Effect of ZnO nanostructures on 2-dimensional random lasing properties

We show results on how the morphology of a ZnO layer can have a big impact on the random lasing threshold of the material. Plasma-enhanced chemical vapor deposition method is used to grow ZnO layers on sapphire substrates. The morphologies and structures of ZnO are observed to undergo transition when growth temperature decreases from 750 to 100°C: the deposited ZnO changes from crystalline films to nanocrystalline films with columnar-shaped grains, then to well-aligned ZnO nanorods, and finally to randomly oriented irregular-shaped grains. ZnO nucleation and surface diffusion rates, coalescence between crystal grains, and preferential growth along c-axis play important roles in this transition from continuous films to nanorods. Random lasing properties of our ZnO films and nanorods are studied. The scattering ability of ZnO is critical to control the lasing properties. The lowest lasing thresholds are observed for ZnO films grown between 500 and 600°C when the films have columnar-shaped grains and not at 750°C when the ZnO layer has a continuous crystalline film. Calculations based on quasi-2D random lasing are consistent with the experimental results of lasing threshold measurements.