We successfully grew Gallium nitride (GaN) films on MgAl 2 O 4 (111) substrates using low temperature pulsed laser deposition (PLD). X-ray diffraction rocking curves revealed the high quality of as-grown GaN with its full width at half maximum (FWHM) along [0002] as small as 0.088. Atomic force microscopy images showed very smooth surface of as-grown GaN and indicated a two-dimensional growth. High-resolution transmission electron microscopy showed high structural perfection of GaN films. The crystalline structure for MgAl 2 O 4 substrate from its body to its top part adjacent to GaN epitaxy kept constant, and the interface between GaN and MgAl 2 O 4 was atomically abrupt. We attribute the high quality of as-grown GaN films to the well-controlled Mg atom evaporation from MgAl 2 O 4 substrate surface owing to the application of low growth temperature in PLD: on the one hand, it avoids the severe interfacial reaction that is very likely to take place in conventional MOCVD or MBE growth, and hence makes the epitaxial growth possible; on the other hand, it conserves the crystalline structure of the substrate surface, and consequently takes good advantage of the small lattice mismatch between GaN and MgAl 2 O 4 .

Zn1-xCoxO thin films on sapphire (0001) substrates were synthesized by laser molecular beam epitaxy (LMBE) method at various temperatures under a work ambient pressure of 5.0 x 10-5Pa condition. X-ray diffraction (XRD) spectra, UV–visible transmission spectra and X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra were employed to characterize the properties of samples. All samples were of wurtzite hexagonal structure with the preferential c-axis-orientation. Co2+ions incorporated into ZnO lattice and substituted for Zn2+ions. ZnLMM Auger spectrum implied Zn interstitials existed in sample. The optical transmission of all samples was relatively high in visible region. Two PL emission peaks located at 418 nm and 490 nm were assigned to the electron transition from the Zn interstitials to the top of the valence band and from the Zn interstitials to the Zn vacancies, respectively.