Thermodynamic Study on Diffusion Growth of Ga³⁺-Doped LiNbO₃ Single Crystal Thin Film for Photonic Application

A thermodynamic study was performed on the growth of Ga³⁺-doped LiNbO₃ (LN) thin film. Some Ga³⁺-doped LN thin films were grown on the LN surface by thermal diffusion of Ga₂O₃ film in the temperature range of 1000-1100 °C. After growth, the Ga³⁺ ion in the grown thin film was profiled, and its diffusion-growth characteristics were studied. The temperature dependences of diffusion-growth coefficient and solubility were quantified. These dependences are crucial to the design and growth of a Ga³⁺-doped LN thin film for various photonic applications. A comparison with the case of Ti⁴⁺ diffusion-growth, which induces an increase of LN refractive index and hence formation of an optical waveguide, shows that Ga³⁺ grows considerably faster than Ti⁴⁺ because of its smaller ionic radius. In addition, Ga³⁺-doping contribution to LN refractive index was studied by measuring and comparing the refractive indices at Ga³⁺-grown and Ga³⁺-free parts of the crystal surface. The results show that the contribution is small.