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In this thesis, a novel method for multi-pulse with equal chirp characteristics and more efficient THz generation through two photon absorption (TPA) are investigated and demonstrated. These are all the first time, for our best knowledge, odd multi-pulses generation and innovative approach for efficient THz through TPA are proposed and studied. By modulating the amplitude and phase of the spatial light modulator with the pulse shaper, the number of multi-pulses can be adjustable without the limitation of even number only. Meanwhile, the chirp properties of the generated pulses are with the same characteristics and tunable also. For the case with bandwidth of 10nm, the generated multi-pulses with equal chirps varying from -20000fs^2 to 20000fs^2 are demonstrated and the results have a good agreement with the theoretical estimation. We also discuss the number of the multi-pulses and inequalities of the amplitude of the pulses are limited by the spectral resolution of SLM within the pulse shaper. Regarding to efficient THz radiation, it can be generated more efficiently from a low-temperature-grown GaAs (LT-GaAs) photoconductive (PC) antenna by taking into account the TPA induced photo-carrier in the photoconductor. A rate-equation-based approach using the Drude-Lorentz model taking into account the band-diagram of LT-GaAs is used for the theoretical analysis. The super-linear power dependent photocurrent clarifies the role of TPA. Previously unnoticed THz pulse features and anomalously increasing THz radiation power rather than saturation were observed. These are in good agreement with the theoretical predictions.

MBE ( Molecular Beam Epitaxy ) technique can obtain high quality of GaSb/GaAs hetero-junction structure and control epilayers precisely.It has 7¢Mlattice mismatch between GaAs ( substrate ) and GaSb ( thin film ), but if we control beam flux ratio (V/III) and substrate temperature exactly, we can obtain high quality of epilayer. The growth mechanisms related to the major factors of (1) Beam flux ratio (V/III)¡B(2) Substrate temperature. The properties of GaSb epilayers are characterized by different methods such as the X-ray diffraction. The optimum growth conditions 500¢J of substrate temperature and the V/III flux ratio about 2.5 have been obtained. On the basis of this condition, We use simulation program for solar cell ( Scaps ), to simulate GaSb/GaAs hetero-junction solar cell structure, to try the possibility of the GaSb/GaAs hetero-junction structure in solar cells. From the simulation result, we know if the doped concentration in the two semiconductor materials and thickness are suitability, and if we can control the concentration of interface states under a suitable value, the efficiency of the solar cell can well. On the basis of this result, the same for thermo-photovoltaic (TPV), its' efficiency will also well.

Abstract The purpose of this thesis is to study the electrical characteristics of ultra-thin high-k gate oxide-semiconductor interfaces. The measured samples are Y2O3/Si¡BGd2O3/GaAs¡BGa2O3(Gd2O3)/GaAs MOS capacitors. An accurate C-V relation has been obtained consistently by using a model that includes both series and shunt parasitic resistances. Using the semiconductor parameters and the oxide parameters, an ideal C-V curve with Dit = 0 is fitted to the accurate capacitance data, and the interface state density is deduced by Terman method. After post - metallization annealing (PMA) at 425¢J, the oxide charge density, interface state density and leakage current were reduced. The results are following : (1) For Y2O3/Si MOS capacitors, we obtained a oxide charge density ~ 7.7 x 1010 cm-2, an interface state density ~ 3.6 x 1010 cm-2ev-1, and an equivalent oxide thickness ~ 52Å; (2) For Gd2O3/GaAs MOS capacitors, we obtained a oxide charge density ~ 9.8 x 1011 cm-2, an interface state density ~ 2 x 1011 cm-2ev-1, and an equivalent oxide thickness ~ 57Å; (3) For Ga2O3(Gd2O3)/GaAs MOS capacitors, we obtained a oxide charge density ~ 4.2 x 1012 cm-2, an interface state density ~ 6 x 1011 cm-2ev-1, and an equivalent oxide thickness ~ 91Å. The dielectric constants obtained from our data are smaller than the reported values. A possible explanation is that an interfacial layer formed at the oxide/semiconductor interface to reduce equivalent dielectric constant.

