optical properties of amorphous [ta o -[tio (x=0.08) thin...

International Journal of Research and Innovations in Science and Technology©SAINTGITS College of Engineering, INDIAwww.journals.saintgits.orgResearch paper

Optical Properties of Amorphous[Ta2O5]1-x-[TiO2]x (x=0.08) Thin Films

Mahesh Kumar Agarwal1*, Aradhana Bhandari2, Y. Singh1, N. S. Panwar2

1GB Pant Engineering College, Pauri Garhwal, Srinagar, India2University Science Instrumentation Centre, HNB Garhwal University, Srinagar, India

*Corresponding author E-mail: [email protected]

Copyright © 2014 IJRIST. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use,distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Thin films of [Ta2O5]1-x-[TiO2]xfilm deposited by RF sputtering method. The spectral transmissions of the [Ta2O5]1-x-[TiO2]x (x = 0.08) films were measured in UV-visible range. Films shows good transmittance for as- deposited andsample annealed at 400, 500 and 600 0C. From the observed transmission spectra, refractive index, optical band gap,absorption coefficient, extinction coefficient, and thickness of prepared [Ta2O5]0.92-[TiO2]0.08 films, have been calculatedby Swanepoel’s envelope technique. Refractive index (n), absorption coefficient (), and extinction coefficient of thefilms were observed, generally, decreasing with increasing photon wavelength. The value of refractive index was found2.29 for sample annealed at 4000C. To understand the associated mechanisms, the energy band gap calculated for all theprepared samples. For the as- deposited films the direct allowed energy band gap was obtained as 4.45 eV; and 4.48,4.52 and 4.45 eV for the films annealed at 400, 500 and 600 0C, respectively. The observed variation of optical bandgap, as a function of annealing temperature, shows that the band gap widens (blue shift) for the films annealed at highertemperatures.

Keywords: Thin films; RF sputtering; transmission spectra; refractive index; absorption coefficient; extinction coefficient

1. Introduction

A thin film material with a high transparency in the visible and soft UV range could widely benefit many fields ofsciences and technology. Tantalum oxide has been identified as such a candidate material based on these electric andoptical properties. Consequently, the charge transport mechanism [1], dielectric function, and optical constants [2] ofTa2O5 have been investigated. Due to its dielectric behavior and good chemical and thermal stability, it has been used asa discrete capacitor insulator in electronic applications, as an oxygen sensor, and as a high-temperature resistor. Currentinterest in optical films still centers on traditional optical components. In addition a variety of new applications hasemerged as the end mirrors for lasers [3], antireflection coatings for solar cells [4,5] films for energy conservationsystem [6], etc., where, by combining various numbers of films of certain thicknesses and refractive indices, it ispossible to achieve a pre- selected wavelength dependence of reflectance or transmittance. The portion of theelectromagnetic spectrum, involving virtually all optical film applications, fall within the span from the ultraviolet (UV)to the infrared (IR), and beyond. In this paper, [Ta2O5]1-x [TiO2] x, (x = 0.08) thin films were prepared on quartz(refractive index, n = 1.55, transmittance = 95 % for visible range) substrates by radio-frequency (RF) magnetronsputtering of [Ta2O5]0.92-[TiO2]0.08 ceramic pellet target, using argon as sputtering gas.

2. Preparation

Thin films of [Ta2O5]1-x-[TiO2]x, (x = 0.08) were deposited by RF magnetron sputtering of bulk [Ta2O5]0.92-[TiO2]0.08pellet target of 2 inch diameter and ~3 mm thickness. Films were deposited onto clean quartz and monocrystallinesilicon (100) surfaces. The target was prepared by conventional solid- state reaction method. The vacuum pumpingsystem gives an ultimate vacuum of 2 10-5 mbar. The pressure was monitored using a pirani-penning gaugecombination. The target disk was mechanically clamped to a water- cooled assembly with magnetic arrangement. The

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system was arranged in sputter down configuration with a substrate- target distance (TSD) of 30 mm. Feed troughswere fitted to insert sputtering gases. We used Argon (99.9%) as sputtering gas. For sputtering, RF sputtering system,from AVAC Engineers, Bangalore, was used. Films were deposited, at the working pressure of 6 10-3 mbar in thepresence of argon for 25 minutes, at room temperature. All the films were deposited at the forward power of 82 W,whereas reverse power was 1 W. All films were deposited at room temperature, maintaining substrate angle at 450. Theroom temperature deposited films were annealed, in the presence of oxygen, at different temperatures, for 1 hr at 400,500 and 6000C. Their structural properties were examined by X- ray diffraction method. Optical parameters of asdeposited [Ta2O5]0.92-[TiO2]0.08films, and films annealed at 400, 500 and 6000C, were measured.

