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Influence of Nb content on the structural and optical properties of anatase TiO2 polycrystalline thin film by e-beam technique
Materials Chemistry and Physics 180 (2016) 383e389
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Influence of Nb content on the structural and optical properties of anatase TiO2 polycrystalline thin film by e-beam technique A. Shah*, Arshad Mahmood, Uzma Aziz, Rashad Rashid, Qaiser Raza, Zahid Ali National Institute of Laser and Optronics (NILOP), P.O Nilore, Islamabad, Pakistan
h i g h l i g h t s The addition of Nb into TiO2 film has strongly influenced its physical properties. Anatase polycrystalline Nb:TiO2 films were grown up to 15% Nb content. The film becomes an amorphous at 20% Nb doping. Band gap energy of TiO2 film was decreased with increasing of Nb content in the film. The Optical constants (n, k) of Nb:TiO2 film were varied as a function of Nb content.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 August 2015 Received in revised form 13 May 2016 Accepted 5 June 2016 Available online 16 June 2016
In this paper, we report the structural and optical properties of Nb-doped TiO2 thin films deposited by ebeam evaporation technique. After post annealing in air at 500 C for 1 h, the samples were characterized by various techniques such as X-ray diffraction (XRD), Raman spectroscopy, UVeVis spectrophotometry and spectroscopic Ellipsometer. Both XRD and Raman analyses indicate that the films were crystallized into the polycrystalline anatase TiO2 structure. However it was observed that the crystallinity of the films decreases with the addition of Nb atoms and tends to become amorphous at 20% Nb content in TiO2 film. Moreover, no new phases such as Nb2O5, NbO2 or Nb metal were observed. The band gap energy was found to decrease with the increasing of Nb concentration which was verified by ellipsometric study. Ellipsomtric measurements also indicate that refractive index (n) of the films decreases while extinction coefficient (k) increases with the increasing of Nb content. All these analyses elucidate that the incorporation of Nb atom into TiO2 may tune the structural and optical properties of TiO2 thin films. © 2016 Elsevier B.V. All rights reserved.
Keywords: Nb-doped TiO2 e-beam evaporation Band gap energy Refractive index Extinction coefficient
1. Introduction Titanium dioxide (TiO2) has received great attention in both fundamental research as well as in various technological applications, due to its promising properties such as wide band gap, high refractive index and high dielectric constant, high transparency throughout the entire visible region, non-toxic behavior, good resistance against water, acids, bases and its moderate price [1e4]. These properties make TiO2 a very attractive material which has been studied in various applications such as optical coatings, dyesensitized solar cells [5,6], optical thin film devices [7], capacitors of microelectronic devices [8], and photo-catalyst for degradation of environmental pollutants [9,10]. In addition, TiO2 thin films and
* Corresponding author. E-mail address: [email protected] (A. Shah). http://dx.doi.org/10.1016/j.matchemphys.2016.06.021 0254-0584/© 2016 Elsevier B.V. All rights reserved.
metal doped TiO2 can also be used as sensors for various gases [11e14]. TiO2 is also an attention-grabbing transparent conducting oxide owing to its three different polymorphs with diverse electronic band structures such as rutile, anatase and brookite phases with band gap energies 3.0 eV, 3.2 eV and 1.9 eV respectively [15e18]. In order to replace indium tin oxide base transparent conducting oxide with TiO2, there is a great need to improve the electronic conductivity of TiO2. Recently, doping TiO2 with Nb atom has been extensively investigated and it is reported that the incorporation of Nb atom in TiO2 lattice greatly enhances the conductivity of TiO2 material while maintaining transparency around 90% [19e21]. Despite great advancement in developing Nb doped TiO2 while using various techniques and post annealing treatments, still detailed investigation of Nb as dopant on the structural, electrical and optical properties of TiO2 [22e26].
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concentrations and carried out a systematic study on the structural and optical properties of the prepared samples. We observed that the crystalline quality, refractive index and band gap of the synthesized films decreases with increasing Nb content. 2. Experimental work
Fig. 1. XRD spectra of Nb-doped TiO2 thin films with different Nb contents.