Photoreflectance (PR) of surface-intrinsic-n+ type GaAs has been measured for various temperatures, then we can get the energy of E0+£G0 transition in various temperatures. The spectra exhibited many Franz-Keldysh oscillations, when probe beam¡¦s energy is larger than energy gap. Electric field (F) and transition energy can be determined from analyzing the Franz-Keldysh oscillation. Further more we can get the surface¡¦s Fermi level from the dependence of surface¡¦s electric field (Fs) on temperature (T).

The incorporation of rare earth-V (RE-V) semimetallic nanoparticles embedded in III-V compound semiconductors is of great interest for applications in solid-state devices including multijunction tandem solar cells, thermoelectric devices, and fast photoconductors for terahertz radiation sources and receivers. With regard to those nanoparticle roles in device applications and material itself, electrical and thermal properties of embedded RE-V nanoparticles, including nanoscale morphology, electronic structure, and electrical and thermal conductivity of such nanoparticles are essential to be understood to engineer their properties to optimize their influence on device performance. To understand embedded RE-V semimetallic nanostructures in III-V compound semiconductors, nanoscale characterization tools are essential for analysis their properties incorporated in compound semiconductors. In this dissertation, we used atomic force microscopy (AFM) with other secondary detection tools to investigate nanoscale material properties of semimetallic RE-V and GaAs heterostructures, grown by molecular beam epitaxy. We used scanning capacitance microscopy and conductive AFM techniques to understand electronic and electrical properties of ErAs/GaAs heterostructures. For the electrical properties, this thesis investigates details of statistical analysis of scanning capacitance and local conductivity images contrast to provide insights into (i) nanoparticle structure at length scales smaller than the nominal spatial resolution of the scanned probe measurement, and (ii) both lateral and vertical nanoparticle morphology at nanometer to atomic length scales, and their influence on electrical conductivity. To understand thermal properties of ErAs nanoparticles, in-plane and cross-sectional plane of ErAs/GaAs superlattice structure were investigated with a scanning probe microscopy technique implemented with 3[omega] method for thermal measurement. By performing detailed numerical modeling of thermal transport between thermal probe tip and employed samples, and estimation of additional phonon scattering induced by ErAs nanoparticles, we could understand influences of ErAs nanoparticles on the host GaAs thermal conductivity. Investigation of ErAs semimetallic nanostructure embedded in GaAs matrix with scanned probe microscopy provided detailed understanding of their electronic, electrical and thermal properties. In addition, this dissertation also demonstrates that an atomic force microscope with secondary detection techniques is promising apparatus to understand and investigate intrinsic properties of nanostructure materials, nanoscale charge transports, when the system is combined with detailed modeling and simulations. / text

Recently laser cooling of semiconductors has received renewed attention, with the hope that a semiconductor cooler might be able to achieve cryogenic temperatures. In order to study semiconductor laser cooling at cryogenic temperatures, it is crucial that the theory include both the effects of excitons and the electron-hole plasma. In this dissertation, I present a theoreticalanalysis of laser cooling of bulk GaAs based on a microscopic many-particle theory of absorptionand luminescence of a partially ionized electron-hole plasma.This theory has been analyzed from a temperature 10K to 500K. It is shown that at high temperatures (above 300K), cooling can be modeled using older models with a few parameter changes. Below 200K, band filling effects dominate over Auger recombination. Below 30K excitonic effects are essential for laser cooling. In all cases, excitonic effects make cooling easier then predicted by a free carrier model.The initial cooling model is based on the assumption of a homogeneous undoped semiconductor. This model has been systematically modified to include effects that are present in real laser cooling experiments. The following modifications have been performed. 1) Propagation and polariton effects have been included. 2) The effect of p-doping has been included. (n-doping can be modeled in a similar fashion.) 3) In experiments, a passivation layer is required to minimize non-radiative recombination. The passivation results in a npn heterostructure. The effect of the npn heterostructure on cooling has been analyzed. 4) The effect of a Gaussian pump beam was analyzed and 5) Some of the parameters in the cooling model have a large uncertainty. The effect of modifying these parameters has been analyzed.Most of the extensions to the original theory have only had a modest effect on the overall results. However we find that the current passivation technique may not be sufficient to allow cooling. The passivation technique currently used appears to be very good at low densities, but loses some of it's effectiveness at the moderately high densities required for laser cooling. We suggest one possible solution that might enable laser cooling. If the sample can be properly passivated, then we expect laser cooling to be possible.

