The doping concentration and resistivity of tin doped Gallium arsenide nanowires (GaAs NWs) have been investigated using Hall effect-, 4-probe-, transmission line-, and field effect measurements. Single nanowires were contacted using electron beam lithography followed by thermal evaporation of Au/Ti (900/100 Å). The Sn precursor (TESn) molar ratios of the investigated nanowires were 8.5·10-7, 1.7·10-6, 3.4·10-6 and 6.8·10-6 resulting in doping concentrations ranging from 4.64·1013 to 2.11·1017 cm-3 and resistivities from ~0.01 to ~1 Ωcm. The yield of the device fabrication was 2.4-7.1 % and evaluation of additional samples should be done in order to establish the validity of the results. The contact material was proved to work well with the higher doped samples but non-ohmic, highly resistive behavior was seen in the lower doped devices. A resistivity gradient along the length of the nanowires was found to be present, most likely the result of a doping gradient. The sample series with TESn molar ratio 1.7·10-6 showed more tapering than the other series possibly leading to a highly doped shell, which was indicated by 4-probe measurements.

Gallium arsenide (GaAs), cadmium selenide (CdSe), and cadmium telluride (CdTe) are semiconductor materials with remarkable opto-electronic properties that make them suitable for a wide variety of applications including, light emitting diodes (LEDs), mobile phones, tablets, and solar panels. Due to the growing demand and the short lifespan of these electronic devices, a remarkable amount of electronic waste (e-waste) has been produced in the last decades. An important fate of e-waste is landfill disposal; therefore, there is an increasing concern about the release of toxic elements into the landfill environment and the potential risks for human health and the environment. Among the elements constituting GaAs, CdTe, and CdSe, tellurium (Te) has gained increasing attention in recent years. Tellurium is a scarce element on the earth’s crust, and a shortage in its supply might compromise the development of new advanced technology, particularly in the energy and defense fields. For these reasons, the US Department of Energy and the European Union regard Te as a critical element, and have urged the need to develop efficient and cost-effective processes to recover Te from waste streams. This thesis dissertation explored different aspects related to the fate and impact of the widely used semiconductor materials, GaAs, CdSe and CdTe in municipal mixed solid waste (MSW) landfills. Furthermore, it investigated the removal of the Te oxyanions, tellurite (Teᴵⱽ, TeO₃²⁻) and tellurate (Teⱽᴵ, TeO₄²⁻), from aqueous streams and the recovery of this strategic metalloid as biogenic, elemental tellurite (Te⁰) nanoparticles (NPs). In the first part this work, the dissolution of GaAs was evaluated under a range of redox conditions, pH levels, ionic strength, and the presence of organic constituents commonly found in landfills. Our results indicated that aerobic conditions and mildly alkaline pH conditions favor the dissolution and release of high levels of soluble arsenic (As) and gallium (Ga) to the synthetic leaching solutions. The rate of As and Ga dissolution in long-term exposure experiments was initially constant but later progressively decreased due to the formation of a passivating layer on the surface of GaAs. The leaching behavior of CdSe and CdTe was also investigated under different pH and redox conditions in solutions simulating landfill leachates. CdTe and CdSe were subjected to two different standardized leaching tests, the federal Toxicity Characteristic Leaching Procedure (TCLP) and the California Waste Extraction Test (WET). CdTe showed a very high leaching potential in both tests and the concentrations of Cd released were 1500- and 260-times higher than the regulatory limit (1 mg Cd L⁻¹), respectively. In contrast, CdSe was relatively stable and dissolved selenium (Se) in both leaching tests was below the regulatory threshold (1 mg Se L⁻¹). Tests performed under different pH and redox conditions confirmed a marked enhancement in CdTe and CdSe dissolution both under acidic pH and aerobic conditions, which is consistent with thermodynamic predictions. Due to the high leaching potential observed for CdTe in the previous batch experiments, leaching studies were designed to investigate the potential release of soluble Cd and Te from a commercially available CdTe thin-film solar panel