Optical properties of undoped and iron doped TiO
2thin films grown by RF magnetron sputtering.
D. Guitoume * 1, R. Saidi 1, K. Chaibainou 1 , A. Guittoum 2, S. Achour 3, M. Saad 2, S.E.H. Abaidia 4, N.
Souami 2
1) Unité de recherche en technologie industrielle URTI/CSC, BP 1037 site université Badji Mokhtar Chaiba, 23000, Annaba, Algerie
2) Nuclear Research Center of Algiers, 2 Bd Frantz Fanon, BP 399, Alger-Gare, Algiers, Algeria 3) Mentouri University, 25000 Constantine, Algeria
4) Lab. of Mineral Materials and Composites, Faculty of Engineering Sciences, Univ. Bourmedes – Algeria
ABSTRACT
Titanium dioxide (TiO2) and iron doped (TiO2: Fe) films have been prepared by direct exposure of Ti and TiFe metallic films to thermal oxidation. Ti and TiFe films were deposited on glass substrates using RF magnetron sputtering technique. In this study we report on the effect of thickness on optical properties of TiO2 and the effect of iron concentration on structural and optical properties of iron doped (TiO2: Fe) films.
Structural properties of the obtained TiO2 were presented in a previous paper [1]. The phase structure of TiO2: Fe thin films was identified by grazing incidence X-ray diffraction (GIXRD). The composition of TiO2: Fe films was extracted from RBS and EDX spectra, the iron concentration in the films varies from 9 % to 35 at %.
From X-ray diffraction spectra, it can be seen that all the films present two phases: rutile and anatase. We note also the presence of iron oxide Fe2O3 for the sample with the highest iron concentration (Fe: 35%).
Optical properties of TiO2 and TiO2:Fe films were studied by means of UV-Visible spectroscopy.
Transmittance spectra of TiO2 films show a good transparency in the visible region with a band gap ranging from 3.44 to 3.66 eV. However Transmittance curves of TiO2: Fe films present a considerable absorption edge shift to long wavelength when the amount of Fe increases from 0 to 35 at %. It was found that the band gap value of TiO2:Fe films decreases from 3.66 to 2.44 eV with iron concentration increasing.
.Keywords: RF magnetron sputtering, Thermal oxidation, TiO2 films,TiO2:Fe films,optical properties.
*) corresponding author: guitoum@gmail.com
1. Introduction
Over the last years titanium dioxide has been the subject of numerous studies due to its remarkable physical properties witch make it suitable for a wide interesting technological applications [2-3]. The bulk material of TiO2 has three main phases: rutile, anatase, and brookite. The rutile phase has a greater density
(ρ=4.25 g/cm3) than anatase phase (ρ =3.89 g/cm3) and it is stable at high temperature with a refractive index (n
=2.75 at 550 nm), while anatase is formed at lower temperature and has a refractive index (n =2.54 at 550 nm) [4].Many techniques have been used to prepare TiO2 films including CVD, sol-gel and rf magnetron sputtering.
In addition to these methods, previous investigation showed that the thermal oxidation of sputtered titanium metal film was found to be a simple technique to prepare Ti02 thin films [4] .In this work undoped and iron doped titanium dioxide thin films were elaborated by thermal oxidation of metallic Ti and TiFe thin films. In the present work we report on the effect of thickness and iron concentration on optical properties of TiO2 and TiO: Fe films.
2. Experimental details
Metallic Ti and TiFe films were deposited on glass substrates using rf magnetron sputtering. The target was a titanium disk with a purity of about 99.96 %. Ti and TiFe films were deposited at a constant power of 250 W with a background pressure of 2.5.10-5 mba. During this operation Argon gas pressure was maintained at 3.10-5 mba .To obtain TiO2 films with different thicknesses, we have varied the deposition time of Ti films.
To prepare metallic TiFe films, small pieces of iron were fixed on the Ti target. In order to get TiFe films with different iron concentrations we have varied the number of the fixed iron pieces. The as deposited Ti and TiFe films were annealed at 520°C for 16 h in air. The crystalline structure the as grown films was studied by using grazing X-ray diffraction (Philips X'pert MPD).The measurements were performed at grazing incidence of (1°) using CuKα radiation (λ =1.54178 Å). The films thicknesses were extracted from RBS spectra. The chemical composition of TiO2: Fe film was measured by means of RBS and EDX. The optical transmittance was measured using Lambda E2210 Perkin Elmer UV-Visible spectrophotometer.
