Intensity dependence and transient dynamics of donor-acceptor pair recombination in ZnO thin films grown on (001) silicon

We report room-temperature time-integrated and time-resolved photoluminescence (PL) measurements on a nominally undoped wurtzite ZnO thin film grown on (001) silicon. A linear and sublinear excitation intensity I-ex dependence of the PL intensity were observed for the 379.48-nm exciton line and the weak broad green band (similar to510 nm), respectively. The green luminescence was found to decay as hyperbolic t(-1), and its peak energy was observed to increase nearly logarithmically with increased I-ex. These results are in an excellent agreement with the tunnel-assisted donor-deep-acceptor pair (DAP) model so that its large blueshifts of about 25 meV per decade increase in I-ex can be accounted for by the screening of the fluctuating impurity potential. Also, the 30-ps fast decay of the exciton emission was attributed to the rapid trapping of carriers at luminescent impurities, while the short lifetime of tau(1/e)=200 ps for the green luminescence may be due to an alternative trapping by deeper centers in the ZnO. Finally, singly ionized oxygen and zinc vacancies have been tentatively invoked to act as donor-deep-acceptor candidates for the DAP luminescence, respectively. (C) 2003 American Institute of Physics.