Electronic band structure and optical properties of zinc monochalcogenides with zinc-blende- and wurtzite-type structures were studied using the ab initio density functional method within the local-density approximation (LDA), generalized-gradient approximation, and $\mathrm{LDA}+U$ approaches. Calculations of the optical spectra have been performed for the energy range $0–20\mathrm{eV}$, with and without including spin-orbit coupling. Reflectivity, absorption and extinction coefficients, and refractive index have been computed from the imaginary part of the dielectric function using the Kramers-Kronig transformations. A rigid shift of the calculated optical spectra is found to provide a good first approximation to reproduce experimental observations for almost all the zinc monochalcogenide phases considered. By inspection of the calculated and experimentally determined band-gap values for the zinc monochalcogenide series, the band gap of ZnO with zinc-blende structure has been estimated.

Structural phase stability, electronic structure, optical properties, and high-pressure behavior of polytypes of ${\mathrm{In}}_2{\mathrm{O}}_3$ in three space group symmetries $I{2}_{1}3$, $Ia\overline{3}$, and $R\overline{3}$ are studied by first-principles density-functional calculations. From structural optimization based on total energy calculations, lattice and positional parameters have been established, which are found to be in good agreement with the corresponding experimental data except for $I{2}_{1}3$, where the symmetry analysis for optimized structure indicates that it arrived at the $Ia\overline{3}$ phase. ${\mathrm{In}}_2{\mathrm{O}}_3$ of space group symmetry $Ia\overline{3}$ is found to undergo a pressure-induced phase transition to the $R\overline{3}$ phase at pressures around $3.8\mathrm{GPa}$. From the analysis of band structure coming out from the calculations within the local density and generalized gradient approximations, it is found that ${\mathrm{In}}_2{\mathrm{O}}_3$ of space group symmetry $I{2}_{1}3$ and $R\overline{3}$ are indirect band gap semiconductors, while the other phase of space group $Ia\overline{3}$ is having direct band gap. The calculated carrier effective masses for all these three phases are compared with available experimental and theoretical values. From charge-density and electron localization function analysis, it is found that these phases have dominant ionic bonding with noticeable covalent interaction between indium and oxygen. The magnitudes of the absorption and reflection coefficients for ${\mathrm{In}}_2{\mathrm{O}}_3$ with space groups $Ia\overline{3}$ and $R\overline{3}$ are small in the energy range $0–5\mathrm{eV}$, indicating that these phases can be regarded and classified as transparent.

In this work we report on photochromism in transparent thin film samples of oxygen-containing yttrium hydride. Exposure to visible and ultraviolet (UV) light at moderate intensity triggers a decrease in the optical transmission of visible and infrared (IR) light. The photo-darkening is colour-neutral. We show that the optical transmission of samples of 500 nm thickness can be reduced by up to 50% after one hour of illumination with light of moderate intensity. The reaction is reversible and samples that are left in the dark return to the initial transparent state. The relaxation time in the dark depends on the temperature of the sample and the duration of the light exposure. The photochromic reaction takes place under ambient conditions in the as-deposited state of the thin-film samples.Comment: Accepted for publication in Solar Energy Materials and Solar Cell

Electronic structure and band characteristics for zinc monochalcogenides with zinc-blende- and wurtzite-type structures are studied by first-principles density-functional-theory calculations with different approximations. It is shown that the local-density approximation underestimates the band gap and energy splitting between the states at the top of the valence band, misplaces the energy levels of the Zn-3d states, and overestimates the crystal-field-splitting energy. The spin-orbit-coupling energy is found to be overestimated for both variants of ZnO, underestimated for ZnS with wurtzite-type structure, and more or less correct for ZnSe and ZnTe with zinc-blende-type structure. The order of the states at the top of the valence band is found to be anomalous for both variants of ZnO, but is normal for the other zinc monochalcogenides considered. It is shown that the Zn-3d electrons and their interference with the O-2p electrons are responsible for the anomalous order. The effective masses of the electrons at the conduction-band minimum and of the holes at the valence-band maximum have been calculated and show that the holes are much heavier than the conduction-band electrons in agreement with experimental findings. The calculations, moreover, indicate that the effective masses of the holes are much more anisotropic than the electrons. The typical errors in the calculated band gaps and related parameters for ZnO originate from strong Coulomb correlations, which are found to be highly significant for this compound. The local-density approximation with multiorbital mean-field Hubbard potential approach is found to correct the strong correlation of the Zn-3d electrons, and thus to improve the agreement between the experimentally established location of the Zn-3d levels and that derived from pure LDA calculations.

Electronic band-structure calculations are performed for zinc-blende III–V (AlP, AlAs, AlSb, GaP, GaAs, GaP, InP, InAs, and InSb) and II–VI (ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe) semiconductors using an ab initio pseudopotential method within a local-density approximation (LDA). Lattice parameters, band gaps, Luttinger parameters, momentum matrix elements and effective masses are studied in detail. It is shown that LDA calculations cannot systematically give accurate band parameters. It is found that LDA band parameters calculated using experimentally determined lattice constants are more accurate than those using LDA lattice constants. We found that inclusion of the d electrons of Group-II atoms in the core gives more accurate band parameters.