Hydrothermally grown ZnO buffer layer for the growth of highly (4 wt%) ga-doped ZnO epitaxial thin films on MgAl₂O₄ (1 1 1) substrates

gallium (4 wt%) doped ZnO (gZO) thin films were deposited on hydrothermally grown ZnO buffered and non-buffered MgAl₂O₄ (1 1 1) substrates by RF magnetron sputtering technique at a growth temperature of 250 °C. The epitaxial ZnO buffer layer was deposited on the MgAl ₂O₄ (1 1 1) substrate by a hydrothermal technique using aqueous solutions of zinc nitrate hexahydrate, ammonium nitrate and ammonium hydroxide at 90 °C. The effect of the ZnO buffer layer on the crystallinity, epitaxial nature, surface morphology, optical and electrical properties of the gZO thin films is investigated. X-ray diffraction and transmission electron microscopy showed that the hydrothermally grown ZnO buffer layer and gZO thin film grown on the hydrothermally grown ZnO buffered substrate were grown epitaxially with an orientation relationship of (0001)[ _112̄0]gZO||(111)[112̄]MgAl₂^O4. However, the gZO thin films grown on the non-buffered substrate are polycrystalline in nature with a hexagonal wurtzite phase. The room temperature photoluminescence spectra of the gZO epitaxial thin films grown on the buffered substrate revealed a sharp near band edge emission peak and a lower broad deep-level emission peak compared to the polycrystalline gZO thin film grown on a non-buffered substrate. The electrical resistivity of the gZO thin films is found to be proved from 4.69×10^-3 to 2.27×10^-3 Ω cm by introducing the hydrothermally grown ZnO buffer layer between the gZO thin film and MgAl₂O₄ (1 1 1) substrate.