Eur. Phys. J. Appl. Phys. 36, 119–124 (2006) DOI: 10.1051/epjap:2006113 T HE EUROPEAN P HYSICAL JOURNAL APPLIED PHYSICS Characterization of Ta 2 O 5 thin films prepared by reactive evaporation M.H. Asghar 1, 2, a , F. Placido 1 , and S. Naseem 2 1 Thin Film Centre, University of Paisley, PA1 2BE, Scotland 2 Centre of Excellence in Solid State Physics, University of Punjab, Lahore, Pakistan Received: 1 March 2006 / Received in final form: 13 July 2006 / Accepted: 1 August 2006 Published online: 11 October 2006 – c  EDP Sciences Abstract. 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 λ = 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 λ = 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 ˚ A respectively, for the studied area of 2.5 μ × 2.5 μ. 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. PACS. 78.20.-e Optical properties of bulk materials and thin films – 78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissiv- ity) 1 Introduction The unique optical properties of tantalum pentoxide (Ta 2 O 5 ) make this material an important candidate for many applications. Indeed, the reasonably high refractive index of Ta 2 O 5 (n ∼ 2.1 at λ = 550 nm), and its low absorption for wavelengths ranging from 300 nm up to 2.0 μm [1–3], are properties that resulted in the emer- gence of tantalum pentoxide as a key material to be used in photovoltaics, optics, and photonic devices. Ta 2 O 5 thin films are used as antireflective coatings for solar cells and a e-mail: mh asghar@yahoo.com charge-coupled devices [3–5]. They are of great interest for optoelectronic applications as propagation and waveguid- ing channels [6]; in optical devices as birefringent coatings and as components of multilayer interference filters [7,8]; and for nonlinear optical applications [9]. Numerous investigations of the optical properties of Ta 2 O 5 coatings have been reported, in which var- ious chemical and physical deposition methods were employed. Such techniques include electron-beam evapo- ration [2,7,8], reactive sputtering [4,6], ion-beam sputter- ing [10], atomic layer deposition [11], reactive low-voltage ion plating [12], sol-gel spin coating [9,13], and pulsed laser deposition (PLD) [14–16]. The tantalum oxide films, Article published by EDP Sciences and available at http://www.edpsciences.org/epjap or http://dx.doi.org/10.1051/epjap:2006113