Preparation of Anatase-TiO2 by Elevated-Temperature Deposition and Annealing in Oxygen Ambient∗

Titanium dioxide (TiO2) films were deposited on silicon substrates using RF sputtering at room temperature (Tsub=R.T.) and elevated temperature (Tsub=300 ◦C). After deposition, TiO2 films were annealed at various temperatures (400-900◦C) for 1 h in Ar and O2 ambient, and phase transition from anatase to rutile was investigated using XRD spectra. The weight fraction of anatase in TiO2 films deposited at Tsub=300 ◦C were larger than that in the films deposited at Tsub=R.T. when they were annealed at temperatures under 600 ◦C. There was no significant difference between Ar and O2 annealing whether the TiO2 films were deposited at Tsub=R.T. or Tsub=300 ◦C. Annealing in O2 ambient was not effective to obtain anatase-TiO2 in the films using the experimental conditions in this study. [DOI: 10.1380/ejssnt.2012.186]


I. INTRODUCTION
Titanium dioxide (TiO 2 ) is useful for various applications such as antireflection coatings (ARC) [1], photocatalysts [2], waveguides [3], metal-oxide-semiconductor (MOS) devices [4], gas sensors [5] and dye-sensitized solar cells [6].TiO 2 has a high refractive index, a high dielectric constant and a wide band gap.For its wide band gap, the highly transparent TiO 2 film is a good candidate for an ARC in solar cells.From an ecological point of view, TiO 2 is a nontoxic material, and its elements (titanium and oxygen) exist abundantly on the earth.TiO 2 crystallizes in three different phases; rutile, anatase and brookite.Rutile TiO 2 is the densest (4.13 g/cm 3 ) [7] and the most thermodynamically stable among these phases.The optical band gap of anatase TiO 2 is larger than that of rutile TiO 2 , which indicates that anatase TiO 2 is more suitable for an ARC because it transmits more visible light than rutile TiO 2 .TiO 2 films have been deposited by a number of methods such as reactive RF magnetron sputtering [6,8], plasma-enhanced chemical vapor deposition (PECVD) [9], atmospheric-pressure chemical vapor deposition (APCVD) [1,10], ion-assisted deposition [11] and metalorganic decomposition (MOD) [12].Structural and optical properties of TiO 2 films are affected by these deposition methods.
In this study, TiO 2 films were deposited on silicon (100) substrates by RF sputtering at room temperature (T sub =R.T.) and elevated temperature (T sub =300 • C).Af-ter deposition, TiO 2 films were annealed at various temperatures (400-900 • C) in Ar and O 2 ambient, and phase transition from anatase to rutile was investigated using X-ray diffraction (XRD) measurement.

