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

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 0.09degrees from 0.2-0.3degrees) upon introduction of a buffer layer of ZnO itself at a low temperature approximately 500 degreesC, 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 similar to90% with a steep falloff at 380 run and a fairly small carrier concentration (0.8 X 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. (C) 2002 Elsevier Science B.V All rights reserved.