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In this thesis we investigate the optical properties of modulation doped GaAs/AlGaAs and strained-layer undoped InGaAs/GaAs multiple quantum well structures (MQWS). The phenomena studied are the effects of carrier, strain, and the electric field on the absorption of excitons. For GaAs/AlGaAs modulation doped MQWS, the quenching of excitons by free carriers has been demonstrated. The comparison of the experimental results with calculations which consider phase space filling, screening, and exchange interaction showed the phase space filling to be the dominant mechanism responsible for the change of oscillator strength and binding energy of excitons associated with partially filled subband. On the other hand, the screening and exchange interaction are equally important to excitons associated with empty subbands. For InGaAs/GaAs strained-layer MQWS, we have demonstrated that the band edges are dramatically modified by strain. We determined the band discontinuities at InGaAs/GaAs interfaces using optical absorption, and showed that in this structure the heavy holes are confined in InGaAs layers while the light holes are in GaAs layers, in contrast to GaAs/AlGaAs MQWS. We also explore applications of GaAs/AlGaAs and InGaAs/GaAs MQWS to opto-electronic devices. The principle of devices investigated is mainly based on the electric field effect on the excitonic absorption in MQWS (the quantum confined Stark effect). Two examples presented in this thesis are the strained-layer InGaAs/GaAs MQWS electroabsorption modulators grown on GaAs substrates and the GaAs/AlGaAs MQWS reflection modulators grown on Si substrates. The large modulation observed in the absorption coefficient by an electric field is expected to facilitate opto-electronic integration.
The last few years have seen rapid advances in nanoscience and nanotechnology, allowing unprecedented manipulation of nanostructures controlling solar energy capture, conversion, and storage. Quantum confined nanostructures, such as quantum wells (QWs) and quantum dots (QDs) have been projected as potential candidates for the implementation of some high efficiency photovoltaic device concepts, including the intermediate band solar cell (IBSC). In this dissertation research, we investigated multiple inter-related themes, with the main objective of providing a deeper understanding of the physical and optical properties of QD structures relevant to the IBSC concept. These themes are: (i) Quantum engineering and control of energy levels in QDs, via a detailed study of the electronic coupling in multilayer QD structures; (ii) Controlled synthesis of well-organized, good quality, high volume density, and uniform-size QD arrays, in order to maximize the absorption efficiency and to ensure the coupling between the dots and the formation of the minibands; and (iii) Characterization of carrier dynamics and development of techniques to enhance the charge transport and efficient light harvesting. A major issue in a QD-based IBSC is the occurrence of charge trapping, followed by recombination in the dots, which results in fewer carriers being collected and hence low quantum efficiency. In order to collect most of the light-generated carriers, long radiative lifetimes, higher mobilities, and a lower probability of non-radiative recombination events in the solar cell would be desirable. QD size-dependent radiative lifetime and electronic coupling in multilayer QD structures were studied using photoluminescence (PL) and time-resolved photoluminescence (TRPL). For the uncoupled QD structures with thick barriers between the adjacent QD layers, the radiative lifetime was found to increase with the QD size, which was attributed to increased oscillator strength in smaller size dots. On the other hand, in the sample with thin barrier and electronically coupled QDs, the radiative lifetime increases and later decreases with the dot size. This is due to the enhancement of the oscillator strength in the larger size, coherently coupled QDs. In order to improve the quality of multi-layer QD structures, strain compensated barriers were introduced between the QD layers grown on off-oriented GaAs (311)B substrate. The QD shape anisotropy resulted from the growth on off-oriented substrate was studied using polarization-dependent PL measurements both on the surface and the edge of the samples. The transverse electric mode of the edge-emitted PL showed about 5° deviation from the sample surface for the dots grown on (311)B GaAs, which was attributed to the tilted vertical alignment and the shape asymmetry of dots resulted from the substrate orientation. Significant structural quality improvements were attained by introducing strain compensated barriers, i.e., reduction of misfit dislocations and uniform dot size formation. Longer lifetime (~1 ns) and enhanced PL intensity at room temperature were obtained, compared to those in conventional multilayer (In, Ga)As/GaAs QD structures. A significant increase in the open circuit voltage (V oc) was observed for the solar cell devices fabricated with the strain compensated structures. A major issue in a QD IBSC is the occurrence of charge trapping, followed by recombination in the dots, which results in fewer carriers being collected, and hence low quantum efficiency. We proposed and studied a novel structure, in which InAs QDs were sandwiched between GaAsSb (12% Sb) strain-reducing layers (SRLs) with various thicknesses. Both short (~1 ns) and long (~4-6 ns) radiative lifetimes were measured in the dots and were attributed to type-I and type-II transitions, respectively, which were induced by the band alignment modifications at the QD/barrier interface in the structures analyzed, due to the quantum confinement effect resulting from different GaAsSb barrier thicknesses. Based on our findings, a structure with type-II QD/barrier interface with relatively long radiative recombination lifetime may be a viable candidate in designing IBSC.
