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The ␣, ␤, and ␥ phases of the GaAs(001)-(2ϫ4) surface have been studied using rocking-curve analysis of reflection high-energy electron diffraction ͑RHEED͒ and reflectance difference spectroscopy ͑RDS͒. We have measured RHEED rocking curves and RD spectra from GaAs͑001͒ under an As flux of 2.5ϫ10 Ϫ7 Torr, and have identified the temperature ranges where each of the ␣, ␤, and ␥ phases is stable. The 2ϫ4 reconstruction was observed in the range of 480-600°C. The ␤ phase, which is stable in the range of 510-550°C, has a ␤ 2(2ϫ4) structure, in good agreement with previous experimental and theoretical results. The ␥ and ␣ phases are stable below 510°C and above 550°C, respectively. While the data from the ␥ phase are well explained by a mixture of ␤ 2(2ϫ4) and c(4ϫ4) phases, we propose that the ␣ phase has a ␤ 2(2ϫ4) structure, the atomic coordinates of which are slightly different from those of the ␤ phase.

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We report on the growth of polarity-controlled ZnO films by plasma-assisted molecular-beam epitaxy. Different polar (Zn- and O-polar) ZnO films on unipolar (Ga-polar) GaN epilayers are selectively grown. Polarity of ZnO films is evaluated by coaxial impact collision ion scattering spectroscopy. Zn preexposure prior to ZnO growth results in Zn-polar ZnO films (Zn face), while O-plasma preexposure leads to the growth of O-polar ZnO films (O face). High-resolution transmission electron microscopy reveals the formation of an interface layer between ZnO and GaN epilayers in O-plasma preexposed samples, while no interface layer is observed in Zn preexposed samples. The interface layer is identified as single crystalline, monoclinic Ga2O3. We propose models for interface configurations at ZnO/GaN heterointerfaces, which can successfully explain the different polarities of the ZnO films.