Characterization of reactively evaporated TiO₂ thin films as high and medium index layers for optical applications

¹ Thin Film Centre, University of Paisley, PA1 2BE, Scotland
² Centre of Excellence in Solid State Physics, University of Punjab, Lahore, Pakistan

Corresponding author: mh_asghar@yahoo.com

Received: 17 March 2006
Revised: 10 May 2006
Accepted: 22 May 2006
Published online: 19 July 2006

Abstract

Reactively evaporated TiO₂ thin films were deposited on the glass substrate by electron beam heating technique. In the first stage evaporation rates are varied keeping substrate temperature fixed at 200 °C and oxygen flow rate at 9.0 sccm for all the samples. The deposition rates were varied from 0.12–0.20 nm/s with a step of 0.02 to optimize the optical constants for the film in the wavelength band of 380–850 nm as a high index material for multilayer optical coatings and filters. The films prepared at higher deposition rates have shown low refractive index values and comparatively higher extinction coefficients for the given substrate temperature in the desired spectral range. The best optical constants were observed for the sample prepared at 0.12 nm/s (at λ = 550 nm: n: 2.39 and k: of the order of 10⁻⁹). However, the variation in refractive index for all the samples was found to be in the range of 2.39–2.21 at 550 nm. In the second stage, films were prepared at a fixed deposition rate of 0.12 nm/s at different substrate temperature (50 °C, 100 °C, 150 °C and 200 °C) to study the optical properties of the films as a function of substrate temperature in order to investigate the potential of TiO₂ film application as medium index layer. It was observed that refractive index decreases with decreasing substrate temperature. However, it was not found true for absorption. Instead, at Ts = 50 °C extremely low absorption (of the order of 10⁻⁵) has been observed with refractive index of 1.83, which increased sharply for next higher substrate temperature. Variation in absorption characteristics is found with varying substrate temperatures. The result shows that TiO₂ films can be used as high and medium index layer in optical coating by varying the deposition parameters during growth. Effectively, one can achieve performance of two materials with only one. This effect could be utilized to produce variable index layers and coatings. Two samples with best high and medium optical constant values, were further investigated using Atomic force microscopy (AFM), X-ray diffraction (XRD) and scanning electron microscopy (SEM) for surface and structure analysis. The results show that the particle size for the film is very small (10–20 nm), and the surface appears to be very compact, smooth and free of pinholes with roughness in angstrom range, extremely suitable for optical applications in single and multilayer structures.