Optical studies of ZnO thin films grown on O- and H-plasma treated Al2O3 substrates by plasma-assisted molecular beam epitaxy

Comparative analysis of the structural and optical properties of ZnO thin films, grown on oxygen (O)- and hydrogen (H)-plasma treated sapphire substrates (denoted by samples O and H, respectively), by plasma-assisted molecular beam epitaxy is reported. We found that sample O has better crystal structure quality as compared to sample H by means of high-resolution X-ray diffraction, atomic force microscopy, and Hall measurements. We observed higher intensity ratio of excitonic emission to deep-level emission, longer radiative recombination lifetime, and lower stimulated emission threshold for sample O compared to sample H. From the results, we conclude that the O-plasma treatment on sapphire substrates may result in a decrease in the number of oxygen vacancies and better lattice matching conditions, thereby leading to an enhancement of the crystal quality and the optical properties of ZnO thin films grown on O-plasma treated sapphire substrates.