Improvement in the crystallinity of ZnO thin films by introduction of a buffer layer

Abstract The influence of pre-deposition of homo-buffer layers on film quality is studied as functions of temperature and duration of pre-deposition, for zinc oxide (ZnO) crystalline films prepared by pulsed laser deposition on sapphire (0 0 0 1) substrates. This preparation technique is necessary to prepare high quality films suitable for the development of ZnO devices. Crystallinity and surface morphology were characterized by X-ray diffraction (XRD), reflection high energy electron diffraction and scanning electron microscopy. The line width of the rocking curve observed for ZnO (0 0 0 2) XRD of ZnO films decreases (to 0.09° from 0.2–0.3°) upon introduction of a buffer layer of ZnO itself at a low temperature approximately 500 °C, indicating the formation of high quality films. The surface morphology and flatness were also improved. The film prepared under optimal conditions shows a high optical transmittance of ∼90% with a steep falloff at 380 nm and a fairly small carrier concentration (1.8×10 17 cm −3 ). These results imply that the buffer layer relaxes the strain due to lattice mismatch between ZnO and sapphire (by 18%) and improves the film crystallinity.