Electronic structures of polycrystalline ZnO thin films probed by electron energy loss spectroscopy

The microstructure of polycrystalline ZnO thin films grown on amorphous fused quartz has been studied by transmission electron microscopy and electron energy loss spectroscopy (EELS). The optical functions of the grain and grain boundary of ZnO acquired from EELS are compared to elucidate the mechanism of the formation of self-assemble laser cavities within this material. It is found that the refractive index of the grain boundary is significantly lower than that of the grain due to the lack of excitonic resonance. This large refractive index difference between the grain and grain boundary substantiates the scenario that the formation of laser cavities is caused by the strong optical scattering facilitated in a highly disordered crystalline structure. In addition, our results also imply that the optical characteristics of ZnO have very high tolerance on defects.