Heteroepitaxy of Group IV and Group III-V semiconductor alloys for photovoltaic applications

Photovoltaic energy conversion represents a economically viable technology for realizing collection of the largest energy resource known to the Earth -- the sun. Energy conversion efficiency is the most leveraging factor in the price of energy derived from this process. This thesis focuses on two routes for high efficiency, low cost devices: first, to use Group IV semiconductor alloy wire array bottom cells and epitaxially grown Group III-V compound semiconductor alloy top cells in a tandem configuration, and second, GaP growth on planar Si for heterojunction and tandem cell applications.

Metal catalyzed vapor-liquid-solid grown microwire arrays are an intriguing alternative for wafer-free Si and SiGe materials which can be removed as flexible membranes. Selected area Cu-catalyzed vapor-liquid solid growth of SiGe microwires is achieved using chlorosilane and chlorogermane precursors. The composition can be tuned up to 12% Ge with a simultaneous decrease in the growth rate from 7 to 1 μm/min^-1. Significant changes to the morphology were observed, including tapering and faceting on the sidewalls and along the lengths of the wires. Characterization of axial and radial cross sections with transmission electron microscopy revealed no evidence of defects at facet corners and edges, and the tapering is shown to be due to in-situ removal of catalyst material during growth. X-ray diffraction and transmission electron microscopy reveal a Ge-rich crystal at the tip of the wires, strongly suggesting that the Ge incorporation is limited by the crystallization rate.

Tandem Ga_1-xIn_xP/Si microwire array solar cells are a route towards a high efficiency, low cost, flexible, wafer-free solar technology. Realizing tandem Group III-V compound semiconductor/Si wire array devices requires optimization of materials growth and device performance. GaP and Ga_1-xIn_xP layers were grown heteroepitaxially with metalorganic chemical vapor deposition on Si microwire array substrates. The layer morphology and crystalline quality have been studied with scanning electron microscopy and transmission electron microscopy, and they provide a baseline for the growth and characterization of a full device stack. Ultimately, the complexity of the substrates and the prevalence of defects resulted in material without detectable photoluminescence, unsuitable for optoelectronic applications.

Coupled full-field optical and device physics simulations of a Ga_0.51In_0.49P/Si wire array tandem are used to predict device performance. A 500 nm thick, highly doped "buffer" layer between the bottom cell and tunnel junction is assumed to harbor a high density of lattice mismatch and heteroepitaxial defects. Under simulated AM1.5G illumination, the device structure explored in this work has a simulated efficiency of 23.84% with realistic top cell SRH lifetimes and surface recombination velocities. The relative insensitivity to surface recombination is likely due to optical generation further away from the free surfaces and interfaces of the device structure.

Finally, GaP has been grown free of antiphase domains on Si (112) oriented substrates using metalorganic chemical vapor deposition. Low temperature pulsed nucleation is followed by high temperature continuous growth, yielding smooth, specular thin films. Atomic force microscopy topography mapping showed very smooth surfaces (4-6 Å RMS roughness) with small depressions in the surface. Thin films (~ 50 nm) were pseudomorphic, as confirmed by high resolution x-ray diffraction reciprocal space mapping, and 200 nm thick films showed full relaxation. Transmission electron microscopy showed no evidence of antiphase domain formation, but there is a population of microtwin and stacking fault defects.

Theoretical model for a periodically driven semiconductor laser subject to optical feedback from a microcantilever

We propose a theoretical model for analyzing the dynamics of a periodically driven semiconductor laser subject to optical feedback from a microcantilever. We numerically investigate the temporal evolution of the light intensity of the semiconductor laser, and we show the interspikes of the light intensity. These interspikes of light intensity are also demonstrated in our experiment. The validity of the theoretical model is verified. The observed phenomenon has a potential application for resonant sensing. (C) 2008 Optical Society of America.

