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.