AlYN Thin Films with High Y Content: Microstructure and Performance

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 Al1xYxN 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 highfrequency
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.