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Structural Characterization of III-V Bismide Materials Grown by Molecular Beam Epitaxy

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III-V-bismide semiconductor alloys are a class of materials with applications in the mid and long wave infrared spectrum. The quaternary alloy InAsSbBi is attractive because it can be grown

III-V-bismide semiconductor alloys are a class of materials with applications in the mid and long wave infrared spectrum. The quaternary alloy InAsSbBi is attractive because it can be grown lattice-matched to commercially available GaSb substrates, and the adjustment of the Bi and Sb mole fractions enables both lattice constant and bandgap to be tuned independently. This dissertation provides a comprehensive study of the surface morphology and the structural and chemical properties of InAsSbBi alloys grown by molecular beam epitaxy.

210 nm thick InAsSbBi layers grown at temperatures from 280 °C to 430 °C on (100) on-axis, (100) offcut 1° to (011), and (100) offcut 4° to (111)A GaSb substrates are investigated using Rutherford back scattering, X-ray diffraction, transmission electron microscopy, Nomarski optical microscopy, atomic force microscopy, and photoluminescence spectroscopy. The results indicate that the layers are coherently strained and contain dilute Bi mole fractions.

Large surface droplets with diameters and densities on the order of 3 µm and 106 cm-2 are observed when the growth is performed with As overpressures around 1%. Preferential orientation of the droplets occurs along the [011 ̅] step edges offcut (100) 1° to (011) substrate. The surface droplets are not observed when the As overpressure is increased to 4%. Small crystalline droplets with diameters and densities on the order of 70 nm and 1010 cm-2 are observed between the large droplets for the growth at 430°C. Analysis of one of the small droplets indicates a misoriented zinc blende structure composed of In, Sb, and Bi, with a 6.543 ± 0.038 Å lattice constant.

Lateral variation in the Bi mole fraction is observed in InAsSbBi grown at high temperature (400 °C, 420 °C) on (100) on-axis and (100) offcut 4° to (111)A substrates, but is not observed for growth at 280 °C or on (100) substrates that are offcut 1° to (011). Improved crystal and optical quality is observed in the high temperature grown InAsSbBi and CuPtB type atomic ordering on the {111}B planes is observed in the low temperature grown InAsSbBi. Strain induced tilt is observed in coherently strained InAsSbBi grown on offcut substrates.

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  • 2020

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Structural and Optical Properties of III-V Semiconductor Materials for Photovoltaics and Power Electronic Applications

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This dissertation focuses on the structural and optical properties of III-V semiconductor materials. Transmission electron microscopy and atomic force microscopy are used to study at the nanometer scale, the structural

This dissertation focuses on the structural and optical properties of III-V semiconductor materials. Transmission electron microscopy and atomic force microscopy are used to study at the nanometer scale, the structural properties of defects, interfaces, and surfaces. A correlation with optical properties has been performed using cathodoluminescence.

The dissertation consists of four parts. The first part focuses on InAs quantum dots (QDs) embedded in a GaInP matrix for applications into intermediate band solar cells. The CuPt ordering of the group-III elements in Ga0.5In0.5P has been found to vary during growth of InAs QDs capped with GaAs. The degree of ordering depends on the deposition time of the QDs and on the thickness of the capping layer. The results indicate that disordered GaInP occurs in the presence of excess indium at the growth front.

The second part focuses on the effects of low-angle off-axis GaN substrate orientation and growth rates on the surface morphology of Mg-doped GaN epilayers. Mg doping produces periodic steps and a tendency to cover pinholes associated with threading dislocations. With increasing miscut angle, the steps are observed to increase in height from single to double basal planes, with the coexistence of surfaces with different inclinations. The structural properties are correlated with the electronic properties of GaN epilayers, indicating step bunching reduces the p-type doping efficiency. It is also found that the slower growth rates can enhance step-flow growth and suppress step bunching.

The third part focuses on the effects of inductively-coupled plasma etching on GaN epilayers. The results show that ion energy rather than ion density plays the key role in the etching process, in terms of structural and optical properties of the GaN films. Cathodoluminescence depth-profiling indicates that the band-edge emission of etched GaN is significantly quenched.

The fourth part focuses on growth of Mg-doped GaN on trench patterns. Anisotropic growth and nonuniform acceptor incorporation in p-GaN films have been observed. The results indicate that growth along the sidewall has a faster growth rate and therefore a lower acceptor incorporation efficiency, compared to the region grown on the basal plane.

