Growth of Heteroepitaxial ZnO Thin Films on GaN-Buffered Al_2O_3 (0001) Substrates by Low-Temperature Hydrothermal Synthesis at 90 ℃

摘要

Heteroepitaxial ZnO films are successfully crown on nondoped GaN-buffered Al_2O_3 (0001) substrates in water at 90 ℃ 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(NO_3)·6H_2O and NH_4NO_3 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(NO_3)·6H_2O and sodium citrate at ca. pH 10.9 for 8 h. Scanning electron microscopy, X-ray diffraction, UV-vis absorption spectroscopy, photolumines-cence 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 )[1120]_(ZnO)‖(0001 )[1120]_(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 tiny significant defect-related deep-level emissions. The electrical property of ZnO film showed n-type behavior with a carrier concentration of 3.5 × 10~(18) cm~(-3) and a mobility of 10.3 cm~2 V~(-1) s~(-1).