Strong correlation of the piezoelectric response and microstructure in AlYN solid solutions

Nitride thin films provide a solid material platform for various applications in modern microtechnology owing to their outstanding physical properties and mechanical robustness, outperforming many oxides. AlN and especially its derivatives such as AlScN are widely employed as the piezoelectric layer in diverse piezo-micro-electro-mechanical systems (piezoMEMS) [1]. Nevertheless, Sc doping is sought to be substituted with more cost-efficient materials such as Yttrium for a manifold production scale-up [2]. In this work, we have investigated high-quality large-scale thin AlYN films with varied Y content deposited on 8’’ Si wafers by means of reactive magnetron co-sputtering. The co-sputtering technique requiring the synchronous sputtering of two, Al and Y, targets via a working gas mixture of Ar and N2 was optimized in a broad parameter space (power, pressure, substrate temperature, target to substrate distance). The films were characterized via SEM, XRD, AFM, EDS, and optical spectroscopy methods. The intricate, non-linear relations between the deposition parameters and the film properties revealed a dominant role of the sputtering power relation in the atomic composition of the films allowing us to obtain a wurtzite lattice with Y concentrations above 25%. Moreover, our results underline a critical role of a buffer interlayer (material) which prevents wurtzite-rock salt phase separations [3] within the nitride film. We show that residual film stress must be considered in large-scale growth as it promotes the formation of unwanted, non-wurtzite phases, deteriorating compositional film homogeneity. Piezoelectric properties of AlYN were characterized using double-beam laser interferometry, the results of which show a direct proportionality between the value of the longitudinal piezoelectric coefficient, d33, and the microstructure of the AlYN films deviating from the theoretical predictions [2]. Understanding the means and performance of Y doping in AlN helps establish Yttrium as a viable substitute for Sc in high-performance piezoMEMS with enhanced electro-mechanical coupling.

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[2] P. M. Mayrhofer, H. Riedl, H. Euchner, M. Stöger-Pollach, P. H. Mayrhofer, A. Bittner, and U. Schmid, Microstructure and Piezoelectric Response of YxAl1−xN Thin Films, Acta Materialia 100, 81 (2015).
[3] C. Höglund, J. Birch, B. Alling, J. Bareño, Z. Czigány, P. O. Å. Persson, G. Wingqvist, A. Zukauskaite, and L. Hultman, Wurtzite Structure Sc1−xAlxN Solid Solution Films Grown by Reactive Magnetron Sputter Epitaxy: Structural Characterization and First-Principles Calculations, Journal of Applied Physics 107, 123515 (2010).