Thesis (Ph.D.)--University of Delaware, 2008. / Principal faculty advisors: Valeria Gabriela Stoleru, Dept. of Materials Science & Engineering; and S. Ismat Shah, Dept. of Materials Science. Includes bibliographical references.

The author explores absorptive and refractive optical nonlinearities at 1.06 [mu]m in bulk, semi-insulating, undoped GaAs with a particular emphasis on the influence of the native deep-level defect known as EL2. Picosecond pump-probe experimental technique is used to study the speed, magnitude, and origin of the absorptive and refractive optical nonlinearities and to characterize the dynamics of the optical excitation of EL2 in three distinctly different undoped, semi-insulating GaAs samples. Intense optical excitation of these materials leads to the redistribution of charge among the EL2 states resulting in an absorptive nonlinearity due to different cross sections for electron and hole generation through this level. This absorptive nonlinearity is used in conjunction with the linear optical properties of the material and independent information regarding the EL2 concentration to extract the cross section ratio [sigma][sub p]/[sigma][sub e] [approx equal]0.8, where [sigma][sub p](e) is the absorption cross section for hole (electron) generation from EL2[sup +] (EL2[sup 0]). The picosecond pump-probe technique can be used to determine that EL2/EL2[sup +]density ratio in an arbitrary undoped, semi-insulating GaAs sample. The author describes the use of complementary picosecond pump-probe techniques that are designed to isolate and quantify cumulative and instantaneous absorptive and refractive nonlinear processes. Numerical simulations of the measurements are achieved by solving Maxwell equations with the material equations in a self-consistent manner. The numerical analysis together with the experimental data allows extraction of a set of macroscopic nonlinear optical parameters in undoped GaAs. The nonlinearities in this material have been used to construct three proof-of-principle nonlinear optical devices for use at 1.06 [mu]m: (1) a weak beam amplifier, (2) a polarization rotation optical switch, and (3) optical limiters.

International Telemetering Conference Proceedings / October 27-30, 1997 / Riviera Hotel and Convention Center, Las Vegas, Nevada / A 10- or 20-Watt, L- or S-band transmitter commonly consumes the majority of the available DC power on a telemetry pack -- often more than all the remaining components combined. A new family of transistors allows a substantial increase in DC to RF efficiency without the use of complex and costly switching regulators. With ever increasing data rates requiring more RF bandwidth (and correspondingly lower receiver sensitivities), transmitters using these transistors offer twice the RF power at little or no increase in DC current. Alternately, in other situations such as observation balloons, the same RF power can be achieved with approximately 40% less current resulting in significantly longer mission life. This paper describes the method for achieving higher efficiency transmitters using new LDMOSFETs.

The development of metal-oxide-semiconductor field effect transistor (MOSFET) technology relies on new channel materials with higher carrier mobilities that allow faster switching but at lower voltages. III-V semiconductors are suitable for channel materials in n-type MOSFETs due to their higher electron mobility. However, the interface between the gate dielectric and the III-V surface shows defects that detriment the electrical performance of the transistor. These defects are attributed to interfacial oxides that create energy states in the band gap. Therefore, III-V oxides must be removed and the surface must be protected from reoxidation for the deposition of other functional layers. In this work, oxide etching and passivation of III-V semiconductors were studied to understand the oxide etching mechanism and to develop passivation techniques that allow the integration of these materials in device manufacturing. The etching of GaAs(100) was studied using aqueous HCl and H₂O₂ mixtures with and without the addition of alpha-hydroxy acids. Oxide etching depends on the strength of the acid. Without the addition H₂O₂, acetic, glycolic, tartaric and hydrochloric acids (pKₐ lower than 5) are able to remove oxides. Upon the addition of H₂O₂, only the stronger acids (glycolic, tartaric and hydrochloric) with a pKₐ lower than 4 are able to compete with H₂O₂ and etch the oxides. Oxide removal leaves an As-rich surface, and in the case of HCl, etching leaves a surface terminated with As-Cl species. As-As dimers are formed when oxides are etched with HCl and organic acids. After oxide removal with HF or HCl, the fresh GaAs and InP surfaces were passivated with a series of alkanethiols (C(n)H(2n+1)SH) to assess their effectiveness in protecting the substrate from reoxidation. Longer C chains provided increased protectiong due to their increased chain-chain interactions that allow them to form a denser and well-ordered monolayer. The surface is chemically passivated through S-X (where X = As, Ga for GaAs, and In for InP) bonding between the alkanethiolate layer and the surface. A layer formed by 1-eicosanethiol protected GaAs for 30 min, but prevented reoxidation of InP for at least 5 hours. Since the thickness of the alkanethiol layer is the same, the difference in protection is a result of the density of the layer and S bonding with the substrate.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / toc / Physics / Master / Master of Philosophy

