AlYN Thin Films with High Y Content: Microstructure and Performance

Abstract

Pseudobinary nitride alloys display enhanced piezoelectric properties compared to their nonalloyed counterparts enabling their wide application in high-performance transducers and acoustic wave resonators. Their fabrication remains challenging because of their inherently stochastic nature, which requires in-depth understanding of the film growth dynamics and the interplay of deposition parameters. Herein, thin Al1−xYxN films are produced with varied yttrium content in the range from x = 0.09 to 0.28 on a gradient seed layer on 200-mm Si substrates and investigated via various X-ray diffraction methods, high-resolution scanning transmission electron microscopy, nanoindentation, and atomic force microscopy. Bulk acoustic wave resonators, solidly mounted on a multilayer acoustic isolation, are fabricated to analyze the piezoelectric performance of the films and to extract corresponding material parameters via fitting of the high-frequency electrical response by 1D Mason's model. The trend of declining coupling is explained by the lattice softening and the increase in electron density, experimentally observed by monitoring reduced elastic modulus and dielectric constant values, respectively. The absence of expected enhancement of the piezoelectric modulus is interpreted by the presence of oxygen impurities, facilitating the inhomogeneous strain of the AlYN lattice, which effectively cancels the energy flattening phenomenon, found in III–V pseudobinary alloys.