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Duke University Dissertation Template GaAsBi Synthesis: From Band Structure Modification to Nanostructure Formation by Kristen N. Collar Department of Physics Duke University Date:______________________ Approved: ___________________________ Harold Baranger, Chair ___________________________ April Brown, Advisor/ Co-Chair ___________________________ Ronen Plesser ___________________________ Maiken Mikkelsen ___________________________ Patrick Charbonneau Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics in the Graduate School of Duke University 2017 i v ABSTRACT GaAsBi Synthesis: From Band Structure Modification to Nanostructure Formation by Kristen N. Collar Department of Physics Duke University Date:_______________________ Approved: ___________________________ Harold Baranger, Chair ___________________________ April Brown, Advisor/Co-Chair ___________________________ Maiken Mikkelsen ___________________________ Ronen Plesser ___________________________ Patrick Charbonneau An abstract of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics in the Graduate School of Duke University 2017 i v Copyright by Kristen N. Collar 2017 Abstract Research and development of bismides have proven bismides to be a promising field for material science with important applications in optoelectronics. However, the development of a complete description of the electrical and material properties of bismide ternaries is not comprehensive or straightforward. One of the main benefits of this ternary system is the opportunity for bandgap tuning, which opens doors to new applications. Tuning the bandgap is achieved by varying the composition; this allows access to a wider energy spectrum with applications in long wavelength emitters and detectors. In addition to bandgap tuning, Bi provides an opportunity to decrease lasing threshold currents, to decrease the temperature sensitivity and to decrease a major loss mechanism of today’s telecom lasers. We propose to characterize the electronic and chemical structure of GaAsBi grown by molecular beam epitaxy. We probe the binding structure using x-ray photoelectron spectroscopy. This provides insights into the antisite incorporation of Bi and the reactivity of the surface. Furthermore, we use x-ray photoelectron spectroscopy to track the energy variation in the valence band with dilute Bi incorporation into GaAs. These insights provide valuable perspective into improving the predictability of bandgaps and of heterostructure band offsets for the realization of bismides in future electronics. iv The stringent growth conditions required by GaAsBi and the surfactant properties of Bi provide a unique opportunity to study nanostructure formation and epitaxial growth control mechanisms. The GaAsBi epitaxial films under Ga-rich growth conditions develop Ga droplets, which seed the Vapor-Liquid-Solid growth of embedded nanowires. We demonstrate a means to direct the nanowires unidirectionally along preferential crystallographic directions utilizing the step-flow growth mode. We mediated the step-flow growth by employing vicinal surfaces and Bi’s surfactant-like properties to enhance the step-flow growth mode. Semiconductor nanostructures are a cornerstone of future optoelectronics and the work presented herein exploits the power of a bottom-up architecture to self-assemble aligned unidirectional planar nanowires. v Abbreviations AFM Atomic force microscopy BE Binding Energy BEP Beam equivalent pressure CB Conduction Band CL Core-Level CP Critical Point DOS Density of States FL Fermi Level FWHM Full-width at half-maximum HH Heavy Hole IMFP Inelastic mean free path IVBA Inter-Valence Band Absorption KE Kinetic energy LCAO Linear Combination of Atomic Orbitals LH Light Hole LT Low Temperature MBE Molecular beam epitaxy NIR Near infrared PL Photoluminescence RSF Relative Sensitivity Factor SE Spectroscopic ellipsometry SO Spin Orbit TEM Transmission electron microscopy UHV Ultrahigh-vacuum VB Valence band VBAC Valence Band Anticrossing XPS X-ray photoelectron spectroscopy XRD X-ray diffraction ZB Zinc Blende vi Contents Abstract ......................................................................................................................................... iv Abbreviations ............................................................................................................................... vi List of Tables ................................................................................................................................. xi List of Figures .............................................................................................................................. xii Acknowledgements ...................................................................................................................xvi 1 Introduction ............................................................................................................................. 1 1.1 III-V Semiconductors .................................................................................................. 1 1.2 Materials in the Near IR .............................................................................................. 4 1.3 Introduction to GaAsBi ............................................................................................... 5 1.4 Motivations: GaAsBi and nanowires ........................................................................ 7 1.5 Summary of work ........................................................................................................ 9 2 III-V Electronic Band Structure ........................................................................................... 13 2.1 Band Structure by Linear Combinations of Atomic Orbitals .............................. 13 2.2 Band Structure of GaAs ............................................................................................ 19 2.3 Band Structure Variations with Bi in GaAs ........................................................... 20 Decreased Bandgap & Increased SO Splitting Energy ..................................... 20 Bandgap Temperature Dependence ................................................................... 30 Bandgap Variations in Strained Films ................................................................ 32 2.4 Comparing Bismides to Nitrides ............................................................................. 35 vii 3 Experimental Techniques .................................................................................................... 41 3.1 Molecular Beam Epitaxy (MBE) .............................................................................. 41 3.2 X-ray Diffraction (XRD) ............................................................................................ 49 3.3 X-ray Photoelectron Spectroscopy (XPS) ............................................................... 51 3.4 Spectroscopic Ellipsometry (SE) .............................................................................. 52 3.5 Electron Microscopy .................................................................................................. 55 3.6 Atomic Force Microscopy (AFM) ............................................................................ 55 4 MBE Growth of GaAsBi ....................................................................................................... 58 4.1 Motivation .................................................................................................................. 58 4.2 Basics of MBE Growth .............................................................................................. 58 4.3 GaAsBi Growth by MBE ........................................................................................... 62 4.4 GaAsBi Growth Specifics .......................................................................................... 65 4.5 Challenges of GaAsBi Growth ................................................................................. 67 Bi as a surfactant in III-V epitaxy ........................................................................ 67 Bi Clusters............................................................................................................... 68 Bi and Ga Droplets ................................................................................................ 69 4.6 Stationary Growth Techniques ................................................................................ 71 4.7 Conclusions ................................................................................................................ 73 5 XPS Theoretical Background .............................................................................................. 74 5.1 XPS Basic Theory ....................................................................................................... 74 Mean Escape Depth .............................................................................................. 75 Born-Haber Cycle .................................................................................................. 77 viii 5.2 XPS Study of Core Electrons .................................................................................... 86 5.3 XPS Study of Valence Electrons ............................................................................... 87 6 XPS Experimental Data on GaAsBi ...................................................................................
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