- Email: [email protected]
Properties of sputtered TiO2 thin films as a function of deposition and annealing parameters
Author’s Accepted Manuscript Properties of sputtered TiO2 thin films as a function of deposition and annealing parameters Dejan Pjević, Marko Obradović, Tijana Marinković, Ana Grce, Momir Milosavljević, Rolf Grieseler, Thomas Kups, Marcus Wilke, Peter Schaaf www.elsevier.com/locate/physb
PII: DOI: Reference:
S0921-4526(15)00070-8 http://dx.doi.org/10.1016/j.physb.2015.01.037 PHYSB308881
To appear in: Physica B: Physics of Condensed Matter Received date: 8 September 2014 Revised date: 26 January 2015 Accepted date: 29 January 2015 Cite this article as: Dejan Pjević, Marko Obradović, Tijana Marinković, Ana Grce, Momir Milosavljević, Rolf Grieseler, Thomas Kups, Marcus Wilke and Peter Schaaf, Properties of sputtered TiO 2 thin films as a function of deposition and annealing parameters, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2015.01.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Properties of sputtered TiO2 thin films as a function of deposition and annealing parameters
Dejan Pjevića*, Marko Obradovića, Tijana Marinkovića, Ana Grcea , Momir Milosavljevića, Rolf Grieselerb , Thomas Kupsb, Marcus Wilkeb, Peter Schaafb
a
VINČA Institute of Nuclear Sciences, University of Belgrade, PO Box 522, Belgrade, Serbia,
b
Institute of Materials Engineering and Institute of Micro- and Nanotechnologies MacroNano,
Ilmenau University of Technology, Gustav-Kirchhoff-Strasse 5, 98693 Ilmenau, Germany
*
Corresponding author: Dejan Pjević,
“Vinča” Institute of Nuclear Sciences, University of Belgrade, P.O.Box 522, 11001 Belgrade, Serbia. Tel/Fax +381 11 6308425, Email: [email protected].
Abstract
The influence of sputtering parameters and annealing on the structure and optical properties of TiO2 thin films deposited by RF magnetron sputtering is reported. A pure TiO2 target was used to deposit the films on Si(100) and glass substrates, and Ar/O2 gas mixture was used for sputtering. It was found that both the structure and the optical properties of the films depend on deposition parameters and annealing. In all cases the as-deposited films were oxygen deficient, which could be compensated by post-deposition annealing. Changes in the Ar/O2 mass flow rate affected the films from an amorphous-like structure for samples deposited without 1
oxygen to a structure where nano-crystalline rutile phase is detected in those deposited with O2. Annealing of the samples yielded growth of both, rutile and anatase phases, the ratio depending on the added oxygen content. Increasing mass flow rate of O2 and annealing are responsible for lowering of the energy band gap values and the increase in refractive index of the films. The results can be interesting towards the development of TiO2 thin films with defined structure and properties. Keywords: TiO2 thin film; RF magnetron sputtering; annealing; structural and optical properties.
1. Introduction
Titanium dioxide films find a very wide range of applications in various fields like gas sensing, photocatalysis, dye sensitized solar cells etc. [1-3]. They show excellent chemical inertness, high photocatalytic activity, optical transmittance with high refractive index in the visible range and good mechanical hardness, the rutile and anatase phase of this material are nontoxic [4-7]. This all makes TiO2 films a suitable candidate for many optical applications. As photocatalyst compound it has a large application in the coating industry due to its efficiency in dissociating organic pollutants under UV irradiation [8, 9]. It is generally considered that TiO2 in the anatase form is the most photoactive and the most practical of the semiconductors for widespread environmental application such as water purification, wastewater treatment, hazardous waste control, air purification, and water disinfection. However, there are reports that in certain circumstances TiO2 in the rutile form exhibits a higher photo-catalytic activity, and in some cases anatase-rutile mixture is more efficient, having some properties not seen in either of the single phases [10-12]. Photo-catalytic
2
oxidation of organic compounds is of interest for environmental applications and in particular for the control and elimination of hazardous wastes. The complete mineralization (i.e., oxidation of organic compounds to CO2, H2O, and associated inorganic components such as HCl, HBr, SO42-, NO3-, etc.) of a variety of aliphatic and aromatic chlorinated hydrocarbons via heterogeneous photo-oxidation on TiO2 has been reported [13]. In addition to organic compounds, a wide variety of inorganic compounds are sensitive to photochemical transformation on semiconductor surfaces. Examples include ammonia, azide, chromium species, copper, cyanide, gold, halide ions, iron species, manganese species, mercury, nitrates and nitrites, nitric oxide and nitrogen dioxide, nitrogen, oxygen, ozone, palladium, platinum, rhodium, silver and sulphur species among others [13]. However, due to a high the band-gap value, ~3 eV for rutile and ~3.2 eV for anatase, the photocatalysis property emerges only for UV radiation. This limits the use of solar light as a sustainable energy source for TiO2 activation, because only 5% of the incoming solar energy on the Earth’s surface is in the UV range [9]. To overcome this constraint one must try to tune the energy band gap by deposition on heated substrates, post annealing of thin layers, doping, implantation and other techniques that can influence the electronic properties of the films. The phase being formed during deposition of TiO2 on unheated substrates strongly depends on mass flow and partial pressure of oxygen in the chamber [14, 15]. Magnetron sputtering of TiO2 films is of special interest, because it is an industrial process applicable to large area deposition, and high quality TiO2 films can be achieved even at low substrate temperatures [16]. In this work, we concentrate on deposition of TiO2 on unheated substrates, of Si and glass, which can extend the range of application on polymers and plastic surfaces. Annealing effects and changes of sputtering parameters on the structure and the optical properties of TiO2/Si thin films deposited by RF magnetron sputtering are studied and discussed. 3
2. Experimental details
Thin films of TiO2 were deposited by RF magnetron sputtering on glass and Si(100) substrates at room temperature from a 99.99% pure TiO2 target. Prior to deposition, the glass and Si substrates were cleaned in acetone, isopropanol and deionized water for 5 minutes. The base pressure in the chamber was about 1.6x10-5 Pa. The target-substrate distance was maintained at 40 cm. The sputtering gas was Ar, and via the same feed-through oxygen could be added to the system. The Ar/O2 ratio was controlled by measuring the mass flow rate. Working pressure during deposition was at 1.1 Pa. The Ar/O2 ratio was varied in the range from 80/0 sccm (no oxygen added to the system) to 30/1 sccm (maximum amount of oxygen added to the system). The sputtering power was kept constant at 200 W and the deposition times were varied between 10 and 22 minutes. Parameters of deposition and the measured thickness of the deposited films are summarized in Table 1. After deposition, the films were annealed in Ar (5.0) atmosphere at 400 °C for 2 hours, in a tube furnace.
Table 1.
