Structure and dielectric function of two- and single-domain ZnO epitaxial films

The differences between two- and single-domain ZnO epitaxial films, grown by reactive pulsed magnetron sputtering, have been studied with respect to their texture development and x-ray coherence length behavior at various substrate temperatures and oxygen partial pressures. The film in-plane ordering depends on the surface pretreatment of the sapphire substrate. After pretreatment in an oxygen radio-frequency plasma, single-domain films form even at a substrate temperature of 100 degrees C in a wide range of oxygen pressures, and at a growth rate up to 1.2 nm/s. The single-domain films show a linear dependence of the x-ray coherence length on the substrate temperature, while a steplike dependence is characteristic of the two-domain films. The ZnO complex dielectric function was obtained using a parametrized semiconductor oscillator model for spectroscopic ellipsometry data analysis. For the films grown at 550 degrees C, the band gap of 3.29 +/- 0.01 eV is independent of the type of in-plane ordering and variation of the texture. The oscillator broadening correlates with the width of (0002) diffraction peak rocking curve. Both parameters increase at high oxygen pressure and low substrate temperature, which is attributed to a higher defect (dislocation) density.