II. EXPERIMENTS
TiO 2 films were deposited on silicon (100) substrates (1-8 Ωcm, n-type, 36 × 36 × 0.5 mm 3 ) using a RF sputtering system (ULVAC, RFS-200).A TiO 2 disk (99.9% purity) of 8 cm diameter was used as a target and the distance between the target and the substrate was fixed at 30 mm.The sputtering chamber was evacuated down to 6.0 × 10 −3 Pa using a diffusion pump, and then backfilled with Ar (6N purity) which was used as a sputtering gas.Before deposition, the target was pre-sputtered for 5 min in order to remove a surface oxide layer on the target.Afterwards, the target was sputtered for 30 min in Ar ambient at a pressure of 3.3-3.6Pa.During sputtering, the RF power was kept at 75W by RF generator operating at 13.56 MHz.The substrate temperature for the film deposition was room temperature (T sub =R.T.) or T sub =300 • C. The thickness of the TiO 2 films deposited with the RF power of 75 W was 100nm.After deposition, each sample was annealed at 400, 500, 600, 700, 800 and 900 • C using an electric furnace for 1 h in Ar and O 2 ambient at a flow rate of 2.0 ℓ/min.
The crystalline structure of the TiO 2 films was characterized by an X-ray diffractometer (Shimadzu, XRD-6000) using CuK α radiation (λ=0.15406nm) at an excitation voltage of 40 kV and a current of 30 mA.The film thickness and optical bandgap of the deposited films was measured using film thickness monitor (Otsuka Electronics, FE-300K ).1(a), as-deposited TiO 2 film deposited at T sub =R.T. was almost amorphous.The intensity of the anatase TiO 2 peak increased as the annealing temperature (T a ) was increased to 700 • C and it began to decrease over T a = 700 • C, whereas that of the rutile TiO 2 peak continued to increase gradually as T a was increased to 900 • C.These results indicate that the TiO 2 films were transformed from anatase to rutile over 700 • C.However, even at T a = 900 • C, the film was not completely transformed to rutile-phase.On the other hand, for the film deposited at T sub =300 • C (Fig. 1(b)), considerably large anatase TiO 2 peak was already observed before annealing.Fig. 1(b) also shows that the intensity of the anatase TiO 2 peak was almost constant under T a = 700 • C and the peak suddenly disappeared over T a = 700 • C, and that the intensity of the rutile TiO 2 peak under T a = 700 • C was smaller than that for the film deposited at T sub =R.T. and it increased steeply in the temperature range over T a = 700 • C. to mean that annealing in O 2 ambient was effective to obtain anatase-rich TiO 2 films.When the films were deposited at T sub =300 • C (Figs. 3(a) and 3(b)), annealing in O 2 ambient was not considered to be effective to obtain anatase-rich TiO 2 films.However, the difference is not exactly clear only from the XRD spectra.
In order to examine the phase transition in detail, the weight fraction of anatase-TiO 2 was calculated from the peak intensities of Gaussian-fitted anatase (101) and rutile (110) peaks using Spurr and Myer's method [13]; where W A is the weight fraction of anatase-TiO 2 , I A and I R are the peak intensities of anatase (101) and rutile (110), respectively.Figure 4 shows the relationship between the annealing temperature and the weight fraction of anatase-TiO 2 .In effect of annealing in O 2 ambient was not shown from the results of calculated W A .However, it was clear from Fig. 4 that the deposition of TiO 2 films at T sub =300 • C is effective to obtain anatase-rich TiO 2 films.
Above results may be understood from the difference in energies required for the transformation.It appears that the energy required for the transformation in the TiO 2 films deposited at T sub =R.T. is relatively low because asdeposited films were almost amorphous and easily crystallized in either anatase-or rutile-phase with thermal energies.Therefore, the films deposited at T sub =R.T. were gradually transformed from anatase to rutile as annealing temperatures were increased from 400 to 900 • C. On the other hand, the films deposited at T sub =300 • C were partially crystallized in anatase-phase just after deposition and they were not easily transformed to rutile-phase under T a = 700 • C.However, when the annealing temperature exceeded the critical temperature, the films obtained http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology  enough energy to form only rutile-phase TiO 2 .Figures 5(a) and (b) show the relationship between T a and the grain sizes of anatase-phase (D A ) and rutile-phase (D R ) TiO 2 in the films, respectively.Symbols in Fig. 5 correspond to those in Fig. 4. The grain size was calculated using the Scherrer's equation [14]; where D is the grain size, λ is the wavelength of an incident X-ray, β is the full width at half maximum of the peak and θ is the center angle of the peak.In this study, D A and D R were calculated using the anatase (101) and rutile (110) peaks in the XRD spectra, respectively.As can be seen in Fig. 5 Figure 6 shows the relationship between the weight fraction of anatase-TiO 2 and optical bandgap of the TiO 2 films.The symbols in Fig. 6 correspond to those in Fig. 4. The optical bandgap of the TiO 2 films was increased with increasing the weight fraction of anatase-TiO 2 and approached the value of anatase-TiO 2 (3.20 eV) [15].
It may be advantageous for the TiO 2 films used as an ARC in solar cells not to be transformed to rutile-phase, because anatase-TiO 2 is preferable as an ARC due to its wide band gap.On the other hand, thermal annealing is necessary for fabrication of solar cells to form a good metal contact on the front side of the cell.It becomes evident that anatase-rich TiO 2 films can be obtained even after thermal annealing up to T a = 600 • C if they are deposited at elevated temperature.