3. Results and discussion
3.1Structural characterizationThe structural study of TiO2 films is presented in a previous paper [1]. Indeed we found that the thickness varies from 87 to 484 nm. All TiO2 films present three phases: anatase, rutile and Brookite. The anatase and rutile phases exhibit a strong preferred orientation along (004) and (210) planes respectively. The analysis of TiO2: Fe RBS spectra coupled with EDX experiments allowed us to determine iron concentration (Fe %) witch varies from 9 to 35 at %.
X-ray diffraction patterns of iron doped TiO2 films with different iron concentrations are illustrated in Fig.1. It can be seen that all the films present two TiO2 phases: rutile and anatase with a preferred orientation along (004) and (210) planes respectively. Iron oxide Fe2O3 phase appeared in the film containing the highest iron concentration (35.24%).
Optical Characterization
The UV-visible transmittance spectra of TiO2 films with different thicknesses are presented in Fig. 2. It is clear that TiO2 films present a good transparency (T %) in the visible region. The direct band gap (Eg) of TiO2 film can be determined by plotting (αhν) 2 versus hv (fig3). According to Tauc relation [5], the absorption coefficient is related to the band gap by the following formula:
12
)
(
h Egh
.The band gap values were equal to: 3.66, 3.57, 3.48, 3.44 and 3.54 for 484, 408, 307, 224 and 87 nm respectively.
UV–Vis spectra of doped TiO2 films are presented in figure 4. The curves demonstrate a significant red shift with iron concentration increasing. Also band gap values (Eg) were calculated by plotting (αhν) 2 versus hν (fig5). We note that Eg of TiO2 films decreases from 3.66 to 2.44 eV when Fe concentration increases from 0 to 35 atm %, i.e., the band gap becomes narrower with more Fe addition. Since Fe2O3 crystal has a band gap of 2.2 eV, Eg of the Fe2O3–TiO2 solid solution (the film with 35 at % iron concentration) becomes narrower in comparison with TiO2,
4. Conclusion
In this work, we have successfully prepared TiO2 and Fe doped TiO2 thin films by annealing Ti and TiFe rf magnetron sputtered films. The obtained TiO2 films with different thicknesses present a good transparency in the visible region with band gap values ranging from 3.44 to 3.66 eV. The iron doped TiO2 films present a considerable absorption edge shift to long wavelength when the amount of Fe increased from 0 to 35 at %.The band gap values of the doped films decreased from 3.66 eV to 2.44 eV when iron concentration increased.
References
[1] D. Guitoume, S. Achour, A. Guittoum and S.E.H. Abaidia, AIP conference proceeding, 1047 (2008) 236- 239.
[2] B.-S. Jeong, D.P.Norton, J.D. Budai, Solid State Electronics, 47 (2003) 2275 – 2278 [3] P.Zeman, S.Takabayashi, Surface and Coating Technology, 153 (2002) 93-99.
[4] Chu-Chi Ting et al., Journal of Applied Physics, 88 (2000) 4628.
[5] J. C. Tauc, Optical Properties of Solids, North-Holland, Amsterdam1972, p. 372.
Figure captions
Figure 1: XRD patterns of TiO2 thin films with different iron concentrations.
Figure 2: transmittance spectra of TiO2 films with different thicknesses.
Figure 3: (αhν) 2 as a function of photon energy for TiO2 films with different thicknesses.
Figure 4: UV-visible spectra of iron doped TiO2 films.
Figure 5: (αhν) 2 as a function of photon energy for TiO2: Fe films.
20 30 40 50 60 70 0
200 400 600 800 1000 1200
in te n s iy ( a ,u )
2
Fe:35,24%
Fe:32%
Fe:23,35%
Fe:9,35%
Fe
2O
3
(104)
R(310) A(004)
R(210)
Figure1
300 400 500 600 700 800 0
20 40 60 80 100
tr a n s mit ta n c e ( %
)wavelength(nm)
484nm 408nm 307,5nm 224nm 87nm
Figure 2
*
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 0,0
0,2 0,4 0,6 0,8
(h)2 /1012 (nm-1 eV)
h(eV) 87nm
224nm 307.5nm 408nm 484nm
Figure 3
300 400 500 600 700 800 0
10 20 30 40 50 60 70 80 90 100
Fe:35,24%
Fe:32%
Fe:23,35%
Fe:9,35%
tr a n s mit ta n c e
(% )
wavelength (nm)
Figure 4
0 1 2 3 4 0.00E+000
5.00E+010 1.00E+011 1.50E+011
h nm eV)2