under different chemical and biogeochemical conditions commonly found in landfills. The solar panel was subjected to two standardized batch leaching tests (i.e., TCLP and WET), and to a continuous column test designed to investigate the dissolution of CdTe under conditions simulating the acidic- and the methanogenic circumneutral phases of a MSW landfill. A negligible amount of Cd and Te was measured in the synthetic leachate of both batch tests. On the other hand, a significant amount of Cd and Te was released from the panel to the synthetic leachate of the column simulating an acid landfill after 30 days (73% and 21% of the total Cd and Te, respectively). In contrast, the amount of Cd or Te detected in the effluent of the column operated at circumneutral pH values was negligible. The marked difference in the leaching behavior of CdTe in both columns is related to the different aqueous pH and redox conditions promoted by the microbial communities in the columns. The microbial toxicity of the soluble species that can be released from CdTe and CdSe was also assessed as a part of this work. The toxicity of cadmium (Cdᴵᴵ), selenite (Seᴵⱽ), selenate (Seⱽᴵ), Teᴵⱽ, and Teⱽᴵ was evaluated in bioassays with different microbial targets, including acetoclastic and hydrogenotrophic methanogenic populations in a mixed microbial culture, similar to those involved in the stabilization of organic waste stabilization in a landfill, and the bioluminescent marine bacterium, 𝐴𝑙𝑖𝑖𝑣𝑖𝑏𝑟𝑖𝑜 𝑓𝑖𝑠𝑐ℎ𝑒𝑟𝑖 (Microtox® test). The acetoclastic methanogens were most sensitive to the presence of the various soluble species, with the toxicity decreasing in the following order: Cdᴵᴵ, Teᴵⱽ, Teⱽᴵ, Seᴵⱽ, while Seⱽᴵ was only toxic at non-environmentally relevant concentration. Hydrogenotrophic methanogens were highly inhibited by Cdᴵᴵ and Seᴵⱽ, but Teᴵⱽ and Teⱽᴵ only had a moderate toxic impact. The bacterium 𝐴. 𝑓𝑖𝑠𝑐ℎ𝑒𝑟𝑖 was very sensitive to inhibition by Cdᴵᴵ and Seᴵⱽ, and Teᴵⱽ. In the last part of this work, the potential recovery of insoluble, elemental Te⁰ NPs from aqueous solutions containing soluble Teᴵⱽ or Teⱽᴵ was investigated in batch- and continuous flow bioreactors inoculated with a methanogenic granular mixed culture. In the batch experiments, the capacity of the culture to catalyze the reduction of the Te oxyanions and to produce Te⁰ NPs internally and externally to the cells was demonstrated. The granular sludge was found to contain enough endogenous substrates to provide the electron equivalents required to reduce both Te oxyanions and the reduction rates were only modestly increased by an exogenous electron-donor (e-donor) such as H₂. The effect of several redox mediators (RM), namely, anthraquinone-2,6-disulfonate (AQDS), hydroxocobalamin, riboflavin, and lawsone, was also tested. Riboflavin and lawsone caused a remarkable increase of the rate of Teᴵⱽ and Teⱽᴵ reduction, respectively, and also enhanced the fraction of Te recovered as extracellular Te⁰ NPs. The morphology and localization of the Te⁰ NPs were also impacted by the presence of a particular RM and e-donor in the system, suggesting that NP production can be tailored for a particular application. Finally, the feasibility of utilizing upflow anaerobic sludge bed (UASB) bioreactors to reduce Teᴵⱽ oxyanions to non-toxic Te⁰ NPs was also investigated. Two reactors were supplied with ethanol as the external e-donor source to promote the biological reduction of Teᴵⱽ. Riboflavin, a redox mediator, was supplied to one of the reactors to enhance Teᴵⱽ bioreduction. Continuous formation of Te⁰ NPs using an UASB was found to be feasible and remarkably improved by addition of riboflavin at the low Teᴵⱽ:riboflavin molar ratio of 4:1. This flavonoid enhanced the conversion rate of Teᴵⱽ and reduced the toxic impact of Teᴵⱽ towards the methanogenic consortium. Overall, the evidence found in this work indicates that recycling of decommissioned devices containing GaAs, CdTe, or CdSe is desirable to prevent the potential environmental release of toxic metals and metalloids in MSW landfills, but also to allow the recovery of critical resources. Microbial processes offer potential for the removal and recovery of soluble metals and metalloid ions leached from decommissioned semiconductor materials. In particular, this study demonstrated the feasibility of utilizing continuous UASB bioreactors for the removal of Teᴵⱽ from aqueous streams and the recovery of this valuable metalloid as biogenic Te⁰ NPs.