Structural and compositional characterization of the thin films was carried out by X-ray diffraction (XRD), glow discharge optical spectroscopy (GDOS) and transmission electron microscopy (TEM). For XRD analysis we used Siemens D5000 X-ray Diffractometer with automatic data acquisition. Diffraction patterns were recorded in the range of diffraction angles 2θ from 20° to 100° at a fixed angle of incidence of 3° in grazing incidence (GIXRD) configuration. Step width for scanning was 0.02° with 2 seconds dwell time. Reference patterns 4
used for identification of reflections were JCPDS cards [021-1272] and [021-1276] for TiO2 and JCPDS card [023-1078] for TiO. GDOS analysis was used to study the elemental depth profiles. GDOS was performed on a GDA 750, Spectruma Analytik GmbH, with RF source of 2.5 mm and with discharge parameters of U = 680V and p = 5 hPa [17]. Before GDOS measurement, calibration was done on sample of well known thickness. The analyzing gas was Ar (5.0). TEM analyses were performed with a Philips XL30 microscope, operated at 200 kV. The samples were prepared for cross-sectional analysis by ion beam thinning. TEM was also used to control the thickness of the deposited films, which was initially measure by a profilometer. Optical properties were studied by UV/VIS measurements, performed on a spectrophotometer Cary 5000 (Varian GmbH). It was used to collect transmittance spectra of both the films and bare substrate. Optical transmittance spectra were measured at normal incidence in the spectral ranges of 250-1500 nm with a scan rate of 300 nm/min, using a bare substrate as a reference. Band gap energies were determined from transmission spectrum employing Tauc plots [18]. Refractive indexes were obtained numerically, using Pointwise Unconstrained Minimization Approach (PUMA) [19, 20]. This method is very useful in obtaining optical data for samples of small thicknesses, for which transmittance doesn’t display a fringe pattern in a highly transparent spectral region. The accuracy of the method has been shown by reconstructing transmittance spectra from calculated thickness (d), refractive index (n) and extinction coefficient (k) by this method and compared to measured transmittance obtained by UV/Vis measurement.
3. Results
5
The results of the film compositional analyses have shown that despite of using a pure TiO2 target the deposited films are oxygen deficient with respect to the Ti:O=1:2 stoichiometry. It is believed that smaller O2 partial pressures, shorter substrate to magnetron distances and greater RF power produce an environment of reduced reaction of sputtered Ti species with O2, and to result in the formation of non-stoichiometric rutile structures [21]. In order to improve this, oxygen was added during sputtering. As can be seen in table 1, this results in a reduced deposition rate as there is a difference in the thicknesses of the deposited films, depending on the Ar/O2 mass flow ratio. Sample which was deposited without introducing oxygen into the chamber, for 600 s, has a thickness of 195 nm. Other samples were deposited with additional oxygen flow into the chamber at different Ar/O2 mass flow ratio. Time of deposition for them was set to 1320 s. The evaluated deposition rates of the TiO2 films as a function of total mass flow of Ar/O2 gases is plotted in Fig. 1. It can be observed that highest deposition rate is when there is no additional oxygen in the system. With introduced oxygen in the system deposition rate significantly drops. For samples deposited with introduced oxygen, deposition rate shows linear dependence of total mass flow rate. Such behavior can be due to a lower concentration of Ar as the sputtering gas, and partial lowering of the target sputtering rate because of a dynamic surface passivation by the introduced oxygen. Similar conclusions were reported by Ohsaki et al. [22]. As an example, the depth profiles deduced by GDOS from as-deposited samples deposited with mass flow ratios of Ar/O2: 80/1 and 60/1 are shown in Fig.2. The profiles indicate that in the initial stage of deposition the composition of the films is close to TiO2 stoichiometry up to around one third of the depth, while in the outer portions of the films there is a lack of oxygen. A similar drop of oxygen concentration towards the surface of the films was observed in all cases. These
6
results suggest a relative change in the sputtering yields of titanium and oxygen from the target with time, or a loss of oxygen in reaction with residual gases in the chamber. More uniform concentration profiles were achieved by annealing the samples. GDOS depth profiles for all annealed samples are shown in Fig. 3. It can be seen that only in the sample deposited without adding oxygen to the system (Ar/O2 = 80/0 sccm) Ti and O profiles are not uniform with depth. In the other samples the ratio is uniform and close to TiO2. Although the samples were annealed in Ar atmosphere, it can be assumed that there was some residual water vapor left in the tube which could contribute to incorporation of oxygen into the layers. It is also seen that there is no significant difference in Ti and O concentration ratio in the samples deposited with different Ar/O2 mass flow ratio, except for the difference in thicknesses. Figure 4 shows GIXRD spectra collected from as-deposited samples deposited with mass flow ratios of Ar/O2: 80/0 and 80/1. Sample deposited without introduced additional oxygen into the system exhibits a feature-less diffraction pattern, which could be assigned either to an amorphous or very fine nano-crystalline structure. By adding oxygen to the system, for mass flow ratio of Ar/O2: 80/1, we get a pattern that resolves (110) and (211) reflections from rutile TiO2 phase. However, having in mind the measured depth profiles, it can be presumed that this phase is grown only partially. In Fig. 5 we present GIXRD spectra collected from the annealed samples. Annealing yielded a more developed crystalline structure, so sharp diffraction peaks could be detected. An interesting behavior with respect to the Ar/O2 mass flow ratio used for deposition of the films was observed. The sample deposited without introduced oxygen into the chamber has different GIXRD pattern compared to all the other samples. It shows two reflections which are rather wide.
7
One of them has a maximum around 2θ ≈ 64° where there are no strong TiO2 reflections, but there is also a strong reflection of TiO ( 2 42). This is in agreement with depth profiles evaluated by GDOS (Fig. 3), which showed a lack of oxygen in this sample much more than in the others. So we can assume that in this sample a mixture of the TiO phase and non-stoichiometric forms of TiO2 are grown. In the annealed samples that were deposited with adding oxygen to the system we only observe reflections arising from TiO2 phases. However, we see that by increasing the O2 content in the Ar/O2 mass flow ratio during layer deposition, the dominant reflections observed by GIXRD change from the anatase (101) to rutile (110) TiO2 phase. Besides these two most intense peaks we also observe reflections from (200), (211) and (204) anatase and (211) rutile TiO2. In the annealed sample D(30/1) we note that the anatase reflections (200) and (204) have practically disappeared. TEM analysis enabled further clarification of the obtained structures. Bright field TEM crosssectional images shown in Fig. 6 were taken from samples: (a) A(80/0) as-deposited, (b) B(80/1) as-deposited, (c) C(60/1) as-deposited, and (d) C(60/1) annealed. Compared to XRD spectra shown in Fig. 4, here we also see an almost featureless contrast in the sample as-deposited without added oxygen to the system (a). However, it should be noted that in the high resolution imaging mode we did observe some nano-crystals (up to ~5 nm) dispersed in an amorphous-like matrix, though their crystallography could not be identified. Contrary to this, in as-deposited samples B(80/1) (b) and C(60/1) (c) one can clearly observe individual crystalline grains formed in the outer regions of the films. After annealing of sample C(60/1) (d) the crystalline structure is observed throughout the whole film. The reason for formation of larger crystalline grains in the surface regions of the samples deposited with additional oxygen flow can be an increase of the substrate temperature during deposition. Namely, sample with no added oxygen in the system, 8
was deposited for 600 s, and the other samples for 1320 s. High resolution TEM images and identification of rutile TiO2 crystal grains in as-deposited sample B(80/1) and annealed sample D(30/1), are shown in Fig.7. Optical measurements were done in the range from 250-1500 nm, on samples deposited on glass substrates. In previous works [23-25], both experimental results and theoretical calculations suggest that TiO2 has a direct forbidden gap (3.03 eV) which is almost degenerated with indirect allowed transitions [23]. Due to a low probability of the direct forbidden transition, the indirect allowed transitions dominate in the optical absorption edge [24]. Then the absorption coefficient above the threshold of fundamental absorption is proportional to (E-Eg)2. So, in high absorption region (α>104cm-1), band gap energy, Eg, may be determined from Tauc relation [25]. For indirect allowed transitions, incident photon energy, h and absorption coefficient, α, are related to each other by:
B(h Eg )2 h
, where B is a constant.
