Non-leaky longitudinal acoustic modes in ScxAl1-xN/sapphire structure for high-temperature sensor applications

Abstract

Multilayered structures based on wide bandgap nitride piezoelectric thin films are very attractive for high-temperature surface acoustic waves (SAW) sensor applications. In this respect, scandium aluminium nitride (ScAlN) films are of particular interest as they combine enhanced piezoelectric properties and slower acoustic waves velocities

Epitaxial Growth of Sc0.09Al0.91N and Sc0.18Al0.82N Thin Films on Sapphire Substrates by Magnetron Sputtering for Surface Acoustic Waves Applications

“…A previous work, including a simulation study, revealed that the three observed modes respectively correspond to a Rayleigh SAW, a shear horizontal (SH) SAW, and a high-velocity longitudinal leaky SAW. The latter can show low attenuation coefficients if a careful choice of the film thickness, as well as the electrode nature and thickness, is made [30]. The experimental velocities of these three modes are of 4680 ms −1 , 6045 ms −1 and 9945 ms −1 respectively.…”

“…The electromechanical coupling k 2 was estimated from the extracted difference between resonant and anti-resonant frequencies. The same numerical technique was previously used to explain the nature of the acoustic modes observed experimentally and predict the existence of low-attenuated high-velocity longitudinal leaky SAWs in ScxAl1-xN/sapphire with optimized film and electrode thicknesses [30]. The electromechanical coupling k 2 of SAW modes propagating in Sc x Al 1−x N/sapphire structure generally increases with the Sc x Al 1−x N thickness, due to the better confinement of displacements in the piezoelectric film.…”

“…The electromechanical coupling k 2 of SAW modes propagating in Sc x Al 1−x N/sapphire structure generally increases with the Sc x Al 1−x N thickness, due to the better confinement of displacements in the piezoelectric film. In a structure with a fixed Sc x Al 1−x N thickness, higher k 2 can be achieved if the Sc content grows [30]. The coupling of the Rayleigh SAW also depends on the wave structure: with increasing the ratio u 3 /u 1 , where u 1 and u 3 are the tangential and vertical motions respectively, accompanying the wave propagation in the film, k 2 also grows because the electrostatic potential is coupled only with u 3 .…”

“…The electromechanical coupling k 2 was estimated from the extracted difference between resonant and anti-resonant frequencies. The same numerical technique was previously used to explain the nature of the acoustic modes observed experimentally and predict the existence of low-attenuated high-velocity longitudinal leaky SAWs in Sc x Al 1−x N/sapphire with optimized film and electrode thicknesses [30]. With increasing mass load of the grating caused by thicker electrodes, or electrode metals with a higher density, the SAW velocity decreases while the wave structure transforms, the ratio u3/u1 grows and k 2 also grows.…”

“…Three SAW modes were observed: a Rayleigh mode, a shear horizontal mode, and a longitudinal leaky mode. These SAW measurements were further studied in our previous work [30]. Though electromechanical coupling of SAW modes in the Sc x Al 1−x N/sapphire bilayer structure generally grows when the Sc content increases up to 40%, the numerical investigation of the coupling coefficient as a function of the electrode metal and thickness, as well as the Sc content, revealed that a trade-off between the degradation of the dielectric properties of ScAlN and the improvement of its piezoelectric properties with Sc content can be achieved, if the intermediate composition between 10 and 20% of scandium is combined with electrode metal with a high density, like Pt for instance.…”