IV. SUMMARY
In this study, titanium dioxide (TiO 2 ) films were deposited on silicon substrates using RF sputtering at room temperature (T sub =R.T.) and elevated temperature (T sub =300 • C).After deposition, TiO 2 films were annealed at various temperatures (400-900 • C) for 1 h in Ar and O 2 ambient, and phase transition from anatase to rutile was investigated using XRD.The weight fraction of anatase in TiO 2 films deposited at T sub =300 • C were larger than that in the films deposited at T sub =R.T. when they were annealed at temperatures under 600 • C.There was no significant difference between Ar and O 2 annealing whether the TiO 2 films were deposited at T sub =R.T. or T sub =300 • C. Annealing in O 2 ambient was not effective to obtain anatase-TiO 2 in the films using the experimental conditions in this study. Fig.1

Figure 2 (Fig. 2 24FIG. 3 :FIG. 4 :
Figures1(a) and 1(b) show XRD spectra for TiO 2 films annealed at various temperatures in Ar ambient.The TiO 2 films used in Figs.1(a) and 1(b) were deposited at T sub =R.T. and T sub =300 • C, respectively.In Figs.1(a) and 1(b), diffraction peaks at 2θ=25.3 • and 2θ=27.5 • correspond to anatase TiO 2 (101) and rutile TiO 2 (110), respectively.As shown in Fig.1(a), as-deposited TiO 2 film deposited at T sub =R.T. was almost amorphous.The intensity of the anatase TiO 2 peak increased as the annealing temperature (T a ) was increased to 700 • C and it began to decrease over T a = 700 • C, whereas that of the rutile TiO 2 peak continued to increase gradually as T a was increased to 900 • C.These results indicate that the TiO 2 films were transformed from anatase to rutile over 700 • C.However, even at T a = 900 • C, the film was not completely transformed to rutile-phase.On the other hand, for the film deposited at T sub =300 • C (Fig.1(b)), considerably large anatase TiO 2 peak was already observed before annealing.Fig.1(b) also shows that the intensity of the anatase TiO 2 peak was almost constant under T a = 700 • C and the peak suddenly disappeared over T a = 700 • C, and that the intensity of the rutile TiO 2 peak under T a = 700 • C was smaller than that for the film deposited at T sub =R.T. and it increased steeply in the temperature range over T a = 700 • C. Figure 2(a) and 2(b) show XRD spectra for TiO 2 films annealed at various temperatures in Ar and O 2 ambient, respectively.The TiO 2 films used in Figs.2(a) and 2(b) were deposited at T sub =R.T. Figure 3(a) and 3(b) show XRD spectra for TiO 2 films annealed at various temperatures in Ar and O 2 ambient, respectively.The TiO 2 films

FIG. 6 :
FIG.6: Relationship between the weight fraction of anatase-TiO2 and optical bandgap of the TiO2 films.
(a), D A increased up to 700 • C and began to decrease over T a = 700 • C although W A in Fig.4began to decrease over 500 • C.These results may be explained by assuming that the phase transition from anatase to rutile (decrease in W A ) and the enhancement of crystallization of anatase-TiO 2 (increase in D A ) occurred simultaneously in the temperature range of T a = 500-700 • C and that only the phase transition occurred over T a = 700 • C. As shown in Fig.5(b), D R became larger with increasing T a .In Figs.5(a) and 5(b), both D A and D R showed no significant dependence on the substrate temperature during deposition and the annealing ambient.