Electronic properties of semiconductors and energy materials from density functional theory
Date
2019
Authors
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Publisher
University of Delaware
Abstract
Computational simulations are extensively used in materials physics and chemistry, to predict novel physical phenomena, understand experimental observations and engineer materials with optimized properties. In this work, using first principles computational methods based on density functional theory, we investigate a wide range of materials properties, with the ultimate goal of guiding materials characterization and design. We studied copper impurities in silicon, surfaces reconstructions and interfaces of Heusler semiconductors, topological properties of Heusler semimetals, and interfaces of chalcopyrite materials for photovoltaic properties. ☐ Motivated by recent experimental studies on Cu impurities in Si, where it was suggested that Cu would unexpectedly act as a shallow acceptor, we initially revisited the problem of Cu impurity in Si. Our calculations accurately predict formation energies and transitions levels associated with several configurations of the Cu-related defects in Si, solving markedly discrepancies between theoretical results and experimental data. ☐ Funded by the U.S. Department of Energy, in a project on emergent phenomena in Heusler materials and their heterostructure, we investigated surface reconstructions, band alignments, formation of two-dimensional electron and holes gases, and topological properties of Heusler compounds. These are a group of promising multi-functional materials, having several technological applications, including thermoelectricity and half-metallicity. From a combined theoretical and experimental approach (from our collaborators at the University of California Santa Barbara and University of Wisconsin Madison), we proposed an electron counting model which explains the observed surface reconstructions in half-Heusler semiconductors. We mapped the phase diagram for stability of various reconstructions, predicted surface atomic structures and proposed driving force behind surface reconstructions. We discussed the applicability and predictions of this model on other half-Heusler materials, paving the way towards engineering the surfaces for a desired property. Based on our understanding of half-Heusler surfaces, we also explored the interfaces between two half-Heusler semiconductors and predicted the formation of two dimensional electron or hole gases (2DEG or 2DHG) at the interface, without any chemical doping. We use CoTiSb / NiTiSn as an example and predict a number of combinations of half-Heusler semiconductors, based on band alignment, interface termination and lattice mismatch, where 2DEG or 2DHG systems form. Novel topological phases in Heusler materials have also been explored in this work. We have identified a family of Heusler compounds Co2MnX (X = Si, Ge or Sn) which show characteristics of magnetic topological Weyl semimetals. We show that these materials host topologically protected band crossings, which have important consequences on the observable transport properties. ☐ Finally, with support from a Summer internship at the Lawrence Livermore National Laboratory, we studied the impact of Cu-deficient layers on the performance of CIGS-based solar cells, motivated by several experimental studies on this topic. We explored the electronic structure, stability of ordered vacancy compounds (OVCs), and band alignment in a number of Cu- and Ag- based chalcopyrite systems. The consequence of formation of OVC layers on the performance of solar cells has been discussed based on device simulations.
Description
Keywords
Semiconductors, Density functional theory