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Structural, optical and magnetic properties of Fe-doped barium stannate thin films grown by PLD
Applied Surface Science 282 (2013) 121–125
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structural, optical and magnetic properties of Fe-doped barium stannate thin films grown by PLD K.K. James, Arun Aravind, M.K. Jayaraj ∗ Nanophotonic and Optoelectronic Devices Laboratory, Department of Physics, Cochin University of Science and Technology, Kerala 682022, India
a r t i c l e
i n f o
Article history: Received 18 August 2012 Received in revised form 19 April 2013 Accepted 16 May 2013 Available online 23 May 2013 Keywords: PLD XRD AFM Raman spectroscopy Ferromagnetism
a b s t r a c t Barium stannate is a wide band gap semiconductor with cubic perovskite structure. Polycrystalline bulk samples of BaSn1−x Fex O3d (BFS), with x = 0.00, 0.02, 0.03, 0.05 and 0.10 were prepared by solid-state reaction. In this paper, we report the growth of undoped and Fe doped barium stannate thin films on fused silica substrate using pulsed laser deposition (PLD) technique at a relatively high substrate temperature and low oxygen pressure. The deposited films have wide bandgap and are transparent in the visible region. The X-ray diffraction analysis of the films confirmed the cubic structure. Microstructural studies were carried out using micro-Raman spectroscopy and AFM analysis. Defect induced Raman shifts were observed in the samples. Magnetic studies revealed an increase in magnetic properties for films doped with 10 at% Fe doped samples. © 2013 Elsevier B.V. All rights reserved.
1. Introduction BaSnO3 is an n-type semiconductor material with perovskite structure [1]. Perovskite materials are well known for their superconducting, magnetic and electro optic properties [2,3]. The optical bandgap reported in the literature for undoped BaSnO3 is 3.1 eV [4]. Thin films of La-doped BaSnO3 with optical transparency in the visible region deposited on MgO (0 0 1) substrates have a bandgap 4.02 eV [5]. Electrical resistivity of Sb-doped barium stannate is of the order of 2.43 m cm [6]. The gas sensing properties of barium stannate have also been explored to a great extent [7,8]. In addition to the physical properties induced by charge carriers, the focus of researchers has now turned towards spin based electronic devices [9]. Materials exhibiting intrinsic ferromagnetism at room temperature attract the attention of the electronic industry. The Fe ion implanted ZnO thin films [10] and Fe doped BaSnO3 films [11] exhibit ferromagnetic properties at room temperature with Curie temperature well below 500 K. Coey et al. [12] have proposed that the ferromagnetic ordering of Fe ions is due to F-centre exchange (FCE) mechanism. BaSnO3 has a cubic perovskite structure. The difference in the ionic radii of the Sn4+ and Fe2+ ions [11] produces an increase in lattice constant. A combination of electrical conductivity and ferromagnetism in the doped state together with optical properties make this material a possible candidate for the future
∗ Corresponding author. Tel.: +91 4842577404; fax: +91 4842577595. E-mail addresses: [email protected] (K.K. James), [email protected] (M.K. Jayaraj). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.05.076
optoelectronic industry. In the present study, the possibility of depositing thin films of Fe doped BaSnO3 on commercially available transparent fused silica substrate which is inexpensive compared with other single crystalline substrates has been explored. A systematic study on the optical properties of thin films in the doped and undoped state has been carried out. A brief analysis of the ferromagnetic behaviour of bulk and thin film samples are incorporated in this work to invoke the potential of Fe doped BaSnO3 as a magnetic material. 2. Experimental details Fe doped BaSnO3 powders were prepared by solid state reaction using high purity BaCO3 , SnO2 and Fe2 O3 . The Fe content in the BaSnO3 powder was made to vary from 0, 2, 3, 5 and 10 at% and they are named as BSO, 2BFS, 3BFS, 5BFS, and 10BFS respectively. Stoichiometric amounts of the starting materials were mixed using agate mortar and pestle with acetone as mixing agent. Calcination of the mixed powder was done for 12 h at 800 ◦ C. The calcined powders were reground and kept for 18 h at 850 ◦ C. The powder was mixed with poly vinyl alcohol as binder and cold pressed into cylindrical pellets of diameter 13 mm and thickness 1–1.5 mm by applying a pressure of 370 MPa. Pellets were sintered in air at a temperature 900 ◦ C for 24 h. The colour changed from white to coral as the Fe concentration was increased. The prepared pellets were polished with fine emery paper and dried at a temperature 200 ◦ C for 1 h. The % porosity was calculated using the relation: % porosity =
Dth − Db × 100 Dth
122
K.K. James et al. / Applied Surface Science 282 (2013) 121–125
Fig. 2. XRD pattern of undoped and Fe doped BSO films deposited on fused silica substrate. Fig. 1. XRD pattern of undoped and Fe doped BSO powder samples.
