Available online at www.sciencedirect.com Optical Materials 30 (2007) 645–651 www.elsevier.com/locate/optmat TiO2 thin films prepared by sol–gel method for waveguiding applications: Correlation between the structural and optical properties R. Mechiakh b a,1 , F. Meriche b, R. Kremer b, R. Bensaha a,1 , B. Boudine a,1 , A. Boudrioua b,* a Laboratoire de céramiques, Université Mentouri de Constantine, Route Ain El-Bey 25000 Constantine, Algeria Laboratoire Matériaux Optique Photonique et Systèmes, CNRS UMR 7132, Université de Metz et Supélec, 2 Rue E. Belin, 57070 Metz, France Received 29 May 2006; received in revised form 25 January 2007; accepted 20 February 2007 Available online 11 April 2007 Abstract Thin films of transparent titanium oxide (TiO2) are prepared by the sol–gel dip-coating technique. Structural and optical properties of TiO2 thin films are investigated for different annealing temperatures and different number of coatings. X-ray diffraction (XRD) and Raman spectroscopy analysis show that the anatase crystalline phase appears beyond 350 C for the four layers TiO2 film. At higher temperatures and for thicker films, we observe in addition to anatase the formation of brookite and rutile phases. The grain size calculated from XRD patterns increases as the temperature of annealing and number of dipping increase, from 11.9 to 17.1 nm for anatase and decreases as the number of dipping increases, from 24.2 to 10.2 nm for brookite. Film thickness, refractive index, and porosity are found to vary with treatment temperature and the number of coating. The obtained films are transparent in the visible range and opaque in the UV region. Waveguiding properties are studied using m-lines spectroscopy. The best results indicate that our films are monomodes TE0 at 632.8 nm with optical losses of 2 dB cm1.  2007 Elsevier B.V. All rights reserved. Keywords: TiO2 thin films; Sol–gel; Planar waveguide; Anatase; Brookite 1. Introduction Titanium dioxide (TiO2) thin films are extensively studied because of their interesting chemical, electrical and optical properties (high bandgap, transparent in the visible range, high refractive index, high dielectric constant, and ability to be easily doped with active ions) which are considered for various optical applications such as high refractive index component of multilayer optical filter, gas sensors, antireflective coating, photocatalysts, planar waveguides [1–3]. Moreover, in recent years, rare-earth (Er, Tb, Eu) doped TiO2 planar optical waveguiding thin films have * Corresponding author. Tel.: +33 (0)3 87 37 85 47; fax: +33 (0)3 87 37 85 59. E-mail address: boudriou@metz.supelec.fr (A. Boudrioua). 1 Tel./fax: +213 31 63 50 18. 0925-3467/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.02.047 attracted much attention due to their potential application in integrated optics: integrated optical amplifiers, up-conversion micro-lasers, planar microcavities, flat panel display [4–6]. Mixed with Silica, sol–gel (SiO2–TiO2) thin films are a good host matrix for semiconductor nanocrystallites (CdS, ZnS, PbS, CdTe) with suitable linear and non-linear optical properties which can be exploited to make all-optical switches, digital signal regenerators [7,8]. TiO2 films have been made by a variety of techniques. Sol–gel method has emerged as one of the most promising process as it is particularly efficient in producing thin, transparent, homogenous, multi component oxide layers of many compositions on various substrates at low cost and it allows the choice of refractive index and thickness of the layer by changing preparation conditions. Furthermore, this method offers the possibility to easily incorporate rare-earth ions as optically active centres and 646 R. Mechiakh et al. / Optical Materials 30 (2007) 645–651 semiconductors nanocrystals into highly transparent glass [4–8]. Crystalline TiO2 film exist in three phases: anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic) [9,10]. Rutile being the most stable and the formation of its phase depends on the starting material, deposition method and temperature treatment. Numerous literature reports on the fabrication of TiO2 thin films by sol–gel dip coating technique using many types of titanium alkoxide as precursors. Nishide et al. [11] used HNO3 as a catalyst to prepare titania sols and TiO2 films in the sol–gel process. As the treatment temperature increases, the transformation from anatase to rutile occurs and the refractive index of the films increases