Determination of linear and non-linear elastic, piezoelectric and dielectric properties of Aluminum-Scandium-Nitride-Films depending on the Scandium-Concentration

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The main objective of the project is to determine a complete set of tensor components of the elastic and piezoelectric properties of thin films of hexagonal Al1-xScxN with Sc concentrations up to x = 0.4 in order to be able to describe the correlation of the linear tensor components with the Sc concentration and its nonlinear course. For this purpose, measurements with samples based on both c-axis oriented and a-plane oriented layers on r-plane sapphire will be used for the first time. In this way, it is possible to experimentally determine a complete set of tensor components for different compositions of the ternary solid solutions. For this purpose, the propagation direction of surface waves in microacoustic resonators is rotated in a controlled manner with respect to the crystal axes of the c- and a-plane Al1-xScxN thin films. Through these measurements, the coupling coefficient of SAW devices and the phase velocity of the waves can be determined as a function of crystal directions. These allow the extraction of piezoelectric and elastic constants.The determination of the linear tensor components forms the basis for the study of nonlinear constants. To achieve this goal, more advanced measurements are envisioned in which an external static electric field is applied and varied to "stiffen" the piezoelectric crystal layers in a controlled manner. In addition, piezoelectric surface acoustic wave resonators and the laser-ultrasonic method will be extended by a bending apparatus to determine the nonlinear elastic constants. The planned experiments and complementary simulations are basic research for which no data are yet available and for which new methods and algorithms for data analysis will be developed by the applicants.To avoid misunderstandings, we would like to point out here that the term "nonlinear" or "nonlinearity" is used in this application with two different meanings. On the one hand, it refers to the nonlinear dependence of the linear material constants (i.e., the second-order elastic and piezoelectric constants and the dielectric constants) on the scandium content x. On the other hand, it refers to the nonlinear material constants, i.e. the elastic and piezoelectric constants of third order, the electrostriction constants, etc.