The effect of ZnO homo-buffer layer on ZnO thin films grown on c-Al 2O 3(0 0 0 1) 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 θ-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 500–600°C, and those grown on the thicker buffer layer or prepared at 400°C or 700°C showed deep-level emission around 510 nm. In Hall measurement, the ZnO films showing deep-level emission gave also carrier concentration higher than 1×10 19/cm 3 and those with better crystalline quality seemed to have high mobility of μ=40–57 cm 2/V s.