A compact optical terahertz (THz) source was demonstrated based on an efficient high-repetition-rate doubly resonant optical parametric oscillator (OPO) around 2 mu m with two type-II phase-matched KTP crystals in the walk-off compensated configuration. The KTP OPO was intracavity pumped by an acousto-optical (AO) Q-switched Nd:YVO4 laser and emitted two tunable wavelengths near degeneracy. The tuning range extended continuously from 2.068 mu m to 2.191 mu m with a maximum output power of 3.29 W at 24 kHz, corresponding to an optical-optical conversion efficiency (from 808 nm to 2 mu m) of 20.69%. The stable pulsed dual-wavelength operation provided an ideal pump source for generating terahertz wave of micro-watt level by the difference frequency generation (DFG) method. A 7.84-mm-long periodically inverted quasi-phase-matched (QPM) GaAs crystal with 6 periods was used to generate a terahertz wave, the maximum voltage of 180 mV at 1.244 THz was acquired by a 4.2-K Si bolometer, corresponding to average output power of 0.6 mu W and DFG conversion efficiency of 4.32x10(-7). The acceptance bandwidth was found to be larger than 0.35 THz (FWHM). As to the 15-mm-long GaSe crystal used in the type-II collinear DFG, a tunable THz source ranging from 0.503 THz to 3.63 THz with the maximum output voltage of 268 mV at 1.65 THz had been achieved, and the corresponding average output power and DFG conversion efficiency were 0.9 mu W and 5.86x10(-7) respectively. This provides a potential practical palm-top tunable THz sources for portable applications.

Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.

Este trabalho apresenta o resultado de investigações sobre efeitos de spin em amostras de poços quânticos simples e duplos de AlGaAs, crescidos em substratos de GaAs por MBE - Molecular Beam Epitaxy. O estudo se concentra na variação do fator g de Landé ao longo da estrutura dos poços, a qual ocorre em virtude da dependência dessa grandeza, com respeito ao conteúdo de Al na liga AlGaAs. Através de cálculos autoconsistentes foram encontradas a distribuição eletrônica nos poços e a penetração da densidade eletrônica nas barreiras. Os cálculos se basearam em valores de densidade superficial de elétrons ns medidos experimentalmente em diversas amostras através de medidas de Hall e Shubnikov-de Haas. O estudo permitiu a determinação do valor esperado do fator g de Landé, em função do deslocamento da densidade eletrônica dentro dos poços devido `a ação de campos elétricos externos arbitrário. Também foi estudada a influência do tunelamento da densidade eletrônica dos poços. / In this work we presents the results of our investigations concerning MBE grown AlGaAs/GaAs single and double quantum well samples. We focused on the variation of the Land´e g factor along the structure of the quantum wells, which occur as a consquence of its dependence on the Al content on the alloy AlGaAs. The electronic distribution on the wells and the penetration of the eletronic density into the barriers of the samples were found through selfconsistent calculations. The calculations were based on the eletronic sheet density ns measured through Hall and Shubinikov-de Haas efects. This research allowed the determination of the expected value of the Landé g-factor, as a function of the displacement of the electronic state inside the wells due to an arbitrary external electric field action. Also the influence of the tunneling effects was also studied.

