Trials to achieve high-quality c-axis-oriented LiNbO3 thin films: Sputter-deposition on a-SiO2, ZnO/SiO2, quartz(0001), and SrTiO3(111) substrates

The crystallographic orientation, crystallinity, and surface morphology of LiNbO3 (LN) thin films sputter deposited on less investigated a-SiO2, ZnO-covered a-SiO2 (ZnO/SiO2), quartz(0001), and SrTiO3(111) substrates were studied with X-ray diffraction and atomic force microscopy. Under precise control of the deposition temperature and O-2 flow rate, c-axis-oriented LN films were obtained on a-SiO2. The same condition was applied to deposition on quartz(0001) substrate that has similar a-axis parameter of LN. Preferentially c-axis-oriented LN films, but not epitaxial, were obtained on quartz(0001). Although Solid-phase crystallization on quartz(0001) improved the c-axis orientation compared with crystallization during deposition, out-grown grains on the LN base layer roughened the surface morphology. The atomically not smooth quartz(0001) surface may hinder close lattice matching with LN crystal. While c-axis oriented ZnO layer on a-SiO2 plays a role of lattice-matched template substrate, the morphology of the ZnO is the key factor controlling crystallinity; sufficiently thick and smooth ZnO layer was effective to obtain highly c-axis-oriented epitaxial-like LN film. Chemical interaction is another factor affecting crystal growth. Only epitaxial LiNb3O8 layer could be obtained on SrTiO3(111) substrate with loss of Li at deposition temperatures between 350 and 550 degrees C. Migrating Li atoms reacted with SrTiO3 resulting in a substantial loss of Li from the LN film.