Experimental determination of the electro-acoustic properties of thin film AlScN using surface acoustic wave resonators

In this work, all electroacoustic material parameters, i.e., the elastic, piezoelectric, and dielectric coefficients, as well as the mass density, were determined experimentally for wurtzite aluminum scandium nitride (Al${}_{1-\mathrm{x}}$ Sc${}_{\mathrm{x}}$ N) for a wide range of Sc concentrations of up to x = 0.32 from the same material source for the first time. Additionally, the mass density and piezoelectric coefficient were determined even up to $\mathrm{x}=0.42$ . Two sets of 1 $\mu$ m-thick AlScN(0001) thin films were deposited on Si(001) using reactive pulsed-DC magnetron cosputtering. One set of thin films was used to determine the $a$ - and $c$ - lattice parameters and the effective relative dielectric coefficient ${\epsilon }_{33,f}$ , using X-ray diffraction and capacitive measurements, respectively. Lattice parameters were then used to extract average internal parameter $u$ , bond length, and bond angle, as well as mass density, as a function of Sc concentration. Density functional theory calculations were performed to provide the equilibrium lattice parameters $a$ , $c$ , and $u$ , as well as the bond angle and the bond lengths for wurtzite-AlN and layered hexagonal-ScN. The second set of films was used to fabricate surface acoustic wave (SAW) resonators with wavelengths $\lambda$ from 2 up to 24 $\mu$ m. The SAW dispersion in conjunction with finite element modeling fitting was used to extract the elastic stiffness as well as the piezoelectric coefficients. The overall evolution of the material parameters and the change of the crystal structure as a function of Sc concentration is discussed in order to provide a possible explanation of the observed behavior.