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  • 2020

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Monte Carlo studies of electron transport in semiconductor nanostructures

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ABSTRACT An Ensemble Monte Carlo (EMC) computer code has been developed to simulate, semi-classically, spin-dependent electron transport in quasi two-dimensional (2D) III-V semiconductors. The code accounts for both three-dimensional (3D)

ABSTRACT An Ensemble Monte Carlo (EMC) computer code has been developed to simulate, semi-classically, spin-dependent electron transport in quasi two-dimensional (2D) III-V semiconductors. The code accounts for both three-dimensional (3D) and quasi-2D transport, utilizing either 3D or 2D scattering mechanisms, as appropriate. Phonon, alloy, interface roughness, and impurity scattering mechanisms are included, accounting for the Pauli Exclusion Principle via a rejection algorithm. The 2D carrier states are calculated via a self-consistent 1D Schrödinger-3D-Poisson solution in which the charge distribution of the 2D carriers in the quantization direction is taken as the spatial distribution of the squared envelope functions within the Hartree approximation. The wavefunctions, subband energies, and 2D scattering rates are updated periodically by solving a series of 1D Schrödinger wave equations (SWE) over the real-space domain of the device at fixed time intervals. The electrostatic potential is updated by periodically solving the 3D Poisson equation. Spin-polarized transport is modeled via a spin density-matrix formalism that accounts for D'yakanov-Perel (DP) scattering. Also, the code allows for the easy inclusion of additional scattering mechanisms and structural modifications to devices. As an application of the simulator, the current voltage characteristics of an InGaAs/InAlAs HEMT are simulated, corresponding to nanoscale III-V HEMTs currently being fabricated by Intel Corporation. The comparative effects of various scattering parameters, material properties and structural attributes are investigated and compared with experiments where reasonable agreement is obtained. The spatial evolution of spin-polarized carriers in prototypical Spin Field Effect Transistor (SpinFET) devices is then simulated. Studies of the spin coherence times in quasi-2D structures is first investigated and compared to experimental results. It is found that the simulated spin coherence times for GaAs structures are in reasonable agreement with experiment. The SpinFET structure studied is a scaled-down version of the InGaAs/InAlAs HEMT discussed in this work, in which spin-polarized carriers are injected at the source, and the coherence length is studied as a function of gate voltage via the Rashba effect.

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  • 2011

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Conversion of a molecular beam epitaxy system for the growth of 6.1 angstrom semiconductors

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A dual chamber molecular beam epitaxy (MBE) system was rebuilt for the growth of 6.1 Angstrom II-VI and III-V compound semiconductor materials that are to be used in novel optoelectronic

A dual chamber molecular beam epitaxy (MBE) system was rebuilt for the growth of 6.1 Angstrom II-VI and III-V compound semiconductor materials that are to be used in novel optoelectronic devices that take advantage of the nearly continuous bandgap availability between 0 eV and 3.4 eV. These devices include multijunction solar cells and multicolor detectors. The MBE system upgrade involved the conversion of a former III-V chamber for II-VI growth. This required intensive cleaning of the chamber and components to prevent contamination. Special features including valved II-VI sources and the addition of a cold trap allowed for the full system to be baked to 200 degrees Celsius to improve vacuum conditions and reduce background impurity concentrations in epilayers. After the conversion, the system was carefully calibrated and optimized for the growth of ZnSe and ZnTe on GaAs (001) substrates. Material quality was assessed using X-ray diffraction rocking curves. ZnSe layers displayed a trend of improving quality with decreasing growth temperature reaching a minimum full-width half-maximum (FWHM) of 113 arcsec at 278 degrees Celsius. ZnTe epilayer quality increased with growth temperature under Zn rich conditions attaining a FWHM of 84 arcsec at 440 degrees Celsius. RHEED oscillations were successfully observed and used to obtain growth rate in situ for varying flux and temperature levels. For a fixed flux ratio, growth rate decreased with growth temperature as the desorption rate increased. A directly proportional dependence of growth rate on Te flux was observed for Zn rich growth. Furthermore, a method for determining the flux ratio necessary for attaining the stoichiometric condition was demonstrated.

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Date Created
  • 2012