Intensity-dependent dynamics of photoexcited carriers in ZnO epilayers studied using pump-probe reflectance

Abstract

The dynamics of carriers/excitons in as grown and annealed ZnO epilayers at room-temperature under high excitation densities were monitored by pump-probe(4.65/3.1 eV) differential reflectance R ∆ transients on a time scale equal to 100 ps. The R ∆ buildup showed a density-independent time delay of about 1.45 ps relative to the pump pulse, indicative of hot phonon effects, i.e., LO-phonon bottleneck, common to the as grown and annealed ZnO epilayers. Both appeared an initial fast R ∆ decay with a time constant of about 20-25 ps due to rapid defect trapping. After the fast decay, the R ∆ transient of the as grown ZnO became of sign reversal and turned to an induced absorption signal from defect levels with a decay time of several hundred picoseconds weakly dependent on excitation intensities. For the annealed ZnO, no change over in sign was observed on the slowly recovering component of R ∆ , of which, in particular, the decay time was found to linearly depend upon carrier density. This finding, as confirmed by photoluminescence measurements, was attributed to a bimolecular recombination in terms of an exciton-exciton scattering in ZnO. These observations suggest that this unique pump-probe technique can provide a useful tool for understanding the defect physics of semiconducting materials.

Size and Morphology Modulation in ZnO Nanostructures for Nonlinear Optical Applications: A Review

“…In order to better understand ZnO’s optical processes under laser excitation, it is necessary to characterize its NLA behavior. Some studies have been conducted on the nonlinear properties of ZnO in a narrow wavelength range or using a megahertz repetition-rate laser. − At present, the dynamic behaviors of ZnO semiconductor materials in different wavebands and pulse time domains have also been reported. − …”