A new optimization technique for SPICE modeling parameters has been developed in this dissertation to increase the accuracy of the circuit simulation. The importance of having accurate circuit simulation models is to prevent the very costly redesign of an Integrated Circuit (IC). This radically new optimization technique utilizes the Taguchi method to improve the fit between measured and simulated I-V curves for GaAs MESFETs. The Taguchi method consists of developing a Signal-to-Noise Ratio (SNR) equation that will find the optimum combination of controllable signal levels in a design or process to make it robust or as insensitive to noise as possible. In this dissertation, the control factors are considered the circuit model curve fitting parameters and the noise is considered the variation in the simulated I-V curves from the measured I-V curves. This is the first known application of the Taguchi method to the optimization of IC curve fitting model parameters. In addition, this method is not technology or device dependent and can be applied to silicon devices as well. Improvements in the accuracy of the simulated I-V curve fit reaching 80% has been achieved between DC test extracted parameters and the Taguchi optimized parameters. Moreover, the computer CPU execution time of the optimization process is 96% less than a commercial optimizer utilizing the Levenberg-Marquardt algorithm (optimizing 31 FETs). This technique does a least square fit on the data comparing measured currents versus simulated currents for various combinations of SPICE parameters. The mean and standard deviation of this least squares fit is incorporated in determining the SNR, providing the best combination of parameters within the evaluated range. Furthermore, the optimum values of the parameters are found without additional simulation by fitting the response curves to a quadratic equation and finding the local maximum. This technique can easily be implemented with any simulator that utilizes simulation modeling parameters extracted from measured DC test data. In addition, two methods are evaluated to obtain the worst case modeling parameters. One method lobks at the correlation coefficients between modeling parameters and the second looks at the actual device parameters that define the +/- 3σ limits of the process. Lastly, an example is given that describes the applicability of the Taguchi methodology in the design of a differential amplifier, that accounts for the effect of offset voltage. / Ph. D.

N-type GaAs is a potentially useful semiconductor in liquid junction type solar cells. Corrosion and corrosion suppression on an n-type GaAs semiconductor in both light and dark has been studied. The application of non-electroactive layers for corrosion suppression on semiconductor electrodes is a relatively new field. GaAs corrodes to form Ga(III) and As(III) solution species during photocurrent generation. The corrosion rate is determined electroanalytically in acidic media by measuring As(III) using differential pulse polarography (DPP). In neutral electrolytes a rotating ring-disc experiment measured the efficiency of hole-transfer to a redox couple. Two protecting processes have been utilized. Silanization and electrochemical polymerization of divinylbenzene and phenol were used to deposit non-electroactive layers on the electrode surface. The polyphenylene oxide coating partially suppressed corrosion in acid electrolytes. However, the coatings did not improve hole transfer efficiency in neutral electrolytes. / Master of Science

Designs for PnP GaAs/InxGa1-xAs/GaAs heterojunction bipolar transistors (HBTs) are proposed and simulated with the aid of commercial software. Band diagrams, Gummel plots and common emitter characteristics are shown for the specific case of x=1, x=0.7, and x linearly graded from 0.75 to 0.7. Of the three designs, it is found that the linearly graded case has the lowest leakage current and the highest current gain. IV curves for all four possible classes of InAs/GaAs heterojunction (nN, nP, pN, pP) are calculated. A pN heterojunction is fabricated and characterized. In spite of the 7% lattice mismatch between InAs and GaAs, the diode has an ideality factor of 1.26 over three decades in the forward direction. In the reverse direction, the leakage current grows exponentially with the magnitude of the bias, and shows an effective ideality factor of 3.17, in stark disagreement with simulation. IV curves are taken over a temperature range of 105 K to 405 and activation energies are extracted. For benchmarking the device processing and the characterization apparatus, a conventional GaAs homojunction diode was fabricated and characterized, showing current rectification ratio of 109 between plus one volt and minus one volt. Because the PnP material for the optimal HBT design was not available, an Npn GaAs/InAs/InAs HBT structure was processed, characterized, and analyzed. The Npn device fails in both theory and in practice; however, by making a real structure, valuable lessons were learned for crystal growth, mask design, processing, and metal contacts. / Master of Science

Extensive Raman-scattering studies have been performed in order to study the structure of ion-implanted GaAs, prior to any anneal. The spectroscopic evidence is consistent with a fine-scale mixture of amorphous and microcrystalline GaAs. Excessive bombardment with 120-keV SiF₃⁺ ions results in a 500-A thick surface layer which is completely amorphous (a-GaAs). A detailed chemical-etch damage depth profile has been completed for 45-keV Be⁺-implanted GaAs, which is not completely amorphized. The damage is characterized using the microcrystalline longitudinal-optical (LO) phonon frequency, line width, and intensity, and the intensity of the a-GaAs component of the Raman spectrum. The damage layer possesses a 1500-A thick surface layer of constant, high damage. This high-damage plateau is followed by a transition region in which the damage level smoothly decreases until the undisturbed crystal is reached near 4000 A. LO intensities were analyzed, within the amorphous/crystalline mixed-phase model, to obtain the volume fractions of the two components. Consistent estimates of the optical absorption in the high-damage plateau were obtained via two independent means. Resonance-Raman experiments were carried out, using laser lines between 1.5 and 2.71 eV. The intensity of the a-GaAs spectral component was found to depend on scattering volume (optical penetration), thus providing an internal intensity standard allowing the effects of scattering volume and scattering efficiencies to be separated. The LO phonon was found to resonate approaching the E₁ electronic transition at 2.9 eV. The strength of the resonance decreases with smaller crystallite size. A new Raman band was observed near 47 cm⁻¹ for photon energies below 2 eV. It resonates at 1.7 eV, near E₀ and not near E₁. I propose that this new feature arises from GaAs acoustic modes made Raman active by defectassisted scattering involving the crystalline/amorphous interface regions. A quantitative analysis is developed, with some success. Intensities of silicon local are observed to remain constant upon annealing, although conductivity increases by several orders of magnitude. The anneal primarily restores the mobility to that of crystalline GaAs. / Ph. D.

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.

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.

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.