Mechanism for LiNb₃O₈ phase formation during thermal annealing of crystalline and amorphous LiNbO₃ thin films

The kinetics of second phase formation during the thermal annealing of crystalline and amorphous LiNbO₃(LN) thin films was investigated. When c-axis-oriented LN texture film was annealed at temperatures higher than 700 °C, Li₂O desorbed from the surface and grain boundaries, which induced epitaxial precipitation of the LiNb₃O₈phase on the LN (0001) plane. While the LiNb₃O₈volume exhibited saturation behavior against annealing time, oxygen atoms continued to diffuse out from the inner part of the film. Arrhenius plots of the rate of LiNb₃O₈formation and oxygen loss were scaled with common activation energy of 30.5 kcal/mol. In terms of phase formation from amorphous LN, annealing at 700 °C in a vacuum induced rapid crystallization into mixtures of LN and LiNb₃O₈, the volume ratio of which can be explained by the phase separation model. Annealing in an O₂atmosphere, however, produced an incubation time before crystallization and a larger volume fraction of LiNb₃O₈after crystallization. Oxygen molecules that penetrated into the film will hinder the migration of atoms and decelerate crystallization. Vacancies produced as a result of Li₂O loss will facilitate the nucleation of LN and LiNb₃O₈pairs while maintaining the epitaxial relation.