We report on the realization of wide band gap (5–6 eV), single-phase, metastable, and epitaxial MgxZn1−xO thin-film alloys grown on sapphire by pulsed laser deposition. We found that the composition, structure, and band gaps of the MgxZn1−xO 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. %) (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 ZnO thin films grown 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 ZnO films. By tuning the oxygen pressure during the initial and the final growth stages, smooth and epitaxial ZnO films 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 MgxZn1−xO. Using pulsed laser deposition technique, Mg0.34Zn0.66O 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 Mg0.34Zn0.66O 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.

Epitaxial films of magnetite (Fe 3 O 4 ) have been grown on SrTiO 3 ͑100͒ and sapphire ͑␣-Al 2 O 3 , 0001͒ substrates by pulsed laser deposition, and they exhibit crystal orientations of ͓100͔ and ͓111͔, respectively. Films of both orientations show a clear Verwey transition near 120 K. The temperature dependence of magnetoresistance of the films is examined at fields up to 8.5 T applied parallel to the film plane. The ͓100͔ oriented films show a pronounced extremum in magnetoresistance ͑MR͒ at the Verwey transition temperature (T v ), while no such feature is observed in the case of the ͓111͔ oriented films. However, the ͓111͔ oriented films exhibit a sudden change in the MR dependence on T at T v and films of both orientations show a rapid increase of MR as the temperature is decreased below about 110 K. An attempt is made to analyze these data in terms of the known anisotropy in magnetostriction and optical magnon-phonon dispersions, and the small polaron band and hopping conductivity mechanisms.