High-quality crystallinity controlled ALD TiO₂ for waveguiding applications

We demonstrate a novel atomic layer deposition (ALD) process to make high-quality nanocrystalline titanium dioxide (TiO₂) with intermediate Al₂O₃ layers to limit the crystal size. The process is based on titanium chloride (TiCl₄) + water and trimethyl aluminum (TMA) + ozone processes at 250°C deposition temperature. The waveguide losses measured using a prism coupling method for 633 and 1551 nm wavelengths are as low as 0.2 ± 0.1 dB/mm with the smallest crystal size, with losses increasing with crystal size. In comparison, plain TiO₂ deposited at 250°C without the intermediate Al₂O₃ layers shows high scattering losses and is not viable as waveguide material. The third-order optical nonlinearity decreases with smaller crystal size as verified by third-harmonic generation microscopy but still remains high for all samples. Crystallinity controlled ALD-grown TiO₂ is an excellent candidate for various optical applications, where good thermal stability and high third-order optical nonlinearity are needed.