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Next-generation telecommunication standards demand compact filters that can offer low acoustic loss, wide bandwidth, and steep filter skirts to enable efficient operation at high frequencies. To meet these requirements, bulk acoustic wave (BAW) resonators that combine a high quality factor (Q) factor with a high effective electro-mechanical coupling coefficient (keff2) are desired. Wurtzite aluminum scandium nitride (AlScN) due to its optimal piezoelectric, electrical, and thermal properties has become the industry standard for these applications. Despite these advantages, the search for alternative alloying elements has accelerated due to concerns related to the availability and cost of Sc. Yttrium (Y) has a similar valence electron configuration as Sc and hence has emerged as a promising alternative. Although mixing enthalpy calculations show that AlYN retains its wurtzite structure up to a Y concentration of 75%, experimentally this has not been achieved yet. [1,2] In this work, we tackle this limitation by engineering the seed layer to incorporate more Y into the AlYN thin film.
We deposit Al1-xYxN on Pt/SiO2/Si substrates using AlN or Al1-xYxN seed layer. Our preliminary results agree with previous reports pointing out the critical role of the seed layer (see Figure 1(a)) [2]. The use of the graded Al1-xYxN seed layer significantly improves the crystalline quality of Al0.85Y0.15N as well as the AlN seed layer of different thicknesses (Figure 1(b)). Based on these findings, we further optimize Al1-xYxN film quality by engineering the concentration and thickness of graded Al1-xYxN seed layer aimed to enhance structural and piezoelectric properties.
Figure 1: (a) XRD 2θ/θ scans of Al1-xYxN thin films grown without a seed layer and (b) ω-FWHM of Al0.85Y0.15N thin films sputtered with varying AlN seed layer thickness on Pt/SiO2/Si substrates.
[1] Žukauskaitė, Agnė, et al. "YxAl1-xN thin films." Journal of Physics D: Applied Physics 45.42 (2012): 422001.
[2] Solonenko, Dmytro, et al. "AlYN thin films with high Y content: Microstructure and performance." physica status solidi (RRL)–Rapid Research Letters 17.10 (2023): 2300193.