Effects of oxygen pressure on optical and crystal properties of ZnO thin films prepared by laser molecular beam epitaxy

C-axis oriented ZnO thin films were epitaxially grown at 350 on sapphire (0001) substrates by laser molecular beam epitaxy(L-MBE) with oxygen pressures of l.lE-4Pa,1.8E-4Pa, 2.3E-4Pa,and 2.1E-3Pa. As oxygen pressure increases,the full-width at half maximum(FWHM) of X ray diffraction(XRD) becomes smaller and the diffraction peak intensity for (0002) planes becomes more intense as well as the symmetry of the peak shape becomes improved. Photoluminescence (PL) spectra of all samples show two emissions of a strong UV near-bandedge(NBE) emission peak at approximately 377 nm and a weak green-yellow deep level emission around 520nm. The samples grown with higher oxygen pressure have higher intensities of luminescence at 377 nm and the ratio of UV emission to deep level emission intensities increases. Reflection high energy electron diffraction(RHEED) pattern of ZnO films changes from spotty pattern to streaky one step by step with higher oxygen pressure.