PLASMA-ASSISTED MOLECULAR-BEAM EPITAXY OF ZNO ON C-PLANE SAPPHIRE - GROWTH AND CHARACTERIZATION

ZnO single crystal thin films were grown on c-plane sapphire using oxy gen microwave plasma assisted molecular beam epitaxy. Atomically flat oxygen-terminated substrate surfaces were obtained by pre-growth clean ing procedures involving an oxygen plasma treatment. A two dimensional nucleation during the initial growth which is followed by a morpholog y transition to three dimensional nucleation was observed by in situ r eflection high energy electron diffraction. X-ray diffraction (XRD) an d photoluminescence investigations suggest that the ZnO epilayer consi sts of a high quality layer on top of a transition layer containing a high density of defects in the interfacial region. A full width at hal f maximum (FWHM) of 0.005 degrees is obtained for the ZnO(0002) diffra ction peak in an XRD rocking curve, while a broad tail extending from the peak can also be observed. The photoluminescence spectra exhibit d ominant bound exciton emission with a FWHM of 3 meV at low temperature s and free exciton emission combined with a very weak deep level emiss ion at room temperature. Recently, these high quality ZnO epilayers ha ve allowed the observation of optically pumped lasing at room temperat ures as well as stimulated emission up to 550 K, both of which are due to an exciton related mechanism. (C) 1998 American Institute of Physi cs. [S0021-8979(98)02619-X].