Nb-doped TiO2 thin films were grown using e-beam evaporation on glass substrate by mixing TiO2 powder (3 N) and Nb2O5 powder (5 N) with 0.0, 0.05, 1.0, 0.15 and 2.0 wt% Nb in TiO2 by means of mechanical mixing with a pestle and mortar for 2 h to obtain uniformly mixed powders. Pellets of all the compositions were made by using 10 mm dyes in mechanical press with load of 5 tons. The glass substrates were cleaned ultrasonically in acetone bath for 20 min, rinsed in iso-propanol for 15 min and then thoroughly washed with DI water. Finally the substrates were dried with air gun before loading into the growth chamber. After reaching the base pressure of ~5 106 torr in chamber, the films were grown at room temperature with a very slow deposition rate of ~0.2 nm/s. All the samples were grown under the same experimental conditions. Then the samples were annealed in air at 500 C for an hour in order to get homogenous crystalline Nb-doped TiO2 films. Structural and phase analyses of the prepared samples were carried out with a Bruker D-8 discover x-ray diffractometer using Cu-Ka-radiation (l ¼ 1.54186 Å). The measurements were performed by q/2q scans in the 2q angular range of 20e90 , with a step size of 0.02 and a scan rate of 2 /min. Raman analyses were performed by Dongwoo Optron’s micro-Raman spectrometer with 532 nm as an excitation laser source. The spectrum was taken in the range from 300 to 1000 cm1, while eliminating the source peak (532 nm) by using 532 cutoff filter. Optical properties such as refractive index and extinction coefficient of the films were measured by spectroscopic ellipsometer (SE 850, Sentech GmbH) and UVeVis (Hitachi U-4001) spectrophotometer was used for the transmission measurements and for energy band gap calculation. 3. Results and discussion 3.1. XRD studies
Fig. 2. Raman spectra of Nb-doped TiO2 thin films with different Nb contents.
In this work, we report the synthesis of Nb-doped anatase TiO2 thin films through e-beam evaporation technique with different Nb
Fig. 1 shows the XRD spectra of un-doped and Nb-doped TiO2 films with different Nb concentrations. It is clear from the figure that the films have polycrystalline nature grown mostly along (101) plane of anatase phase of TiO2 [27,28]. The initial broad peak (hump) in the spectra is due to the glass substrate. The relatively sharp and intensive (1 0 1) diffraction peak indicates a high crystallinity of the films. Moreover, there is slight shift of the (101) peak towards the lower 2q value in the doped samples. This is because
Fig. 3. EMA model for Ellipsometry measurements.
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385
Fig. 4. Ellipsometry curve fittings of Nb-doped TiO2 thin films with different Nb contents.
that Nb atom of larger radii (Nb5þ ¼ 0.64 Å) may get substituted
Table 1 Thicknesses of Nb-doped TiO2 thin films deposited by e-beam evaporation technique. S. no
Nb-doped TiO2 thin films
Film thickness (nm)
1 2 3 4 5
Pure TiO2 05% Nb:TiO2 10% Nb:TiO2 15% Nb:TiO2 20% Nb:TiO2
249.01 250.20 252.54 249.47 238.51
into the Ti site of smaller radii (Ti4þ ¼ 0.61 Å) [29]. In addition, there is no peaks related to Nb metal or its oxide phases such as Nb2O5, or NbO2 etc were observed in the Nb-doped TiO2 films, which confirm that the anatase structure is retained after doping up to 15% Nb. However, above 10% Nb-doped TiO2 the crystallinity of the film decreases and become an amorphous film at 20% Nb-doped TiO2 film. 3.2. Raman spectroscopy To further elucidate the structural investigation of Nb-doped TiO2 thin films obtain in XRD, Raman measurement were carried
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Fig. 5. The variation of the (a) Refractive index (n) and (b) Extinction coefficient (k) with wavelength.
Fig. 6. The variation of the (a) Refractive index (n) and (b) Extinction coefficient (k) as a function of Nb concentration.
