The ex situ sputtered AlN buffer and GaN epilayer grown on top of it by metalorganic chemical vapor deposition were studied comprehensively by a variety of techniques including atomic force microscope, high resolution x-ray diffraction, Raman and x-ray photoelectron spectroscopy characterizations. It exhibited that the AlN buffer deposited by using sputtering technique could be oxidized with exposure in atmosphere. Such oxidation phenomenon significantly influences the characteristics of GaN epilayer, for example leading to poor surface morphology, high dislocation density, and large compressive stress. This study demonstrated the effect of oxygen impurities on GaN growth and has an important guiding significance for the growth of high-quality III-nitride related materials.

A method using vacuum evaporation and hot-pressing to prepare Cu2Se thin films on Cu substrates is developed. The effects of the heating temperature, heating time, and pressure on the crystallinities of the Cu2Se thin films were studied. The structure, morphology, and composition properties of the Cu2Se thin films were characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results showed that the crystallinities of the Cu2Se thin films improved as the heating temperature was increased from 200 degrees C to 280 degrees C. A low pressure of 2 MPa and a short time of 5 min were sufficient to obtain highly crystalline Cu2Se thin films at 280 degrees C. The fabricated Cu2Se/Cu structure could directly convert heat to electricity with a thermoelectric potential of 12.6 mV at 400 K and 17.8 mV at 500 K. Compared with conventional hot-pressing of bulk materials, this method requires lower temperatures and smaller pressures.