abstract: 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. / Dissertation/Thesis / Ph.D. Electrical Engineering 2011

abstract: The research described in this dissertation has involved the use of transmission electron microcopy (TEM) to characterize the structural properties of II-VI and III-V compound semiconductor heterostructures and superlattices. The microstructure of thick ZnTe epilayers (~2.4 µm) grown by molecular beam epitaxy (MBE) under virtually identical conditions on GaSb, InAs, InP and GaAs (100) substrates were compared using TEM. High-resolution electron micrographs revealed a highly coherent interface for the ZnTe/GaSb sample, and showed extensive areas with well-separated interfacial misfit dislocations for the ZnTe/InAs sample. Lomer edge dislocations and 60o dislocations were commonly observed at the interfaces of the ZnTe/InP and ZnTe/GaAs samples. The amount of residual strain at the interfaces was estimated to be 0.01% for the ZnTe/InP sample and -0.09% for the ZnTe/GaAs sample. Strong PL spectra for all ZnTe samples were observed from 80 to 300 K. High quality GaSb grown by MBE on ZnTe/GaSb (001) virtual substrates with a temperature ramp at the beginning of the GaSb growth has been demonstrated. High-resolution X-ray diffraction (XRD) showed clear Pendellösung thickness fringes from both GaSb and ZnTe epilayers. Cross-section TEM images showed excellent crystallinity and smooth morphology for both ZnTe/GaSb and GaSb/ZnTe interfaces. Plan-view TEM image revealed the presence of Lomer dislocations at the interfaces and threading dislocations in the top GaSb layer. The defect density was estimated to be ~1 x107/cm2. The PL spectra showed improved optical properties when using the GaSb transition layer grown on ZnTe with a temperature ramp. The structural properties of strain-balanced InAs/InAs1-xSbx SLs grown on GaSb (001) substrates by metalorganic chemical vapor deposition (MOCVD) and MBE, have been studied using XRD and TEM. Excellent structural quality of the InAs/InAs1-xSbx SLs grown by MOCVD has been demonstrated. Well-defined ordered-alloy structures within individual InAs1-xSbx layers were observed for samples grown by modulated MBE. However, the ordering disappeared when defects propagating through the SL layers appeared during growth. For samples grown by conventional MBE, high-resolution images revealed that interfaces for InAs1-xSbx grown on InAs layers were sharper than for InAs grown on InAs1-xSbx layers, most likely due to a Sb surfactant segregation effect. / Dissertation/Thesis / Ph.D. Physics 2012

abstract: 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. / Dissertation/Thesis / M.S. Electrical Engineering 2012

A isolação elétrica de estruturas semicondutoras de InP e GaAs através de bombardeamento com íons foi estudada. Amostras de InP semi-isolante e tipo-p foram implantadas com íons P+ para produzir excesso de átomos de fósforo na ordem de 0,1%. Recozimento em ambiente de argônio no intervalo de temperatura de 400ºC a 600ºC e tempo de 30s foram feitos em sistema de tratamento térmico rápido. Medidas de efeito Hall com temperatura variável (12-300K) foram usadas para a caracterização elétrica. Uma alta concentração de portadores livres negativos foi observada. A existência destes elétrons foi atribuída a criação de anti-sítios PIn com energia acima do mínimo da banda de condução. A dependência da resistência de folha de camadas δ tipo-p de GaAs com a dose irradiada de íons de He+ foi investigada. A dose de limiar (Dth) para isolação elétrica depende da energia dos íons implantados e da concentração inicial da camada condutiva. A estabilidade térmica da isolação aumenta com o aumento da dose de irradiação e depende da razão entre a concentração de armadilhas criadas durante o bombardeamento e a concentração inicial de portadores livres. A isolação observada foi atribuída principalmente a introdução de anti-sítios AsGa e defeitos relacionados. A máxima estabilidade térmica para camadas δ tipo-p em GaAs foi de 550ºC, para doses maiores do que 10Dth. Os resultados deste trabalho podem ser usados para isolação de diferentes dispositivos discretos e integrados de InP e GaAs. / Electrical isolation of InP and GaAs structures by ion bombardment was studied. Semi-insulating and p-type InP were implanted with P+ ions to produce an excess of phosphorous atoms in the order of 0.1 at. %. Subsequent annealing in Ar ambient in the temperature interval 400ºC – 600ºC were performed for 30s in rapid thermal annealing system. Room temperature and variable temperature (12-300 K) Hall-effect measurements have been used for electrical characterization. A large amount of negative free carriers have been observed after the thermal treatments. These electrons are contributed to the creation of PIn anti-site with energy level above the minimum of the conduction band. The dose dependence of sheet resistance of p-type δ-doped GaAs structures irradiated by He+ ions was investigated. It was found that the threshold dose (Dth) for electrical isolation of the structures was determined by the energy of ions and by the concentration of initial doping. The thermal stability of isolation rises as the irradiation dose increases and is dependent on the ratio of trap centers concentration, created during the bombardment, to the initial concentration of free carriers. The isolation was attributed mainly to the introduction of AsGa anti-site related defects. The maximum thermal stability for p-type δ conductive layers in GaAs was of 550 0C for doses higher than 10 Dth. The results of this work can be used for isolation of different InP and GaAs discrete and integrated devices.

