Compositional dependence of Schottky barrier heights for Au on chemically etched In_(x)Ga_(1-x)P surfaces

Measurements of the Au Schottky barrier height were carried out on thin films of n‐In_(x)Ga_(1−x)P, of various compositions epitaxially grown on n‐GaAs substrates. Conventional C–V, I–V, and photo response techniques were used. The junction was formed by evaporating Au in an ion‐pumped vacuum system onto a In_(x)Ga_(1−x)P surface which had been chemically etched (5H_(2)SO_(4):1H_(2)O_(2):1H_(2)O at 40 °C for 90 s). Barrier heights determined from the I–V and photoresponse were found to be in good agreement while the C–V measurement encountered difficulties. The Au barrier, ϕ_p, to p‐In_(x)Ga_(1−x)P was found to be independent of composition. The barrier, ϕ_p, was determined by the relation ϕ_(p) + ϕ_(n)=ϕ_(g) where ϕ_(g) is the bandgap energy and ϕn is the measured barrier to n‐In_(x)Ga_(1−x)P. It has been observed that the Au barrier height to p‐type material for most compound semiconductors is determined by the anion, thus p‐InP and p‐GaP have the same Au barrier, about 0.76 eV. This dependence on the anion of the compound has now been seen to extend to the alloy system In_(x)Ga_(1−x)P measured here. While chemically etched specimens yielded diodes with reproducible barrier heights, diodes formed on surfaces which were untreated or cleaned only with organic solvents were of poor quality with varying barrier heights or even ohmic contacts

Investigating the influence of sea ice cover on benthic community and trophic structure

ThesisPublisher PD

USING CONCEPT MAPS TO ASSESS SCIENCE KNOWLEDGE OF PRE-SERVICE ELEMENTARY METHODS STUDENTS

The purpose of this study was observing the initial state of the pre-service elementary students’ content knowledge in science. Concept maps are designed to demonstrate the prior knowledge students have acquired around a variety of topics (Ruiz-Primo & Shavelson, 1996) as well as providing a valid assessment tool that is respectful of nontraditional learners who may have previously avoided the topics being assessed due to lack of understanding (Hough et al. 2007). Pre-service students were given a blank sheet of paper and asked to make a concept map with science as the central node. The results indicate that elementary pre-service students have less breadth and less organized science content knowledge than their secondary counterparts

USING CONCEPT MAPS TO ASSESS SCIENCE KNOWLEDGE OF PRE-SERVICE ELEMENTARY METHODS STUDENTS

The purpose of this study was observing the initial state of the pre-service elementary students’ content knowledge in science. Concept maps are designed to demonstrate the prior knowledge students have acquired around a variety of topics (Ruiz-Primo & Shavelson, 1996) as well as providing a valid assessment tool that is respectful of nontraditional learners who may have previously avoided the topics being assessed due to lack of understanding (Hough et al. 2007). Pre-service students were given a blank sheet of paper and asked to make a concept map with science as the central node. The results indicate that elementary pre-service students have less breadth and less organized science content knowledge than their secondary counterparts

Nanoscale Voltage Enhancement at Cathode Interfaces in Li-ion Batteries

Interfaces are ubiquitous in Li-ion battery electrodes, occurring across compositional gradients, regions of multiphase intergrowths, and between electrodes and solid electrolyte interphases or protective coatings. However, the impact of these interfaces on Li energetics remains largely unknown. In this work, we calculated Li intercalation-site energetics across cathode interfaces and demonstrated the physics governing these energetics on both sides of the interface. We studied the olivine/olivine-structured LixFePO4/LixMPO4 (x=0 and 1, M=Co, Ti, Mn) and layered/layered-structured LiNiO2/TiO2 interfaces to explore different material structures and transition metal elements. We found that across an interface from a high- to low-voltage material the Li voltage remains constant in the high-voltage material and decays approximately linearly in the low-voltage region, approaching the Li voltage of the low-voltage material. This effect ranges from 0.5-9nm depending on the interfacial dipole screening. This effect provides a mechanism for a high-voltage material at an interface to significantly enhance the Li intercalation voltage in a low-voltage material over nanometer scale. We showed that this voltage enhancement is governed by a combination of electron transfer (from low- to high-voltage regions), strain and interfacial dipole screening. We explored the implications of this voltage enhancement for a novel heterostructured-cathode design and redox pseudocapacitors

Application of selective epitaxy to fabrication of nanometer scale wire and dot structures

The selective growth of nanometer scale GaAs wire and dot structures using metalorganic vapor phase epitaxy is demonstrated. Spectrally resolved cathodoluminescence images as well as spectra from single dots and wires are presented. A blue shifting of the GaAs peak is observed as the size scale of the wires and dots decreases

HgTe/CdTe heterojunctions: A lattice-matched Schottky barrier structure

HgTe-CdTe lattice-matched heterojunctions were formed by the epitaxial growth of HgTe on CdTe substrates using a low-temperature metal organic chemical vapor deposition technique. These heterojunctions combine features of the Schottky barrier structure, due to the high carrier concentrations found in the semimetallic HgTe, with the structural perfection present in a lattice-matched heterojunction. The measured Schottky barrier height varied from 0.65 to 0.92 eV depending on the details of the heterojunction growth procedure used. This dependence may be due to the formation of an inversion layer in the CdTe at the interface. Presence of such an inversion layer suggests that the valence band discontinuity between HgTe and CdTe is small, in agreement with previous theoretical estimates

Transition state redox during dynamical processes in semiconductors and insulators

Activation barriers associated with ion diffusion and chemical reactions are vital to understand and predict a wide range of phenomena, such as material growth, ion transport, and catalysis. In the calculation of activation barriers for non-redox processes in semiconductors and insulators, it has been widely assumed that the charge state remains fixed to that of the initial electronic ground state throughout a dynamical process. In this work, we demonstrate that this assumption is generally inaccurate and that a rate-limiting transition state can have a different charge state from the initial ground state. This phenomenon can significantly lower the activation barrier of dynamical process that depends strongly on charge state, such as carbon vacancy diffusion in 4H-SiC. With inclusion of such transition state redox, the activation barrier varies continuously with Fermi level, in contrast to the step-line feature predicted by the traditional fixed-charge assumption. In this study, a straightforward approach to include the transition state redox effect is provided, the typical situations where the effect plays a significant role are identified, and the relevant electron dynamics are discussed