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 degrees 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 degrees 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. (C) 2013 Optical Society of America