The results of optical transmittance measurements obtained from annealed samples are shown in Fig. 8. We can notice that the integral transparency is about 91% in the 400-1500nm wavelength range and the absorption edge is at about 360-380 nm. It can be seen that in the samples deposited with added oxygen (B(80/1), C(60/1), and D(30/1)) the measured absorption edge has very close values grouped around 360 nm, while the absorption edge is shifted to ~380 nm for the sample deposited without oxygen. This behavior can be correlated to structural analysis, where for sample deposited without any additional oxygen in the system, we assume a formation of TiO nano-particles, and in the other samples we detect formation of TiO2 particles. Also, reconstructed transmittance functions, from calculated values of d, n and k by PUMA method, are plotted on Fig.8. It can be seen that correlation between measured and calculated 9
transmittance data is very good, and that calculated values by PUMA can be considered as correct. From the optical transmittance data, absorption coefficients were calculated and then energy band gap values were extrapolated. For as-deposited samples it was found that the energy band gap values vary between 3.30-3.36 eV and are dependant of mass flow ratio of added O2. The lowest Eg was found for sample A(80/0) which was deposited without introduced oxygen in the system. For samples deposited with additional oxygen in the system, lowest Eg was found in sample D(30/1) where rutile phase is dominant, and highest for sample B(80/1) where anatase phase dominates. With increasing of mass flow rate of oxygen in the system energy gap values are decreasing. Such behavior can be based on the results of phase formation, where the content of the rutile phase in the films increases with increasing added oxygen. In Fig. 9 we plot the (αhν)1/2 versus photon energy (E) data for annealed samples. The evaluated band gap values are close to those evaluated for as-deposited samples, varying between 3.29-3.34 eV. Again a clear difference for the sample deposited without oxygen, where by GIXRD we detect TiO phase, could be seen. For the films deposited with oxygen, where TiO2 phases were detected, the band gap vary between 3.33-3.34 eV. From these measurements one can conclude that annealing at 400 °C for 2 hours did not affect the band gap significantly, although the values for all annealed samples are slightly lower than for as-deposited ones. Calculation of refractive indexes from transmittance data was done by PUMA method. Again sample deposited without introduced oxygen in the system show different behavior in comparison to others samples, and calculated refractive index was 3.13 for as deposited sample and 3.18 for annealed one. In Fig.10 calculated values for n were plotted against wavelength of
10
incident light for samples deposited with added oxygen on to the system. It can be seen that with increasing oxygen in the system, refractive index is increasing. That increase is significant, and for λ=550 nm it goes from 2.52 for sample B(80/1) to 2.62 for sample D(30/1). For annealed samples refraction index is increased in comparison with as deposited thin films, except for sample C(60/1) for which it is around same values as for as deposited one, and it goes as high as 2.69 for mass flow rate of m = 30/1 sccm. This increasing of refractive index corresponds with increase of rutile phase fraction in the sample and with increment of O2 mass flow rate during deposition.
4. Discussion
In their studies of TiO2 crystalline phase formation, Howitt and Harker [26] suggested an amorphous-anatase transformation model for obtaining stabile anatase structure in thin films. Some researchers also found high dependence between the amorphous-anatase transformation and the existence of crystalline seeds of the appropriate phase within the amorphous matrix [27]. At temperatures below 600 °C, growth of anatase phase in amorphous matrix is favored and annealing leads to large anatase grains upon annealing, with a presence of small rutile grains [4,16]. This can be understood from differences in the density of the involved phases. Since the density of an amorphous film (3.2~3.65 g·cm-3) is much lower than the density of rutile (4.26 g·cm-3), the formation of rutile from an amorphous matrix means a stronger densification and costs more elastic energy then the formation of anatase, which has a density of 3.84 g·cm-3 [16].
11
In the case presented here, only the presence of rutile nano-particles in as-deposited samples, embedded in an amorphous non-stoichiometric Ti-O mixture, could be detected. Upon annealing these rutile grains grow larger, but we also detect the anatase phase, which may imply that some of these nano-grains were seeded in the as-deposited films. It is interesting to observe sample B(80/1), deposited with the lowest oxygen content added to the system, (Figs. 4 and 5): in the asdeposited sample GIXRD shows only distinct reflections from the rutile phase, while after annealing we detect even more intense reflections from the anatase phase. As more oxygen is added (Fig. 5), the presence of rutile increases and the presence of anatase decreases. However, in the sample deposited without oxygen, the TiO2 stoichiometry was not reached, so presence of the TiO phase was detected. The optical measurements have shown that the sample containing monoxide has distinctly lower band gap value, compared to the other samples that contain only the dioxide. Annealing temperature has an effect in slightly decreasing the band gap in the dioxide films. A decrease in the band gap energy with increasing the annealing temperature is due to the lowering of interatomic spacing (amorphous-crystalline) [28], release of defects and growing larger crystalline grains. This conclusion is in agreement with the observed GIXRD and TEM results for crystalline structure of the samples. Values for energy gap obtained by the used method are higher than values for bulk material (Eg for rutile is approximately 3 eV, and for anatase 3.2 eV). There are two possible reasons for a large band gap value of the films: (1) presumably due to an axial strain effect from lattice deformation as has been pointed out for ZnO films; and (2) probably due to a change in the density of charge carriers [29].
12
5.
Conclusions
The presented results show that the structure and properties of TiO2 films can be varied and adjusted by altering the deposition parameters and by post-deposition thermal treatment of the samples. As-deposited films were found to be oxygen deficient, which could be compensated by post-deposition annealing. Changes in the mass flow rate between Ar and O2 affected the structure of the films in the way such that we got an amorphous-like structure for samples deposited without oxygen to a structure where nano-crystalline rutile phase could be detected in those deposited with O2. Annealing yielded larger grains, where both rutile and anatase phases were detected. An interesting observation is the change of the dominant phase from anatase to rutile in the annealed samples with increasing the O2 content during their deposition. Optical measurements showed that increase of O2 mass flow rate and annealing are responsible for lowering energy gap values and increase in refractive index of thin layers, which is due to the increase of the rutile phase content in the films. All measured band gap values are higher than for bulk rutile and anatase phases, which is assigned to the nano-crystalline nature and to structural irregularities of the films. The results can be interesting towards exploiting magnetron sputtering techniques for deposition of TiO2 thin films of desired structure and optical properties.
Acknowledgments
This research was supported by the German-Serbian bilateral collaborative project (DAAD PPP 50752549) and the Serbian Ministry of Education, Science and Technological Development (Project ON 171023).