The concentration of tin and palladium in catalysts used in electroless copper plating have been determined by Rutherford backscattering spectrometry with high energy (2-5) MeV ⁴He⁺. The tin:palladium ratio in the catalyst decreases when exposed to an alkaline solution. X-ray photoelectron spectroscopy has confirmed this result and has shown the palladium in the catalyst is present as palladium metal and the tin is present, probably as an oxidized species, to a depth of about 30 Å. Catalysts for the electroless plating of copper are obtained by the reaction of Pd(II) and Sn(II). The extent of the reaction and the concentrations of the reaction products depend on the solution conditions. Conflicting results obtained in previous investigations of tin-palladium catalysts can be explained on this basis. Single crystals of gallium arsenide (GaAs(100)) were found to dissolve in synthetic lung fluid (Gamble solution). The concentrations of arsenic and gallium in the Gamble solution as well as the arsenic:gallium ratio on the GaAs surface increased continuously as the time of exposure to the Gamble solution increased. X-ray photoelectron spectroscopic studies of the gallium arsenide surface showed that arsenic migrated to the surface and it was oxidized to a species resembling As₂O₃ and finally solubilized by the Gamble solution. The solubility of gallium was governed primarily by the formation of stable complexes with the citrate and phosphate ions in the Gamble solution. Zinc that was present in the single crystals of gallium arsenide also migrated to the surface.

The high balance-of-system costs of photovoltaic installations indicate that reductions in absorber cost alone are likely insufficient for photovoltaic electricity to reach grid parity unless energy conversion efficiency is also increased. Technologies which both yield high-efficiency cells (>25%) and maintain low costs are needed. GaAs and related III-V semiconductors are used in the highest-efficiency single- and multi-junction photovoltaics, but the technology is too expensive for non-concentrated terrestrial applications. This is due in part to the limited scalability of traditional syntheses, which rely on expensive reactors and employ toxic and pyrophoric gas-phase precursors such as arsine and trimethyl gallium. This work describes GaAs films made by close-spaced vapor transport, a potentially scalable technique which is carried out at atmospheric pressure and requires only bulk GaAs, water vapor, and a temperature gradient to deposit crystalline films with similar electronic properties to GaAs prepared using traditional syntheses. Although close-spaced vapor transport of GaAs was first developed in 1963, there were few examples of GaAs photovoltaic devices made using this method in the literature at the onset of this project. Furthermore, it was unclear whether close-spaced vapor transport could produce GaAs films appropriate for use in photovoltaics. The goal of this project was to create and study GaAs devices made using close-spaced vapor transport and determine whether the technique could be used for production of grid-connected GaAs photovoltaics. In Chapter I the design of the vapor transport reactor, the chemistry of crystal growth, and optoelectronic characterization techniques are discussed. Chapter II focuses on compositional measurements, doping, and improved electronic quality in CSVT GaAs. Chapter III describes several aspects of the interplay between structure and electronic properties of photoelectrochemical devices. Chapter IV addresses heteroepitaxial growth of GaAs on "virtual" Ge-on-Si substrates. This is a topic of importance for the broader III-V community as well as the photovoltaic community, as Si is the substrate of choice in many areas of industry. This dissertation includes unpublished and previously published co-authored material.

Thesis (Ph.D.)--Chinese University of Hong Kong ,2001. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

Hui Yat Wai. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1993. / Includes bibliographical references (leaves 106-108). / Acknowledgments / Abstract / Chapter 1. --- Introduction --- p.1 / Chapter 2. --- Theory / Chapter 2.1 --- Optical Waveguide --- p.4 / Chapter 2.1.1 --- Optical Waveguide Classification / Chapter 2.1.2 --- Theoretical Analysis of 2-dimensional Step Index Waveguides / Chapter 2.2 --- Optical Waveguides Measurement --- p.18 / Chapter 2.2.1 --- Refractive Index Measurement / Chapter 2.2.2 --- Loss Measurement / Chapter 2.3 --- Ion Implantation and Annealing --- p.36 / Chapter 2.4 --- Refractive Index Change --- p.40 / Chapter 3. --- Equipments and Their Experimental Setup / Chapter 3.1 --- Light Source-Laser Diode --- p.42 / Chapter 3.2 --- Ellipsometry Measurement System --- p.45 / Chapter 3.2.1 --- Ellipsometry Measurement System and its Existing Problems / Chapter 3.2.2 --- Improvement of the Original System / Chapter 3.2.3 --- System Calibration / Chapter 3.3 --- Reflectance Measurement System --- p.51 / Chapter 3.3.1 --- System Design and Setup / Chapter 3.3.2 --- System Calibration / Chapter 3.4 --- End-Coupling Measurement System --- p.56 / Chapter 3.4.1 --- System Setup / Chapter 3.4.2 --- System Calibration / Chapter 4. --- Experiment / Chapter 4.1 --- Samples Preparation --- p.77 / Chapter 4.2 --- Refractive Index Measurement by Ellipsometer --- p.80 / Chapter 4.3 --- Refractive Index Measurement by Reflectance --- p.84 / Chapter 4.4 --- Waveguide Measurement --- p.88 / Chapter 4.4.1 --- Fiber-Waveguide Coupling / Chapter 4.4.2 --- Lens-Waveguide Coupling / Chapter 5. --- Results and Discussion / Chapter 5.1 --- Refractive Index Change and Waveguide Formation --- p.94 / Chapter 5.2 --- Mechanism of Refractive Index Change --- p.100 / Chapter 6. --- Conclusion --- p.103 / Chapter 7. --- Improvement and Extension --- p.105 / Reference --- p.106 / Appendices / Chapter A. --- Thick.m --- p.VI / Chapter B. --- Distrib.m --- p.IX