Estudamos a adsorção de Te em superfícies de GaAs(001) e InAs(001) com periodicidades 1x1, 1x2, 2x1 e 2x2, para as concentrações de telúrio Tc= 0, 1/4, 1/2, 3/4 e 1. Realizamos cálculos dentro do formalismo do funcional da densidade empregando pseudopotenciais de norma conservada. Para a relaxação das estruturas empregamos a dinâmica molecular de Car e Parrinello. Nossos resultados apontam para uma redução na estabilidade das superfícies na proporçã em que aumenta a concentração de Te na uperfície. A cobertura de As (Tc = 0) é energeticamente mais favorável que as recobertas com qualquer concentração de Te, tanto na superfície de GaAs(001) quanto na de InAs(001). Observouse ainda nas superfícies com Tc = 0 Tc = 1, que a dimerização dos átomos de As e da ordem de 30% mais intensa que dos átomos e Te. Comparando as dimerizações do Te nas superfícies com concentrações Tc 1/2, observamos que elas são maiores sobre o InAs(001) (célula terminada em In) que sobre o GaAs(001) terminada em Ga). Outra tendência verificada e a \"flutuação\" do Te sobre as superfícies. Para uma mesma concentração verificamos a \"preferência de adsorção\" em sítios fora da cadeia de dímeros indicando uma adsorção monoatômica. Para concentração Tc = 1 na superfície GaAs(001):Te2x2 observamos duas geometrias possíveis: uma com cadeias de dímeros seguindo o modelo \"dimerrowmissing\" e outra com dois tipos de dímeros em posições alternadas ao longo da dir~ao (110). Analisando as energias de adsorção nas duas superfícies verificamos que a adsorção sobre o InAs é mais favorável que sobre o GAs. Analisamos ainda a estrutura eletrônica das superfícies em todas as reconstruções e concentrações consideradas e verificamos anda o caráter semicondutor das superfícies com concentração Tc = 1/2. / We have carried out ab initio density functional calculations to investigate the adsorption of Te on GaAs(001) and lnAs(001) surfaces as a function of Te surface coverage: Tc = 0, 1/4, 1/2, 3/4, 1. In order to determine the equilibrium atomic positions, the geometry was relaxed according he calculated total energy and forces following the Car Parrinello approach for molecular dynamics. Our calculations indicate that a full monolayer of As (Tc = 0) is energetically more favourable than any of the studied coverage of Te (Tc = 1/4, 1/2, 3/4, 1), where the stability is educed with increasing Te coverage. The dimerization of surface As atoms is about 30% more intense than surface Te atoms. Comparing the Te dimerization on InAs(001), In terminated, and aAs(OOI), Ga terminated, we observed that the Te atoms dimerize more over lnAs than GaAs surface. Another observation is the tendency of the Te atoms to \'l1oat\" from the surface with increasing coverage. For the same concentrations of Te the atoms \"prefer\" to be adsorbed on the offchain sites indicating a monoatomic adsorption. The adsorption energy of Te on InAs(001) is more favorable than GaAs(001). We also determined the surface band structure for all reconstructions and Te concentrations, veryfying the semiconductor nature for Tc = 1/2.

Estudamos a adsorção de Te em superfícies de GaAs(001) e InAs(001) com periodicidades 1x1, 1x2, 2x1 e 2x2, para as concentrações de telúrio Tc= 0, 1/4, 1/2, 3/4 e 1. Realizamos cálculos dentro do formalismo do funcional da densidade empregando pseudopotenciais de norma conservada. Para a relaxação das estruturas empregamos a dinâmica molecular de Car e Parrinello. Nossos resultados apontam para uma redução na estabilidade das superfícies na proporçã em que aumenta a concentração de Te na uperfície. A cobertura de As (Tc = 0) é energeticamente mais favorável que as recobertas com qualquer concentração de Te, tanto na superfície de GaAs(001) quanto na de InAs(001). Observouse ainda nas superfícies com Tc = 0 Tc = 1, que a dimerização dos átomos de As e da ordem de 30% mais intensa que dos átomos e Te. Comparando as dimerizações do Te nas superfícies com concentrações Tc 1/2, observamos que elas são maiores sobre o InAs(001) (célula terminada em In) que sobre o GaAs(001) terminada em Ga). Outra tendência verificada e a \"flutuação\" do Te sobre as superfícies. Para uma mesma concentração verificamos a \"preferência de adsorção\" em sítios fora da cadeia de dímeros indicando uma adsorção monoatômica. Para concentração Tc = 1 na superfície GaAs(001):Te2x2 observamos duas geometrias possíveis: uma com cadeias de dímeros seguindo o modelo \"dimerrowmissing\" e outra com dois tipos de dímeros em posições alternadas ao longo da dir~ao (110). Analisando as energias de adsorção nas duas superfícies verificamos que a adsorção sobre o InAs é mais favorável que sobre o GAs. Analisamos ainda a estrutura eletrônica das superfícies em