Reactively evaporated thin films of tantalum oxide are prepared on glass
substrate, using electron beam heating, for optical applications. Firstly,
the deposition was carried out at 0.20 nm/s, with substrate temperature of
200 °C, and oxygen flow rate was varied from 0.0 to 30.0 sccm to study
the effect of flow rate on optical constants. The optical constants
evaluated by using transmission data of the samples, with curve fitting,
show a strong dependence on oxygen flow rate. Oxygen flow rate of 10.0 sccm
has been found to give reasonably high index (n:2.11 at $\lambda =500$ nm)
and low absorption of the order of 10−3 in most part of the desired
spectrum (380–850 nm). However, for oxygen flow rates below 10.0 sccm and
above 20.0 sccm the films have exhibited low index and comparatively high
absorption. In the next step, deposition rates were varied from 0.10–0.30
nm/s with steps of 0.01, keeping oxygen flow rate and substrate temperature
constant at 10.0 sccm and 200 °C respectively, to optimize the film
properties. A variation in refractive index and extinction coefficient
values is observed with varying deposition rates. An increase in refractive
index (n:2.125 at $\lambda =500$ nm) with reduced absorption (“k” of the
order of 10−4) is achieved over the entire spectrum for the film
deposited at 0.10 nm/s. The film was found to be highly adherent to the
substrate as revealed by qualitative adhesive tape peel test. Keeping in
view the application of the work, calculation of optical constants was
extended up to 1100 nm for the sample deposited at 0.10 nm/s. The results
have shown nearly constant optical constant values over the extended range
making the film useful over a broad spectral region. AFM studies show that
the surface is extremely smooth and compact, giving average and rms
roughness values of 5.51 and 7.174 Å respectively, for the studied area
of $2.5~\mu \times2.5~\mu$. XRD and SEM studies carried out for structural
analysis show that the film is generally amorphous with a sign that
crystalline structure could be achieved at little higher substrate
temperatures. The surface of the film is very smooth, free of voids, with
the particle size in the range of 17–35 nm, calculated from SEM studies. The
film has exhibited compact and dense structure. The work reported in this
paper will be useful in tailoring the optical properties of Ta2O5
as per requirement.