**1. Introduction**

Surface acoustic wave (SAW) devices leverage the piezoelectric effect to generate and detect electroacoustic signals [1,2]. These devices are fabricated with cutting-edge clean room technologies, which significantly shrinks their unitary costs. The possibility of controlling phonons and monitoring their interaction with electrons, photons, or magnetic spins have attracted the attention of the research community. Their applications have increased from the delay lines of the early 1960s to the nowadays state-of-the-art sensors or the emerging quantum technologies [3,4]. Within the scope of mobile telecommunication, the operating frequencies of these devices have been steadily increasing from 450 MHz to 6 GHz. Furthermore, SAW devices based on polycrystalline piezoelectric thin films are a cost-effective MEMS solution for the 5G technological constraints due to their outstanding capabilities such as power handling or thermal stability [5,6].

Among polycrystalline piezoelectric thin films, AlN has been extensively studied for its high SAW propagation velocity due to the stiffness of the compound, as well as its thermal and chemical stability [7,8]. However, its relatively low piezoelectric constant d<sup>33</sup> restrains its applicability where large electromechanical coupling coefficients (k 2 ) are

**Citation:** Sinusia Lozano, M.; Fernández-García, L.; López-Romero, D.; Williams, O.A.; Iriarte, G.F. SAW Resonators and Filters Based on Sc0.43Al0.57N on Single Crystal and Polycrystalline Diamond. *Micromachines* **2022**, *13*, 1061. https://doi.org/10.3390/ mi13071061

Academic Editor: Agne˙ Žukauskaite˙

Received: 6 June 2022 Accepted: 29 June 2022 Published: 30 June 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

required. In this regard, the introduction of Sc atoms into the wurtzite AlN structure increases the piezoelectric response of the thin film [9]. The maximum increase is reported to occur at an Sc concentration of 43%. However, experimentally, the synthesis of this compound has been challenging for the competitive synthesis of the rock−salt phase (non-piezoelectric) of ScAlN, which is more energetically favorable at Sc concentrations above 55% [10–12]. Furthermore, the inclusion of Sc does not only increase the piezoelectric response of the compound but reduces its elastic constant and alters the optoelectronic properties, such as the bandgap [13,14].

In the case of loss-less materials, the electromechanical coupling coefficient (k 2 ) is a measure of the conversion efficiency between the mechanical and electrical energies and vice versa, and it is directly related to the piezoelectric response of the thin film [15]. Therefore, the ScAlN compound and its increase of the piezoelectric constant with the Sc concentration is a promising material to fabricate high-frequency SAW devices with outstanding performances.

In this work, SAW devices were fabricated with Sc0.43Al0.57N/diamond-based heterostructures. Highly c-axis oriented Sc0.43Al0.57N thin films were synthesized on polycrystalline and single crystal diamond substrates. The electroacoustic properties of these layered structures were assessed. From the 1-port resonators, the effective electromechanical coupling coefficients (K 2 eff) and effective propagation velocities were extracted. Finally, SAW filters working at frequencies above 4.70 GHz are presented.