3. Characterization

To evaluate the crystalline structure of the [Ta2O5]1-x-[TiO2]x (x= 0.08) film, we used XRD to analyze the as-depositedsample and the samples with annealing treatment at 400, 500and 600 0C, respectively, as shown in Fig.1. The structureof [Ta2O5]1-x-[TiO2]x, (x = 0.08) thin films, deposited on silicon substrates at room temperature, and annealed at 400,500and 600 0C, was studied by X- ray diffraction (XRD) method. Fig. 1 shows the XRD patterns, of as- deposited andannealed films at different temperatures. There is no diffraction peak were observed at any angle. Dobal et al [7] showsthat the [Ta2O5]1-x-[TiO2]x films shows crystalline structure only when annealed at higher annealing temperature (>8000C). However, it has been reportedly observed that oxide films deposited at room temperature, generally, showamorphous nature and crystalline structure is set at higher temperatures and further annealing at higher temperatures, inatmosphere, the film becomes more crystalline [8,9].

Figure 1: XRD patterns of as- deposited and annealed Figure 2: Transmission spectra of as- deposited and annealedfilms at different temperatures. films at different temperatures

4. Transmission Spectra

The optical transmissions of the [Ta2O5]0.92-[TiO2]0.08 films, deposited on quartz substrates, were measured as a functionof photon energy, in wavelength range 190 – 900 nm, using a UV- visible spectrophotometer (X’plorer, model XP3001). The observed transmission spectra of the films, deposited at room temperature, and annealed at 400, 500and 6000C, are shown in Fig. 2.

The optical transmittance of the films was high in the wavelength range above 400 nm. Generally, it increased with theincrease of annealing temperature. These sample shows the high transmittance for the sample deposited at roomtemperature and slight increase for sample annealed at different temperatures. As the annealing temperature increasedthe density of defects centers decreased by increase in optical transmittance [9]. Generally, the as-deposited films showthe low optical transmittance which may be due to the presence of unreacted tantalum along with Ta2O5 in the films[10]. When the annealing temperature increased, the optical transmittance of the films increased. A sharp absorptionedge was observed at about 260 nm. The optical absorption edge of the films shifted towards the lower wavelength sidewith the increase of annealing temperature. This is known as the Burstein- Moss shift [11]. Sufficient maxima andminima were observed in the transmission spectra of the present samples, which indicate that the films of hightransmittance and uniform thickness were deposited. The increase in absorption in the films, when annealed at highertemperatures, 500 0C, may be due to the increasing oxygen deficiency with increasing annealing temperature [9].However several previous studies observed that the as-deposited film is amorphous and became crystalline as annealingtemperature increased [9]. The amorphous film can be condensed during annealing.

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International Journal of Research and Innovations in Science and TechnologyVolume 1 : Issue 1 : 2014

5. Optical Parameters

From the observed transmission spectra, Fig. 2, optical parameters, such as, refractive index (n), optical band gap (Eg),absorption coefficient (), extinction coefficient (k), and thickness of [Ta2O5]0.92-[TiO2]0.08 films, deposited on quartzsubstrate (n = 1.55, transmittance = 95%), have been calculated, using Swanepoel’s envelope technique [12].Refractive index of the films was observed, generally, increasing with increasing photon energy, Fig. 3. The low valueof packing density in the samples may be one of the reasons for the decrease in ‘n’ with increasing wavelength. At 550nm wavelength, value of refractive index (n) of films deposited at RT, and annealed at 400, 500 and 600 0C wasobserved 2.19, 2.29, 2.18and 2.17, respectively. Generally refractive index of [Ta2O5]1-x-[TiO2]x (x = 0.08) filmsincreases with increase of annealing temperature due to the improvement in packing density. Furthermore, defectsinside the films are reduced after annealing, which is helpful to decrease the absorption of visible light of Ta2O5 thinfilms [13]. The significant dependence of refractive index of the film on impurities, imperfections, and voids in thefilms, which may have been incorporated during the film growth process, etc. may be held responsible [14] for theobserved behavior, Fig. 3. The combined effect of crystal structure, voids, imperfections, film- substrate interaction,processing conditions, etc., may be responsible for the wide variation in the observed trend of the refractive indicesvalues of the deposited films. In general, refractive index of the films increased with the increase of photon energy [9].Value of refractive index for Ta2O5 single crystal has been defined 2.02 in case of film formed at RT due to the lowpacking density, at wavelength (>550 nm) [9].

Figure 3: Absorption coefficient of as- deposited and annealed Figure 4: Extinction coefficient of as- deposited andfilms at different temperatures. annealed films at different temperatures

The excellent transmission of dielectric materials in the visible region of the spectrum was found terminating at shortwavelengths with the onset of the UV absorption edge. The critical radiation wavelength c, at which this occurs, wasobserved at 259, 260, 260 and 260 nm for the as- deposited and films annealed at 400, 500 and 600 0C, respectively.These values physically correspond to electronic transitions from the filled valence band levels, across the energy bandgap Eg, to the unfilled conduction band states. At longer wavelengths the high optical transmission is limited byabsorption due to the vibration of lattice ions in resonance with the incident radiation.

Figure 5: Extinction coefficient of as- deposited and annealed films at different temperatures.

