Recombination dynamics of excitons in phosphorus-doped ZnO nanostructures

The optical properties of an undoped and a P-doped ZnO nanostructures have been studied by means of photoluminescence (PL), time-resolved PL, and spatially-resolved cathodoluminescence (CL) spectroscopy. The luminescence bands due to the radiative recombination of biexcitons and the exciton-exciton scattering process were observed. The binding energy of excitons and biexcitons was estimated to be 60 and 15 meV, respectively. Radiative and nonradiative recombination lifetimes of free excitons were estimated from temperature dependence of the PL lifetime and the time-integrated PL intensity. Although the radiative recombination lifetime for each sample was almost equal, the nonradiative recombination lifetime for the P-doped sample was longer than that for the undoped sample. This result suggested that the thermal activation of nonradiative recombination process was suppressed by the P doping. CL images revealed that the intensity of the side surface was much stronger than that of the interior in the P-doped sample. This result indicated that the P impurities were distributed around the surface of the nanostructures.