Confinement effects in ultra-thin ZnO polymorph films: electronic and optical properties

TL;DR

This study uses first-principles simulations to analyze how ultra-thin ZnO films' electronic and optical properties are affected by confinement and surface states across various orientations, providing insights for experimental characterization.

Contribution

It offers a comprehensive computational analysis of electronic and optical properties of ultra-thin ZnO films with different orientations, highlighting quantum confinement and surface effects.

Findings

Band gap increases in thin films due to quantum confinement.

Surface states influence optical absorption features.

Dielectric constants are orientation-dependent.

Abstract

Relying on generalized-gradient and hybrid first-principles simulations, this work provides a complete characterization of the electronic properties of ZnO ultra-thin films, cut along the Body-Centered-Tetragonal(010), Cubane(100), h-BN(0001), Zinc-Blende(110), Wurtzite(10$\bar{1}$0) and (0001) orientations. The characteristics of the local densities of states are analyzed in terms of the reduction of the Madelung potential on under-coordinated atoms and surface states/resonances appearing at the top of the VB and bottom of the CB. The gap width in the films is found to be larger than in the corresponding bulks, which is assigned to quantum confinement effects. The components of the high frequency dielectric constant are determined and the absorption spectra of the films are computed. They display specific features just above the absorption threshold due to transitions from or to the surface resonances. This study provides a first understanding of finite size effects on the electronic properties of ZnO thin films and a benchmark which is expected to foster experimental characterization of ultra-thin films via spectroscopic techniques.