Abstract
The effects of biaxial in-plane stress on the elastic, dielectric, and piezoelectric (PE) properties of c-axis textured thin film wurtzite phase scandium aluminum nitride (w-ScxAl1−xN) alloys have been calculated with density functional perturbation theory. The in-plane stress σR was kept below 1 GPa covering compressive and tensile values and applied to alloy supercells represented with special quasi-random structures. An increasingly tensile biaxial stress (σR > 0) produces higher displacement-response internal-strain coefficients for the constituent atoms of the alloy and the related PE properties are more sensitive to σR when the fraction x increases. A significant rise of the relative dielectric permittivity ϵr,33η and softening of the stiffness coefficient c33E are also reported with σR > 0. The effective thin film PE strain coefficient d33,f and coupling coefficient k33,f2 show a relative increase of 22% and 26%, respectively, at σR = 1 GPa and x = 0.438. Both tensile σR and x tend to decrease the c/a cell parameter ratio of the wurtzite structure with a significant impact on the PE coefficients. Based on the decomposition of the stiffness, dielectric, and PE coefficients as well as the structural data, it is suggested that tensile biaxial stress enhances the hexagonal character of w-ScxAl1−xN in a qualitatively similar manner as the scandium nitride fraction x does. The manufacture and PE characterization of a beneficially stressed thin film of w-ScxAl1−xN on a substrate of w-InyAl1−yN with adjusted x, y values are suggested to confirm the calculated values of d33,f.
Highlights
The effects of biaxial in-plane stress on the elastic, dielectric, and piezoelectric (PE) properties of c-axis textured thin film wurtzite phase scandium aluminum nitride (w-ScxAl1−xN) alloys have been calculated with density functional perturbation theory
The in-plane stress σR was kept below 1 GPa covering compressive and tensile values and applied to alloy supercells represented with special quasi-random structures
Piezoelectric (PE) aluminum nitride (AlN) in the wurtzite structure (w) is a promising material for the microelectromechanical system (MEMS) industry.1,2 w-AlN is often grown in thin film form with a c-axis texture to exploit the effective longitudinal PE stress coefficient d33, f = 5.5 pm V−1
Summary
Incorporating scandium to form substitutional wurtzite-like scandium aluminum nitride (w-ScxAl1−xN) alloys has been found experimentally to improve the PE properties of w-AlN, for example, by increasing the d33, f by as much as 490%,3,4 and leads to higher electromechanical coupling coefficients in MEMS devices. No dielectric data are provided, which are important to establish the coupling coefficients of w-ScxAl1−xN alloys These superlattices are not typical of the sputtered w-ScxAl1−xN alloys found in PE devices, which can possess fractions of ScN x < 0.5, and ordering has been shown to greatly affect the PE properties of w-ScxAl1−xN.. A significant increase in the PE figures of merit d33, f , e31, f and k233, f is found to be proportional to positive values of σR for x ≥ 0.313 This is interpreted as a consequence of the sensitivity to σR of the atomic displacement-response internal-strain coefficients in w-ScxAl1−xN alloys. We suggest a method to fabricate tensile stressed PE materials and examine the possible benefits for devices
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