**1. Introduction**

Thin film-based surface acoustic wave (SAW) devices are enunciated to be exploited for their use in communication devices, acousto-optic devices, optoelectronics, automotive sensors and biosensors, etc., as these are efficient, compact, economical and provide the advantage of tailoring the material properties as per the need of the application relative to single crystal SAW device [1–5]. And with the advent of technology, SAW devices have become an integral part of current LTE and 5G wireless devices [6]. These devices are increasingly finding applications in the domain of life sciences and microfluidics (acoustofluidics), producing 'lab-on-a-chip' (LOC) or micro total analysis systems (*µ*TAS) [6,7]. The important parameters that gauge the competence and use of SAW device are its phase velocity, electromechanical coupling coefficient and temperature coefficient of delay [7]. Until now, various SAW layered structures, like SiO2/LiNbO3, LiNbO3/Sapphire, ZnO/Diamond, etc., have been investigated for their potential as acoustic wave devices [8]. It is evident from the available literature that for high frequency applications, temperature stable SAW devices with appreciable SAW phase velocity and good electromechanical coupling coefficient are required [8]. Diamond based SAW devices are reported to provide the advantage of high velocity. But these devices are expensive and one needs to compromise with coupling coefficient [8]. On the other hand, widely used LiNbO<sup>3</sup> based SAW devices have reasonable SAW velocity and good coupling coefficients [8]. Efforts are still being made to find an alternative that is suitable for high frequency applications. Beryllium Oxide (BeO) single crystal is reported to be piezoelectric material with very high acoustic velocities for bulk longitudinal and shear waves and is studied for its application in SAW device [9]. Although, there are various reports on the deposition of the crystalline BeO over layer on crystal and amorphous substrates [10,11], yet no attempt to date has been made to study the use of BeO thin films in SAW device applications. BeO is reported to have unique mechanical and thermal properties, such as hardness, high melting point, high thermal conductivity, and large elastic constants, making it suitable for large number of applications in microwave and nano devices [11,12]. All these properties make BeO a sturdy material.

So, in the present work, an attempt has been made to study the use of thin films of BeO in acoustic wave devices. The SAW propagation properties of BeO/128◦ YX LiNbO<sup>3</sup> layered structure have been found using the theoretical tool developed by Farnell and Alder [13,14]. The SAW software used in the present analysis has been exercised earlier by many workers, like Zhou et al., Benetti et al., etc., to find the optimum values of thickness of various layers used in the multilayered acoustic devices [15–17]. The theoretical results are in close proximity with the experimentally obtained results [15–17]. Moreover, the experimental realization of the proposed layered structure seems to be possible and supported by the report on the growth of crystalline BeO thin films irrespective of the substrate type [10]. This suggests that BeO/128◦YX LiNbO<sup>3</sup> layered structure can be experimentally realized without lattice mismatch.

In the present study, the authors have considered widely used 128◦ YX LiNbO<sup>3</sup> SAW substrate to investigate the effect of adding BeO and subsequently TeO<sup>3</sup> thin films on it theoretically. The BeO over layer thickness is optimized and it is found that with the integration of 0.08 λ thick BeO over layer in BeO/128◦YX LiNbO<sup>3</sup> layered structure, an efficient SAW device with appreciable phase velocity (∼4500 ms−<sup>1</sup> ) along with a very high electromechanical coupling coefficient (∼10%) can be realized. The bilayer BeO (0.08 λ)/128◦YX LiNbO<sup>3</sup> SAW device is temperature unstable and has a high positive value of temperature coefficient of delay (TCD ~ 66 ppm ◦C −1 ). The device can be made temperature stable by integrating it with negative TCD over layer. TeO<sup>3</sup> films are reported to exhibit negative temperature coefficient of delay [1,18–21] and, thus, can be used in the present layered structure to make it temperature stable. The proposed BeO (0.08 λ)/128◦YX LiNbO<sup>3</sup> bilayer SAW structure is integrated with ∼0.026 λ thick TeO<sup>3</sup> over layer to realize a temperature stable device and moreover, the values of SAW phase velocity and electromechanical coupling coefficient remain essentially untouched. Thus, the authors present the first-ever optimized SAW device based on BeO thin film owing to its potential use in acoustic wave device applications.