linearly. Kim et al. [12] studied the influence of the temperature on the optical and structural properties of TiO2 thin films, used HCl as a catalyst to prepare titanium solutions and thin films of TiO2 in their sol–gel process. Their thin films crystallise, starting from 400 C, into anatase, and are transformed into anatase– rutile starting from 1000 C; the refractive index increases with temperature while porosity decreases due to shrinkage and densification of the films. These studies indicate that the optical and structural properties of TiO2 films depend on the process conditions and the materials used in this process. In this paper, we report the study of the structural and optical properties of TiO2 thin films deposited on indium tin oxide (ITO) coated glass by sol–gel dip coating technique as a function of the preparation conditions. Structural evolution with annealing temperature are investigated by X-ray Diffraction (XRD) and confirmed by Raman Spectroscopy measurements. Transmittance, refractive index and porosity of the films are also studied. Finally, the waveguiding properties of the obtained thin films are investigated using m-lines spectroscopy. The main objective of this work is to establish the best synthesis conditions of TiO2 sol–gel films in order to obtain high quality transparent thin film waveguides that could be used as a host matrix for rare-earth and/or semiconductor nanocrystals for fluorescence and non linear integrated optical applications. 2. Experiments The TiO2 thin films were prepared by the sol–gel process, which is based on the hydrolysis of alkoxydes in alcoholic solutions in the presence of an acid catalyst. The procedure of preparation includes the dissolution of one mole of butanol (C4H9OH) as solvent and four moles of acetic acid (C2H4O2), one mole of distilled water is added as well as one mole of tetrabutyl–orthotitanate (C4H9O)4Ti (Fig. 1); this solution is transparent, of yellowish color and is ready for the deposit. The ITO glass (with refractive index equal to 1.517 and thickness 20 Å) substrates carefully cleaned are dipped into the solution and are pulled up at a constant rate of 6.25 cm/s. After each dipping, these thin films are dried for 30 min at a distance of 40 cm from a (C4H9O)4Ti Tetrabutylorthotitanate C4H9OH C2H4O2 (H2O) Mixing for 10 minutes Mixing for 1 hour (Sol) Gel Substrates Dipping + Withdraw Drying (100˚C) Xerogel Heating Oxide Fig. 1. Sol–gel process of TiO2 thin film preparation. 500 W-light source (drying at 100 C). The TiO2 thin films were annealed in the temperature range of 300–450 C with increasing temperature rate of 5 C min1 for 2 h in furnace. To determine the transformation points, we have analyzed the powder obtained from the xerogel by Differential Scanning Calorimetry (DSC) using a SETARAM DSC–92 analyzer equipped with a processor and a measuring cell. The thermal cycle applied consists of heating from room temperature to 520 C, holding for 5 min at this temperature and finally cooling back to room temperature with the same rate (5 C/min). The variations of the lattice parameter and the crystalline structure are determined by an automated powder diffractometer (Siemens D5005) using a copper anticathode at 40 kV, 20 mA over the 2h range 10–70 (0.1/s). The UV transmittance studies are carried out using UV–Vis double–beam spectrophotometer SHIMADZU (UV3101PC). Its useful range is between 190 and 3200 nm. The treatment of the spectra is performed using the UVPC software. A surface profiler DEKTAK 3ST AUTO1 (VEECO) is used to determine film thicknesses. Raman spectra were recorded in a back scattering configuration with a Jobbin Yvon micro Raman spectrometer coupled to a DX40 Olympus microscope. The samples of the TiO2 thin films are excited with a 632.8 nm wavelength with an output of 20 mw. Finally, the waveguiding properties (refractive index, guided modes and optical losses) are investigated by dark lines spectroscopy with a totally automated experimental set-up. A right-angle rutile prism is used for coupling light of a He–Ne laser with a wavelength k = 632.8 nm into the waveguide. More details of the experimental arrangement and theoretical analysis have been already discussed by 647 R. Mechiakh et al. / Optical Materials 30 (2007) 645–651 205 (101) . 4 layers 3. Results and discussion Exo 80 Intensity (a.u.) * (b) 350ºC * (c) 400ºC (a) xerogel 0 20 30 40 50 60 70 2θ (degree) . (110) T from 400ºC (101) Intensity (a.u.) . 