Fig. 7. Transmission spectra of Nb-doped TiO2 thin films with different Nb contents.
out at room temperature. Fig. 2 shows Raman spectra of the samples under investigation as a function of Nb concentration. Typically, there are four Raman-active vibrational phonon modes in TiO2 anatase phase, named as A1g, B1g and Eg modes [30]. All these vibrational modes were observed in our samples, which further confirm that the films were grown in anatase phase. In case of Nbdoped TiO2 films, the spectra still show the same phonon modes with slightly shifted towards lower frequency side because of the stresses produced in the TiO2 lattice due to the addition of heavier Nb atoms. Also the figure describes that the width of the Raman modes increases whereas its intensity decreases with the increasing of Nb doping concentration, indicating a degradation of crystalline quality of TiO2 film with the addition of Nb atoms. This is due to the fact that Nb atom (radius Nb > rTi) creates lattice distortion and this damage increases by increasing the doping concentration. Therefore the crystal quality decreases or amorphization in the film get started and finally converted into amorphous phase for the TiO2 film doped with 20% Nb as shown Fig. 2 (top
A. Shah et al. / Materials Chemistry and Physics 180 (2016) 383e389
387
Fig. 8. Energy band gap of Nb-doped TiO2 thin films determined from (a) transmission spectra (b) k-spectra.
curve). These results are in consistence with the XRD analyses. 3.3. Ellipsometric studies Spectroscopic ellipsometer was used to evaluate the film thickness and optical constants (n, k) of Nb-doped TiO2 thin films grown by e-beam evaporation technique. The ellipsometric parameters psi (J) and delta (D) were recorded at an incident angle 70 in a spectral wavelength range from 300 to 900 nm. This is taken as an experimental data. Since an ellipsometry is an indirect way to determine different material properties as various mathematical models can be used to fit the model data with the experimental one in order to extract such properties [31e33]. Therefore in this case Bruggemann EMA (effective medium approximation) model was applied to compute the theoretical psi (J) and delta (D) values and try to fit with the experimental data. EMA model is very useful for the calculation of the optical constant spectra of mixture of two materials, provided the volume fractions of the constituent materials and physical geometry of the mixture are known. In this
model TiO2 is taken as host material and Nb atoms are taken as small inclusions in the host layer. Whereas for pure TiO2 sample (0% Nb), we used dense TiO2 layer and a thin EMA over layer for surface roughness. In this over layer we took voids as inclusions in the host TiO2 layer. the multilayer stacking is composed of air, EMA layer (TiO2 þ Nb) and a glass (Bk7) substrate as shown in Fig. 3. The fitted results of the Nb-doped TiO2 films with various Nb contents are shown in the Fig. 4, which seems clearly overlapped and well fitted. Using these curve fittings, film thickness and optical constants were determined. The thicknesses of each film were summarized in the Table 1. The refractive index (n) and extinction coefficient (k) of Nbdoped TiO2 films as a function of wavelength are shown in Fig. 5(a, b) and the variation of n and k as a function of Nb concentration are shown in Fig. 6(a, b). It is clear from these figures that refractive index of the TiO2 film decreases whereas extinction coefficient increases with the increasing of Nb content in the film. The variation of the optical constants of TiO2 thin film with the addition of Nb atoms may be associated with the fact that Nb promoted the
Fig. 9. The variation of Eg with Nb concentration in TiO2 thin films.
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degradation of crystallinity of TiO2 as already observed in XRD as well as in Raman analyses [34]. 3.4. Spectrophotometric studies Fig. 7 shows optical transmission spectra of the un-doped and Nb-doped TiO2 thin films in the visible spectral region. The transmission of the un-doped TiO2 film is ~88% and it tends to decrease with addition Nb in the TiO2 film because of the creation of structural defects in the host lattice. Also the spectra exhibit an oscillatory behavior with sharp absorption edge in the measured wavelength range. Such oscillatory behaviors are related to an interference effect between the reflected lights at the surface and at the interface. Therefore, it indicates that our Nb-doped TiO2 grown films on glass are of high quality with uniform thickness as well as clean interface. Moreover, the absorption edges seem to shift towards higher wavelength side as the Nb content increasing in the films. Therefore it suggests that doping of Nb metal in TiO2 film causes to decrease the band gap energy of the film. The band gap energy (Eg) of these thin films was calculated from transmission spectra as well as from ellipsometry data using the following Tauc’s relation for direct band gap material.
