Temperature-induced changes in optical properties of thin film TiO_2 ～Al_2O_3 bi-layer structures grown by atomic layer deposition

摘要

We investigate the optical properties and corresponding temperature-induced changes in highly uniform thin amorphous films and their bi-layer stacks grown by Atomic Layer Deposition (ALD). The environmentally driven conditions such as temperature, humidity and pressure have a significant influence on optical properties of homogeneous and heterogeneous bi-layer stacked structures of TiO_2-Al_2O_3 and subsequently affect the specific sensitive nature of optical signals from nano-optical devices. Owing to the super hydrophilic behavior and inhibited surface defects in the form of hydrogenated species, the thermo-optic coefficient (TOC) of ～100 nm thick ALD-TiO_2 films vary significantly with temperature, which can be used for sensing applications. On the other hand, the TOC of ～100 nm thick ALD-Al_2O_3 amorphous films show a differing behavior with temperature. In this work, we report on reduction of surface defects in ALD-TiO_2 films by depositing a number of ultra-thin ALD-Al_2O_3 films to act as impermeable barrier layers. The designed and fabricated heterostructures of ALD-TiO_2/Al_2O_3 films with varying ALD-Al_2O_3 thicknesses are exploited to stabilize the central resonance peak of Resonant Waveguide Gratings (RWGs) in thermal environments. The temperature-dependent optical constants of ALD-TiO_2/Al_2O_3 bi-layer films are measured by a variable angle spectroscopic ellipsometer (VASE), covering a wide spectral range 380 ≤ λ ≤ 1800 nm at a temperature range from 25 to 105 °C. The Cauchy model is used to design and retrieve refractive indices at these temperatures, measured with three angles of incidence (59°, 67°, and 75°). The optical constants of 100 nm thick ALD-TiO_2 and various combinational thicknesses of ALD-Al_2O_3 films are used to predict TOCs using a polynomial fitting algorithm.