We report on the realization of wide band gap (5–6 eV), single-phase, metastable, and epitaxial ${\mathrm{Mg}}_x{\mathrm{Zn}}_{\text{1−x}}\mathrm{O}$ thin-film alloys grown on sapphire by pulsed laser deposition. We found that the composition, structure, and band gaps of the ${\mathrm{Mg}}_x{\mathrm{Zn}}_{\text{1−x}}\mathrm{O}$ thin-film alloys depend critically on the growth temperature. The structural transition from hexagonal to cubic phase has been observed for (Mg content greater than 50 at. %) $\text{(1⩾x⩾0.5)}$ which can be achieved by growing the film alloys in the temperature range of 750 °C to room temperature. Interestingly, the increase of Mg content in the film has been found to be beneficial for the epitaxial growth at relatively low growth temperature in spite of a large lattice mismatch between sapphire and cubic MgZnO alloys.

ZnO thin films have been grown heteroepitaxially on epi-GaN/sapphire (0001) substrates. Rutherford backscattering spectroscopy, ion channeling, and high resolution transmission electron microscopy studies revealed high-quality epitaxial growth of ZnO on GaN with an atomically sharp interface. The x-ray diffraction and ion channeling measurements indicate near perfect alignment of the ZnO epilayers on GaN as compared to those grown directly on sapphire (0001). Low-temperature cathodoluminescence studies also indicate high optical quality of these films presumably due to the close lattice match and stacking order between ZnO and GaN. Lattice-matched epitaxy and good luminescence properties of ZnO/GaN heterostructures are thus promising for ultraviolet lasers. These heterostructures demonstrate the feasibility of integrating hybrid ZnO/GaN optoelectronic devices.

Influence of oxygen pressure on the epitaxy,surface morphology, and optoelectronic properties has been studied in the case of ZnOthin filmsgrown on sapphire (0001) by pulsed-laser deposition. Results of Rutherford backscattering and ion channeling in conjunction with atomic force microscopy clearly indicate that the growth mode, degree of epitaxy, and the defect density strongly depend on the oxygen background pressure during growth. It is also found that the growth mode and the defects strongly influence the electron mobility, free-electron concentration, and the luminescence properties of the ZnOfilms. By tuning the oxygen pressure during the initial and the final growth stages, smooth and epitaxialZnOfilms with high optical quality, high electron mobility, and low background carrier concentration have been obtained. The implication of these results towards the fabrication of superlattices and controlled n- and p-type doping is discussed.

We report on the fabrication and characterization of visible blind ultraviolet photodetectors based on ${\mathrm{Mg}}_x{\mathrm{Zn}}_{\text{1−x}}\mathrm{O}.$ Using pulsed laser deposition technique, ${\mathrm{Mg}}_{\mathrm{0.34}}{\mathrm{Zn}}_{\mathrm{0.66}}\mathrm{O}$ thin films with a bandgap of 4.05 eV were epitaxially grown on c-plane sapphire substrates. The structural, electrical, and optical properties of epilayers were characterized using various techniques. Based on the ${\mathrm{Mg}}_{\mathrm{0.34}}{\mathrm{Zn}}_{\mathrm{0.66}}\mathrm{O}$ films, planar geometry photconductive type metal–semiconductor–metal photodetectors were fabricated. At a 5 V bias, a high responsivity of 1200 A/W was achieved at 308 nm, and the visible rejection (R308 nm/R400 nm) was more than four orders of magnitude. The 10%–90% rise and fall time were 8 ns and 1.4 μs, respectively.

ZnO thin films with near perfect crystallinity have been grown epitaxially on sapphire (001) by pulsed laser deposition technique. The ω-rocking curve full width at half-maximum of the ZnO(002) peak for the films grown at 750 °C, oxygen pressure ${10}^{\text{−5}}\hspace{0.17em}\mathrm{Torr}$ was 0.17°. The high degree of crystallinity was confirmed by ion channeling technique providing a minimum Rutherford backscattering yield of 2%–3% in the near-surface region $\text{(∼2000\hspace{0.17em}Å).}$ The atomic force microscopy revealed smooth hexagonal faceting of the ZnO films. It has been possible to deposit epitaxial AlN films of thickness 1000 Å on epi-ZnO/sapphire. Excellent crystalline properties of these epi-ZnO/sapphire heterostructures are, thus, promising for lattice-matched substrates for III–V nitride heteroepitaxy and optoelectronics devices.