Electronic properties of topological semimetals, and wide-gap oxide semiconductors using hybrid density functional theory
Date
2022
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Publisher
University of Delaware
Abstract
Computational materials science is at the forefront of discovering new materials and predicting novel material properties with much improved efficiency. In this work we used first principles methods based on density functional theory to explore a wide range of material properties with an objective to develop and optimize materials properties for a range of applications. We studied the electronic structure, carrier densities and band alignments in rare-earth monopnictides (RE-V), effects of hydrostatic pressure and epitaxial strain on their band structures, the formation of 2D hole gas at the interface between LuSb and GaSb, defect-controlled Fermi-level tuning in half-Heusler topological semimetals (LuPtSb, LuPtBi), and electronic properties of corundum-structured Ir2O3, Ga2O3 and their alloys for high power electronic devices and heterojunctions. ☐ The electronic and magnetic properties of rare-earth monopnictide (RE-V) have long been studied both experimentally and by theory, but there have been notable contradictions in the experimental characterization of the electronic properties of these materials. Previous theoretical work was able to clarify only a few specific properties of some RE-V compounds, yet general agreement with experiments for the complete series was not satisfactory. Motivated by this, we focused on RE-V compounds, with RE=La, Gd, Er, and Lu, and V=As, Sb, and Bi, and analyzed the effects of spin-orbit coupling and treating the RE 4ƒ electrons as valence electrons. Our calculations predict that all the RE-V compounds are semimetals with electron pocket at X point and hole pocket at Γ point. The predicted carrier densities are in good agreement with available experimental data. Based on our understanding of rare-earth monopnictide (RE-V), we explored the effects of hydrostatic pressure on the electronic properties of LaAs. We find that in DFT-GGA, under the calculated equilibrium lattice parameter, LaAs displays a crossing between the highest As pband and the lowest La d band near the X point due to the overestimated p-d band overlap. Such crossing does not occur when the band overlap is corrected in the HSE06 hybrid functional calculation, in agreement with experiments. However, we find that the p-d crossing can be induced in LaAs under hydrostatic pressure, showing a topological phase transition at ∼7 GPa. The rocksalt crystal structure of LaAs is predicted to be stable under applied pressure up to 20 GPa, in good agreement with experimental observations. We also showed that non-trivial topological phase can be introduced in LaSb under the effect of epitaxial strain. We show that under compressive epitaxial strain, the La d band crosses the Sb p band near the Z point in the Brillouin zone, stabilizing a topologically nontrivial phase, opening unique opportunities to probe epitaxially strained thin films. ☐ In a joint project with an experimental group at the University of California Santa Barbara, we explored emergent phenomena in rare-earth monopnictide (RE-V) thin films via quantum confinement. We show that quantum confinement lifts carrier compensation and differentially affects the carrier density of the electron and hole-like carriers resulting in a strong modification in its large, non-saturating magnetoresistance behavior. We predicted a 2D interfacial hole gas due to the bonding mismatch at hetero-epitaxial interface of the semi-metal (LuSb) and a semiconductor (GaSb) which is accompanied by a charge transfer across the interface creating opportunity to tune magnetoresistance and engineer hetero-epitaxial interface. ☐ Motivated from the recent experimental work on half-Heusler topological semimetals, we investigate how point defects impact the Fermi level position in two representative half-Heusler topological semimetals, PtLuSb and PtLuBi; we explore how intrinsic defects can be used to tune the Fermi level, and explain recent observations based on Hall measurements in bulk and thin films. Under typical growth conditions we show that Pt vacancies are the most abundant intrinsic defects, leading to excess hole densities that place the Fermi level significantly below the expected position in the pristine material. Suggestions for tuning the Fermi level by tuning chemical potentials are discussed. ☐ Finally, we worked on corundum phase of Ir2 O3, Ga2O3 and their alloys. α-Ga2O3 is ultra-wide band gap semiconductor that can be easily doped n-type, but not p-type. Finding a lattice matched p-type material is highly desirable for device applications. In this context, we studied Ir2O3 and its alloys with α-Ga2O3.The stability and electronic structure of α-(IrxGa1-x)2O3 alloys are studied along with variations of band edge positions with Ir/Ga concentrations. Our results indicate that Ir2O3 can be made p-type, and the predicted band alignment at the Ir2O3/Ga2O3 interface is in good agreement with experimental data, opening up opportunities for p-type Ir2O3-based heterojunctions for high power electronic devices.
Description
Keywords
Rare-earth monopnictide, Effects of hydrostatic pressure, Fermi-level tuning