ðahnÞ2 ¼ A hn Eg
(1)
In case of transmission data, the value of the absorption coefficient a was calculated using the relation
a¼
lnT nm1 t
(2)
where T is the normalized transmittance value at a particular wavelength and t is the thickness of the film. Similarly in case of ellipsometry data, the absorption coefficient a was calculated from k-spectra of the ellipsometer by using the following relation
a¼
4pk
l
(3)
Now to estimate the band gap energy, (ahn)2 was plotted against hn for each films as shown in Fig. 8(a) & (b). Extrapolation of the linear portion to the (ahn)2 ¼ 0 axis gives the value of band gap energy. The variation of Eg as a function of Nb content in the films is shown in Fig. 9(a) & (b). Clearly one can see that Eg value decreases from 3.55 eV to 3.35 eV as the Nb content varies from 0% to 20% in TiO2 films. In supporting to the results obtained from transmission data, the band gap obtained from ellipsometric data also shows a linear decrease in Eg as a function of Nb concentration in the TiO2 films. As observed in the structural analyses that the incorporation of Nb in TiO2 films produces lattice damages or structural defects which form defective energy level below the conduction band causes to reduce the band gap energy of the films. Since the lattice defects are proportional the Nb concentration and hence Eg decreases gradually with the increasing of Nb content in TiO2 thin films. 4. Conclusion Nb-doped TiO2 thin films with varying Nb concentration were deposited by e-beam evaporation on glass substrate. The influence of Nb content on structural and optical properties of TiO2 thin films were investigated. The structural analyses reveal that the films are uniform and Nb atoms are properly doped by substituting Ti sites. The films have polycrystalline in nature with preferential
orientation along (101) plane corresponding to anatase phase of TiO2. Furthermore, no secondary phases or Nb precipitates were detected in XRD as well as Raman measurements. However, the addition of Nb atoms produces lattice damage which causes an amorphization in the films at higher Nb contents. It is also confirmed from optical analyses that the films have grown uniformly with sharp interface. The transmission of TiO2 films decreases upon Nb doping and the band gap energy of the films decreases from 3.55 down to 3.35 eV on Nb doping up to 20%. These results were confirmed with Ellipsometric measurements. In addition it is also observed that refractive index (n) decreases whereas extinction coefficient (k) increases with the increasing of Nb content in TiO2 thin films. References [1] N.E. Stankova, I.G. Dimitrov, T.R. Stoyanchov, P.A. Atanasov, Optical and gas sensing properties of thick TiO2 films grown by laser deposition, Appl. Surf. Sci. 254 (2007) 1268e1272. [2] Xiaohui Yua, Changsheng Li, Yun Ling, Ting-Ao Tang, Qiong Wu, Junjie Kong, First principles calculations of electronic and optical properties of Mo-doped rutile TiO2, J. Alloys Compd. 507 (2010) 33e37. [3] O. Legrini, E. Oliveros, A.M. Braun, Chem. Rev. 2 (1993) 671. [4] Young Ug Ahn, Eui Jung Kim, Hwan Tae Kim, Sung Hong Hahn, Variation of structural and optical properties of sol-gel TiO2 thin films with catalyst concentration and calcination temperature, Mater. Lett. 57 (2003) 4660e4666. [5] A. Dakka, J. Lafait, C. Sella, S. Berthier, M. Abd-Lefdil, J.C. Martin, M. Maaza, Appl. Opt. 39 (2000) 2745. [6] K. Hara, T. Horiguchi, T. Kinoshita, K. Sayama, H. Arakawa, Sol. Energy Mater. Sol. Cells 70 (2001) 151. [7] Y. Tachibana, H. Ohsaki, A. Hayashi, A. Mitsui, Y. Hayashi, Vacuum 59 (2000) 836. [8] P. Alexandrov, J. Koprinarova, D. Todorov, Vacuum 47 (1996) 1333. [9] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [10] J. Jun, M. Dhayal, J.H. Shin, J.C. Kim, N. Getoff, Radiat. Phys. Chem. 75 (5) (2006) 583. [11] L.L.W. Chow, M.M.F. Yuen, P.C.H. Chan, A.T. Cheung, Sens. Actuators B 76 (2001) 310. [12] T.Y. Yang, N.M. Lin, B.Y. Wei, C.Y. Wu, C.K. Lin, Rev. Adv. Mater. Sci. 4 (2003) 48. [13] H. Tang, K. Prasad, R. Sanjines, F. Levy, Sens. Actuators B 26e27 (1995) 71e75. [14] E. Comini, G. Faglia, G. Sberveglieri, Y.X. Li, W. Wlobarski, M.K. Ghantasala, Sens. Actuators B 64 (2000) 169e174. [15] F.A. Grant, Rev. Mod. Phys. 31 (1959) 646. [16] R. Zallen, M.P. Moret, Solid State Commun. 