Nucleation and crystallization of Li2O-Nb2O5 ternary compound thin films co-sputtered from LiNbO3 and Li2O targets

Li2O-Nb2O5 ternary compound thin films were deposited by dual sputtering from LiNbO3 and Li2O targets to cover a wide range of Li/Nb molar ratios. When all parts of the films were crystallized during sputtering at elevated temperatures, pyrochlore phase crystallites of Li3NbO4, LiNbO3, and LiNb3O8 were preferentially oriented toward their respective (222), (006), and (6¯02) directions. These closely packed crystallographic planes having the lowest surface free energies were initially created under exposure to the plasma and the crystallization that followed proceeded exclusively on these seed planes. The volume fraction of each crystalline component as a function of the Li/Nb molar ratio obeyed the established phase diagram. In contrast, solid-phase crystallization of pre-deposited amorphous films through post annealing exhibited a non-equilibrium nature, and the volume fraction of each crystalline component depended on the crystallization rate rather than the Li/Nb molar ratio. Solid-phase crystallization decelerated when excess Li2O was contained in the film. Specifically, LiNbO3 crystallites with near-stoichiometric compositions were randomly oriented without the operating mechanism of preferential orientation toward c-planes. In certain situations, crystalline domains were spatially of inhomogeneous composition, resulting in the simultaneous coexistence of Li3NbO4, LiNbO3, and LiNb3O8.