Near Band-Edge and Mid-Gap Photoluminescence of a ZnO Thin Film Grown on a (001) Silicon Substrate

A nominally undoped wurtzite ZnO thin film of highly c-axis orientation was successfully grown on (001) silicon by metal-organic chemical vapor deposition, and its photoluminescence was measured as a function of excitation intensity at room temperature. The ZnO sample exhibited a strong near band-edge (NBE) line at 379.48 nm (3.267 eV) and a weak broad green band around ～ 510 nm (2.43 eV), showing a linear and sublinear excitation dependence of the luminescence intensity, respectively. No discernable intensity dependence of lineshape and emission peak was found for the NBE line. On the other hand, the peak energy of the green luminescence was found to increase nearly logarithmically with the increasing excitation intensity. The above results clearly indicate that in the ZnO epilayer, the NBE line was due to an excitonic spontaneous emission, while the mid-gap green luminescence can be assigned to the tunnel-assisted donor-acceptor pair (DAP) radiative recombination. Moreover, we obtained an energy depth β ～ 11.74 meV for the potential wells due to the fluctuating distribution of the unintentional impurities/defects responsible for the tunnel-assisted DAP emission.