Electron spin relaxation and carrier recombination were investigated in gallium indium nitride arsenide (GaInNAs) multiple quantum wells, using picosecond optical pulses. Pump-probe experiments were carried out at room temperature, using pulses produced by a Ti:sapphire pumped optical parametric oscillator. The peak wavelengths of the excitonic resonances for the quantum well samples were identified using linear absorption measurements, and were found to be in the range 1.25µm-1.29µm. Carrier recombination times were measured for three samples of varying nitrogen content, and were observed to decrease from 548 to 180ps as nitrogen molar fractions were increased in the range 0.45-1.24%. Carrier recombination times were also measured for samples which had undergone a post-growth annealing process, and were found to be signicantly shorter compared to times measured for as-grown samples. Electron spin relaxation time was investigated for samples with quantum well widths in the range 5.8-8nm, and was found to increase with increasing well width, (i.e. decreasing quantum confinement energy), a trend predicted by both D'Yakonov-Kachorovskii and Elliott-Yafet models of spin relaxation in quantum wells. In a further study, longer spin relaxation times were exhibited by samples containing higher molar fractions of nitrogen, but having nominally constant quantum well width. Spin relaxation times increased from 47ps to 115ps for samples containing nitrogen concentrations in the range 0.45-1.24%. Decreases in spin relaxation time were observed in the case of those samples which had been annealed post-growth, compared to as-grown samples. Finally, all-optical polarisation switching based on spin relaxation of optically generated carriers in GaInNAs multiple quantum wells was demonstrated.

É feito um resumo dos principais modelos criados para se explicar as intrigantes propriedades do centro DX e atualizar o problema. O decaimento da fotocondutividade persistente (PPC) é medido em AlxGa1-x As dopado com Si e se discute a validade dos modelos em função da cinética de captura dos elétrons pelos centros DX. Boa concordância com o modelo de Chadi e Chang é encontrada desde que se postule a existência de um nível doador mais raso. O crescimento por MBE assim como todo o processamento de amostras para os experimentos realizados é descrito sinteticamente. É discutido também o problema dos contatos a baixa temperatura e a possível influência dos centros DX nos desvios do comportamento ôhmico observados. Inclui-se também a descoberta da. fotocondutividade persistente em AlxGa1-xAs dopado com Pb, que também é relacionado à existência dos centros DX. / A short discussion about the main models created to explain the striking properties of the DX center is done in order to bring the problem up-to-date. The decay of persistent photoconductivity is measured and it is analyzed as a function of the kinetics of electron trapping by DX centers in Si-doped AlxGa1-xAs, according to these models. Good agreement with Chadi and Chang\'s model is found as long as we postulate the existence of a shallower donor. The M.B.E. growth as well as the whole sample processing is shortly described. It in siso diacussed the problem of low temperature contacts and the possible influence of DX centers in the deviation from ohmic behavior. Persistent Photoconductivity has been found in Pb-doped AlxGa1-xAs and it is also related to the DX center existence.

