LiNbO3 film with a new epitaxial orientation on R-cut sapphire

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Abstract
It is argued that epitaxial films of ionically bonded materials are more easily achieved than those of covalently bonded materials. Good epitaxy can be achieved despite a relatively large lattice mismatch with the substrate. However, a strong influence of interfacial energy can result in difficulty in controlling the orientation of epitaxial films. The crystallographic orientation of ionically bonded LiNbO3 films was therefore studied. Growth orientation was controlled by paying attention to the sharing between octahedra in the structure and to the formation of the octahedra containing lithium and niobium ions. The orientation of the film on R-cut (011̄2) sapphire changed from (011̄2) to (101̄0) via (112̄0) by increasing the Li concentration in the film. Lithium concentration could be increased by increasing the rf power, O2 partial pressure and total gas pressure, and decreasing the substrate temperature during sputtering deposition. The (011̄2) and (101̄0) films were epitaxial with respect to the substrate. The (101̄0) epitaxial film, in particular, was of excellent quality. Detailed studies on the crystallinity and the formation mechanism of LiNbO3 film with a new epitaxial orientation (101̄0) on (011̄2) sapphire was performed. Based on the degree of freedom in sharing of octahedra containing Li and Nb, sufficient Li concentration and effects of the interfacial restriction are considered to promote the formation of the (101̄0) epitaxial film. If deposition is performed under the conditions by which (101̄0) epitaxial film is formed, excellent epitaxial film can be formed because the interface between (101̄0) LiNbO3 and (011̄2) sapphire has a larger cohesive energy.

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