Reduction mechanism of loss tangent of scandium-doped aluminum nitride thin film by post-deposition annealing

Scandium-doped aluminum nitride thin films are key materials for MEMS applications including bulk acoustic wave devices for communication. Although one drawback is the increase in the loss tangent with increasing Sc concentration, the loss tangent is reported to decrease after post-deposition annealing. However, the underlying mechanisms remain unclear. In this study, we propose the hypothesis that a low-resistivity thin layer near the surface of a substrate is one of the main reasons for the high loss tangent, and that annealing enhances the resistivity, eventually decreasing the loss tangent. The reasonability of the hypothesis was successfully confirmed by analyzing the frequency response of the loss tangent using an equivalent circuit with current–voltage characteristics, cathodoluminescence, etc. This achievement represents a significant step toward advanced methods for reducing the loss tangent and its application to other thin-film materials.