GaN and AlN Graded Crystal – change nearest neighbor sites

UIC Physics FEFF9 Graphs

UIC Physics FEFF9 Graphs

UIC Physics Comparison to STEM Images