Instabilities in a grating feedback external cavity semiconductor laser

Output power fluctuations in a grating external cavity diode laser with Littman configuration are described, showing peculiar chaotic behaviors of self-pulsation at the L-I curve kink points. Different spectral characteristics with multiple peaks are observed at upper and lower state of the self-pulsation. It is found also that P-N junction voltage jumps in a same pace with the pulsation. The observed phenomena reflect competition between different longitudinal modes, and transient variation of transverse modes in addition. These experimental results may contain information about the mechanisms of the chaotic instability in strong filtered feedback semiconductor lasers. (C) 2008 Optical Society of America

A mathematical and experimental investigation of microcycle spectral estimates of semiconductor flicker noise

The experimental portion of this thesis tries to estimate the density of the power spectrum of very low frequency semiconductor noise, from 10^-6.3 cps to 1. cps with a greater accuracy than that achieved in previous similar attempts: it is concluded that the spectrum is 1/f^α with α approximately 1.3 over most of the frequency range, but appearing to have a value of about 1 in the lowest decade. The noise sources are, among others, the first stage circuits of a grounded input silicon epitaxial operational amplifier. This thesis also investigates a peculiar form of stationarity which seems to distinguish flicker noise from other semiconductor noise.

In order to decrease by an order of magnitude the pernicious effects of temperature drifts, semiconductor "aging", and possible mechanical failures associated with prolonged periods of data taking, 10 independent noise sources were time-multiplexed and their spectral estimates were subsequently averaged. If the sources have similar spectra, it is demonstrated that this reduces the necessary data-taking time by a factor of 10 for a given accuracy.

In view of the measured high temperature sensitivity of the noise sources, it was necessary to combine the passive attenuation of a special-material container with active control. The noise sources were placed in a copper-epoxy container of high heat capacity and medium heat conductivity, and that container was immersed in a temperature controlled circulating ethylene-glycol bath.

Other spectra of interest, estimated from data taken concurrently with the semiconductor noise data were the spectra of the bath's controlled temperature, the semiconductor surface temperature, and the power supply voltage amplitude fluctuations. A brief description of the equipment constructed to obtain the aforementioned data is included.

The analytical portion of this work is concerned with the following questions: what is the best final spectral density estimate given 10 statistically independent ones of varying quality and magnitude? How can the Blackman and Tukey algorithm which is used for spectral estimation in this work be improved upon? How can non-equidistant sampling reduce data processing cost? Should one try to remove common trands shared by supposedly statistically independent noise sources and, if so, what are the mathematical difficulties involved? What is a physically plausible mathematical model that can account for flicker noise and what are the mathematical implications on its statistical properties? Finally, the variance of the spectral estimate obtained through the Blackman/Tukey algorithm is analyzed in greater detail; the variance is shown to diverge for α ≥ 1 in an assumed power spectrum of k/|f|^α, unless the assumed spectrum is "truncated".

Comparison of Yb-doped fiber laser with one-end and double-end pumping configuration

A high-power Ytterbium-doped fiber laser (YDFL) with homemade double clad fiber (DCF) is introduced in this paper. The output power characteristics of a linear cavity fiber laser have been studied theoretically by solving the rate equations and experimentally tested with single- and double-end-pumping configurations. When both ends of the fiber are pumped by two high-power laser diodes with a launched power of similar to 300 W each, a maximum CW output of 444 W is obtained with a slope efficiency of similar to 75%. (c) 2006 Elsevier Ltd. All rights reserved.

Synthesis and characterizations of Al-doped Zn0.95Ni0.05O nanocrystals

Nanocrystalline Zn0.95-xNi0.05AlxO (x = 0.01, 0.02, 0.05 and 0.10) diluted magnetic semiconductors have been synthesized by an autocombustion method. X-ray absorption spectroscopy, high-resolution transmission electron microscopy, energy-dispersive spectrometry and Ni 2p core-level photoemission spectroscopy analyses revealed that some of the nickel ions were substituted for Zn2+ into the ZnO matrix while others gave birth to NiO nanoclusters embedded in the ZnO particles. The Zn0.95Ni0.05O sample showed no enhancement of room-temperature ferromagnetism after Al doping. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.