120 (d) 450ºC . The thermal curve in Fig. 2 shows two singularities: firstly, an endothermic peak spreading from 50 to 250 C, which corresponds to the evaporation of water, the thermal decomposition of butanol as well as the carbonization or the combustion of the acetic acid and certain elements which constitute our alkoxyde and secondly, an exothermic peak spreading from 290 to 410 C, which corresponds to the crystallization of titanium oxide. This analysis shows that an annealing at a temperature equal or higher than 400 C would be largely sufficient to form titanium oxide completely. Fig. 3a and b shows the XRD patterns of the xerogel and the thin films of oxide obtained after 4 dipping and various annealing temperatures at 350, 400 and 450 C (a) and for various dipping number (from 5 to 10) at annealing temperature of 400 C (b). We observe that the crystallization occurs from amorphous phase beyond 350 C. These spectra, show a peak corresponding to the (1 0 1) plane, which is attributed to the presence of anatase regardless of the annealing temperature. At higher temperatures (400 and 450 C) and for a number of coating layers increasing from 4 to 7, we observe in addition to anatase the formation of brookite which crystallizes with the (1 2 1) plane parallel to the surface. These thin films deposited on ITO substrates annealed at 350, 400 and 450 C, are stoechiometric. We also observe that the intensities corresponding to the lines characteristic of anatase (1 0 1) and brookite (1 2 1) increase as the annealing temperature increases. This increase in the intensity of the peaks leads us to deduce the amount of titanium oxide as a function of the annealing temperature. Moreover, Fig. 3b shows that for annealing temperature at 400 C and 10 layers, the crystalline structure changes from ana- Heat Flow (W/g) * * (121) 3.1. Structural properties 100 : Anatase :brookite . Tien and Ulrich [13]. The optical losses were determined with a CCD camera by measuring the scattered light from the surface film along the propagation direction. .* * anatase brookite rutile (121) * (d) 10 layers (c) 7 layers (a) 5 layers (b) 6 layers 0 20 30 40 50 60 70 2θ (degree) Fig. 3. X-ray diffraction pattern of TiO2 xerogel (a), and TiO2 thin films on ITO glass obtained after 4 dippings and various annealings 350 C (b), 400 C (c) and 450 C (d). X-ray diffraction pattern of TiO2 thin films obtained after annealing at 400 C for various number of dippings 5 (a), 6 (b), 7 (c), and 10 layers (d). tase–brookite to rutile, which normally does not appear below 800 C as reported in the literature [14]. This phase crystallizes with the (1 1 0) plane parallel to the surface. The Raman spectra of Fig. 4a and b shows different peaks related to the presence of titanium oxide in anatase and brookite phases. In particular, Fig. 4b confirms the presence of titanium oxide starting from the temperature 350 C. These spectra exhibit bands at around 153 and 193 cm1 which are assigned to TiO2 anatase [15]. 60 3.2. Surface morphology and grain size 40 The crystallite size L of TiO2 thin films can be deduced from XRD line broadening using the Scherrer equation [16]: 20 0 Xerogel 0 100 200 300 400 500 600 T(ºC) Fig. 2. The thermal curve of the xerogel recorded with 5 C/min. 0:94  k 1 L ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi cos h ðD2hkl  D2instr Þ ð1Þ 648 R. Mechiakh et al. / Optical Materials 30 (2007) 645–651 Table 1 Crystallite size L (nm) of the TiO2 thin films for different annealing temperatures and different numbers of dipping 5000 T from 450ºC . 7 layers . 3000 * 2000 . Intensity (a. u.) 4000 Phases L (nm) (hkl) Xerogel – Amorphous – 4 layers at 350 C 4 layers at 400 C Anatase Anatase Brookite 11.9 12.3 24.2 (1 0 1) (1 0 1) (1 2 1) 4 layers at 450 C Anatase Brookite 13.3 25 (1 0 1) (1 2 1) 5 layers at 400 C Anatase Brookite 15.7 12.4 (1 0 1) (1 2 1) 6 layers at 400 C Anatase Brookite 13.3 14.7 (1 0 1) (1 2 1) 7 layers at 400 C Anatase Brookite Rutile 17.1 10.2 5.4 (1 0 1) (1 2 1) (1 1 0) 6 layers 5 layers 1000 Samples : Anatase : brookite . * 0 200 400 600 800 Raman Shift (cm-1) 10 layers at 400 C 5000 400ºC 4 layers 100 (c) Transmittance . 2000 . 1000 80 350ºC 3000 . Intensity (a. u.) 4000 : Anatase (d) (b) (e) 60 (a) 40 (a) 4 layers at 400ºC (b) 5 layers at 400ºC (c) 6 layers at 400ºC (d) 7 layers at 400ºC (e) 10 layers at 400ºC 20 0 200 400 600 800 Raman Shift (cm-1) 0 400 Fig. 4. The Raman spectra of TiO2 thin films obtained after 4 dippings and various annealings. The Raman spectra of a 450 C heat-treated TiO2 thin films obtained for various numbers of dippings. 