137 (2006) 154. [17] A. Welte, C. Waldauf, C. Brabec, P. Wellmann, Thin Solid Films 516 (2008) 7256. [18] T. Hitosugi, H. Kamisaka, K. Yamashita, H. Nogawa, Y. Furubayashi, S. Nakao, N. Yamada, A. Chikamatsu, H. Kumigashira, M. Oshima, Y. Hirose, T. Shimada, T. Hasegawa, Appl. Phys. Exp. 1 (2008) 111203. [19] Y. Sato, H. Akizuki, T. Kamiyama, Y. Shigesato, Thin Solid Films 516 (2008) 5758. [20] N.L.H. Hoang, N. Yamada, T. Hitosugi, J. Kasai, S. Nakao, T. Shimada, T. Hasegawa, Appl. Phys. Expr. 1 (2008) 115001. [21] Y. Sato, Y. Sanno, C. Tasaki, N. Oka, T. Kamiyama, Y. Shigesato, J. Vac. Sci. Technol. A 28 (2010) 851. [22] J. Jun, J.H. Shin, M. Dhayal, Appl. Surf. Sci. 252 (10) (2005) 3871. [23] B. ORegan, M. Gratzel, Nature 353 (1991) 737. [24] J.M. Herrmann, J. Disdier, P. Pichat, Chem. Phys. Lett. 108 (1984) 618. [25] H. Yamashita, M. Harada, J. Misaka, H. Nakao, M. Takeuchi, M. Anpo, Nucl. Instrum. Meth. Phys. Res. B 206 (2003) 889. [26] J. Jun, J.H. Shin, J.S. Choi, M. Dhayal, J. Biomed. Nanotechnol. 2 (2006) 152. [27] T. Modes, B. Scheffel, C. Metzner, O. Zywitzki, E. Reinhold, Surf. Coat. Technol. 200 (2005) 306e309. [28] J. Domaradzki, D. Kaczmarek, E.L. Prociow, A. Borkowska, D. Schmeisser, G. Beuckert, Thin Solid Films 513 (2006) 269e274. [29] Xujie Lu, Xinliang Mou, Jianjun Wu, Dingwen Zhang, Linlin Zhang, Fuqiang Huang, Fangfang Xu, Sumei Huang, Improved-performance dyesensitized solar cells using Nb-Doped TiO2 electrodes: efficient electron injection and transfer, Adv. Funct. Mater 20 (2010) 509e515. [30] Huolin Huang, Yannan Xie, Zifeng Zhang, Feng Zhang, Qiang Xu, Zhengyun Wu, Growth and fabrication of sputtered TiO2 based ultraviolet detectors, Appl. Surf. Sci. 293 (2014) 248e254. [31] G. He, L.D. Zhang, G.H. Li, M. Liu, L.Q. Zhu, S.S. Pan, Q. Fang, Spectroscopic ellipsometry characterization of nitrogen-incorporated HfO2 gate dielectrics grown by radio-frequency reactive sputtering, Appl. Phys. Lett. 86 (2005) 232901. [32] J.W. Zhang, G. He, L. Zhou, H.S. Chen, X.S. Chen, X.F. Chen, B. Deng, J.G. Lv, Z.Q. Sun, Microstructure optimization and optical and interfacial properties
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Influence of Nb content on the structural and optical properties of anatase TiO2 polycrystalline thin film by e-beam technique A. Shah*, Arshad Mahmood, Uzma Aziz, Rashad Rashid, Qaiser Raza, Zahid Ali National Institute of Laser and Optronics (NILOP), P.O Nilore, Islamabad, Pakistan
h i g h l i g h t s The addition of Nb into TiO2 film has strongly influenced its physical properties. Anatase polycrystalline Nb:TiO2 films were grown up to 15% Nb content. The film becomes an amorphous at 20% Nb doping. Band gap energy of TiO2 film was decreased with increasing of Nb content in the film. The Optical constants (n, k) of Nb:TiO2 film were varied as a function of Nb content.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 August 2015 Received in revised form 13 May 2016 Accepted 5 June 2016 Available online 16 June 2016
In this paper, we report the structural and optical properties of Nb-doped TiO2 thin films deposited by ebeam evaporation technique. After post annealing in air at 500 C for 1 h, the samples were characterized by various techniques such as X-ray diffraction (XRD), Raman spectroscopy, UVeVis spectrophotometry and spectroscopic Ellipsometer. Both XRD and Raman analyses indicate that the films were crystallized into the polycrystalline anatase TiO2 structure. However it was observed that the crystallinity of the films decreases with the addition of Nb atoms and tends to become amorphous at 20% Nb content in TiO2 film. Moreover, no new phases such as Nb2O5, NbO2 or Nb metal were observed. The band gap energy was found to decrease with the increasing of Nb concentration which was verified by ellipsometric study. Ellipsomtric measurements also indicate that refractive index (n) of the films decreases while extinction coefficient (k) increases with the increasing of Nb content. All these analyses elucidate that the incorporation of Nb atom into TiO2 may tune the structural and optical properties of TiO2 thin films. © 2016 Elsevier B.V. All rights reserved.