todas as reconstruções e concentrações consideradas e verificamos anda o caráter semicondutor das superfícies com concentração Tc = 1/2. / We have carried out ab initio density functional calculations to investigate the adsorption of Te on GaAs(001) and lnAs(001) surfaces as a function of Te surface coverage: Tc = 0, 1/4, 1/2, 3/4, 1. In order to determine the equilibrium atomic positions, the geometry was relaxed according he calculated total energy and forces following the Car Parrinello approach for molecular dynamics. Our calculations indicate that a full monolayer of As (Tc = 0) is energetically more favourable than any of the studied coverage of Te (Tc = 1/4, 1/2, 3/4, 1), where the stability is educed with increasing Te coverage. The dimerization of surface As atoms is about 30% more intense than surface Te atoms. Comparing the Te dimerization on InAs(001), In terminated, and aAs(OOI), Ga terminated, we observed that the Te atoms dimerize more over lnAs than GaAs surface. Another observation is the tendency of the Te atoms to \'l1oat\" from the surface with increasing coverage. For the same concentrations of Te the atoms \"prefer\" to be adsorbed on the offchain sites indicating a monoatomic adsorption. The adsorption energy of Te on InAs(001) is more favorable than GaAs(001). We also determined the surface band structure for all reconstructions and Te concentrations, veryfying the semiconductor nature for Tc = 1/2.

InAs/GaSb superlattices are an attractive material system for infrared light emitting diodes, due to the ability to tune the band gap throughout most of the infrared regime. A key consideration in the epitaxial growth of these heterostructures is crystalline material quality. In developing thick layers of epitaxially grown material, there are moderate amounts of elastic strain that can be incorporated into a heterostructure, beyond which deformations will form that will alleviate the lattice mismatch. This thesis investigates the optical and electronic properties of lattice-mismatched and strained materials through the study of thick dual-color light emitting diodes, broadband light emitting diodes, and InAs/GaSb superlattice devices developed on GaAs substrates and GaAs integrated circuits. A dual-color infrared light emitting diode is demonstrated emitting in the mid-wave infrared band at 3.81 μm and 4.72 μm. The design of the device stacks two independently operable InAs/GaSb superlattices structures on top of one another, so that 10 μm of material is grown with molecular beam epitaxy. Each layer is lattice-matched to a GaSb substrate. At quasi-continuous operation, radiances of 5.48 W/cm2-sr and 2.67 W/cm2-sr are obtained. A broadband light emitting diode spanning the mid-wave infrared is demonstrated with eight stages of InAs/GaSb superlattices individually tuned to a different color. The performance of the device is compared with an identical eight stage device emitting in the middle of the mid-wave infrared. The emission of the fabricated broadband device spans from 3.2 μm to 6 μm with peak radiance of 137.1 mW/cm2-sr. Growth of antimonide-based devices on GaAs is desirable to the relative transparency of semi-insulating substrates throughout the infrared, and as semi-insulating GaSb substrates are not available. The growth of bulk GaSb on GaAs is explored through different techniques in order to confine relaxation due to lattice mismatch strain to the GaSb/GaAs interface. A low temperature nucleation technique with a thin GaSb wetting layer is found to have the best overall surface morphology, although screw dislocations are a prominent feature on all samples. The dislocations and overall surface roughness are not found to destructively impact the overall device quality, as four stage InAs/GaSb superlattice devices grown on GaAs substrates are found to have superior electroluminescent emission and external quantum efficiency compared to an identical device grown on a GaSb substrate due to the higher substrate transparency and superior thermal properties. Epitaxy on electronics growth techniques on GaAs integrated circuits are developed to bypass the hybridization process in light emitting diode development. Chips obtained from Quorvo, Inc. are found to endure ultra-high vacuum molecular beam epitaxy environment at higher temperatures with silicon nitride encapsulation, and a low temperature oxide removal technique is developed using an atomic hydrogen source. Chemical-mechanical polishing techniques are developed to create an “epi-ready” substrate surface. Ultimately, no photoluminescent emission is observed from InAs/GaSb superlattices grown on GaAs integrated circuits, although electroluminescent emission is still possible.

