Characterization of Ta2O5 thin films prepared by reactive evaporation

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 degrees 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 degrees 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 angstrom respectively, for the studied area of 2.5 mu x 2.5 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.