This thesis presents the design and theoretical modeling of an aluminum gallium arsenide/gallium arsenide heterostructure from which suspended nanoscale mechanical resonators with embedded two-dimensional electron gas (2DEG) can be made. The mechanical characteristics of the resonator and the piezoelectric actuation scheme are investigated using finite-element modeling. For a 836 nm-long, 250 nm-wide and 164 nm-thick beam with gold electrodes on top, out-of-plane flexural vibrations are verified to be piezoelectrically excited at the beam's fundamental frequency of 925.6 MHz. Fabrication recipes for the making of ohmic contacts to the 2DEG, Hall bars and suspended structures are developed using the designed crystal structure. Electrical properties of the 2DEG are evaluated in both large, unsuspended structures as well as in sub-micron size suspended structures. It is found that the 2DEG has a reasonable electron density of 7.04E11 cm^-2 and electron mobility of 1.72E5 cm^2/(V s). / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2007-09-28 10:13:56.094

This thesis consists of a study of several thin film semiconductor structures of practical technological use either presently or in the near future. The first system studied is an ultra thin film single crystal gallium arsenide layer. The absorption spectra of these layers are measured and transitions at both the F- point and L-point of the Brillouin Zone are observed, the latter are not normally measurable in thicker layers. The observed shift in the F-point absorption edge is attributed to contributions from the Franz-Keldysh Effect and the Moss-Burstein Effect. The temperature dependence of the L-point energy gap is measured and compared with previous data. The next system investigated is an n-type porous silicon layer coated with p-type polyaniline. Both photoluminescence and electroluminescence spectra and the electrical characteristics have been measured for this system. The interface between the two layers is found to be a rectifying junction consistent with a potential barrier formed at the interface. In forward bias, it is possible to generate electroluminescence in the visible and near infra red regions. The final structure studied is a thin film cadmium sulphide-cadmium telluride solar cell structure. The cells are found to have a low efficiency of around 1% as grown, but a process of treatment with cadmium chloride and annealing in air improves this by a factor of approximately ten. Photoluminescence measurements on the back surface of the cadmium telluride revealed three major emission bands at 1.59 eV, 1.55 eV and 1.45 eV. By varying temperature and incident laser power, attempts at assigning the bands to specific impurity centres in the cadmium telluride is made Using a novel bevelling etch technique to prepare samples, depth dependent measurement of the photoluminescence is possible. This reveals that the major changes associated with the improvement in efficiencies occurs at the interface between the CdS and the CdTe.

Neste trabalho foram processados e caracterizados dispositivos emissores de luz (LED’s) baseados em estruturas p-i-n a partir de filmes de GaAs dopados unicamente com Silício e crescidos através da técnica de Epitaxia de Feixes Moleculares sobre substratos de GaAs orientados nas superfícies (311)A e (211)A. Nas superfícies (311)A e (211)A, o Si tem comportamento anfótero, ou seja, pode ocupar tanto o sítio do Ga como o do As, o que resulta em filmes com portadores tipo n e p, respectivamente. As características elétricas dos filmes dependem das seguintes condições de crescimento: i) razão entre os fluxos de Ga e As; e ii) temperatura do substrato. As técnicas de caracterização utilizadas foram fundamentalmente: Fotoluminescência, IxV (corrente-tensão) e Efeito Hall. Os dispositivos p-i-n foram processados a partir de técnicas convencionais de fotolitografia e caracterizados a partir das técnicas de IxV e Eletrolumi-nescência. Os dispositivos estudados foram produzidos a partir de camadas de GaAs e AlGaAs crescidas nas direções [311]A e [211]A. Os dispositivos obtidos a partir de filmes dopados de GaAs apresentaram uma boa eficiência de emissão, porém foram observadas perdas ôhmicas ocasionadas pela natureza da junção obtida nestes planos, ocasionada por defeitos, principalmente nos filmes dopados tipo n que são crescidos em condições extremas de temperatura e pressão de Arsênio. Os dispositivos a base de AlGaAs apresentaram baixa eficiência e altas perdas, relacionadas com a alta compensação apresentada pelos filmes. Foram testados também dispositivos com heteroestruturas na camada intrínseca, apresentando resultados satisfatórios.