O processo de isolação elétrica por implantação de prótons é estudado para os casos do semicondutor GaAs e de estruturas DBR - Distributed Bragg Reflectors - de AlGaAs. A obtenção da energia crítica de implantação em camadas DBR de dispositivos VCSELs, Vertical Cavity Surface Emmiting Lasers, é exemplificada. Medidas da evolução da resistência destas estruturas com a dose de prótons irradiada, tanto para amostras tipo-n quanto para tipo-p, revelaram comportamentos qualitativamente semelhantes para os transportes eletrônicos paralelo e perpendicular às heterojunções. O mesmo valor de dose de limiar para isolação pode ser usado em ambos os casos. A estabilidade térmica da isolação de camadas DBR é estimada em ~150oC. O uso de um modelo simples de conservação de carga é sugerido para a simulação do processo de isolação por implantação em semicondutores, empregando como entrada a distribuição de níveis introduzidos pela irradiação. A comparação dos resultados dessa simulação com dados experimentais para o caso específico do GaAs revela que os modelos de defeitos previamente associados a antisítios na literatura não são suficientes para descrever a isolação. Os dados obtidos até então sobre a estrutura de níveis introduzida pela implantação com prótons também não reproduzem completamente o comportamento observado. Um sistema para medidas de Espectroscopia de Transientes de Níveis Profundos (DLTS) foi desenvolvido para a realização de experimentos visando identificar os níveis responsáveis pela isolação e determinar seus parâmetros relevantes. Quatro contribuições principais foram observadas em GaAs tipo-n (n1, n2, n3 e n4) implantado com prótons, e outras cinco para GaAs tipo-p (p1, p2, p3, p4 e p5). O nível p1, com energia aparente de 0,05 eV e secção de choque de captura de lacunas de 1x10-16 cm2, havia sido identificado previamente em amostras irradiadas com elétrons, mas pela primeira vez foi medido em amostras GaAs tipo-p implantadas com prótons. Medidas de variação de taxas de emissão com o campo elétrico foram realizadas em n1, n2, n3, p1, p2 e p3. O efeito Poole-Frenkel foi identificado como o responsável pelo comportamento observado em p2. Já para n3 e n1, mostrou-se que o responsável é o efeito de tunelamento auxiliado por fônons. O nível p3 não apresentou variação considerável em sua taxa de emissão com o campo; enquanto que os níveis n2 e p1revelaram comportamentos que indicam a presença de ambos os efeitos (Poole-Frenkel e tunelamento auxiliado por fônons). Esses dados, combinados com informações sobre as taxas de introdução obtidas a partir dos espectros DLTS, nos levam a parâmetros para os defeitos introduzidos pela implantação que conseguem descrever, de maneira aceitável, a curva de isolação experimental para doses menores que aproximadamente o dobro da dose de limiar, sugerindo que vacâncias de arsênio e anti-sítios de GaAs desempenhem um papel importante nesta etapa do processo. / The electrical isolation by proton implantation process is studied for the specific cases of AlGaAs Distributed Bragg Reflectors (DBR) and bulk GaAs. The critical implant energy evaluation is exemplified for AlGaAs DBR layers on Vertical Cavity Surface Emmiting Lasers devices. Sheet resistance measurements as a function of the proton fluence on both p-type and n-type samples presented similar behaviors for the electronic transports parallel and perpendicular to the heterojunctions. The same dose threshold value can be used in both cases. The DBR structures isolation thermal stability is estimated as ~150oC. A simple charge neutrality calculation is employed to simulate the implant isolation process in semiconductors using as input data on the deep levels introduced during irradiation. For the case of bulk GaAs, comparison between simulation results and experimental sheet resistance versus dose curves pointed out that models previously suggested for anti-site defects are not able, by their own, to reproduce the actual isolation behavior. Simulation using information previously obtained on proton implant related deep levels also didn’t reproduce the experimental curves. A Deep Level Transient Spectroscopy (DLTS) system was developed in order to properly identify the deep levels introduced by proton implantation and obtain the relevant parameters regarding each one of these levels. Four main contributions were measured in n-type proton bombarded GaAs (n1, n2, n3 and n4), and other five in p-type samples (p1, p2, p3, p4 and p5). p1 level, with 0.05 eV apparent energy and 1x10-16 cm2 apparent capture cross section, was measured for the first time in proton implanted GaAs. Electric field related emission rate enhancement measurements were carried out on n1, n2, n3, p1, p2 and p3. Poole-Frenkel effect was identified as the responsible for the behavior obtained for p2. Concerning the cases of n3 and n1, the effect responsible for the measured emission rate enhancement was found to be the phonon assisted tunneling. Level p3 presented no appreciable emission rate enhancement with the electric field, and levels n2 and p1 revealed behaviors pointing out the presence of both Poole-Frenkel and phonon assisted tunneling effects. Introduction rates of the measured levels were also obtained. All these data and other previous information on defects in proton bombarded GaAs could then be compiled together, producing a more complete and accurate input for the simulation, which leads to results reproducing the experimental curve in an acceptable manner for doses lower than twice the threshold value. This result points to a very active participation of As vacancies and GaAs in the GaAs implant isolation process.