13
References
[1] B. Karunagaran, P. Uthirakumar, S.J. Chung, S. Velumani, E.K. Suh, TiO2 thin film gas sensor for monitoring ammonia, Mater. Characterization. 58 (2007) 680-684. [2] J. Yu, X. Zhao, Q. Zhao, Photocatalytic activity of nanometer TiO2 thin films prepared by the sol–gel method, Mater. Chem. Phys. 69 (2001) 25–29. [3] K.H. Ko, Y.C. Lee, Y.J. Jung, Enhanced efficiency of dye-sensitized TiO2 solar cells (DSSC) by doping of metal ions, Journal of Colloid Interface 283 (2005) 482–487. [4] P. Lobl, M. Huppertz, D. Margel, Nucleation and growth in TiO2 films prepared by sputtering and evaporation, Thin Solid Films 251 (1994) 72. [5] P. Wang, P. S. Yap, T. T. Lim, C-N-S tridoped TiO2 for photocatalytic degradation of tetracycline under visible-light irradiation, Appl. Catal. A 399 (2011) 252-261. [6] F. Lapostolle, A. Billard, J. Stebut, Structure/mechanical properties relationship of titanium-oxygen coatings reactively sputter-deposited, Surf. Coat. Technol. 135 (2000) 1. [7] S. Serio, M.E. Melo Jorge, Y. Nunes, N.P. Barradas, E. Alves, F. Munnik, Incorporation of N in TiO2 films grown by DC-reactive magnetron sputtering, Nucl. Instrum. Methods B 273 (2012) 109. [8] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37. [9] M. Kitano, M. Matsuoka, M. Ueshima, M. Anpo, Recent developments in titanium oxide-based photocatalysts, Appl. Catal. A 325(1) (2007) 1-14. [10] D.F. Ollis, H. Al-Ekabi, Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993, pp. 337-351. 14
[11] J. Peral, X. Domenech, Photocatalytic cyanide oxidation from aqueous coppercyanide solutions over TiO2 and ZnO, J. Chem. Technol. Biotechnol. 53 (1992) 93-96. [12] J. Peral, J. Munoz, J. Domenech, Photosensitized CN-oxidation over TiO2, Photochem. Photobiol. 55 (1990) 251. [13] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69-96. [14] M.C. Liao, H. Niu, G.S. Chen, Effect of sputtering pressure and post-annealing on hydrophilicity of TiO2 thin films deposited by reactive magnetron sputtering, Thin Solid Films 518 (2010) 7259. [15] J. Šicha, J. Musil, M. Meissner, R. Čerstvy, Nanostructure of photocatalytic TiO2 films sputtered at temperatures below 200 °C, App.Surf.Sci. 254 (2008) 3793-3800. [16] W. Zhang, Y. Li, F. Wang, Properties of TiO2 Thin Films Prepared by Magnetron Sputtering, J. Mater. Sci. Technol. 18(02) (2002) 101-107. [17] M. Wilke, G. Teichert, R. Gemma, A. Pundt, R. Kirchheim, H. Romanus, P. Schaaf, Glow discharge optical emission spectroscopy for accurate and well resolved analysis of coatings and thin films, Thin Solid Films 520 (2011) 1661. [18] J. Tauc, R. Grigorovich, A. Vancu, Optical properties and electronic structure of amorphous germanium, Phys. Stat. Sol. 15 (1966) 627. [19] E. G. Birgin, I. Chambouleyron, J. M. Martínez, Estimation of the optical constants and the thickness of thin films using unconstrained optimization, Journal of Computational Physics 151 (1999) 862-880. [20] M. Mulato, I. Chambouleyron, E. G. Birgin, J. M. Martínez, Determination of thickness and optical constants of amorphous silicon films from transmittance data, Applied Physics Letters 77 (2000) 2133-2135. 15
[21] N. Kawashima, Q. Y. Zhu, A. R. Gerson, The selective growth of rutile thin films using radio-frequency magnetron sputtering, Thin Solid Films 520 (2012) 3884. [22] H. Ohsaki, Y. Tachibana, A. Mitsui, T. Kamiyama, Y. Hayashi, High rate deposition of TiO2 by DC sputtering of the TiO2-X target, Thin Solid Films 392 (2001) 169-173. [23] K.M. Glassford, J.R. Chelikovsky, Structural and electronic properties of titanium dioxide, Phys. Rev. B 46 (1992) 1284. [24] N. Martin, C. Rousselot, D. Rondot, F. Palmino, R. Mercier, Microstructure modification of amorphous titanium oxide thin films during annealing treatment, Thin Solid Films 300 (1997) 113-121. [25] J. Tauc, Amorphous and Liquid Semiconductors, Plenum Press, New York, 1974, pp. 159. [26] D.G. Howitt, A.B. Harker, The oriented growth of anatase in thin films of amorphous titania, J.Mater.Res. 2 (1987) 201. [27] L.S. Hsu, R. Rujkorakarn, J.R. Sites, C.Y. She, Thermally induced crystallization of amorphous-titania films, J. Appl. Phys. 59 (1986) 3475. [28] C.V.R. Vasantkumar, A. Mansingh, Seventh IEEE International Symposium on Application of Ferroelectrics, IEEE, New York, 1990, pp. 713-716. [29] M. Horprathum, P. Eiamchai, P. Chindaudom, A. Pokaipisit, P. Limsuwan, Oxygen partial pressure dependence of the properties of TiO2 thin films deposited by DC reactive magnetron sputtering, Procedia Engineering 32 (2012) 676 – 682.
16
Figure captions
Fig.1. Deposition rates of TiO2 films as a function of the total mass flow rate of Ar/O2 gases. Fig.2. Elemental depth profiles as obtained from GDOS measurements for as-deposited TiO2 samples: (a) for a sample deposited with m (Ar/O2) = 80/1 and (b) for a sample deposited with m (Ar/O2) = 60/1. Fig.3. Elemental depth profiles obtained from GDOS measurements for annealed TiO2 samples deposited with different mass flow ratios (Ar/O2): (a) m = 80/0 sccm, (b) m = 80/1 sccm, (c) m = 60/1 sccm and (d) m = 30/1 sccm. Fig.4. GIXRD patterns for the as-deposited TiO2 samples, with different Ar/O2 flow ratios. Fig.5. GIXRD pattern from TiO2 samples annealed on 400 °C for 2 hours. Flow ratios are stated in the graph. Fig.6. Bright field TEM cross-sectional images taken from samples: (a) A(80/0) as-deposited, (b) B(80/1) as-deposited, (c) C(60/1) as-deposited, (d) C(60/1) annealed. Fig.7. HRTEM image and the corresponding FFT pattern taken from as-deposited sample B(80/1) (a) and annealed sample D(30/1) (b), identifying rutile TiO2 crystal grains. Fig.8. Transmittance spectra for annealed TiO2 films at different mass flow ratios Ar/O2, and reconstructed spectra by PUMA. Fig.9. Square root of (αhν) versus energy plots of TiO2 films annealed on 400 °C for 2 hours. The fitted lines were used for determining the band-gap.
17
Fig.10. Calculated values of refractive indexes by PUMA for as deposited and annealed samples: B(80/1), C(60/1) and D(30/1).
Tables:
Table 1: Deposition and annealing parameters for TiO2/Si thin films, measured thicknesses and thicknesses calculated by PUMA. Sample A(80/0) B(80/1) C(60/1) D(30/1)
m(Ar/O) (sccm) 80/0 80/1 60/1 30/1
t(s)
d(nm)
600 1320 1320 1320
195 120 105 80
18
d(nm) PUMA 180 112 108 80
Tan 2h (°C) 400 400 400 400
Figure1
Fig.1. Deposition rates of TiO2 films as a function of the total mass flow rate of Ar/O2 gases.
Figure2
Fig.2. Elemental depth profiles as obtained from GDOS measurements for as-deposited TiO2 samples: (a) for a sample deposited with ݉ሶ(Ar/O2) = 80/1 and (b) for a sample deposited with ݉ሶ(Ar/O2) = 60/1.
Figure3
Fig.3. Elemental depth profiles obtained from GDOS measurements for annealed TiO2 samples deposited with different mass flow ratios (Ar/O2): (a) ݉ሶ = 80/0 sccm, (b) ݉ሶ = 80/1 sccm, (c) ݉ሶ = 60/1 sccm and (d) ݉ሶ = 30/1 sccm.
Figure4
Fig.4. GIXRD patterns for the as-deposited TiO2 samples, with different Ar/O2 flow ratios.
Figure5
Fig.5. GIXRD pattern from TiO2 samples annealed on 400 °C for 2 hours. Flow ratios are stated in the graph.
Figure6
Fig.6. Bright field TEM cross-sectional images taken from samples: (a) A(80/0) as-deposited, (b) B(80/1) as-deposited, (c) C(60/1) as-deposited, (d) C(60/1) annealed.
Figure7
Fig.7. HRTEM image and the corresponding FFT pattern taken from as-deposited sample B(80/1) (a) and annealed sample D(30/1) (b), identifying rutile TiO2 crystal grains.
Figure8
Fig.8. Transmittance spectra for annealed TiO2 films at different mass flow ratios Ar/O2, and reconstructed spectra by PUMA.
Figure9
Fig.9. Square root of (αhν) versus energy plots of TiO2 films annealed on 400 °C for 2 hours. The fitted lines were used for determining the band-gap.
