Growth of ZnO/sapphire heteroepitaxial thin films by radio-frequency sputtering with a raw powder target

Epitaxial ZnO thin films were deposited by radio-frequency sputtering. In contrast to typical sputter growth, in which a ZnO sintered target was used, raw ZnO powder with a particle size smaller than 1 μm is used as the source material. In order to verify this approach, ZnO thin films were deposited on sapphire(0001) substrates and characterized by X-ray diffraction, atomic force microscopy, ultraviolet-visible-near-infrared (near-IR) transmission spectroscopy, and photoluminescence spectroscopy. The as-deposited ZnO thin films grew epitaxially on the sapphire(0001) substrate. A crossover in the growth mode from an initial 2-dimensional planar layer to later 3-dimensional islands was observed, which is consistent with the results obtained using a ZnO sintered target. The ZnO films showed band-edge emission with a bandgap energy of 3.27 eV and a high optical transmittance > 80% from visible to near-IR region. This shows that ZnO powder targets can be an alternative to relatively expensive sintered ones in the fabrication of ZnO nano-structures and doped ZnO.