Photoluminescence and Optically Pumped Ultraviolet Lasing fromNanocrystalline ZnO Thin Films Prepared by Thermal Oxidation of High-Quality ZnS Thin Films

Abstract

We present a simple and useful method for preparing high-qualitynanocrystalline ZnO thin films, i.e. the thermal oxidation ofhigh-quality ZnS films prepared by the low-pressure metal-organicchemical vapour deposition technique. The x-ray diffractionmeasurements reveal that the nanocrystalline ZnO has a hexagonalwurtzite structure. Raman spectra show that the longitudinal opticalphonon with the E1-mode appears at 578 cm-1. The multiplephonon scattering process is also observed, indicating the formationof a high-quality nanocrystalline ZnO thin film. Thephotoluminescence spectrum has a single emission peak at 3.264 eVfrom the free-exciton mission, under the condition of low excitationpower at room temperature. However, when excitation intensitiesexceed the threshold of 150 kW/cm2, a new and narrow peakemerges at lower energies, which are attributed to exciton-excitoncollisions, and is called the P line. The intensity of this peakincreases superlinearly with the pumping power over a thresholdvalue. This supplies strong evidence of stimulated emission. Themultiple longitudinal cavity modes observed in the stimulatedemission spectrum indicate the successful realization of opticallypumped lasing from nanocrystalline ZnO films at room temperature.