Epitaxy, optical and acoustical properties of X-, Y-, and Z-axis oriented LiNbO3 thin films grown on sapphire substrates

LiNbO3 (LN) single crystals are highly applied in acoustical, pyroelectric, acousto-optic, electro-optic, and non-linear optical devices [1]. LN thin films are attracting interest due to possibility to miniaturize, to integrate and to ameliorate the performance of these devices. Optical and surface acoustic wave (SAW) devices require the LN films with single crystallographic out-of-plane and in-plane orientations. The epitaxial growth of X-, Y- and Z-oriented films on sapphire substrates was already reported in literature. However, these LN films usually contained growth domains with different in-plane orientations. Moreover, little is known about the residual stresses and their relaxation mechanisms including twinning in these films. In this work, the epitaxial growth of X-, Y- and Z-axis oriented LN films was optimized on A-, M- and C-sapphire by Pulsed-Injection Metal Organic Chemical Vapor Deposition (PIMOCVD), respectively. In order to obtain films with smooth surface and consisting of pure LN phase with controlled Li concentration, the deposition parameters such as growth rate, deposition temperature, solution concentration and injection frequency were tuned. A particular effort was done to obtain films with single in-plane orientation. The epitaxial quality, Li2O nonstoichiometry, in-plane orientation, residual stresses and twinning were studied by means of X-ray diffraction and Raman spectroscopy. The LN films with nearly stoichiometric composition were obtained. The thick films tend to crack and to form the twins in order to relax the high thermal stresses [2]. The cooling rate was optimized in order to avoid the relaxation of the stresses. The differences in relaxation mechanisms and in ability to withstand high stresses of X-, Y- and Z-LN films will be discussed. Energy gap and refractive indexes of LN films, measured by spectroscopic ellipsometry, were similar to those of LN single crystal. The simulations indicated that a high phase velocity and a high electromechanical coupling can be attained in LN films on sapphire. Acoustic resonance at 5.5 GHz in single-port resonators based on LN/sapphire films was demonstrated experimentally.