Growth of Heteroepitaxial ZnO Thin Films on GaN‐Buffered Al₂O₃ (0001) Substrates by Low‐Temperature Hydrothermal Synthesis at 90 °C

Abstract Heteroepitaxial ZnO films are successfully grown on nondoped GaN‐buffered Al₂O₃ (0001) substrates in water at 90 °C using a two‐step process. In the first step, a discontinuous ZnO thin film (ca. 200 nm in thickness) consisting of hexagonal ZnO crystallites is grown in a solution containing Zn(NO₃)·6 H₂O and NH₄NO₃ at ca. pH 7.5 for 24 h. In the second step, a dense and continuous ZnO film (ca. 2.5 μm) is grown on the first ZnO thin film in a solution containing Zn(NO₃)·6 H₂O and sodium citrate at ca. pH 10.9 for 8 h. Scanning electron microscopy, X‐ray diffraction, UV‐vis absorption spectroscopy, photoluminescence spectroscopy, and Hall‐effect measurement are used to investigate the structural, optical, and electrical properties of the ZnO films. X‐ray diffraction analysis shows that ZnO is a monocrystalline wurtzite structure with an epitaxial orientation relationship of (0001)[11$ \bar 2 $0]_ZnO∥(0001)[11$ \bar 2 $0]_GaN. Optical transmission spectroscopy of the two‐step grown ZnO film shows a bandgap energy of 3.26 eV at room temperature. A room‐temperature photoluminescence spectrum of the ZnO film reveals only a main peak at ca. 380 nm without any significant defect‐related deep‐level emissions. The electrical property of ZnO film showed n‐type behavior with a carrier concentration of 3.5 × 10¹⁸ cm^–3 and a mobility of 10.3 cm² V^–1 s^–1.