*3.2. Filters*

The transmission coefficient of the SAW filter fabricated using the PCD heterostructure showed that the Rayleigh and Sezawa modes propagated with their center frequency (fc) at 2.66 GHz and 4.88 GHz, respectively (Figure 3A). Both modes presented a maximum gain above −6 dB, whereas their cut−off frequencies were below −12 dB. The −3-dB bandwidth of the Rayleigh mode was above 100 MHz, whereas the Sezawa mode bandwidth was above 180 MHz (Table 3).

**Figure 3.** Transmission coefficient of a ladder-type SAW filter fabricated on Sc0.43Al0.57N/PCD (**A**) and Sc0.43Al0.57N/SCD (**B**) heterostructures.

On the other hand, the Rayleigh mode generated in the SAW filter fabricated using the SCD substrate had a band pass maximum of −2.5 dB (Figure 3B). The center frequency of this mode was 2.17 GHz. whereas the Sezawa mode f<sup>c</sup> was at 4.74 GHz, and its band

pass maximum was −5 dB. The cut-off of both the lower (f1) and higher (f2) frequencies in the Rayleigh mode were below −18 dB, whereas those corresponding to the Sezawa mode were below −10 dB.

**Table 3.** Lower (f1) and higher (f2) cut-off frequencies, center frequencies (fc), and bandwidth in the Rayleigh and Sezawa modes for SCD and PCD filters.

