Abstract
While increasing the Cd content in ZnCdO can lead to a very large nominal change in bandgap, incorporating high Cd concentrations has shown to be extremely difficult and can enhance the formation of different types of defects. Indeed, the tendency that Cd has to segregate and form clusters can yield alloy disorder which in turn can impact the carrier recombination paths, thus limiting the total radiative efficiency. In order to analyze the different recombination mechanisms and their dynamics we have used time-resolved photoluminescence (TRPL) as a function of temperature in dense ensembles of ZnCdO nanowires grown by MOCVD [1]. The nanowires were 1-3 µm-long and 100-200 nm-wide and contain different Cd contents, ranging from 0 to 45%, and were analyzed as-grown and after exposure to different rapid thermal annealing (RTA) cycles [2]. The comparison between the decay time of as-grown and thermally annealed ZnO nanowires revealed a higher lifetime after annealing, providing clear evidence of a reduction of non-radiative channels. In the case of the ternary ZnCdO nanowires, the carrier lifetime is larger than in the Cd-free case. This larger lifetime may be explained to arise from the localization of the photogenerated excitons at potential fluctuations produced by alloy disorder. As in the case of ZnO, annealing also yields a higher carrier lifetime in ZnCdO, indicative of the removal of non-radiative recombination paths. Temperature-dependent TRPL confirmed this exciton localization behaviour in the ternary nanowires. Indeed radiative emission was observed at higher temperatures in the ternaries, especially after thermal annealing, due to the enhanced carrier lifetime. In the case of the ZnO, even after thermal annealing where the non-radiative paths have been largely removed, increasing the temperature produces a rapid collapse of the PL likely due to the absence of exciton localization, as in the case of the ZnCdO nanowires. References: [1] K. Yamamoto, T. Tsuboi, T. Ohashi, T. Tawara, H. Gotoh, A. Nakamura, and J. Temmyo, J. Cryst. Growth 312, 10 (2010). [2] T. Ohasi, K. Yamamoto, A. Nakamura and J. Temmyo, J. J. of Applied Physics, vol.47, pp.2961 (2008).