Time and spatially resolved luminescence spectroscopy of ZnO nanostructures

The optical properties of undoped, P-doped, and Sb-doped ZnO nanostructures (NSs) have been studied by means of photoluminescence (PL), time-resolved PL, and spatially resolved cathodoluminescence (CL) spectroscopy. The temperature dependence of the PL spectra of the P-doped and Sb-doped ZnO NSs was analyzed, and the binding energies of the P-acceptor- and the Sb-acceptor-bound excitons were estimated to be 15 and 11 meV, respectively. This indicated that the Sb impurities formed a shallower acceptor level than the P impurities in ZnO. PL lines due to the radiative recombination of biexcitons and the inelastic scattering processes of excitons were clearly observed in the undoped ZnO NSs, which enabled us to evaluate the binding energies of the excitons and biexcitons as 60 and 15 meV, respectively. These values were identical to the values in bulk ZnO. The radiative and nonradiative recombination lifetimes were estimated from the temperature dependence of the PL lifetime and the time-integrated PL intensity. Although the radiative recombination lifetimes for the undoped and P-doped ZnO NSs were almost equal, the nonradiative recombination lifetime for the P-doped ZnO NSs was longer than that for the undoped ZnO NSs. This suggested that the P doping suppressed the thermal activation of the nonradiative recombination processes. CL images revealed that the intensity of the side surface was much stronger than that of the interior in the P-doped ZnO NSs. On the other hand, the CL intensity was distributed almost uniformly in the Sb-doped.