Electronic structure and optical properties of ZnX (X=O, S, Se, Te)

Abstract: 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 LDA, GGA, and LDA+U approaches. Calculations of the optical spectra have been performed for the energy range 0-20 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.

Insights into the Electronic Structure and Optical Properties of Zinc Monochalcogenides

Zinc monochalcogenides, denoted as Zn$X$ ($X$ = O, S, Se, Te), are prototype II-VI semiconductors, each exhibiting zinc-blende and wurtzite crystalline structures. These compounds are foundational to optical device technology, serving critical roles in applications such as visual displays and solar cells. The paper by Karazhanov et al. presents an in-depth study of the electronic band structure and optical properties of these zinc monochalcogenides using density functional theory (DFT) approximations within LDA (local-density approximation), GGA (generalized-gradient approximation), and LDA+$U$.

Electronic Structure Analysis

The authors provide a comprehensive examination of the band dispersion characteristics of zinc monochalcogenides. They illustrate that the conduction band (CB) minima are significantly more dispersive compared to the valence band (VB) maxima across both zinc-blende and wurtzite phases. Such dispersion profiles indicate heavier hole carriers than electron carriers, potentially influencing electron mobility and conductivity behavior—key considerations for semiconductor applications.

Spin-orbit (SO) coupling effects are particularly noted for their critical role in altering band dispersion and chemical bonding attributes. A systematic study reveals appreciable Coulomb correlation effects in ZnO phases, whereas in ZnS, ZnSe, and ZnTe, such effects are discernible primarily at energies exceeding 10 eV. The authors also address the well-known band gap underestimation problem inherent to DFT-LDA/GGA, estimating the band gap of the ZnO-z phase through extrapolation.

Optical Properties Observations

Optical properties, influenced by electronic structure, were calculated over an energy range from 0 to 20 eV. The authors explore reflectivity, absorption, extinction coefficients, and refractive indices derived from dielectric functions using Kramers-Kronig transformations. They emphasize the need for a rigid shift to reconcile theoretical predictions with experimental observations, particularly concerning optical spectra—a crucial correction for semiconductors with strong Coulomb correlation effects.

Overall, qualitative agreement between calculated and experimental optical spectra is achieved, especially for low-energy peak locations. However, discrepancies remain concerning intensity predictions for higher-energy peaks, possibly attributed to neglected considerations such as exciton effects and local-field influences.

Implications and Future Directions

This study extends implications for both theoretical understanding and practical exploitation of zinc monochalcogenides for optoelectronic devices. The findings could inform refinements in computational methods (e.g., incorporating electron-hole interactions) and guide experimental work targeting unexplored Zn$X$ phases, such as ZnSe-w and ZnTe-w. Future developments could include employing approaches that systematically account for many-body perturbations, thereby enhancing predictive accuracy for strong correlated systems.

The work highlights the importance of accurate electronic and optical property calculations in optimizing semiconductor materials for technological applications. As zinc monochalcogenides further integrate into next-generation optoelectronic devices, studies like these furnish essential insights into material design and function.