The optical properties of a high quality bulk ZnO, thermally post treated in a forming gas environment are investigated by temperature dependent continuous wave and time-resolved photoluminescence (PL) measurements. Several bound and free exciton transitions along with their first excited states have been observed at low temperatures, with the main neutral-donor-bound exciton peak at 3.3605 eV having a linewidth of 0.7 meV and dominating the PL spectrum at 10 K. This bound exciton transition was visible only below 150 K, whereas the A-free exciton transition at 3.3771 eV persisted up to room temperature. A-free exciton binding energy of 60 meV is obtained from the position of the excited states of the free excitons. Additional intrinsic and extrinsic fine structures such as polariton, two-electron satellites, donor-acceptor pair transitions, and longitudinal optical-phonon replicas have also been observed and investigated in detail. Time-resolved PL measurements at room temperature reveal a biexponential decay behavior with typical decay constants of ϳ170 and ϳ864 ps for the as-grown sample. Thermal treatment is observed to increase the carrier lifetimes when performed in a forming gas environment.

ZnO is considered as a promising substrate for GaN epitaxy because of stacking match and close lattice match to GaN. Traditionally, however, it suffered from poor surface preparation which hampered epitaxial growth in general and GaN in particular. In this work, ZnO substrates with atomically flat and terrace-like features were attained by annealing at high temperature in air. GaN epitaxial layers on such thermally treated basal plane ZnO with Zn and O polarity have been grown by molecular beam epitaxy, and two-dimensional growth mode was achieved as indicated by reflection high-energy electron diffraction. We observed well-resolved ZnO and GaN peaks in the high-resolution x-ray diffraction scans, with no Ga 2 ZnO 4 phase detectable. Low-temperature photoluminescence results indicate that high-quality GaN can be achieved on both O-and Zn-face ZnO.

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Electronic behavior of the Zn-and O-polar ZnO surfaces studied using conductive atomic force microscopy Facets evolution and surface electrical properties of nonpolar m -plane ZnO thin films

The authors report on the development of a molecular beam epitaxy production process for the epitaxial growth of high quality, single crystal, single phase SrTiO3 (STO) films on Si substrates with diameter up to 8in. Reflection high-energy electron diffraction indicated that the STO growths proceeded two dimensionally with excellent stoichiometric control, as confirmed by Rutherford backscattering spectroscopy measurement. Excellent crystalline quality has been confirmed by x-ray diffraction rocking curves of the STO (200) reflection with narrow full width at half maximum of 0.06° for a 1200Å thin film. Atomic force microscopy images show smooth, defect-free STO surface with a root-mean-square roughness value as low as ∼0.6Å. Cross-sectional transmission electron microscope images reveal an abrupt interface between STO and Si, with a very thin SiO2 interfacial layer.