Ultrafast exciton dynamics in highly excited bulk ZnO

Large bandgap semiconductors like ZnO are currently of interest as light emitters in the blue-UV spectral range. Advances in these technologies rely on a fundamental understanding of carrier dynamics and excitonic effects at high excitation densities. Researchers have described highly photo-excited ZnO using the polariton model. However, at high excitation, one would expect screening effects to come into play, such that the polariton model would no longer hold. With high carrier densities induced by intense, ultrashort laser pulses, we are able to monitor exciton dynamics through our micro-cryostat pump-probe reflectometry setup, and determine the regimes for which the polariton model is not valid. We use an amplified Ti:Sapph laser system to generate upconverted 266nm fs-laser pulses to pump m-plane and c-plane bulk ZnO above the bandgap resonance. A femtosecond broadband probe pulse generated by focusing the fs-laser pulse through a CaF2 crystal monitors the ZnO reflectivity changes at different pump-probe delay times. The pump and probe pulses are collinear and are focused through a micro-cryostat system at normal incidence to the bulk samples. The samples are cooled down to 4K with liquid He to resolve all excitonic features and accurately monitor exciton dynamics. From the reflectivity data, we find that the response of the exciton resonance of ZnO dramatically changes immediately after fs-laser excitation. The reflectivity data show a damping of the excitonic resonances that vary with pump excitation fluence, which we can model very well with theoretical calculations. For pump fluences close to ZnO’s damage threshold of 3 kJ/m2, the damping of the excitonic resonances becomes so strong that the exciton is completely bleached for several tens-of-picoseconds after excitation. This observation indicates that there is a regime post-excitation when the density of electron-hole pairs is so high that the polariton model cannot accurately describe the dynamics inside ZnO. We also discuss how the exciton dynamics provide insight into ZnO device optimization.