Keywords: Nb-doped TiO2 e-beam evaporation Band gap energy Refractive index Extinction coefficient
1. Introduction Titanium dioxide (TiO2) has received great attention in both fundamental research as well as in various technological applications, due to its promising properties such as wide band gap, high refractive index and high dielectric constant, high transparency throughout the entire visible region, non-toxic behavior, good resistance against water, acids, bases and its moderate price [1e4]. These properties make TiO2 a very attractive material which has been studied in various applications such as optical coatings, dyesensitized solar cells [5,6], optical thin film devices [7], capacitors of microelectronic devices [8], and photo-catalyst for degradation of environmental pollutants [9,10]. In addition, TiO2 thin films and
* Corresponding author. E-mail address: [email protected] (A. Shah). http://dx.doi.org/10.1016/j.matchemphys.2016.06.021 0254-0584/© 2016 Elsevier B.V. All rights reserved.
metal doped TiO2 can also be used as sensors for various gases [11e14]. TiO2 is also an attention-grabbing transparent conducting oxide owing to its three different polymorphs with diverse electronic band structures such as rutile, anatase and brookite phases with band gap energies 3.0 eV, 3.2 eV and 1.9 eV respectively [15e18]. In order to replace indium tin oxide base transparent conducting oxide with TiO2, there is a great need to improve the electronic conductivity of TiO2. Recently, doping TiO2 with Nb atom has been extensively investigated and it is reported that the incorporation of Nb atom in TiO2 lattice greatly enhances the conductivity of TiO2 material while maintaining transparency around 90% [19e21]. Despite great advancement in developing Nb doped TiO2 while using various techniques and post annealing treatments, still detailed investigation of Nb as dopant on the structural, electrical and optical properties of TiO2 [22e26].
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A. Shah et al. / Materials Chemistry and Physics 180 (2016) 383e389
concentrations and carried out a systematic study on the structural and optical properties of the prepared samples. We observed that the crystalline quality, refractive index and band gap of the synthesized films decreases with increasing Nb content. 2. Experimental work
Fig. 1. XRD spectra of Nb-doped TiO2 thin films with different Nb contents.
Nb-doped TiO2 thin films were grown using e-beam evaporation on glass substrate by mixing TiO2 powder (3 N) and Nb2O5 powder (5 N) with 0.0, 0.05, 1.0, 0.15 and 2.0 wt% Nb in TiO2 by means of mechanical mixing with a pestle and mortar for 2 h to obtain uniformly mixed powders. Pellets of all the compositions were made by using 10 mm dyes in mechanical press with load of 5 tons. The glass substrates were cleaned ultrasonically in acetone bath for 20 min, rinsed in iso-propanol for 15 min and then thoroughly washed with DI water. Finally the substrates were dried with air gun before loading into the growth chamber. After reaching the base pressure of ~5 106 torr in chamber, the films were grown at room temperature with a very slow deposition rate of ~0.2 nm/s. All the samples were grown under the same experimental conditions. Then the samples were annealed in air at 500 C for an hour in order to get homogenous crystalline Nb-doped TiO2 films. Structural and phase analyses of the prepared samples were carried out with a Bruker D-8 discover x-ray diffractometer using Cu-Ka-radiation (l ¼ 1.54186 Å). The measurements were performed by q/2q scans in the 2q angular range of 20e90 , with a step size of 0.02 and a scan rate of 2 /min. Raman analyses were performed by Dongwoo Optron’s micro-Raman spectrometer with 532 nm as an excitation laser source. The spectrum was taken in the range from 300 to 1000 cm1, while eliminating the source peak (532 nm) by using 532 cutoff filter. Optical properties such as refractive index and extinction coefficient of the films were measured by spectroscopic ellipsometer (SE 850, Sentech GmbH) and UVeVis (Hitachi U-4001) spectrophotometer was used for the transmission measurements and for energy band gap calculation. 3. Results and discussion 3.1. XRD studies
Fig. 2. Raman spectra of Nb-doped TiO2 thin films with different Nb contents.