Figure10
Fig.10. Calculated values of refractive indexes by PUMA for as deposited and annealed samples: B(80/1), C(60/1) and D(30/1).
PII: DOI: Reference:
S0921-4526(15)00070-8 http://dx.doi.org/10.1016/j.physb.2015.01.037 PHYSB308881
To appear in: Physica B: Physics of Condensed Matter Received date: 8 September 2014 Revised date: 26 January 2015 Accepted date: 29 January 2015 Cite this article as: Dejan Pjević, Marko Obradović, Tijana Marinković, Ana Grce, Momir Milosavljević, Rolf Grieseler, Thomas Kups, Marcus Wilke and Peter Schaaf, Properties of sputtered TiO 2 thin films as a function of deposition and annealing parameters, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2015.01.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Properties of sputtered TiO2 thin films as a function of deposition and annealing parameters
Dejan Pjevića*, Marko Obradovića, Tijana Marinkovića, Ana Grcea , Momir Milosavljevića, Rolf Grieselerb , Thomas Kupsb, Marcus Wilkeb, Peter Schaafb
a
VINČA Institute of Nuclear Sciences, University of Belgrade, PO Box 522, Belgrade, Serbia,
b
Institute of Materials Engineering and Institute of Micro- and Nanotechnologies MacroNano,
Ilmenau University of Technology, Gustav-Kirchhoff-Strasse 5, 98693 Ilmenau, Germany
*
Corresponding author: Dejan Pjević,
“Vinča” Institute of Nuclear Sciences, University of Belgrade, P.O.Box 522, 11001 Belgrade, Serbia. Tel/Fax +381 11 6308425, Email: [email protected].
Abstract
The influence of sputtering parameters and annealing on the structure and optical properties of TiO2 thin films deposited by RF magnetron sputtering is reported. A pure TiO2 target was used to deposit the films on Si(100) and glass substrates, and Ar/O2 gas mixture was used for sputtering. It was found that both the structure and the optical properties of the films depend on deposition parameters and annealing. In all cases the as-deposited films were oxygen deficient, which could be compensated by post-deposition annealing. Changes in the Ar/O2 mass flow rate affected the films from an amorphous-like structure for samples deposited without 1
oxygen to a structure where nano-crystalline rutile phase is detected in those deposited with O2. Annealing of the samples yielded growth of both, rutile and anatase phases, the ratio depending on the added oxygen content. Increasing mass flow rate of O2 and annealing are responsible for lowering of the energy band gap values and the increase in refractive index of the films. The results can be interesting towards the development of TiO2 thin films with defined structure and properties. Keywords: TiO2 thin film; RF magnetron sputtering; annealing; structural and optical properties.
1. Introduction
Titanium dioxide films find a very wide range of applications in various fields like gas sensing, photocatalysis, dye sensitized solar cells etc. [1-3]. They show excellent chemical inertness, high photocatalytic activity, optical transmittance with high refractive index in the visible range and good mechanical hardness, the rutile and anatase phase of this material are nontoxic [4-7]. This all makes TiO2 films a suitable candidate for many optical applications. As photocatalyst compound it has a large application in the coating industry due to its efficiency in dissociating organic pollutants under UV irradiation [8, 9]. It is generally considered that TiO2 in the anatase form is the most photoactive and the most practical of the semiconductors for widespread environmental application such as water purification, wastewater treatment, hazardous waste control, air purification, and water disinfection. However, there are reports that in certain circumstances TiO2 in the rutile form exhibits a higher photo-catalytic activity, and in some cases anatase-rutile mixture is more efficient, having some properties not seen in either of the single phases [10-12]. Photo-catalytic
2
oxidation of organic compounds is of interest for environmental applications and in particular for the control and elimination of hazardous wastes. The complete mineralization (i.e., oxidation of organic compounds to CO2, H2O, and associated inorganic components such as HCl, HBr, SO42-, NO3-, etc.) of a variety of aliphatic and aromatic chlorinated hydrocarbons via heterogeneous photo-oxidation on TiO2 has been reported [13]. In addition to organic compounds, a wide variety of inorganic compounds are sensitive to photochemical transformation on semiconductor surfaces. Examples include ammonia, azide, chromium species, copper, cyanide, gold, halide ions, iron species, manganese species, mercury, nitrates and nitrites, nitric oxide and nitrogen dioxide, nitrogen, oxygen, ozone, palladium, platinum, rhodium, silver and sulphur species among others [13]. However, due to a high the band-gap value, ~3 eV for rutile and ~3.2 eV for anatase, the photocatalysis property emerges only for UV radiation. This limits the use of solar light as a sustainable energy source for TiO2 activation, because only 5% of the incoming solar energy on the Earth’s surface is in the UV range [9]. To overcome this constraint one must try to tune the energy band gap by deposition on heated substrates, post annealing of thin layers, doping, implantation and other techniques that can influence the electronic properties of the films. The phase being formed during deposition of TiO2 on unheated substrates strongly depends on mass flow and partial pressure of oxygen in the chamber [14, 15]. Magnetron sputtering of TiO2 films is of special interest, because it is an industrial process applicable to large area deposition, and high quality TiO2 films can be achieved even at low substrate temperatures [16]. In this work, we concentrate on deposition of TiO2 on unheated substrates, of Si and glass, which can extend the range of application on polymers and plastic surfaces. Annealing effects and changes of sputtering parameters on the structure and the optical properties of TiO2/Si thin films deposited by RF magnetron sputtering are studied and discussed. 3
2. Experimental details
Thin films of TiO2 were deposited by RF magnetron sputtering on glass and Si(100) substrates at room temperature from a 99.99% pure TiO2 target. Prior to deposition, the glass and Si substrates were cleaned in acetone, isopropanol and deionized water for 5 minutes. The base pressure in the chamber was about 1.6x10-5 Pa. The target-substrate distance was maintained at 40 cm. The sputtering gas was Ar, and via the same feed-through oxygen could be added to the system. The Ar/O2 ratio was controlled by measuring the mass flow rate. Working pressure during deposition was at 1.1 Pa. The Ar/O2 ratio was varied in the range from 80/0 sccm (no oxygen added to the system) to 30/1 sccm (maximum amount of oxygen added to the system). The sputtering power was kept constant at 200 W and the deposition times were varied between 10 and 22 minutes. Parameters of deposition and the measured thickness of the deposited films are summarized in Table 1. After deposition, the films were annealed in Ar (5.0) atmosphere at 400 °C for 2 hours, in a tube furnace.
Table 1.
Structural and compositional characterization of the thin films was carried out by X-ray diffraction (XRD), glow discharge optical spectroscopy (GDOS) and transmission electron microscopy (TEM). For XRD analysis we used Siemens D5000 X-ray Diffractometer with automatic data acquisition. Diffraction patterns were recorded in the range of diffraction angles 2θ from 20° to 100° at a fixed angle of incidence of 3° in grazing incidence (GIXRD) configuration. Step width for scanning was 0.02° with 2 seconds dwell time. Reference patterns 4
used for identification of reflections were JCPDS cards [021-1272] and [021-1276] for TiO2 and JCPDS card [023-1078] for TiO. GDOS analysis was used to study the elemental depth profiles. GDOS was performed on a GDA 750, Spectruma Analytik GmbH, with RF source of 2.5 mm and with discharge parameters of U = 680V and p = 5 hPa [17]. Before GDOS measurement, calibration was done on sample of well known thickness. The analyzing gas was Ar (5.0). TEM analyses were performed with a Philips XL30 microscope, operated at 200 kV. The samples were prepared for cross-sectional analysis by ion beam thinning. TEM was also used to control the thickness of the deposited films, which was initially measure by a profilometer. Optical properties were studied by UV/VIS measurements, performed on a spectrophotometer Cary 5000 (Varian GmbH). It was used to collect transmittance spectra of both the films and bare substrate. Optical transmittance spectra were measured at normal incidence in the spectral ranges of 250-1500 nm with a scan rate of 300 nm/min, using a bare substrate as a reference. Band gap energies were determined from transmission spectrum employing Tauc plots [18]. Refractive indexes were obtained numerically, using Pointwise Unconstrained Minimization Approach (PUMA) [19, 20]. This method is very useful in obtaining optical data for samples of small thicknesses, for which transmittance doesn’t display a fringe pattern in a highly transparent spectral region. The accuracy of the method has been shown by reconstructing transmittance spectra from calculated thickness (d), refractive index (n) and extinction coefficient (k) by this method and compared to measured transmittance obtained by UV/Vis measurement.
