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.

Room-temperature spectral and temporal behaviors of UV and visible emissions in ZnO epilayers grown onto (100) silicon substrates have been investigated by means of time-integrated and time-resolved photoluminescence(TIPL & TRPL). The PL lifetimes as short as 25-50 ps for the excitonic UV lines peaked at ~380 nm were found, indicative of ultrafast trapping of excitons by defects states inside the band gap. Compared to its as-grown precursor, the ZnO epilayer subjected to postgrowth thermal annealing in air showed enhanced intensities of both UV as well green emissions by nearly the same factor of ~3.1, in accompany with complete disappearance of the impurity luminescence peaked at 2.83 eV. More importantly, the green luminescence in the post-annealed ZnO was observed to decay as hyperbolic 1 − t and logarithmically shift its peak emission toward higher energies with increased excitation intensity, in excellent agreement with the tunnel-assisted donor-acceptor pair(DAP) recombination model. The possible mechanism of compensation between intrinsic impurities was also discussed.