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The frequency of maximum absorption is related to the force constant and mass of vibrating anions and cations. Thevalue of Eg has been calculated at and beyond the absorption edge. The variation of the absorption coefficient with theincident photon energy has been plotted for the estimation of energy gap. The band gap of the dielectric films dependson its composition [15-17]. The photon- electron interaction leads to direct transition probability and this interactionsatisfies, with m = ½. Fig(s) 6(a), 7(a), 8(a) and 9(a), show 2vs. h curves, for the films deposited at RT, annealed at400, 500 and 600 0C, respectively. The plot of 2 versus the h presents a characteristic linear behavior of band to banddirect transition which means the direct allowed energy band gap of the film samples. Extrapolation of the linear portionof the curves intercepts the absorption edge, in the 2vs. h plot (the straight line portion), with the energy axis. Theobtained values of direct allowed energy band gap were given in Table 1. The band to band transition reported by I.Porqueras et al [2] reported the value of direct allowed energy gap is 4.71 and 4.53 for Ta2O5 thin films.

Table 1: Obtained values of direct allowed energy band gap

Temperature 2vs. h 1/2vs. h 2/3vs. h 1/3vs. hRT 4.45 3.94 4.10 3.78

400 0C 4.48 4.06 4.15 3.56500 0C 4.52 4.08 4.17 3.85600 0C 4.45 3.94 4.10 3.78

The photon- phonon interaction leads to indirect allowed transition probability and is shown in Fig(s) 6(b), 7(b), 8(b)and 9(b) show the variation of 1/2 with h. The direct allowed energy band gap, from 1/2vs.h plots, is given in Table1. The previous reported value for this type of transition is 4.18 eV by C. Corbella et al. [18], and concluded that thistype of transition occurs for the amorphous structure of RF sputtered Ta2O5 films [19].

Figure 6: Variation of (a) 2, (b) 1/2, (c) 2/3 and (d) 1/3vs. h for the films deposited at RT

In addition to the photon assisted electronic transitions there are phonon assisted interaction probabilities (2/3vs. h)that give rise to the inter-band absorptions in the films corresponding to the energy levels in forbidden band. Excitonsmay be formed by direct photons only or by photon- phonon assistance. This direct absorption probability is shown inFig(s) 6(c), 7(c), 8(c) and 9(c), for films prepared at different conditions. The values of direct forbidden gap energy(associated with this mechanism) were shown for the films deposited at room temperature, and for the films annealed atdifferent temperatures.

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Figure 7: Variation of (a) 2, (b) 1/2, (c) 2/3 and (d)1/3 vs. h for the films annealed at 400 0C

Figure 8: Variation of (a) 2, (b) 1/2, (c) 2/3 and (d) 1/3 vs. h for the films annealed at 500 0C

Generally the optical band gap of the films decreased with the increase of annealing temperature. The decrease ofoptical band gap with the increase of annealing temperature is also due to the improvement in the packing density andthe increase in the crystallinity of the films [9]. The observed variation of optical band gap, as a function of annealingtemperature [20], shows that the band gap widens (red shift) for higher annealing temperatures, i.e., blue shift for lowerannealing temperature. This broadening effect can be understood based on the Burstein [11] effect.

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Burstein pointed out that an increase in the Fermi level in the conduction band of degenerate semiconductors leads towidening of the energy band gap (blue shift). The blue shift, for prepared thin film samples, is proportional to the thirdpower of the carrier concentration [20] and hence shows high transmission. The effect of post annealing on film bandgap can be explained by considering both the total change in the carrier concentration and mobility. The mobilityincreases with increasing annealing temperature and decreases the carrier concentration in conduction band. It seemsthat during post annealing process of the samples, intensification of diffusion processes [21] enhances significantly thepossibility of escape of highly volatile alkali metal ions from the lattice. An increase in the A-site vacancies will capturemore electrons and hence reduce the number of electrons in the conduction band. From these measurements the mostsatisfactory representation was observed for 2 vs. h plot, which suggests the dominating direct transition probability,in the deposited [Ta2O5]1-x-[TiO2]x (x = 0.08) films, by photon- electron interaction.

Figure 9: Variation of (a) 2, (b) 1/2, (c) 2/3 and (d) 1/3 vs. h for the films annealed at 600 0C

6. Conclusion

We have deposited thin films of [Ta2O5]1-x-[TiO2]xfilm by RF sputtering method. We have measured the spectraltransmissions of the [Ta2O5]1-x-[TiO2]x (x = 0.08) films in UV-visible range. We have calculated refractive index,optical band gap, absorption coefficient, extinction coefficient, and thickness of prepared [Ta2O5]0.92-[TiO2]0.08 filmsfrom the observed transmission spectra by Swanepoel’s envelope technique. We have observed that refractive index (n),absorption coefficient (), and extinction coefficient of the films were, generally, decreasing with increasing photonwavelength. The band gap widens (blue shift) for the films annealed at higher temperatures from the observed variationof optical band gap as a function of annealing temperature.

Acknowledgements

Authors are thankful to the Department of Science & Technology (DST), New Delhi for the financial support to carryout these investigations.

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optical properties of amorphous [ta o -[tio (x=0.08) thin...

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