Confinement effects in ultrathin ZnO polymorph films: Electronic and optical properties

Relying on generalized-gradient and hybrid first-principles simulations, this work provides a complete characterization of the electronic properties of ZnO ultrathin films, cut along the body-centered-tetragonal(010), cubane(100), hexagonal boron nitride(0001), zinc-blende(110), and wurtzite (10 (1) over bar0) and (0001) orientations. The characteristics of the local densities of states are analyzed in terms of the reduction of the Madelung potential on undercoordinated atoms and surface states/resonances appearing at the top of the valence band and bottom of the conduction band. 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 ultrathin films via spectroscopic techniques.