**5. Conclusions**

SAW devices have been fabricated using Sc0.43Al0.57N as a piezoelectric thin film, synthetized over polycrystalline and single crystalline diamond substrates. The admittance characteristics confirm that Sc0.43Al0.57N thin films can be employed in the fabrication of SAW devices with low insertion losses above 2 GHz. The higher insertion loss and attenuation of the SAW in the PCD structure revealed the damaging influence of the electrical performance of the scattering in the grain boundaries, as well as the defects within the crystal structure.

The Sc0.43Al0.57N thin films employed in these devices were synthesized by reactive magnetron sputtering without the intentional heating of the substrate. The energy required to minimize the defects within the thin film is provided by the plasma conditions. The Rayleigh and Sezawa mode K 2 eff values substantially increased with the devices comprising the Sc0.43Al0.57N thin films, as compared to those values obtained for the modes propagating within the Sc0.27Al0.73N thin films shown in our previous works [18,39]. These two propagating modes are efficiently excited where the reflection coefficient of the Sezawa mode propagating in the SCD heterostructure outstands, with an attenuation close to −50 dB. Additionally, SAW filters with –3-dB bandwidth above 180 MHz have been fabricated at 4.7 GHz resonance frequencies with insertion losses below –5 dB with a SCD based device, revealing their potential application to 5G technology.

Furthermore, the devices presented in this work showed promising electrical performances for sensing applications where the AlN and ScAlN thermal, chemical, or high stiffness of the compound were exploited. Therefore, the applicability of these devices

will not only be constrained by 5G technology, but these results reveal the potential of the versatile ScAlN compound for the next-generation of SAW devices.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/mi13071061/s1, Figure S1: SEM inspection of the SAW filter IDT and ground reflector; Figure S2: RBS spectra of a Sc0.43Al0.57N thin film [40].

**Author Contributions:** Conceptualization, M.S.L. and G.F.I.; methodology, M.S.L.; software, M.S.L.; validation, M.S.L. and D.L.-R.; formal analysis, M.S.L. and D.L.-R.; investigation, M.S.L. and D.L.-R.; resources, G.F.I. and O.A.W.; data curation, M.S.L.; writing—original draft preparation, M.S.L. and L.F.-G.; writing—review and editing, M.S.L., L.F.-G. and G.F.I.; visualization, M.S.L., D.L.-R. and L.F.-G.; supervision, G.F.I.; project administration, G.F.I.; funding acquisition, G.F.I. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