Since a decade, spintronics and related physics have attracted considerable attention due to the massive research conducted in these areas. The main reason for growing interest in these fields is the expectation to use the electrons spin instead of or in addition to the charge for the applications in spin-based electronics, quantum information, and quantum computation. A prime concern for these spins to be possible candidates for carrying information is the ability to coherently control them on the time scales much faster than the decoherence times. This thesis reports on the spin dynamics in two-dimensional electron gases hosted in artificially grown III-V semiconductor quantum wells. Here we present a series of experiments utilizing the techniques to optically control the spin polarization triggered by either optical or electrical methods i.e. well known pump-probe technique and current-induced spin polarization. We investigated the spin coherence in high mobility dense two-dimensional electron gas confined in GaAs/AlGaAs double and triple quantum wells, and, it\'s dephasing on the experimental parameters like applied magnetic field, optical power, pump-probe delay and excitation wavelength. We have also studied the large spin relaxation anisotropy and the influence of sample temperature on the long-lived spin coherence in triple quantum well structure. The anisotropy was studied as a function sample temperature, pump-probe delay time, and excitation power, where, the coherent spin dynamics was measured in a broad range of temperature from 5 K up to 250 K using time-resolved Kerr rotation and resonant spin amplification. Additionally, the influence of Al concentration on the spin dynamics of AlGaAs/AlAs QWs was studied. Where, the composition engineering in the studied structures allows tuning of the spin dephasing time and electron g-factor. Finally, we studied the macroscopic transverse drift of long current-induced spin coherence using non-local Kerr rotation measurements, based on the optical resonant amplification of the electrically-induced polarization. Significant spatial variation of the electron g-factor and the coherence times in the nanosecond scale transported away half-millimeter distances in a direction transverse to the applied electric field was observed. / Há uma década, a spintrônica e outras áreas relacionadas vêm atraindo considerável atenção, devido a enorme quantidade de pesquisa conduzidas por elas. A principal razão para o crescente interesse neste campo é a expectativa da aplicação do controle do spin do elétron no lugar ou em adição à carga, em dispositivos eletrônicos e informação e computação quânticas. A possibilidade destes spins carregarem informação depende, primeiramente, da habilidade de controlá-los coerentemente, em uma escala de tempo muito mais rápida do que o tempo de decoerência. Esta tese trata da dinâmica de spins em gases de elétrons bidimensionais, em poços quânticos de semicondutores III-V, crescidos artificialmente. Nós apresentamos uma série de experimentos, utilizando técnicas para o controle ótico da polarização de spin, desencadeadas por métodos óticos ou eletrônicos, ou seja, técnicas conhecidas de bombeio e prova e polarização de spin induzida por corrente. Nós investigamos a coerência de spin em gases bidimensionais, confinados em poços quânticos duplos e triplos de GaAs/AlGaAs e a dependência da defasagem com parâmetros experimentais, como campo magnético externo, potência ótica, tempo entre os pulsos de bombeio e prova e comprimento de onda da excitação. Também estudamos a grande anisotropia de relaxação de spin como função da temperatura da amostra, potência de excitação e defasagem entre bombeio e prova, medidos para uma vasta gama de temperatura, entre 5K e 250K, usando Rotação de Kerr com Resolução Temporal (TRKR) e Amplificação Ressonante de Spin (RSA). Além disso estudamos a influência da concentração de Al na dinâmica dos poços de AlGaAs/AlAs, para o qual a engenharia da composição da estrutura permite sintonizar o tempo de defasagem de spin e o fator $ g $ do elétron. Por fim, estudamos a deriva transversal macroscópica da longa coerência de spin induzida por corrente, através de medidas de Rotação de Kerr não-locais, baseadas na amplificação ressonante ótica da polarização eletricamente induzida. Observamos uma variação espacial significante do fator $ g $ e do tempo de vida da coerência, na escala de nanosegundos, deslocada distâncias de meio milímetro na direção transversa ao campo magnético aplicado.

Arsenic contamination of groundwater associated with mining operations is a widespread problem across the globe. The release of arsenic (As) into the environment occurs naturally by oxidation of exposed sulfide minerals. In the case of gold ores, the mining and beneficiation may also produce As-bearing wastes and this can accelerate the natural mechanisms of As mobilization. The Bakyrchik is the largest gold deposit in Kazakhstan and one of the largest in the world. Gold (Au) is dispersed in pyrite and arsenopyrite in the form of microscopic inclusions. Despite the fact that only 10% of gold ore has been mined to date, it has left behind a dangerous As-containing legacy. Speciation of As has been determined for samples from Bakyrchik to understand the post-processing environment and the mobility of arsenic in the mining-influenced area. As(III) and As(V) have been detected in water samples using HPLC-ICP-MS. The variability of As species across the narrow pH and Eh range indicates that biogeochemical processes can play a role in the speciation of As in water at the study site. In order to understand processes controlling As mobilization in water, the solid phase speciation of As in Bakyrchik sediments, soil, and metallurgical processing products has been investigated using XRD and EPMA. This revealed arsenopyrite, As-bearing pyrite, and their alteration products containing up to 25% As, iron oxides and oxyhydroxides (with up to 2.5% As), haidingerite, and calcium arsenate in studied samples. Sequential extraction demonstrated that in soil and sediment samples As is associated mainly with Al and amorphous Fe oxyhydroxides. Results suggest that the main mechanisms controlling As mobility in Bakyrchik are dilution with regional waters, adsorption onto iron and aluminium oxyhydroxides, and co-precipitation of dissolved As with alteration products of sulfide minerals. Assessment of As bioaccessibility through inhalation demonstrates its strong dependence on the mineralogy rather than on total As content of the solid samples. Calculated cancer and non-cancer risks of inhalation exposure imply that all samples are highly hazardous for human health. With the depletion of the oxide lode ore deposits, gold extraction is moving towards the mining of technologically difficult ores, such as those found at Bakyrchik. A new method of Au recovery from double refractory Au-As-bearing concentrates has been developed, allowing recovery of 97% of Au, and the conversion of up to 95% As into iron-arsenic alloy. Fe-As alloy can contain up to 40% As, and do not require further solidification/stabilisation prior to disposal. The method has been published as a patent with the Patent Office of the Republic of Kazakhstan. Quantitative phase composition of the Fe-As alloy, has been determined by EPMA, QEMSCAN, X-ray and neutron diffraction. Toxicity and solubility of Fe-As alloy in aqueous solutions have been characterised. High-temperature structural behaviour of Fe-As alloy and Fe$_{2}$As in inert atmosphere has been determined by $\textit{in situ}$ synchrotron XRD. Results of the study support the development of the new method as an efficient alternative for processing double refractory Au-As-bearing concentrates.

