Growth of ZnO epitaxial films by metal organic chemical vapor deposition at various temperatures

ZnO epitaxial films are grown on sapphire (0001) substrates by metal organic chemical vapor deposition (MOCVD) at temperatures from 200 to 500 °C. The half width (FWHM) of rocking curves from these films is greatly improved with increasing the growth temperature, T_g, with a sudden decrease at about T_g= 300 °C. The in-plane orientation of ZnO unit cells changes with temperature: no-twist for T_g = 200 °C and 30 °twist for T_g ≥300 °C with respect to the unit cell of the substrate. The surface morphology is strongly dependent on growth temperature, with smooth surfaces at low T_g. The emission spectra from the ZnO films are dominated by bound exciton transitions and better emission properties are obtained for Tg > 375 °C. Acceptor-related emissions were observed from ZnO films grown at Tg < 300 °C, indicating the importance of growth at low temperatures in future applications of ZnO-related oxides.