Shear SAW piezoelectricity of single-crystalline $\mathrm{LiNbO_3}$ films grown on $42^{\circ}$ YX-$\mathrm{LiTaO_3}$ substrates by DLI-CVD
Direct growth of single-crystalline $\mathrm{LiNbO_3}$ films with technologically relevant nonpolar orientations and a single ferroelectric domain state remains challenging. Here, we demonstrate the epitaxial growth of a 320-nm-thick, single-domain $42°$ YX-oriented $\mathrm{LiNbO_3}$ film on a congruent $42°$ YX-cut $\mathrm{LiTaO_3}$ substrate by direct liquid injection chemical vapor deposition. The deposition temperature was limited to $600°\mathrm{C}$ to preserve the ferroelectric domain state of the $\mathrm{LiTaO_3}$ substrate. X-ray diffraction confirmed that the $\mathrm{LiNbO_3}$ film inherited the substrate orientation and domain state and exhibited no detectable secondary phases. The LN rocking-curve full width at half maximum was $0.011°$, approaching the instrumental resolution and demonstrating its high crystalline quality. Single-port leaky surface acoustic wave (LSAW) resonators with a wavelength of $4\,μ\mathrm{m}$ were fabricated on the $\mathrm{LiNbO_3/LiTaO_3}$ heterostructure and on a bare $\mathrm{LiTaO_3}$ reference substrate. At approximately 1 GHz, the $\mathrm{LiNbO_3/LiTaO_3}$-based resonators exhibited an electromechanical coupling coefficient of about 13%, compared to 8.5% for the reference $\mathrm{LiTaO_3}$-based resonators. These results demonstrate that direct epitaxial growth can produce high-quality, strongly piezoelectric $\mathrm{LiNbO_3}$ films and provide a route toward thickness-controlled layered substrates for high-performance acoustic devices.