In this work, we report the synthesis of Nb-doped anatase TiO2 thin films through e-beam evaporation technique with different Nb
Fig. 1 shows the XRD spectra of un-doped and Nb-doped TiO2 films with different Nb concentrations. It is clear from the figure that the films have polycrystalline nature grown mostly along (101) plane of anatase phase of TiO2 [27,28]. The initial broad peak (hump) in the spectra is due to the glass substrate. The relatively sharp and intensive (1 0 1) diffraction peak indicates a high crystallinity of the films. Moreover, there is slight shift of the (101) peak towards the lower 2q value in the doped samples. This is because
Fig. 3. EMA model for Ellipsometry measurements.
A. Shah et al. / Materials Chemistry and Physics 180 (2016) 383e389
385
Fig. 4. Ellipsometry curve fittings of Nb-doped TiO2 thin films with different Nb contents.
that Nb atom of larger radii (Nb5þ ¼ 0.64 Å) may get substituted
Table 1 Thicknesses of Nb-doped TiO2 thin films deposited by e-beam evaporation technique. S. no
Nb-doped TiO2 thin films
Film thickness (nm)
1 2 3 4 5
Pure TiO2 05% Nb:TiO2 10% Nb:TiO2 15% Nb:TiO2 20% Nb:TiO2
249.01 250.20 252.54 249.47 238.51
into the Ti site of smaller radii (Ti4þ ¼ 0.61 Å) [29]. In addition, there is no peaks related to Nb metal or its oxide phases such as Nb2O5, or NbO2 etc were observed in the Nb-doped TiO2 films, which confirm that the anatase structure is retained after doping up to 15% Nb. However, above 10% Nb-doped TiO2 the crystallinity of the film decreases and become an amorphous film at 20% Nb-doped TiO2 film. 3.2. Raman spectroscopy To further elucidate the structural investigation of Nb-doped TiO2 thin films obtain in XRD, Raman measurement were carried
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Fig. 5. The variation of the (a) Refractive index (n) and (b) Extinction coefficient (k) with wavelength.
Fig. 6. The variation of the (a) Refractive index (n) and (b) Extinction coefficient (k) as a function of Nb concentration.
Fig. 7. Transmission spectra of Nb-doped TiO2 thin films with different Nb contents.
out at room temperature. Fig. 2 shows Raman spectra of the samples under investigation as a function of Nb concentration. Typically, there are four Raman-active vibrational phonon modes in TiO2 anatase phase, named as A1g, B1g and Eg modes [30]. All these vibrational modes were observed in our samples, which further confirm that the films were grown in anatase phase. In case of Nbdoped TiO2 films, the spectra still show the same phonon modes with slightly shifted towards lower frequency side because of the stresses produced in the TiO2 lattice due to the addition of heavier Nb atoms. Also the figure describes that the width of the Raman modes increases whereas its intensity decreases with the increasing of Nb doping concentration, indicating a degradation of crystalline quality of TiO2 film with the addition of Nb atoms. This is due to the fact that Nb atom (radius Nb > rTi) creates lattice distortion and this damage increases by increasing the doping concentration. Therefore the crystal quality decreases or amorphization in the film get started and finally converted into amorphous phase for the TiO2 film doped with 20% Nb as shown Fig. 2 (top
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387
Fig. 8. Energy band gap of Nb-doped TiO2 thin films determined from (a) transmission spectra (b) k-spectra.
