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A full-epitaxial bulk acoustic wave (BAW) resonator is attractive because of its high Q and high-power handling capability. An epitaxial technique is difficult to be employed due to the amorphous SiO2 low acoustic impedance layer in the solidly mounted resonator (SMR), which consists of a piezoelectric thin film on an acoustic Bragg reflector. In this study, we report a full-epitaxial ScAlN or MgZnO SMR based on an epitaxial Ti/Pt or ZnO/Pt acoustic Bragg reflector. The pole figure of epitaxial ScAlN and MgZnO piezoelectric layers showed clear sixfold symmetry. The epitaxial Sc0.43Al0.57N SMR exhibits keff2 of 13.6%. Moreover, the full-epitaxial metal Bragg reflector can act as a thick bottom electrode. This thick electrode is attractive for high frequency operation above 10 GHz in which BAW filter suffers from Q degradation due to the increase in the resistivity of extremely thin electrode.
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Tokai et al. (2024) studied this question.
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