Effect of Annealing Temperature on the Optical Properties of TiO 2 Thin Films Prepared by Pulse Laser Deposition

. TiO 2 thin films were prepared by pulse laser deposition technique on glass substrates with laser power 700 mJ and 900 shot at distance 1cm under vacuum of 10 -2 mbar with different annealing temperature (273, 373, 423)K. The influences of the annealing temperature on the optical properties of TiO 2 thin films were mainly investigated. TiO 2 is a wide band gap n-type semiconductor that has a wide range of applications. It was found that the optical properties of TiO 2 thin films were dependent on the annealing temperature. The values of optical energy gap decreases from (3.4 to 3.2) eV when increasing annealing temperature. The optical constants such as refractive index, extinction coefficient, real and imaginary dielectric constants as a function of wavelength were determined.


INTRODUCTION
Titanium dioxide (TiO 2 )(titania) is a cheap, non-toxic and one of the most efficient semiconductor photocatalysts for extensive environmental applications because of its strong oxidizing power, high photochemical corrosive resistance and cost effectiveness [1]. Due to these inherent properties, TiO 2 is the most suitable candidate for degradation and complete mineralization of toxic organic pollutants in water [1,2]. It is well known that TiO 2 exists in three crystalline structures: rutile, anatase and brookite [3,4]. The anatase phase is especially adequate for those applications due to its crystal structure and a higher band gap of 3.2 eV compared to the 3 eV in rutile. Anatase and rutile have properties of interest for sensing applications [5]. In principle, transition metal with proper oxidation state replace some of the Ti (IV) from lattice producing an impurity state that reduces the band gap of TiO 2 . Titanium dioxide (TiO 2 ) has attracted much attention in recent years due to its great potential for applications in optical elements, electrical insulation, capacitors or gates in microelectronic devices, photovoltaic solar cells, anti reflection coatings, optical waveguides, photonic crystals, devices based on metal etc [6]. Many deposition methods have been used to prepare TiO 2 thin films, such as electron-beam evaporation [7], ion-beam assisted deposition [8], DC reactive magnetron sputtering [9], RF reactive magnetron sputtering [10], sol-gel dip coating method [11], sol-gel spin coating method [12], chemical vapor deposition [13], plasma enhanced chemical vapor deposition [14] and pulsed laser deposition (PLD) [15]. Among these methods, PLD technique has been widely used for growing oxide films because it allows for stoichiometry of the synthesized material. In this work, we study the effect of annealing temperature on the optical properties of TiO 2 .

EXPERIMENTAL
The deposition was carried out using a Q switched Nd:YAG laser at 1064 nm (900 shot and laser fluency 700 mJ/cm 2 ). The studied films were prepared from pure TiO 2 target film were grown by pulsed laser deposition on glass substrates, kept distance of 1 cm from the TiO 2 target. For film deposition, all substrates (the glass substrates 8 × 2.5 cm 2 with 1 mm thickness) were cleaned in acetone and methanol solutions in an ultrasonic bath for 30 minutes. Then compress the mixture under 5 Ton (homemade compressor) to get the final pellet of TiO 2 powder of 2.5 cm diameter. The optical properties of the films were studied by recording the absorption spectra from UV-VIS spectrophotometer at room temperature in the wavelength range of 200-1100 nm.

RESULTS AND DISCUSSION
The optical properties of TiO 2 thin film prepared by PLD method with various annealing temperature (273, 373, 423)K and average thickness 500 nm where investigated using UV-Visible spectrophotometer in the region of (200-1100) nm. From Fig.1 it can observe that the transmittance of the TiO 2 thin film decreases with increasing annealing temperature and then dropped, this may be due to the increase in film homogeneity, with the increase of the surface roughness promoting the increase of the surface scattering of the light by increasing the columnar growth with needle and rod like shape [7,16]. In addition, the transmittance increase with increasing wavelength for all prepared thin films. The absorbance spectra of the TiO 2 thin film deposited on glass substrate measured with various annealing temperature (273, 373, 423) K is shown in Fig.2. In high-energy spectral range, where the film is strongly absorbent, and the absorbance of TiO 2 thin film increases with increasing annealing temperature for all prepared thin films. The absorption coefficient  was determined from the region of high absorption at the fundamental absorption edge of the film. The variation of the absorption coefficient versus the wavelength for TiO 2 with various annealing temperature (273, 373, 423) K is shown in the Fig.3. The absorption coefficient (α) is increased with the increasing annealing temperature. This can be linked with increase in grain size and it may be attributed to the light scattering effect for its high surface roughness [17]. In order to calculate the band gap energies of TiO 2 thin films, the plot of (αhν) 2 as a function of the energy of incident radiation has been ploted in Fig.4. The energy band gap is obtained from intercept of the extrapolated linear part of the curve with the energy axis at (αhν) n = 0 the intercept with x-axis equal to E g . it can be observe that direct band gap of the TiO 2 films decreases from (3.4 eV -3.2 eV) as the annealing temperature increases from RT to 423 K. From Table (1) and the figure, it can be observed that (E g ) was decreased with the increasing of annealing temperature for all films. Annealing led to increased levels of localized near valence band and conduction band and these levels ready to receive electrons and generate tails in the optical energy gap and tails is working toward reducing the energy gap, or can be attributed decrease energy gap to the increased size of particles in the films [18].  Optical constants included refractive index (n), extinction coefficient (k), and real ( r ) and imaginary ( i ) parts of dielectric constant. The refractive index can be calculated from the following equation [19].
Where R is the reflectance and given by the equation [20].
The variation of the refractive index versus wavelength in the range (200-1100) nm for TiO 2 thin films with various annealing temperature (273, 373, 423) K, is shown in Fig. 5. It can notice from the figure that the refractive index, in general, increases slightly with increasing of T a . This behavior can be explained on the basis of that increasing of T a to make less density (lowering the packing density) which in turn increases propagation velocity of light through them which results in increasing of n values, since n represent the ratio of light velocity through vacuum to velocity through any medium. The behavior of the extinction coefficient of TiO 2 films at different annealing temperatures shown in Fig.6. It can observe from the figure that the extinction coefficient, in general, increases with increasing of annealing temperature. The variation of the real ( r ) and imaginary ( i ) parts of dielectric constant values versus wavelength in the range (200-1100) nm for TiO 2 with various annealing temperature (273, 373, 423)K as shown in Figs. (7,8). The behavior of  r is similar to that of the refractive index because of the smaller value of k 2 compared with n 2 according by relation ɛ r = (n 2 − k 2 ) while  i mainly depends on the k values. It is found that  r and  i increased with increasing of annealing temperature.

CONCLUSIONS
TiO 2 thin films were deposited on glass substrate by applying a versatile PLD technique with synchronized laser (Nd:YAG). The dependences of optical properties of TiO 2 thin films in RT and different annealing temperatures were investigated. Optical properties of TiO 2 thin films deposited at different temperature are presented. Band gap obtained for these films is in the range of 3.4-3.2 eV. With the temperature increased for (273, 373 and 423) K, the optical transmittance reduced and the band gap decreased. The refractive index and extinction coefficient increased slightly with increasing of annealing temperatures for all samples. The variation of the real and imaginary parts of the dielectric constant have similar trend as for n and k respectively.