Crystallography and structural evolution of LiNbO₃ and LiNb_1-xTa_xO₃ films on sapphire prepared by high-rate thermal plasma spray chemical vapor deposition

The structure and the crystallography of lithium niobate and lithium niobate-tantalate thin films (0.2-1.0 μm in thickness) with the tantalum composition range of 0 ≤ x ≤ 0.5 grown on (0001) sapphire substrate by thermal plasma spray chemical vapor deposition have been studied by means of cross-sectional high-resolution transmission electron microscopy and x-ray diffraction. The tantalum composition in the films shows a minor effect on the rocking curve full width at half maximum values. The narrowest rocking curve width was obtained for the LiNb_0.5Ta_0.5O₃ film to be as low as 0.25° λ. The films are under compressive strain along the c direction; c- and a-axis lattice parameters are correspondingly smaller and higher than those of the bulk single crystal. Under optimized growth conditions, the LiNbO₃ and LiNb_1-xTa_xO₃ films are 97% c-axis oriented. The film out-of-plane orientation changes from the [0001] to the [0112] direction by either decreasing the growth rate or increasing the substrate temperature. Particular attention has been paid to the orientation of individual grains in the partly c-axis-oriented films. The results demonstrate that their orientations are not random and specific orientation relationships are preferred for the film nucleation. The surface of as-received sapphire substrate reveals polishing defects with the well-defined surface ledges of 1-2 nm in height with smooth terraces of 25 nm in width. In the case of columnar growth, the terrace width becomes a limiting factor controlling the lateral crystallite size in the film. Finally, the film growth mechanism is discussed.