Os compostos semicondutores ternários, dentre eles o Ga1-xInxSb, têm sido objeto de interesse de pesquisadores e da indústria microeletrônica devido à possibilidade de ajuste da constante de rede, assim como a correspondente modificação da banda proibida de energia e do intervalo de emissão e absorção óptica, com a variação da fração molar de x. A flexibilidade destas propriedades estruturais torna este composto apropriado como substratos para epitaxias de outros compostos ternários e quaternários, na formação de mono e heterojunções. A maneira mais econômica para obtenção de substratos de materiais semicondutores é através do crescimento de cristais a partir da fase líquida. Porém, os parâmetros que regem a obtenção de lingotes de Ga1-xInxSb com qualidade comercial, a partir da fase líquida, ainda não estão bem definidos. O índio tende a segregar para o líquido, pois seu coeficiente de segregação é menor que a unidade (k < 1), resultando num perfil composicional variado ao longo do lingote. Como os binários GaSb e InSb apresentam configurações de defeitos intrínsecos que originam condutividades de tipos opostos, tipo p e tipo n, respectivamente, a mudança na composição da liga, durante o crescimento, provavelmente resulta na modificação da concentração de cada um destes defeitos. A dopagem com telúrio consiste numa alternativa para minimizar a segregação do índio e diminuir a densidade dos defeitos pontuais, melhorando a qualidade estrutural de cristais de Ga1-xInxSb obtidos através do método Bridgman convencional. Desta forma foram crescidos cristais ternários Ga1-xInxSb, com e sem agitação do líquido durante a síntese, com fração molar inicial de índio de 10% e 20%, alguns deles dopados com 1020 átomos/cm3 de telúrio, pelo método Bridgman vertical. A caracterização estrutural em termos de formação de defeitos lineares, interfaciais e volumétricos foi realizada através de imagens obtidas por microscopia óptica, eletrônica de varredura e de transmissão. A homogeneidade composicional e distribuição de fases foi avaliada através de medidas de espectroscopia por dispersão de energia. Medidas de resistividade e efeito Hall foram utilizadas para a caracterização elétrica, enquanto que a transmitância óptica e a banda proibida de energia foram avaliadas por espectrometria FTIR. Os padrões de difração obtidos através da microscopia eletrônica de transmissão foram utilizados para avaliar a cristalinidade das amostras e determinar o parâmetro de rede. Os resultados obtidos indicam que o telúrio atua de forma compensatória, minimizando a segregação de índio e contribuindo para a homogeneidade composicional e redução de defeitos, principalmente de discordâncias. Além disso, altera a condutividade do Ga1-xInxSb para tipo n, mesmo em frações molares de In inferiores a x = 0,5, diminuindo o número de cargas positivas na rede atribuídas aos defeitos tipo GaSb e VGaGaSb e, desta forma, aumenta a concentração de portadores de carga e reduz a resistividade. Na condição de alta dopagem, reduz a transmitância óptica no infravermelho e aumenta a banda proibida de energia através do efeito Burstein-Moss. A avaliação de cristais de Ga1-xInxSb, dopados e não dopados, crescidos pelo método Bridgman convencional contribuiu