3.3. Optical properties 3.3.1. UV transmittance analysis Fig. 5a and b shows the diffused scattering UV–Vis transmittance spectra of TiO2 thin films for different annealing temperatures and different number of dipping in wavelength range 300–1000 nm. The transmission of the titanium oxide thin films decreases with the increase 800 1000 Wavelength (nm) 100 80 Transmittance k is the wavelength of X-ray beam (Cu Ka = 1.5406 Å), Dhkl is the full width at half maximum (FWHM) of the (hkl) diffraction peak, Dinstr is the FWHM corresponding the instrumental limit, and h is the Bragg angle. The computed values of the grain sizes are given in Table 1. We have calculated the grain sizes of the thin films for different temperatures of annealing and different numbers of dippings. We found that the crystallinity of the obtained anatase particles increased from 11.9 to 17.1 nm as the temperature of annealing and the number of dipping increase, whereas the size of brookite crystallites decreases with increasing dippings from 24.2 to 10.2 nm. 600 60 40 4 layers at 350ºC 4 layers at 400ºC 4 layers at 450ºC 20 0 400 600 800 1000 Wavelength (nm) Fig. 5. UV–Vis transmittance spectra of the TiO2 thin film, for various layers annealed at 400 C. UV–Vis transmittance spectra of 4 layers TiO2 thin film, annealed at various temperatures. in annealing temperature and in the number of dipping. This can be linked with the formation stage of anatase and with the increase in the grain size [14]. The bands due to the interference color of the film appeared in the wavelength range of 350–800 nm. 649 R. Mechiakh et al. / Optical Materials 30 (2007) 645–651  1 2 T max ðkÞ  T min ðkÞ n ðkÞ þ n2S ðkÞ þ 2n0 nS 2 0 T max ðkÞ  T min ðkÞ 3.3.2. m-Lines measurements m-Lines spectroscopy is a useful method to determine the optogeometric parameters of waveguiding thin films, such as thickness and refractive index [13]. It uses a prism coupling method to launch the laser light into the optical layer. In optical planar waveguides, light propagation can occur within a thin layer of a transparent material when its refractive index is higher than that of surrounding layers and when the film has sufficient thickness to support at least one guided mode. Besides, the waveguiding properties of the film strongly depend on the microstructure of the material: surface roughness, porosity, grain size and grain boundaries which are connected to the fabrication process parameters such as withdrawal rate, sol concentration, treatment temperature and number of coating layers [20]. This study showed that almost of our TiO2 thin films support only one guided TE polarized mode (transverse Refractive Index 40 2.2 30 2.1 20 2.0 10 Refractive Index Porosity 1.9 0 1.8 300 350 400 450 Temperature (ºC) 60 2.5 10 layers 2.4 50 2.3 40 2.2 2.1 30 2.0 Porosity (%) Where nd is the refractive index of pore–free anatase (nd = 2.52 [19]), and n is the refractive index of the porous thin films. For instance, Fig. 6 shows results of the calculation of the refractive index (n) and porosity (p) of the thin films of oxide obtained after 8 and 10 layers for different annealing temperatures. It is noted that the refractive index of the thin films of titanium oxide increases with increasing treatment temperature and number of dipping (from 1.94 to 2.44 for 10 layers and from 1.82 to 2.37 for 8 layers). In addition, porosity decreases from 48.5% to 7.7% for 10 layers and from 57.1% to 13.6% for 8 layers. This can be connected with the change in the crystalline structure (anatase, anatase–brookite, and rutile), the increase in the size of the grains and/or the density of the layers. 50 2.3 ð3Þ where n0 is the refractive index of air, ns is the refractive index of substrate, Tmax is the maximum envelope, and Tmin is the minimum envelope. The thickness of the films was adjusted to provide the best fits to the measured spectra. In this study, all the deposited films are assumed to be homogeneous. The porosity of the thin films is calculated using the following equation [18]:   n2  1 porosity ¼ 1  2  100ð%Þ ð4Þ nd  1 8 layers 2.4 Refractive Index S¼ 60 2.5 Porosity (%) The refractive index of the prepared TiO2 thin films was calculated from the measured UV–Vis transmittance spectrum. The evaluation method used in this work is based on the analysis of the UV–Vis transmittance spectrum of a weakly absorbing film deposited on a non–absorbing substrate [17]. The