Optics and photonics technologies are ubiquitous: they are responsible for the displays on smart phones and computing devices, optical fiber that carries the information in the internet, advanced precision manufacturing, enhanced defense capabilities, and a plethora of medical diagnostics tools. The opportunities arising from optics and photonics offer the potential for even greater societal impact in the next few decades, including solar power generation and new efficient lighting that could transform the nation's energy landscape and new optical capabilities that will be essential to support the continued exponential growth of the Internet. As described in the National Research Council report Optics and Photonics: Essential Technologies for our Nation, it is critical for the United States to take advantage of these emerging optical technologies for creating new industries and generating job growth. The report assesses the current state of optical science and engineering in the United States and abroad-including market trends, workforce needs, and the impact of photonics on the national economy. It identifies the technological opportunities that have arisen from recent advances in, and applications of, optical science and engineering. The report also calls for improved management of U.S. public and private research and development resources, emphasizing the need for public policy that encourages adoption of a portfolio approach to investing in the wide and diverse opportunities now available within photonics. Optics and Photonics: Essential Technologies for our Nation is a useful overview not only for policymakers, such as decision-makers at relevant Federal agencies on the current state of optics and photonics research and applications but also for individuals seeking a broad understanding of the fields of optics and photonics in many arenas.
Strained alloy InGaAs/GaAs multiple quantum well structures (MQWs), on GaAs substrates, are being investigated' for use in optical modulators, low-threshold diode lasers, photodetectors and other opto-electronic devices operating near 1 um. Attempts are being made to cover the 0.9-1.1 am spectral range by varying well-widths and/or alloy mole-fraction and by growing such structures on superlattice or alloy buffer layers. Special problems, however, are posed in growing these strained ternary alloy quantum wells with high quality by epitaxial techniques. Alloy concentration is difficult to reproduce, and alloy-disorder introduces an additional line-broadening contribution and non-uniformity into the materials. The critical thickness parameter places an upper limit on the indium mole-fraction (and thereby the strain) for growth directly on GaAs, restricting the flexibility in varying the well width for increased spectral coverage. Typically, this mole-fraction must be less than 0.2, and the lattice-parameter mismatch below 2%, for well-widths -10 nm. Here, we focus on structures in which each quantum well consists of an ordered InAs/GaAs short-period superlattice as an attractive alternative to the random InGaAs alloy structures. These all-binary MQWs are highly-strained (7% lattice parameter mismatch) and can accommodate high average indium mole-fraction (30-40%) in wide wells (10-20 nm) without evidence of strain relaxation due to misfit dislocation formation.
The aim of the contributions in this volume is to give a current overview on the basic properties and applications of semiconductor and nonlinear optical materials for optoelectronics and integrated optics. They provide a cross-linkage between different materials (III-V, II-VI, Si-Ge, glasses, etc.), various sample dimensions (from bulk crystals to quantum dots), and a range of techniques for growth (LPE to MOMBE) and for processing (from surface passivation to ion beams). Major growth techniques and materials are discussed, including the sophisticated technologies required to exploit the exciting properties of low dimensional semiconductors. These proceedings will prove an invaluable guide to the current state of optoelectronic and nonlinear optical materials development, as well as indicating trends and also future markets for optoelectronic devices.
Semiconductor devices based on lattice mismatched heterostructures have been the subject of much study. This volume focuses on the physics, technology and applications of strained layer quantum wells and superlattices, featuring chapters on aspects ranging from theoretical modeling of quantum-well lasers to materials characterization and assessment by the most prominent researchers in the field. It is an essential reference for both researchers and students of semiconductor lasers, sensors and communications.
This book contains detailed descriptions and associated discussions regarding different generation, detection and signal processing techniques for the electrical and optical signals within the THz frequency spectrum (0.3–10 THz). It includes detailed reviews of some recently developed electronic and photonic devices for generating and detecting THz waves, potential materials for implementing THz passive circuits, some newly developed systems and methods associated with THz wireless communication, THz antennas and some cutting-edge techniques associated with the THz signal and image processing. The book especially focuses on the recent advancements and several research issues related to THz sources, detectors and THz signal and image processing techniques; it also discusses theoretical, experimental, established and validated empirical works on these topics. The book caters to a very wide range of readers from basic science to technological experts as well as students.
Gallium Arsenide and Related Compounds 1993 covers III-V compounds from crystal growth of materials to their device applications. Focusing on the fields of optical communications and satellite broadcasting, the book describes the practical applications for GaAs and III-V compounds in devices and circuits, both conventional and those based on quantum effects. It also discusses ultrafast GaAs transistors and integrated circuits, novel laser diodes, and tunneling devices, and considers the direction for future technologies. In addition, this volume addresses the increasing demands of ultra high speed systems that require careful selection of III-V materials to optimize the performance of electronic and optoelectronic components. It is ideal reading for physicists, materials scientists, electrical, and electronics engineers investigating III-V compound materials, properties, and devices.