High-quality indium phosphide on gallium arsenide

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.Includes bibliographical references (leaves 173-181).In addition to traditional telecommunication applications, devices based on InP have received increased attention for high-performance electronics. InP growth on GaAs is motivated by the fact that InP wafers are smaller, more expensive, and utilize older fabrication equipment than GaAs. High-quality InP on GaAs may also serve as a step towards bringing high-quality InP onto the Si platform. Integrating high-quality InP onto bulk GaAs has proven to be challenging, however. While a number of commercial Molecular Beam Epitaxy (MBE) growth foundries offer InP on GaAs for M-HEMT (Metamorphic High-Electron-Mobility Transistor) applications, the successful demonstration of InP-based, minority-carrier devices on bulk GaAs remains elusive. In this work InP on GaAs suitable for minority carrier devices is demonstrated exhibiting a threading dislocation density of 1.2x1 06/cm2 determined by plan-view transmission electron microscopy. To further quantify the quality of this InP on GaAs, a photoluminescence (PL) structure was grown to compare the quality to bulk InP. Comparable room and low (20K) temperature PL was attained. (The intensity from the PL structure grown on the InP on GaAs was -70% of that on bulk InP at both temperatures.)(cont.) To achieve this, graded buffers in the InGaAs, InGaP, InAlAs and InGaAlAs materials systems were explored. In each of these systems, under certain growth conditions, microscopic compositional inhomogeneities along the growth direction blocked dislocations leading to dislocation densities sometimes > 109/cm2. Using scanning-transmission electron microscopy, composition variations were observed. These composition variations are caused by surface-driven phase separation leading to Ga-rich regions. As the phase separation blocked dislocation glide and led to high threading dislocation densities, conditions for avoiding phase separation were explored and identified. Composition variations could be prevented in InxGal-,As graded buffers grown at 725 °C to yield low dislocation densities of 9x105/cm2 for x {1 10}. This secondary-slip system has a Burgers vector typical in semiconductors of a/2. Unlike the primary-slip system, where dislocations glide on { 111 }-type planes, the secondary-slip system dislocations glide on { 110}-type planes. Relaxation via the secondary-slip system was found to be a function of stress and temperature.(cont.) A critical stress, ec, appears to be required for dislocations to glide via the secondary-slip system otherwise all relaxation occurs by the primary-slip system. For e > ec and at all temperatures studied, both the primary- and secondary-slip systems are active with apparent cross-slip from one system to the other. At low temperatures, nearly all of the relaxation was accomplished through the secondary-slip system, however. The amount of relaxation via the primary- and secondary-slip systems at three different temperatures was quantified; the resulting Arrhenius plot suggests a difference in the activation energy for glide between the two systems is 1.5 eV.by Nathaniel Joseph Quitoriano.Ph.D

Aqueous-Based Binary Sulfide Nanoparticle Inks for Cu2ZnSnS4 Thin Films Stabilized with Tin(IV) Chalcogenide Complexes

Cu2ZnSnS4 (CZTS) is a promising semiconductor material for photovoltaic applications, with excellent optical and electronic properties while boasting a nontoxic, inexpensive, and abundant elemental composition. Previous high-quality CZTS thin films often required either vacuum-based deposition processes or the use of organic ligands/solvents for ink formulation, which are associated with various issues regarding performance or economic feasibility. To address these issues, an alternative method for depositing CZTS thin films using an aqueous-based nanoparticle suspension is demonstrated in this work. Nanoparticles of constituent binary sulfides (CuxS and ZnS) are stabilized in an ink using tin(IV)-based, metal chalcogenide complexes such as [Sn2S6]4−. This research paper provides a systematic study of the nanoparticle synthesis and ink formulation via the enabling role of the tin chalcogenide complexing power, the deposition of high-quality CZTS thin films via spin coating and annealing under sulfur vapor atmosphere, their structural characterization in terms of nanocrystal phase, morphology, microstructure, and densification, and their resultant optoelectronic properties

Interpreting Kelvin probe force microscopy under an applied electric field: local electronic behavior of vapor–liquid–solid Si nanowires

Kelvin probe force microscopy (KPFM) is used to characterize the electrical characteristics of vapor-liquid-solid (VLS) Si nanowires (NWs) that are grown in-place between two predefined electrodes. KPFM measurements are performed under an applied bias. Besides contact potential differences due to differing materials, the two other primary contributions to measured variations on Si NWs between electrodes are: trapped charges at interfaces, and the parallel and serial capacitance, which are accounted for with voltage normalization and oxide normalization. These two normalization processes alongside finite-element-method simulations are necessary to characterize the bias-dependent response of Si NWs. After applying both normalization methods on open-circuit NWs, which results in a baseline of zero, we conclude that we have accounted for all the major contributions to CPDs and we can isolate effects due to applied bias such as impurity states and charged carrier flow, as well as find open connections when NWs are connected in parallel. These characterization and normalization methods can also be used to determine that the specific contact resistance of electrodes to the NWs is on the order of μΩ cm². Thus, the VLS growth method between predefined electrodes overcomes the challenge of making low-resistance contacts to nanoscale systems. Thereby, the experiments and analysis presented outline a systematic method for characterizing nanowires in parallel arrays under device operation conditions

Ideal, constant-loss nanophotonic mode converter using a Lagrangian approach

Coupling light between an optical fiber and a silicon nanophotonic waveguide is a challenge facing the field of silicon photonics to which various mode converters have been proposed. Inverted tapers stand out as a practical solution enabling efficient and broadband mode conversion. Current design approaches often use linearly-shaped tapers and two dimensional approximations; however, these approaches have not been rigorously verified and there is not an overarching design framework to guide the design process. Here, using a Lagrangian formulation, we propose an original, constant-loss framework for designing shape-controlled photonic devices and apply this formalism to derive an ideal constant-loss taper (CLT). We specifically report on the experimental demonstration of a fabrication-tolerant, 15-\ub5m-long CLT coupler, that produces 0.56 dB fiber-chip coupling efficiency, the highest efficiency-per-length ratio ever reported.Peer reviewed: YesNRC publication: Ye