3. Results
5
The results of the film compositional analyses have shown that despite of using a pure TiO2 target the deposited films are oxygen deficient with respect to the Ti:O=1:2 stoichiometry. It is believed that smaller O2 partial pressures, shorter substrate to magnetron distances and greater RF power produce an environment of reduced reaction of sputtered Ti species with O2, and to result in the formation of non-stoichiometric rutile structures [21]. In order to improve this, oxygen was added during sputtering. As can be seen in table 1, this results in a reduced deposition rate as there is a difference in the thicknesses of the deposited films, depending on the Ar/O2 mass flow ratio. Sample which was deposited without introducing oxygen into the chamber, for 600 s, has a thickness of 195 nm. Other samples were deposited with additional oxygen flow into the chamber at different Ar/O2 mass flow ratio. Time of deposition for them was set to 1320 s. The evaluated deposition rates of the TiO2 films as a function of total mass flow of Ar/O2 gases is plotted in Fig. 1. It can be observed that highest deposition rate is when there is no additional oxygen in the system. With introduced oxygen in the system deposition rate significantly drops. For samples deposited with introduced oxygen, deposition rate shows linear dependence of total mass flow rate. Such behavior can be due to a lower concentration of Ar as the sputtering gas, and partial lowering of the target sputtering rate because of a dynamic surface passivation by the introduced oxygen. Similar conclusions were reported by Ohsaki et al. [22]. As an example, the depth profiles deduced by GDOS from as-deposited samples deposited with mass flow ratios of Ar/O2: 80/1 and 60/1 are shown in Fig.2. The profiles indicate that in the initial stage of deposition the composition of the films is close to TiO2 stoichiometry up to around one third of the depth, while in the outer portions of the films there is a lack of oxygen. A similar drop of oxygen concentration towards the surface of the films was observed in all cases. These
6
results suggest a relative change in the sputtering yields of titanium and oxygen from the target with time, or a loss of oxygen in reaction with residual gases in the chamber. More uniform concentration profiles were achieved by annealing the samples. GDOS depth profiles for all annealed samples are shown in Fig. 3. It can be seen that only in the sample deposited without adding oxygen to the system (Ar/O2 = 80/0 sccm) Ti and O profiles are not uniform with depth. In the other samples the ratio is uniform and close to TiO2. Although the samples were annealed in Ar atmosphere, it can be assumed that there was some residual water vapor left in the tube which could contribute to incorporation of oxygen into the layers. It is also seen that there is no significant difference in Ti and O concentration ratio in the samples deposited with different Ar/O2 mass flow ratio, except for the difference in thicknesses. Figure 4 shows GIXRD spectra collected from as-deposited samples deposited with mass flow ratios of Ar/O2: 80/0 and 80/1. Sample deposited without introduced additional oxygen into the system exhibits a feature-less diffraction pattern, which could be assigned either to an amorphous or very fine nano-crystalline structure. By adding oxygen to the system, for mass flow ratio of Ar/O2: 80/1, we get a pattern that resolves (110) and (211) reflections from rutile TiO2 phase. However, having in mind the measured depth profiles, it can be presumed that this phase is grown only partially. In Fig. 5 we present GIXRD spectra collected from the annealed samples. Annealing yielded a more developed crystalline structure, so sharp diffraction peaks could be detected. An interesting behavior with respect to the Ar/O2 mass flow ratio used for deposition of the films was observed. The sample deposited without introduced oxygen into the chamber has different GIXRD pattern compared to all the other samples. It shows two reflections which are rather wide.
7
One of them has a maximum around 2θ ≈ 64° where there are no strong TiO2 reflections, but there is also a strong reflection of TiO ( 2 42). This is in agreement with depth profiles evaluated by GDOS (Fig. 3), which showed a lack of oxygen in this sample much more than in the others. So we can assume that in this sample a mixture of the TiO phase and non-stoichiometric forms of TiO2 are grown. In the annealed samples that were deposited with adding oxygen to the system we only observe reflections arising from TiO2 phases. However, we see that by increasing the O2 content in the Ar/O2 mass flow ratio during layer deposition, the dominant reflections observed by GIXRD change from the anatase (101) to rutile (110) TiO2 phase. Besides these two most intense peaks we also observe reflections from (200), (211) and (204) anatase and (211) rutile TiO2. In the annealed sample D(30/1) we note that the anatase reflections (200) and (204) have practically disappeared. TEM analysis enabled further clarification of the obtained structures. Bright field TEM crosssectional images shown in Fig. 6 were taken from samples: (a) A(80/0) as-deposited, (b) B(80/1) as-deposited, (c) C(60/1) as-deposited, and (d) C(60/1) annealed. Compared to XRD spectra shown in Fig. 4, here we also see an almost featureless contrast in the sample as-deposited without added oxygen to the system (a). However, it should be noted that in the high resolution imaging mode we did observe some nano-crystals (up to ~5 nm) dispersed in an amorphous-like matrix, though their crystallography could not be identified. Contrary to this, in as-deposited samples B(80/1) (b) and C(60/1) (c) one can clearly observe individual crystalline grains formed in the outer regions of the films. After annealing of sample C(60/1) (d) the crystalline structure is observed throughout the whole film. The reason for formation of larger crystalline grains in the surface regions of the samples deposited with additional oxygen flow can be an increase of the substrate temperature during deposition. Namely, sample with no added oxygen in the system, 8
was deposited for 600 s, and the other samples for 1320 s. High resolution TEM images and identification of rutile TiO2 crystal grains in as-deposited sample B(80/1) and annealed sample D(30/1), are shown in Fig.7. Optical measurements were done in the range from 250-1500 nm, on samples deposited on glass substrates. In previous works [23-25], both experimental results and theoretical calculations suggest that TiO2 has a direct forbidden gap (3.03 eV) which is almost degenerated with indirect allowed transitions [23]. Due to a low probability of the direct forbidden transition, the indirect allowed transitions dominate in the optical absorption edge [24]. Then the absorption coefficient above the threshold of fundamental absorption is proportional to (E-Eg)2. So, in high absorption region (α>104cm-1), band gap energy, Eg, may be determined from Tauc relation [25]. For indirect allowed transitions, incident photon energy, h and absorption coefficient, α, are related to each other by:
B(h Eg )2 h
, where B is a constant.
