Polymorphism, band-structure, band-lineup, and alloy energetics of the group II oxides and sulfides MgO, ZnO, CdO, MgS, ZnS, CdS

Abstract

Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 5/10/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use Polymorphism, band-structure, band-lineup, and alloy energetics of the group II oxides and sulfides MgO, ZnO, CdO, MgS, ZnS, CdSStephan Lany* a a National Renewable Energy Laboratory, 15013 Denver West Blvd, Golden, CO, USA 80401 ABSTRACTThe group II chalcogenides are an important class of functional semiconductor materials exhibiting a remarkable diversity in terms of structure and properties. In order to aid the materials design, a consistent set of electronic structure calculations is presented, including data on the polymorphic energy ordering, the band-structures, the band-lineups relative to the vacuum level, surface energies, as well as on the alloy energetics. To this end, current state-of-the-art electronic structure tools are employed, which, besides standard density functional theory (DFT), include totalenergy calculation in the random phase approximation and GW quasiparticle energy calculations. The ionization potentials and electron affinities are obtained by combining the results of bulk GW and surface DFT calculations. Considering both octahedral and tetrahedral coordination symmetries, exemplified by the rock-salt and zinc-blende lattices, respectively, this data reveals both the chemical and structural trends within this materials family.

Metastable rocksalt ZnO is p -type dopable

“…Concerning the ionized impurity scattering in the Li-doped rocksalt ZnO, the shallow nature of Li Zn with the localized polaronic state appearing as a resonance deep in the valence band could be considered less detrimental to transport than the deep defects with states in the band gap [29]. In regard to the optical transparency, we calculate the band gap of rocksalt ZnO to be indirect and close to 3.1 eV, in good agreement with many-body GW calculations of S. Lany [21]. Hence, all direct interband transitions lay above the visible part of the solar spectrum.…”

“…The high-pressure rocksalt phase forms at about ∼9 GPa, and can be stabilized at ambient conditions in the nanocrystalline form [16][17][18] or as thin film grown on the MgO substrate with about 15 % of MgO alloyed [19,20]. This phase of ZnO is specifically relevant for our discussion because of its octahedral coordination of atoms, smaller volume (about 18 % reduction) relative to the ground state wurtzite structure and predicted higher valence band edge (∼ 1 eV closer to vacuum) than the wurtzite [21]. Furthermore, previous demonstration of activated p-type transport in rocksalt MgO with Li as an external dopant [22], supports the prospects of the rocksalt structure for alleviating self-compensation of Li impurities.…”

Negative-pressure polymorphs made by heterostructural alloying

“…The nonideal component of mixing enthalpy Δ H Ω ( x ) depends on the crystal structure of the alloy: Lower-density structures tend to have smaller alloy interaction parameters Ω owing to less competition for space between the atoms [that is, less steric hindrance ( 19 )]. In turn, the evolution of the enthalpy as a function of composition is different for each polymorph, with the lower-density structure showing a smaller bowing of the mixing enthalpy.…”

Zinc-Stabilized Manganese Telluride with Wurtzite Crystal Structure

“…As the Zn concentration in Mn 1−x Zn x Te increases the optical band gap decreases from ∼3.5 eV in MnTe to ∼2.5 eV in ZnTe, which correlates well with our experimental results (Figure 2 which is not commonly observed in semiconductors due to bandgap bowing caused by alloy interaction effects. 36 The small bowing also implies that the electronic band structure of the alloys can be understood as a linear combination of the end-points, in this case WZ-ZnTe and hypothetical Zn-free WZ-MnTe. The nearly linear change of the band gap in Mn 1-x Zn x Te alloys should make it easy to precisely tune this material for various optoelectronic applications.…”