Next, the research team studied electrode materials from the viewpoint of k2. Figure 3 shows the relationship obtained from the calculation by the FEM between electrode film thickness and k2 using various electrode materials in a ZnO/electrode/27ºY-X quartz structure. k2 was the largest in Al and the smallest in Pt. In the range illustrated, with Al, Ni and Cu, k2 did not change much when the film thickness changed, but with Pt, as the film thickness became larger, k2 became smaller.
Based on these results, the research team eliminated Cu, Ni and Pt from the study.
Regarding the Al electrode and the Au electrode, Fig. 4 shows the relationship between the metallization ratio and the reflection coefficient in a ZnO/electrode/27ºY-X quartz structure. In Al, the polarity of the reflection coefficient inverted at the metallization ratio of 0.37, with monotone decreasing. In Au, on the other hand, though the metallization ratio was 0.3 or greater in a decreasing trend, the reflection coefficient was in the positive domain, up to a metallization ratio of 0.7. Figure 5 shows the relationship between the metallization ratio and k2 in a ZnO/electrode/27ºY-X quartz structure. In the range illustrated, k2 of the Al electrode increased to its peak value near the metallization ratio of 0.45, and k2 of the Au electrode reached its peak value near 0.3.
Fig. 3 Relationships between electrode film thickness and electro-mechanical coupling coefficient in ZnO/various electrodes/27ºY-X quartz structure
Fig. 4 Relationships between metallization ratio and reflection coefficient in ZnO/electrode/27ºY-X quartz structure
Fig. 5 Relationships between metallization ratio and electro-mechanical coupling coefficient in ZnO/electrode/27ºY-X quartz structure