The results of optical transmittance measurements obtained from annealed samples are shown in Fig. 8. We can notice that the integral transparency is about 91% in the 400-1500nm wavelength range and the absorption edge is at about 360-380 nm. It can be seen that in the samples deposited with added oxygen (B(80/1), C(60/1), and D(30/1)) the measured absorption edge has very close values grouped around 360 nm, while the absorption edge is shifted to ~380 nm for the sample deposited without oxygen. This behavior can be correlated to structural analysis, where for sample deposited without any additional oxygen in the system, we assume a formation of TiO nano-particles, and in the other samples we detect formation of TiO2 particles. Also, reconstructed transmittance functions, from calculated values of d, n and k by PUMA method, are plotted on Fig.8. It can be seen that correlation between measured and calculated 9
transmittance data is very good, and that calculated values by PUMA can be considered as correct. From the optical transmittance data, absorption coefficients were calculated and then energy band gap values were extrapolated. For as-deposited samples it was found that the energy band gap values vary between 3.30-3.36 eV and are dependant of mass flow ratio of added O2. The lowest Eg was found for sample A(80/0) which was deposited without introduced oxygen in the system. For samples deposited with additional oxygen in the system, lowest Eg was found in sample D(30/1) where rutile phase is dominant, and highest for sample B(80/1) where anatase phase dominates. With increasing of mass flow rate of oxygen in the system energy gap values are decreasing. Such behavior can be based on the results of phase formation, where the content of the rutile phase in the films increases with increasing added oxygen. In Fig. 9 we plot the (αhν)1/2 versus photon energy (E) data for annealed samples. The evaluated band gap values are close to those evaluated for as-deposited samples, varying between 3.29-3.34 eV. Again a clear difference for the sample deposited without oxygen, where by GIXRD we detect TiO phase, could be seen. For the films deposited with oxygen, where TiO2 phases were detected, the band gap vary between 3.33-3.34 eV. From these measurements one can conclude that annealing at 400 °C for 2 hours did not affect the band gap significantly, although the values for all annealed samples are slightly lower than for as-deposited ones. Calculation of refractive indexes from transmittance data was done by PUMA method. Again sample deposited without introduced oxygen in the system show different behavior in comparison to others samples, and calculated refractive index was 3.13 for as deposited sample and 3.18 for annealed one. In Fig.10 calculated values for n were plotted against wavelength of
10
incident light for samples deposited with added oxygen on to the system. It can be seen that with increasing oxygen in the system, refractive index is increasing. That increase is significant, and for λ=550 nm it goes from 2.52 for sample B(80/1) to 2.62 for sample D(30/1). For annealed samples refraction index is increased in comparison with as deposited thin films, except for sample C(60/1) for which it is around same values as for as deposited one, and it goes as high as 2.69 for mass flow rate of m = 30/1 sccm. This increasing of refractive index corresponds with increase of rutile phase fraction in the sample and with increment of O2 mass flow rate during deposition.
4. Discussion
In their studies of TiO2 crystalline phase formation, Howitt and Harker [26] suggested an amorphous-anatase transformation model for obtaining stabile anatase structure in thin films. Some researchers also found high dependence between the amorphous-anatase transformation and the existence of crystalline seeds of the appropriate phase within the amorphous matrix [27]. At temperatures below 600 °C, growth of anatase phase in amorphous matrix is favored and annealing leads to large anatase grains upon annealing, with a presence of small rutile grains [4,16]. This can be understood from differences in the density of the involved phases. Since the density of an amorphous film (3.2~3.65 g·cm-3) is much lower than the density of rutile (4.26 g·cm-3), the formation of rutile from an amorphous matrix means a stronger densification and costs more elastic energy then the formation of anatase, which has a density of 3.84 g·cm-3 [16].
11
In the case presented here, only the presence of rutile nano-particles in as-deposited samples, embedded in an amorphous non-stoichiometric Ti-O mixture, could be detected. Upon annealing these rutile grains grow larger, but we also detect the anatase phase, which may imply that some of these nano-grains were seeded in the as-deposited films. It is interesting to observe sample B(80/1), deposited with the lowest oxygen content added to the system, (Figs. 4 and 5): in the asdeposited sample GIXRD shows only distinct reflections from the rutile phase, while after annealing we detect even more intense reflections from the anatase phase. As more oxygen is added (Fig. 5), the presence of rutile increases and the presence of anatase decreases. However, in the sample deposited without oxygen, the TiO2 stoichiometry was not reached, so presence of the TiO phase was detected. The optical measurements have shown that the sample containing monoxide has distinctly lower band gap value, compared to the other samples that contain only the dioxide. Annealing temperature has an effect in slightly decreasing the band gap in the dioxide films. A decrease in the band gap energy with increasing the annealing temperature is due to the lowering of interatomic spacing (amorphous-crystalline) [28], release of defects and growing larger crystalline grains. This conclusion is in agreement with the observed GIXRD and TEM results for crystalline structure of the samples. Values for energy gap obtained by the used method are higher than values for bulk material (Eg for rutile is approximately 3 eV, and for anatase 3.2 eV). There are two possible reasons for a large band gap value of the films: (1) presumably due to an axial strain effect from lattice deformation as has been pointed out for ZnO films; and (2) probably due to a change in the density of charge carriers [29].
12
5.
Conclusions
The presented results show that the structure and properties of TiO2 films can be varied and adjusted by altering the deposition parameters and by post-deposition thermal treatment of the samples. As-deposited films were found to be oxygen deficient, which could be compensated by post-deposition annealing. Changes in the mass flow rate between Ar and O2 affected the structure of the films in the way such that we got an amorphous-like structure for samples deposited without oxygen to a structure where nano-crystalline rutile phase could be detected in those deposited with O2. Annealing yielded larger grains, where both rutile and anatase phases were detected. An interesting observation is the change of the dominant phase from anatase to rutile in the annealed samples with increasing the O2 content during their deposition. Optical measurements showed that increase of O2 mass flow rate and annealing are responsible for lowering energy gap values and increase in refractive index of thin layers, which is due to the increase of the rutile phase content in the films. All measured band gap values are higher than for bulk rutile and anatase phases, which is assigned to the nano-crystalline nature and to structural irregularities of the films. The results can be interesting towards exploiting magnetron sputtering techniques for deposition of TiO2 thin films of desired structure and optical properties.
Acknowledgments
This research was supported by the German-Serbian bilateral collaborative project (DAAD PPP 50752549) and the Serbian Ministry of Education, Science and Technological Development (Project ON 171023).