The coupling between longitudinal-optical (LO) phonons and neutral excitons in two different kinds of InGaAs pyramidal quantum dots embedded in either AlGaAs or GaAs barriers is experimentally examined. We find a slightly weaker exciton-LO-phonon coupling and increased linewidth of the phonon replicas for the quantum dots with GaAs barriers compared to the ones with AlGaAs barriers. These results, combined with the fact that the LO-phonon energy of the exciton is the same for both kinds of dots, are taken as evidence that the excitons mainly couple to LO-phonons within the QDs. / Original Publication:Daniel Dufåker, L. O. Mereni, Fredrik K. Karlsson, V. Dimastrodonato, G. Juska, Per-Olof Holtz and E. Pelucchi, Exciton-phonon coupling in single quantum dots with different barriers, 2011, Applied Physics Letters, (98), 25, 251911.http://dx.doi.org/10.1063/1.3600781Copyright: American Institute of Physicshttp://www.aip.org/

This investigation examines band gap engineering of the GaAsN alloy by means of plasma ion implantation. The strong redshift of the alloy's band gap is suitable for telecommunication applications and thus stimulated much interest in recent years. Nitrogen (N) ion implantation into gallium arsenide (GaAs) results in a thin shallow N-rich layer below the surface. However, the violent implantation process also modifies the concentrations of gallium and arsenide. The core of this thesis is a novel method for prediction of the band gap from the conditions in the processing plasma.<p>The first important variable, the number of implanted ions, is obtained from the Lieberman model for the current during high-voltage Plasma Ion Implantation (PII). A review of the model's assumptions is provided as well as a comprehensive discussion of the implantation which includes error boundaries. The predicted and measured ion currents agree within error boundaries. The number of implanted ions can therefore be obtained from the prediction.<p>The distribution of the implanted ions was subsequently explored by simulations such as TRIM and TRIDYN. It was found that the nitrogen content in GaAs is limited by the sputtering of the surface atoms. Furthermore, the content of gallium increases near the surface while the content of arsenic decreases. The predicted ratios of the constituents in the implanted layer is such that the alloy cannot form by ion implantation alone; it could be reconciled by annealing.<p>Preliminary samples were produced and tested for the formation of the GaAsN alloy by Raman spectroscopy. No evidence for bonds between N and either Ga or As was found in the as-implanted samples. The thesis concludes with a discussion of the necessary steps to synthesize the GaAsN alloy.