This dissertation addresses the design of monolithically integrated phase shifters at S- and L- frequency bands using a commercially available GaAs process from Triquint. The focus of the design is to operate over a wide bandwidth with full 360° phase shift capability, 50 Ω input/output impedance match and low RMS phase and gain errors. The first version of the design is based on passive all-pass phase shifters integrated with wideband amplifiers to compensate for insertion loss. This design uses a 4-bit system to achieve the required phase shift and each bit consists of 3 sections of all-pass filters designed at separate frequencies within the 0.8 – 3 GHz band. Simulation results show a complete 360° phase shift with RMS gain error of less than 0.6 dB and RMS phase error of less than 2.5°. The system is also shown to achieve good input and output impedance matching characteristics. However, the fabricated prototype fails to perform with full functionality due to the excessive number of passive inductors in the design and the resulting mutual coupling. The mutual coupling issue could be solved by spacing out the layout to allow more separation among the inductors. Unfortunately, in the S- and L-bands, this is not an option for this research work as the fabricated design already uses the maximum allowed chip size as determined by the foundry. In addition, larger chip sizes considerably increase the cost in practical applications. To address the challenging needs of small size, wide bandwidth and low frequency applicability, the second design introduced in this dissertation proposes a novel phase shifter implementation that utilizes tunable active differential inductors within all-pass networks. The inductor tuning is used to achieve phase shifts up to 180⁰. A switchable active balanced to unbalanced transition circuit (balun) is included in front of the all-pass network to complement its phase shift capability by another 180°. In addition, the all-pass network is followed by a variable gain amplifier (VGA) to correct for gain variations among the phase shifting states and act as an output buffer. Although active inductors have been previously used in the design of various components, to the best of our knowledge, this is the first time that they have been used in an all-pass phase shifter. The approach is demonstrated with an on-chip design and implementation exhibiting wideband performance for S and L band applications by utilizing the 0.5 µm TriQuint pHEMT GaAs MMIC process. Specifically, the presented phase shifter exhibits 1 × 3.95 mm2 die area and operates within the 1.5 GHz to 3 GHz band (i.e. 2:1 bandwidth) with 10 dB gain, less than 1.5 dB RMS gain error and less than 9° RMS phase error. Comparison with the state-of-the-art MMIC phase shifters operating in S and L bands demonstrates that the presented phase shifter exhibits a remarkable bandwidth performance from a very compact footprint with low power consumption. Consequently, it presents an important alternative for implementation of wideband phase shifters where all-passive implementations will consume expensive die real estate.

In this dissertation, we study properties of quantum interaction phenomena in two-dimensional (2D) and one-dimensional (1D) electronic systems in p-GaAs micro- and nano-scale devices. We present low-temperature magneto-transport data from three forms of low-dimensional systems 1) 2D hole systems: in order to study interaction contributions to the metallic behavior of 2D systems 2) Bilayer hole systems: in order to study the many body, bilayer quantum Hall state at nu = 1 3) 1D hole systems: for the study of the anomalous conductance plateau G = 0.7 ???? 2e2/h The work is divided into five experimental studies aimed at either directly exploring the properties of the above three interaction phenomena or the development of novel device structures that exploit the strong particle-particle interactions found in p-GaAs for the study of many body phenomena. Firstly, we demonstrate a novel semiconductor-insulator-semiconductor field effect transistor (SISFET), designed specifically to induced 2D hole systems at a ????normal???? AlGaAs-on-GaAs heterojunction. The novel SISFETs feature in our studies of the metallic behavior in 2D systems in which we examine temperature corrections to ????xx(T) and ????xy(T) in short- and long-range disorder potentials. Next, we shift focus to bilayer hole systems and the many body quantum Hall states that form a nu = 1 in the presence of strong interlayer interactions. We explore the evolution of this quantum Hall state as the relative densities in the layers is imbalanced while the total density is kept constant. Finally, we demonstrate a novel p-type quantum point contact device that produce the most stable and robust current quantization in a p-type 1D systems to date, allowing us to observed for the first time the 0.7 structure in a p-type device.