13
References
[1] B. Karunagaran, P. Uthirakumar, S.J. Chung, S. Velumani, E.K. Suh, TiO2 thin film gas sensor for monitoring ammonia, Mater. Characterization. 58 (2007) 680-684. [2] J. Yu, X. Zhao, Q. Zhao, Photocatalytic activity of nanometer TiO2 thin films prepared by the sol–gel method, Mater. Chem. Phys. 69 (2001) 25–29. [3] K.H. Ko, Y.C. Lee, Y.J. Jung, Enhanced efficiency of dye-sensitized TiO2 solar cells (DSSC) by doping of metal ions, Journal of Colloid Interface 283 (2005) 482–487. [4] P. Lobl, M. Huppertz, D. Margel, Nucleation and growth in TiO2 films prepared by sputtering and evaporation, Thin Solid Films 251 (1994) 72. [5] P. Wang, P. S. Yap, T. T. Lim, C-N-S tridoped TiO2 for photocatalytic degradation of tetracycline under visible-light irradiation, Appl. Catal. A 399 (2011) 252-261. [6] F. Lapostolle, A. Billard, J. Stebut, Structure/mechanical properties relationship of titanium-oxygen coatings reactively sputter-deposited, Surf. Coat. Technol. 135 (2000) 1. [7] S. Serio, M.E. Melo Jorge, Y. Nunes, N.P. Barradas, E. Alves, F. Munnik, Incorporation of N in TiO2 films grown by DC-reactive magnetron sputtering, Nucl. Instrum. Methods B 273 (2012) 109. [8] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37. [9] M. Kitano, M. Matsuoka, M. Ueshima, M. Anpo, Recent developments in titanium oxide-based photocatalysts, Appl. Catal. A 325(1) (2007) 1-14. [10] D.F. Ollis, H. Al-Ekabi, Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993, pp. 337-351. 14
[11] J. Peral, X. Domenech, Photocatalytic cyanide oxidation from aqueous coppercyanide solutions over TiO2 and ZnO, J. Chem. Technol. Biotechnol. 53 (1992) 93-96. [12] J. Peral, J. Munoz, J. Domenech, Photosensitized CN-oxidation over TiO2, Photochem. Photobiol. 55 (1990) 251. [13] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69-96. [14] M.C. Liao, H. Niu, G.S. Chen, Effect of sputtering pressure and post-annealing on hydrophilicity of TiO2 thin films deposited by reactive magnetron sputtering, Thin Solid Films 518 (2010) 7259. [15] J. Šicha, J. Musil, M. Meissner, R. Čerstvy, Nanostructure of photocatalytic TiO2 films sputtered at temperatures below 200 °C, App.Surf.Sci. 254 (2008) 3793-3800. [16] W. Zhang, Y. Li, F. Wang, Properties of TiO2 Thin Films Prepared by Magnetron Sputtering, J. Mater. Sci. Technol. 18(02) (2002) 101-107. [17] M. Wilke, G. Teichert, R. Gemma, A. Pundt, R. Kirchheim, H. Romanus, P. Schaaf, Glow discharge optical emission spectroscopy for accurate and well resolved analysis of coatings and thin films, Thin Solid Films 520 (2011) 1661. [18] J. Tauc, R. Grigorovich, A. Vancu, Optical properties and electronic structure of amorphous germanium, Phys. Stat. Sol. 15 (1966) 627. [19] E. G. Birgin, I. Chambouleyron, J. M. Martínez, Estimation of the optical constants and the thickness of thin films using unconstrained optimization, Journal of Computational Physics 151 (1999) 862-880. [20] M. Mulato, I. Chambouleyron, E. G. Birgin, J. M. Martínez, Determination of thickness and optical constants of amorphous silicon films from transmittance data, Applied Physics Letters 77 (2000) 2133-2135. 15
[21] N. Kawashima, Q. Y. Zhu, A. R. Gerson, The selective growth of rutile thin films using radio-frequency magnetron sputtering, Thin Solid Films 520 (2012) 3884. [22] H. Ohsaki, Y. Tachibana, A. Mitsui, T. Kamiyama, Y. Hayashi, High rate deposition of TiO2 by DC sputtering of the TiO2-X target, Thin Solid Films 392 (2001) 169-173. [23] K.M. Glassford, J.R. Chelikovsky, Structural and electronic properties of titanium dioxide, Phys. Rev. B 46 (1992) 1284. [24] N. Martin, C. Rousselot, D. Rondot, F. Palmino, R. Mercier, Microstructure modification of amorphous titanium oxide thin films during annealing treatment, Thin Solid Films 300 (1997) 113-121. [25] J. Tauc, Amorphous and Liquid Semiconductors, Plenum Press, New York, 1974, pp. 159. [26] D.G. Howitt, A.B. Harker, The oriented growth of anatase in thin films of amorphous titania, J.Mater.Res. 2 (1987) 201. [27] L.S. Hsu, R. Rujkorakarn, J.R. Sites, C.Y. She, Thermally induced crystallization of amorphous-titania films, J. Appl. Phys. 59 (1986) 3475. [28] C.V.R. Vasantkumar, A. Mansingh, Seventh IEEE International Symposium on Application of Ferroelectrics, IEEE, New York, 1990, pp. 713-716. [29] M. Horprathum, P. Eiamchai, P. Chindaudom, A. Pokaipisit, P. Limsuwan, Oxygen partial pressure dependence of the properties of TiO2 thin films deposited by DC reactive magnetron sputtering, Procedia Engineering 32 (2012) 676 – 682.
16
Figure captions
Fig.1. Deposition rates of TiO2 films as a function of the total mass flow rate of Ar/O2 gases. Fig.2. Elemental depth profiles as obtained from GDOS measurements for as-deposited TiO2 samples: (a) for a sample deposited with m (Ar/O2) = 80/1 and (b) for a sample deposited with m (Ar/O2) = 60/1. Fig.3. Elemental depth profiles obtained from GDOS measurements for annealed TiO2 samples deposited with different mass flow ratios (Ar/O2): (a) m = 80/0 sccm, (b) m = 80/1 sccm, (c) m = 60/1 sccm and (d) m = 30/1 sccm. Fig.4. GIXRD patterns for the as-deposited TiO2 samples, with different Ar/O2 flow ratios. Fig.5. GIXRD pattern from TiO2 samples annealed on 400 °C for 2 hours. Flow ratios are stated in the graph. Fig.6. Bright field TEM cross-sectional images taken from samples: (a) A(80/0) as-deposited, (b) B(80/1) as-deposited, (c) C(60/1) as-deposited, (d) C(60/1) annealed. Fig.7. HRTEM image and the corresponding FFT pattern taken from as-deposited sample B(80/1) (a) and annealed sample D(30/1) (b), identifying rutile TiO2 crystal grains. Fig.8. Transmittance spectra for annealed TiO2 films at different mass flow ratios Ar/O2, and reconstructed spectra by PUMA. Fig.9. Square root of (αhν) versus energy plots of TiO2 films annealed on 400 °C for 2 hours. The fitted lines were used for determining the band-gap.
17
Fig.10. Calculated values of refractive indexes by PUMA for as deposited and annealed samples: B(80/1), C(60/1) and D(30/1).
Tables:
Table 1: Deposition and annealing parameters for TiO2/Si thin films, measured thicknesses and thicknesses calculated by PUMA. Sample A(80/0) B(80/1) C(60/1) D(30/1)
m(Ar/O) (sccm) 80/0 80/1 60/1 30/1
t(s)
d(nm)
600 1320 1320 1320
195 120 105 80
18
d(nm) PUMA 180 112 108 80
Tan 2h (°C) 400 400 400 400
Figure1
Fig.1. Deposition rates of TiO2 films as a function of the total mass flow rate of Ar/O2 gases.
Figure2
Fig.2. Elemental depth profiles as obtained from GDOS measurements for as-deposited TiO2 samples: (a) for a sample deposited with ݉ሶ(Ar/O2) = 80/1 and (b) for a sample deposited with ݉ሶ(Ar/O2) = 60/1.
Figure3
Fig.3. Elemental depth profiles obtained from GDOS measurements for annealed TiO2 samples deposited with different mass flow ratios (Ar/O2): (a) ݉ሶ = 80/0 sccm, (b) ݉ሶ = 80/1 sccm, (c) ݉ሶ = 60/1 sccm and (d) ݉ሶ = 30/1 sccm.
Figure4
Fig.4. GIXRD patterns for the as-deposited TiO2 samples, with different Ar/O2 flow ratios.
Figure5
Fig.5. GIXRD pattern from TiO2 samples annealed on 400 °C for 2 hours. Flow ratios are stated in the graph.
Figure6
Fig.6. Bright field TEM cross-sectional images taken from samples: (a) A(80/0) as-deposited, (b) B(80/1) as-deposited, (c) C(60/1) as-deposited, (d) C(60/1) annealed.
Figure7
Fig.7. HRTEM image and the corresponding FFT pattern taken from as-deposited sample B(80/1) (a) and annealed sample D(30/1) (b), identifying rutile TiO2 crystal grains.
Figure8
Fig.8. Transmittance spectra for annealed TiO2 films at different mass flow ratios Ar/O2, and reconstructed spectra by PUMA.
Figure9
Fig.9. Square root of (αhν) versus energy plots of TiO2 films annealed on 400 °C for 2 hours. The fitted lines were used for determining the band-gap.
Figure10
Fig.10. Calculated values of refractive indexes by PUMA for as deposited and annealed samples: B(80/1), C(60/1) and D(30/1).