para o entendimento do comportamento de dopantes em compostos semicondutores ternários. / Ternary compound semiconductors, including Ga1-xInxSb, have been subject of interest of researchers and microelectronics industry because of the possibility of adjusting the lattice constant, as well as the corresponding modification in the band gap energy, and in the optical absorption and emission range, by varying the mole fraction x. The flexibility of their structural properties makes this compound suitable as substrates for epitaxy of other ternary and quaternary compounds, in the formation of mono- and heterojunctions. The most economical way to obtain semiconductor substrates is by crystal growth from the liquid phase. However, the parameters governing the outcoming of Ga1-xInxSb ingots with commercial quality, from liquid phase, are not well defined. Indium tends to segregate to the liquid, since its segregation coefficient is less than the unity (k < 1), resulting in a varied compositional profile along the ingot. As the binary GaSb and InSb have intrinsic defects configurations that originate opposite conductivities, type p and type n, respectively, the change in the alloy composition, while growing, probably results in a modification of the concentration on each of these defects. Doping with tellurium is an alternative to minimize the indium segregation and decrease the density of point defects, therefore improving the structural quality of Ga1-xInxSb crystals obtained through the conventional Bridgman method. Thus, ternary Ga1-xInxSb crystals were grown by vertical Bridgman method with and without stirring the melt during the synthesis, with 10% and 20% initial molar fraction of indium and some of them were tellurium-doped at 1020 atoms/cm3. The structural characterization regarding linear, interfacial, and volumetric defects formation was performed by using images obtained through optical, scanning and transmission electron microscopy. The compositional homogeneity and phase distribution was assessed by energy-dispersive spectroscopy measurements. Resistivity and Hall Effect measurements were used for the electrical characterization, while the optical transmittance and the band gap energy were examined by FTIR spectroscopy. Diffraction patterns obtained by transmission electron microscopy were used to evaluate the crystallinity of the samples and determine the lattice parameter. The results indicate that tellurium acts in a compensatory way, minimizing indium segregation and contributing to the compositional homogeneity and defect reduction, especially in dislocations. In addition, it changes the conductivity of Ga1-xInxSb to n-type, even in mole fraction of In lower than x = 0.5, reducing the number of positive charges on the network assigned to GaSb and VGaGaSb defects, thus increasing the concentration of charge carriers and reducing the resistivity. In high doping condition, it reduces the optical transmittance in the infrared region and increases the energy of the band gap by the Burstein-Moss Effect. The evaluation of Ga1-xInxSb crystals, doped and undoped, grown by the conventional Bridgman method contributed to the understanding of dopants behavior in ternary compound semiconductors.

