Integration of epitaxial LiNbO₃thin films with silicon technology

Development of bulk acoustic wave filters with ultra-wide pass bands and operating at high frequencies for 5^thand 6^thgeneration telecommunication applications and micro-scale actuators, energy harvesters and sensors requires lead-free piezoelectric thin films with high electromechanical coupling and compatible with Si technology. In this paper, the epitaxial growth of 36°Y-X and 30°X-Y LiNbO₃films by direct liquid injection chemical vapour deposition on Si substrates by using epitaxial SrTiO₃layers, grown by molecular beam epitaxy, has been demonstrated. The stability of the interfaces and chemical interactions between SrTiO₃, LiNbO₃and Si were studied experimentally and by thermodynamical calculations. The experimental conditions for pure 36°Y-X orientation growth have been optimized. The piezoelectricity of epitaxial 36°Y-X LiNbO₃/SrTiO₃/Si films was confirmed by means of piezoelectric force microscopy measurements and the ferroelectric domain inversion was attained at 85 kV.cm^-1as expected for the nearly stoichiometric LiNbO₃. According to the theoretical calculations, 36°Y-X LiNbO₃films on Si could offer an electromechanical coupling of 24.4% for thickness extension excitation of bulk acoustic waves and a comparable figure of merit of actuators and vibrational energy harvesters to that of standard PbZr_1-xTi_xO₃films.