refractive index n(k) over the spectral range is calculated by using the envelopes that are fitted to the measured extreme: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi nðkÞ ¼ S þ ðS 2  n20 ðkÞn2S ðkÞÞ ð2Þ 20 1.9 Refractive Index Porosity 1.8 300 350 400 10 450 Temperature (ºC) Fig. 6. Refractive index and porosity of 8 layers TiO2 thin film, annealed at various temperatures (from 300 to 450 C). Refractive index and porosity of 10 layers TiO2 thin film, annealed at various temperatures (from 300 to 450 C). electric mode: TE0). For instance, Fig. 7 shows that the best result is obtained in case of 9 layers annealed at 450 C. This curve gives the variation of the reflected intensity versus the angle of incidence at the base of the prism coupler. The intensity dip is synonymous of the excitation of guided mode from which we can measure the effective indices. By using this value and that of thickness measured by the profilometer (the films thickness varies between 20 and 292.1 nm) in the guided mode dispersion equation, we calculate the film refractive index nTE. The results are reported in Table 2. Refractive index for 4–9 layers at 450 C is found to be 2.064. However, one has to note that we did not observe any TM modes. That is very likely due to the coupling conditions which strongly depend on the surface roughness of the films. Similar phenomenon has been observed by Boudiombo et al. [21]. It was suggested that the undetected TM modes could be also due to the crystallographic configuration of the films. Thus, TM modes are difficult to excite than TE ones and therefore they might be considered as ‘‘missing’’ modes [22]. Besides, several reports indicated controversial results concerning waveguiding properties of TiO2 thin films pre- 650 R. Mechiakh et al. / Optical Materials 30 (2007) 645–651 Finally, propagation loss measurements at 632.8 nm have been estimated from the guided mode spectra. Indeed, optical losses are connected to the width of reflectivity dips. Moreover, the scattered light from the guide surface has been collected by using a CCD camera and analysed by using specific software as reported in reference [24]. The losses were found to be about of 2 dB cm1. This result emphasizes the interest of using our TiO2 thin films as a waveguiding structure. Moreover, it is believed that a careful preparation process as well as an adequate post-deposition heat treatment is very likely to decrease the optical losses to less than 1 dB cm1. 1.450 Reflected Intensity (a. u.) 1.445 1.440 1.435 1.430 1.425 9 layers 450ºC 2h Pol. TE 1.420 TE0 4. Conclusion 1.415 0 5 10 15 20 25 Angle of Incidence (º) 1.26 Reflected Intensity (a. u.) 1.24 1.22 TE0 1.20 4 layers 400ºC 2h Pol. TE 1.18 1.16 0 5 10 15 20 25 Angle of Incidence (º) Fig. 7. TE mode spectra obtained by measuring the reflected intensity vs the angle of incidence for (a) 9 layers annealed at 450 C and (b) 4 layers annealed at 400 C. Table 2 Measured TE effective indexes, refractive indexes and thickness of the TiO2 layers at 450 C Number of layers Annealing temperature (C) Film thickness d (±0.1 nm) Effective index Nm ± 4 · 104 Refractive index no ± 4 · 104 4 6 7 9 450 450 450 450 177.3 215.0 277.0 292.1 1.8037 1.8573 1.9168 1.9207 2.0640 2.0653 2.0667 2.0600 layers layers layers layers pared by sol–gel method. For instance, Mugnier et al. [23] reported TiO2 monomode waveguide (thickness d  80 nm) with 1 layer as well as multilayers (4 layers) waveguide which were TE and TM multimode. This emphasizes that waveguiding property of TiO2 thin films strongly depend on sol elaboration, deposition conditions and heat treatment. In this work we reported the investigation of TiO2 thin films prepared by sol–gel method for optical waveguiding applications. X-ray diffraction and Raman spectroscopy analysis show that the thin films obtained crystallize into tetragonal titanium oxide anatase starting from the annealing at 350 C. At higher temperatures (400–450 C), we also note the formation of the orthorhombic brookite phase. Besides, for the sample with ten layers annealed at 400 C, only tetragonal rutile is obtained. The calculation of the grain size by Scherrer’s formula, gives us sizes varying from 5.4 to 25 nm for all structures. The analysis of the UV–Vis transmission spectra shows that TiO2 thin films are transparent in the visible range and opaque in the UV region, irrespective of the treatment temperature and the number of dipping. 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