Growth of high-quality epitaxial ZnO films on (10-10) sapphire by atomic layer deposition with flow-rate interruption method

A novel process in an atomic layer deposition system with "flow-rate interruption" (FRI) was developed to obtain epitaxial ZnO films of high quality. The m-plane ZnO thin films were grown on m-plane sapphire substrates by atomic layer deposition with FRI or a conventional continuous-flow method at the temperature in the range of 25-260°C; 200°C appeared optimal. Measurements of X-ray reflectivity indicated that the thickness of ZnO films with FRI is almost twice than that grown with the continuous-flow method. The structural, optical and electrical properties were investigated with X-ray diffraction (XRD), photoluminescence and Hall measurements. The diffraction results indicated that the interrupted flow might extend the reaction of diethylzinc and water through an increased duration to improve the crystallographic quality of the films. According to the results from XRD at high resolution, to substantiate the epitaxial relation between the thin film and the substrate, an off-normal azimuthal scan along ZnO (201) demonstrated two-fold symmetry that indicated the ZnO films to be in epitaxial growth on sapphire. The photoluminescence results showed a strongly enhanced near-band-edge emission of an FRI sample, and the donor-bond exciton appearing in films also indicated superior crystalline qualities. The Hall mobility of the FRI method was up to 64.7cm²V^-1s^-1. The FRI method evidently improved the structural, optical and electrical properties of the ZnO films with small consumption of precursors.