Determination of the polarities of ZnO thin films on polar and nonpolar substrates using scanning nonlinear dielectric microscopy

Using scanning nonlinear dielectric microscopy (SNDM) which we developed, we determined the polarities of ZnO thin films on various substrates including polar materials. SNDM is a useful tool for investigating the anisotropy of polar materials by measuring the nonlinear dielectric constant, whose sign changes in accordance with the polarity of the specimen. In conventional methods based on detecting the piezoelectric and pyroelectric responses, it is very difficult to determine the polarities of thin films, particularly in the case of laying these films on the polar substrates, because the detected signals from thin films are very small and those from the substrates are large. Our SNDM method, however, enables us to determine the polarities of thin films on polar substrates easily. Using SNDM, the polarities of ZnO thin films on several kinds of polar and nonpolar substrates were determined. We also determined experimentally that ZnO thin films grew with a sign opposite to the substrate polarity and it was suggested that pyroelectric effects mainly governed the polarity of ZnO films.