Photoluminescence of ZnO films grown by plasma-assisted molecular beam epitaxy

Photoluminescence (PL) of homoepitaxial and heteroepitaxial ZnO films grown by plasma-assisted molecular beam epitaxy is studied. Homoepitaxial ZnO layers were grown on an O-face melt-grown ZnO (0 0 0 1) substrate. Heteroepitaxial ZnO layers were grown on an epitaxial GaN template predeposited by metalorganic chemical vapor deposition on a c-plane sapphire substrate. The low-excitation PL spectra of ZnO epilayers excited by a He-Cd laser exhibit only bound-exciton emission with phonon replicas. There are green luminescence from the ZnO substrate but not from the ZnO epilayers. However, under high-excitation by a N2 pulse laser, the emission due to exciton-exciton scattering dominates the PL spectrum from the heteroepitaxial ZnO layer but is not observed from the homoepitaxial ZnO layer. The difference is probably due to the different quality of the ZnO substrate and GaN template.