Luminescence of bound excitons in epitaxial ZnO thin films grown by plasma-assisted molecular beam epitaxy

Luminescence properties of ZnO films, which have been epitaxially grown on c-sapphire (0001) substrates by plasma-assisted molecular beam epitaxy, are investigated by means of different excitation sources and their measurement conditions. With the increase of measurement temperature, photoluminescence spectra clearly present, the appearance of different bound-exciton peaks (I-10 line) with an abrupt increase of emission intensity at the measurement temperature of 30-50 K. Hypothetical explanations on the basis of thermalization effects, vibronic/rotational resonance states, and the involvement of the B-valence level in emission are given. In cathodoluminescence (CL), the deep level emission intensity was enlarged with the electron beam penetration depth due to the higher defect density near the interface between ZnO and the sapphire. From the CL image of the ZnO film, the dislocation density was estimated as 6x10(8)-3x10(9)/cm(2). The lasing phenomenon was observed at the threshold power density of 1.3 MW/cm(2) at 300 K. (c) 2006 American Institute of Physics.