The present work is concerned with the theory of optical and magnetic properties of deeo centers in various III-V semiconductors.

abstract: III-V multijunction solar cells have demonstrated record efficiencies with the best device currently at 46 % under concentration. Dilute nitride materials such as GaInNAsSb have been identified as a prime choice for the development of high efficiency, monolithic and lattice-matched multijunction solar cells as they can be lattice-matched to both GaAs and Ge substrates. These types of cells have demonstrated efficiencies of 44% for terrestrial concentrators, and with their upright configuration, they are a direct drop-in product for today’s space and concentrator solar panels. The work presented in this dissertation has focused on the development of relatively novel dilute nitride antimonide (GaNAsSb) materials and solar cells using plasma-assisted molecular beam epitaxy, along with the modeling and characterization of single- and multijunction solar cells. Nitrogen-free ternary compounds such as GaInAs and GaAsSb were investigated first in order to understand their structural and optical properties prior to introducing nitrogen. The formation of extended defects and the resulting strain relaxation in these lattice-mismatched materials is investigated through extensive structural characterization. Temperature- and power-dependent photoluminescence revealed an inhomogeneous distribution of Sb in GaAsSb films, leading to carrier localization effects at low temperatures. Tuning of the growth parameters was shown to suppress these Sb-induced localized states. The introduction of nitrogen was then considered and the growth process was optimized to obtain high quality GaNAsSb films lattice-matched to GaAs. Near 1-eV single-junction GaNAsSb solar cells were produced. The best devices used a p-n heterojunction configuration and demonstrated a current density of 20.8 mA/cm2, a fill factor of 64 % and an open-circuit voltage of 0.39 V, corresponding to a bandgap-voltage offset of 0.57 V, comparable with the state-of-the-art for this type of solar cells. Post-growth annealing was found to be essential to improve Voc but was also found to degrade the material quality of the top layers. Alternatives are discussed to improve this process. Unintentional high background doping was identified as the main factor limiting the device performance. The use of Bi-surfactant mediated growth is proposed for the first time for this material system to reduce this background doping and preliminary results are presented. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

The present work is concerned with the theory of optical and magnetic properties of deeo centers in various III-V semiconductors.

A isolação elétrica de estruturas semicondutoras de InP e GaAs através de bombardeamento com íons foi estudada. Amostras de InP semi-isolante e tipo-p foram implantadas com íons P+ para produzir excesso de átomos de fósforo na ordem de 0,1%. Recozimento em ambiente de argônio no intervalo de temperatura de 400ºC a 600ºC e tempo de 30s foram feitos em sistema de tratamento térmico rápido. Medidas de efeito Hall com temperatura variável (12-300K) foram usadas para a caracterização elétrica. Uma alta concentração de portadores livres negativos foi observada. A existência destes elétrons foi atribuída a criação de anti-sítios PIn com energia acima do mínimo da banda de condução. A dependência da resistência de folha de camadas δ tipo-p de GaAs com a dose irradiada de íons de He+ foi investigada. A dose de limiar (Dth) para isolação elétrica depende da energia dos íons implantados e da concentração inicial da camada condutiva. A estabilidade térmica da isolação aumenta com o aumento da dose de irradiação e depende da razão entre a concentração de armadilhas criadas durante o bombardeamento e a concentração inicial de portadores livres. A isolação observada foi atribuída principalmente a introdução de anti-sítios AsGa e defeitos relacionados. A máxima estabilidade térmica para camadas δ tipo-p em GaAs foi de 550ºC, para doses maiores do que 10Dth. Os resultados deste trabalho podem ser usados para isolação de diferentes dispositivos discretos e integrados de InP e GaAs. / Electrical isolation of InP and GaAs structures by ion bombardment was studied. Semi-insulating and p-type InP were implanted with P+ ions to produce an excess of phosphorous atoms in the order of 0.1 at. %. Subsequent annealing in Ar ambient in the temperature interval 400ºC – 600ºC were performed for 30s in rapid thermal annealing system. Room temperature and variable temperature (12-300 K) Hall-effect measurements have been used for electrical characterization. A large amount of negative free carriers have been observed after the thermal treatments. These electrons are contributed to the creation of PIn anti-site with energy level above the minimum of the conduction band. The dose dependence of sheet resistance of p-type δ-doped GaAs structures irradiated by He+ ions was investigated. It was found that the threshold dose (Dth) for electrical isolation of the structures was determined by the energy of ions and by the concentration of initial doping. The thermal stability of isolation rises as the irradiation dose increases and is dependent on the ratio of trap centers concentration, created during the bombardment, to the initial concentration of free carriers. The isolation was attributed mainly to the introduction of AsGa anti-site related defects. The maximum thermal stability for p-type δ conductive layers in GaAs was of 550 0C for doses higher than 10 Dth. The results of this work can be used for isolation of different InP and GaAs discrete and integrated devices.