curve). These results are in consistence with the XRD analyses. 3.3. Ellipsometric studies Spectroscopic ellipsometer was used to evaluate the film thickness and optical constants (n, k) of Nb-doped TiO2 thin films grown by e-beam evaporation technique. The ellipsometric parameters psi (J) and delta (D) were recorded at an incident angle 70 in a spectral wavelength range from 300 to 900 nm. This is taken as an experimental data. Since an ellipsometry is an indirect way to determine different material properties as various mathematical models can be used to fit the model data with the experimental one in order to extract such properties [31e33]. Therefore in this case Bruggemann EMA (effective medium approximation) model was applied to compute the theoretical psi (J) and delta (D) values and try to fit with the experimental data. EMA model is very useful for the calculation of the optical constant spectra of mixture of two materials, provided the volume fractions of the constituent materials and physical geometry of the mixture are known. In this
model TiO2 is taken as host material and Nb atoms are taken as small inclusions in the host layer. Whereas for pure TiO2 sample (0% Nb), we used dense TiO2 layer and a thin EMA over layer for surface roughness. In this over layer we took voids as inclusions in the host TiO2 layer. the multilayer stacking is composed of air, EMA layer (TiO2 þ Nb) and a glass (Bk7) substrate as shown in Fig. 3. The fitted results of the Nb-doped TiO2 films with various Nb contents are shown in the Fig. 4, which seems clearly overlapped and well fitted. Using these curve fittings, film thickness and optical constants were determined. The thicknesses of each film were summarized in the Table 1. The refractive index (n) and extinction coefficient (k) of Nbdoped TiO2 films as a function of wavelength are shown in Fig. 5(a, b) and the variation of n and k as a function of Nb concentration are shown in Fig. 6(a, b). It is clear from these figures that refractive index of the TiO2 film decreases whereas extinction coefficient increases with the increasing of Nb content in the film. The variation of the optical constants of TiO2 thin film with the addition of Nb atoms may be associated with the fact that Nb promoted the
Fig. 9. The variation of Eg with Nb concentration in TiO2 thin films.
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degradation of crystallinity of TiO2 as already observed in XRD as well as in Raman analyses [34]. 3.4. Spectrophotometric studies Fig. 7 shows optical transmission spectra of the un-doped and Nb-doped TiO2 thin films in the visible spectral region. The transmission of the un-doped TiO2 film is ~88% and it tends to decrease with addition Nb in the TiO2 film because of the creation of structural defects in the host lattice. Also the spectra exhibit an oscillatory behavior with sharp absorption edge in the measured wavelength range. Such oscillatory behaviors are related to an interference effect between the reflected lights at the surface and at the interface. Therefore, it indicates that our Nb-doped TiO2 grown films on glass are of high quality with uniform thickness as well as clean interface. Moreover, the absorption edges seem to shift towards higher wavelength side as the Nb content increasing in the films. Therefore it suggests that doping of Nb metal in TiO2 film causes to decrease the band gap energy of the film. The band gap energy (Eg) of these thin films was calculated from transmission spectra as well as from ellipsometry data using the following Tauc’s relation for direct band gap material.
ðahnÞ2 ¼ A hn Eg
(1)
In case of transmission data, the value of the absorption coefficient a was calculated using the relation
a¼
lnT nm1 t
(2)
where T is the normalized transmittance value at a particular wavelength and t is the thickness of the film. Similarly in case of ellipsometry data, the absorption coefficient a was calculated from k-spectra of the ellipsometer by using the following relation
a¼
4pk
l
(3)
Now to estimate the band gap energy, (ahn)2 was plotted against hn for each films as shown in Fig. 8(a) & (b). Extrapolation of the linear portion to the (ahn)2 ¼ 0 axis gives the value of band gap energy. The variation of Eg as a function of Nb content in the films is shown in Fig. 9(a) & (b). Clearly one can see that Eg value decreases from 3.55 eV to 3.35 eV as the Nb content varies from 0% to 20% in TiO2 films. In supporting to the results obtained from transmission data, the band gap obtained from ellipsometric data also shows a linear decrease in Eg as a function of Nb concentration in the TiO2 films. As observed in the structural analyses that the incorporation of Nb in TiO2 films produces lattice damages or structural defects which form defective energy level below the conduction band causes to reduce the band gap energy of the films. Since the lattice defects are proportional the Nb concentration and hence Eg decreases gradually with the increasing of Nb content in TiO2 thin films. 4. Conclusion Nb-doped TiO2 thin films with varying Nb concentration were deposited by e-beam evaporation on glass substrate. The influence of Nb content on structural and optical properties of TiO2 thin films were investigated. The structural analyses reveal that the films are uniform and Nb atoms are properly doped by substituting Ti sites. The films have polycrystalline in nature with preferential
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