Fabrication of stable wide-band-gap ZnO/MgO multilayer thin films

We report the fabrication of stable wide-band-gap (∼6 eV) ZnO/MgO multilayer thin films using pulsed-laser deposition on $c$-plane $Al2O3$ substrates. The thickness of ZnO layers was varied in the range of 0.75–2.5 nm inside the MgO host with a constant MgO thickness of 1 nm. With a decrease in the thickness of ZnO sublayers, abrupt structural transition from hexagonal to cubic phase was observed. The band gap of the films was found to be influenced by the crystalline structure of multilayer stacks. Thin films with hexagonal and cubic structure exhibited band-gap values of 3.5 and 6 eV, respectively. The x-ray photoelectron spectroscopy analysis confirmed that the Mg content of the films was about 40% and 60% in the hexagonal and cubic phases, respectively. Annealing at 750 °C did not influence the structural and optical properties of the ZnO/MgO multilayers.