The quantum Spin Hall Insulator (QSHI) is a two-dimensional variant of a novel class of materials characterized by topological order, whose unique properties have recently triggered much interest and excitement in the condensed matter community. Most notably, the topological properties of these systems hold great promise in mitigating the difficult problem of decoherence in implementations of quantum computers. Although QSHI has been theoretically predicted in a few different materials, prior to the work presented in this thesis, only the HgTe/CdTe semiconductor system has shown direct evidence for the existence of this phase. Ideally insulating in the bulk, QSHI is characterized by one-dimensional channels at the sample perimeter, which have a helical property, with carrier spin tied to the carrier direction of motion, and protected from elastic back-scattering by time-reversal symmetry. In this thesis we present low temperature transport measurements, showing strong evidence for the existence of proposed helical edge channels in InAs/CaSb quantum wells, which thus emerge as an important alternate to HgTe/CdTe quantum wells in studies of two-dimensional topological insulators and superconductors. Surprisingly, edge modes persist in spite of comparable bulk conduction of non-trivial origin and show only weak dependence on magnetic field in mesoscopic devices. We elucidate that the seeming independence of edge on bulk transport comes due to the disparity in Fermi wave-vectors between the bulk and the edge, leading to a total internal reflection of the edge modes. Furthermore, low Schottky barrier of this material system and good interface to superconductors allows us to probe topological properties of helical channels in Andreev reflection measurements, opening a promising route towards the realization of topologically superconducting phases hosting exotic Majorana modes.

This investigation examines band gap engineering of the GaAsN alloy by means of plasma ion implantation. The strong redshift of the alloy's band gap is suitable for telecommunication applications and thus stimulated much interest in recent years. Nitrogen (N) ion implantation into gallium arsenide (GaAs) results in a thin shallow N-rich layer below the surface. However, the violent implantation process also modifies the concentrations of gallium and arsenide. The core of this thesis is a novel method for prediction of the band gap from the conditions in the processing plasma.<p>The first important variable, the number of implanted ions, is obtained from the Lieberman model for the current during high-voltage Plasma Ion Implantation (PII). A review of the model's assumptions is provided as well as a comprehensive discussion of the implantation which includes error boundaries. The predicted and measured ion currents agree within error boundaries. The number of implanted ions can therefore be obtained from the prediction.<p>The distribution of the implanted ions was subsequently explored by simulations such as TRIM and TRIDYN. It was found that the nitrogen content in GaAs is limited by the sputtering of the surface atoms. Furthermore, the content of gallium increases near the surface while the content of arsenic decreases. The predicted ratios of the constituents in the implanted layer is such that the alloy cannot form by ion implantation alone; it could be reconciled by annealing.<p>Preliminary samples were produced and tested for the formation of the GaAsN alloy by Raman spectroscopy. No evidence for bonds between N and either Ga or As was found in the as-implanted samples. The thesis concludes with a discussion of the necessary steps to synthesize the GaAsN alloy.

The objective of this research is to demonstrate the feasibility of the implementation of low-cost, small-size, and high power RF front ends using CMOS technology which has been known not to be suitable for high-power applications due to its material characteristic. One part of this research focuses on developing GaAs switches for multi band and multi mode high power applications. The development of RF front end switches for high power applications using CMOS technology is very challenging in that the characteristics of CMOS technology such as low breakdown voltages, slow electron mobility and existence of substrate junction diodes are limiting power handling capability of CMOS technology. Various topologies of CMOS switches have been employed in implementing high power RF front end CMOS switches in order to overcome material limitations of CMOS technology in high power applications. Based on measurement data such as power handling capability and S-parameters of fabricated CMOS switches, the feasibility of use of CMOS technology in high power RF antenna switch design has been studied, and novel methods of designing CMOS switches to improve the power handling capability without compensating S-parameter performance are proposed. As a part of this research, multi-band and multi-mode power switches using GaAs technology are fabricated and tested for use of the commercial applications such as handsets covering GSM, PCS/DCS, and UMTS bands. Current commercial RF switch products demand small size, low cost and low voltage control as the number of wireless standards integrated in a single application increases. This research provides a solution for commercial products which can meet all the specifications as well as needs required in the wireless market.

The objective of this work was to study the noise in semiconductors and relate the transport mechanisms in the devices with the noise from the devices. The main part of the work was to set up a system for the measurement of noise in semiconductor devices. To establish the sensitivity of the system, it was calibrated at different temperatures. Some of the results from GaAs pn-junction showed some anomaly from that available in the literature. But certain points are yet to be clarified. This requires certain developments in the measurement system. In the case of QWIPS structures, studies on some samples with varying number of wells are required and in order to study the GR noise spectra and other activated processes, we need to study the temperature dependence of the noise and a larger bias variation for studying the low frequency current noise.