The effect of ZnO homo-buffer layer on ZnO thin films grown on c-Al2O3(0001) by plasma assisted molecular beam epitaxy

Abstract

ZnO thin films were deposited on c-plane sapphire substrates on the ZnO homo-buffer layer with different thickness and growth temperature by plasma assisted molecular beam epitaxy. The effects of buffer-layer growth variables on the properties of the ZnO films were investigated and discussed on a collective basis compared with other reports. RHEED patterns were taken over different buffer layer surfaces and the initial growth mode of ZnO buffer layer was recognized as Stranski-Krastanov mode by the direct observation of a streaky pattern superimposed with a spotty pattern of thicker than 8 nm. Through examining the XRD theta-rocking curve of ZnO (0 0 0 2) peak, it seems that the crystalline quality of the ZnO thin film grown on the ZnO buffer layer was gradually improved with the increase of the buffer layer thickness. Strong near band-edge emission at 378 nm was well observed without deep-level emission at the ZnO films grown on the 15 nm, buffer layer prepared at 500-600degreesC, and those grown on the thicker buffer layer or prepared at 400degreesC or 700degreesC showed deep-level emission around 510 nm. In Hall measurement, the ZnO films showing deep-level emission gave also carrier concentration higher than 1 x 10(19)/cm(3) and those with better crystalline quality seemed to have high mobility of mu = 40-57 cm(2)/V S. (C) 2004 Elsevier B.V. All rights reserved.