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

Multilayered structures based on wide bandgap nitride piezoelectric thin films are very attractive for high-temperature surface acoustic wave (SAW) sensor applications. In this respect, scandium aluminum nitride (ScAlN) films are of particular interest as they combine enhanced piezoelectric properties and slower acoustic wave velocities when the Sc content steadily increases up to 40%. This property offers the possibility to combine slow ScAlN films on fast substrates like sapphire, to generate higher-order SAW modes which often show a better electromechanical coupling coefficient k2 compared to zero-order modes. In this letter, we show that low-attenuated longitudinal SAW can be generated in the ScxAl1-xN/sapphire structure, for the x parameter varying in a large range. This theoretical result is then confirmed by the experimental investigation of SAW resonators based on highly textured (002) Sc0.09Al0.91N films sputtered on c-cut sapphire substrates. It is finally shown that the use of electrodes based on metals with high density can lead to SAW structures offering a unique combination between a large bandgap over 5 eV, a k2 value beyond 1%, and a high SAW velocity near 10 000 m/s.