where Dth is the theoretical density and Db is the measured density. The theoretical density of the undoped BaSnO3 is 7.2 g/cm3 [13]. For samples sintered at 1300 ◦ C, the density reported is 5.6 g/cm3 . In the present study, the estimated density of undoped sample is 5.4 g/cm3 . Porosity of the sample was 21%. The Fe doped samples showed an increase in porosity as the doping concentration increased. Porosity of 5 and 10 at% Fe doped BaSnO3 was 22 and 25% respectively. These pellets were used as the target for pulsed laser deposition. The deposition was carried out using fourth harmonic (266 nm) of Nd-YAG laser with repetition rate of 10 Hz and pulse width 6–7 ns. The ablation was carried out at laser energy density of 1.1 J cm−2 . The target was kept rotating during the ablation for uniform ablation and to avoid pitting of the target surface. The target to substrate distance was fixed at 4.5 cm and duration of deposition was 1 h for all samples. Before depositing the film, the base pressure of the chamber was brought down to 5 × 10−4 Pa. The deposition temperature and oxygen pressure inside the chamber was optimized and maintained at 630 ◦ C and 5 × 10−2 Pa respectively. Deposition of BaSnO3 on a substrate like fused silica is more economical than other single crystalline substrates currently being used. The thicknesses of the films were measured using stylus profiler (Dektak6M). The average thickness of deposited films was 200 nm. For VSM measurements, Fe doped BaSnO3 films were deposited on Si/SiO2 /TiO2 /Pt (PtSi) wafers. The X-ray diffractometer (PANalytical X’Pert Pro) with Cu K␣ ˚ radiation was used for recording the diffraction pat( = 1.5418 A) tern. Microstructural studies were carried out using Horiba Jobin Yvon LabRam HR Raman spectrometer and Agilent 5500 series AFM.
Fig. 2 shows the X-ray diffraction pattern of Fe doped BaSnO3 films deposited on fused silica substrate. Reflections from secondary phases, Fe or its oxides were absent in the XRD patterns of bulk and thin film samples. XRD reflections (2 values) are shifted to lower angle for 5BFS sample. Lattice constant was similar to that obtained for La doped samples except for 5BFS. Minimum FWHM of 5BFS film indicates better crystallinity of the deposited film (Fig. 3). ˚ which decreased Lattice constant of the undoped film was 4.189 A, as the Fe doping concentration increased and the minimum was for 5BFS. 10 at% Fe doped film showed an increase in lattice constant and FWHM with respect to 5BFS. The ionic radii of Sn4+ and Fe2+ ions are 0.69 A˚ and 0.78 A˚ respectively. As a result of Fe incorporation, the lattice of BaSnO3 expands. The increase in volume of the unit cell contributes to the shift in 2 values towards lower angle. The grain size of the powder sample from the X-ray diffraction data was calculated using the Debye–Scherrer formula D = 0.9/ˇ cos, where D is the grain size of the crystallite, is the wavelength of the X-rays used, ˇ is the broadening of diffraction line measured at the half of its maximum intensity in radians and is the angle of diffraction. The grain size is nearly 21 nm for all bulk samples. The variation of grain size and FWHM with Fe doping concentration was found to be very small. The deposited films have an average grain size 17 nm. 3.2. Optical properties Diffuse reflectance spectra (DRS) spectra of undoped barium stannate and Fe-doped BaSnO3 powders using UV Vis NIR spectroscopy are shown in Fig. 4. BaSO4 powder was used as reference
3. Results 3.1. Crystal structure The phase purity of the prepared powders was examined using X-ray diffraction (XRD) measurements (Fig. 1). The reflections from the (1 1 0), (2 0 0), (2 1 1) planes of cubic BaSnO3 powder were identified in comparison with jcpds 15-0780. The lattice parameter reported by Bevillon et al., obtained from the density functional cal˚ culations [14] is 4.156 A˚ and the experimental value [15] is 4.116 A. The lattice constant for the undoped powder sample in the present ˚ which is in very good agreement study was found to be 4.121 A, with the theoretical and the previous experimental results. Lattice constant slightly increased with Fe concentration. 5 and 10 at% Fe doped samples have lattice constant 4.122 and 4.124 A˚ respectively. Minimum value for full width at half maximum (FWHM) was obtained for 5 at% Fe doped sample. Reflections from impurity or Fe2 O3 phases were absent in the recorded XRD pattern.
Fig. 3. Variation of lattice constant and FWHM of undoped and Fe doped films with Fe concentration.
K.K. James et al. / Applied Surface Science 282 (2013) 121–125
Fig. 4. Plot of [(k/s)h]2 vs energy of undoped and Fe doped BaSnO3 . (Inset shows the variation of bandgap for Fe doped BSO powder for various Fe concentrations.)
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Fig. 6. Micro-Raman spectra of undoped and Fe doped BaSnO3 powder samples.
3.3. Microstructural analysis for measuring relative diffuse reflectance. From the reflectance, the absorbance was calculated using Kubelka–Munk equation [16,17]. The bandgap estimated for undoped BaSnO3 from the plot of [(k/s) h]2 vs h is 3.3 ± 0.04 eV, where k and s are the absorption and scattering coefficients, and h the photon energy. As doping concentration of Fe increased, the bandgap exhibits decreasing tendency (inset of Fig. 4). The sample doped with 10 at% Fe has a bandgap 3.1 ± 0.04 eV. Fe doping produces defect states and tail states extending into the bandgap. As a result, the sharp band edge disappears and the bandgap decreases. A similar behaviour of splitting of energy levels of Mn 3d orbitals was observed in Mn-doped BaSnO3 [4]. All the films have a transmission more than 85% in the visible region. The transmittance spectra of Fe doped BSO film for various Fe concentrations deposited on fused silica substrate at 630 ◦ C are given in Fig. 5. (˛h)2 vs h plots indicate that the material is of direct bandgap (inset a of Fig. 5). The bandgap for undoped and Fe doped BaSnO3 thin films was 3.7 ± 0.03 eV. Inset b of Fig. 5 shows the variation of bandgap of BSO thin films with Fe doping. It was found that bandgap of Fe doped BaSnO3 thin films was larger than bulk samples. A slight increase in bandgap of Fe doped BSO thin films was also observed with respect to undoped film. This may be attributed to the shifting of absorption edge towards higher energy with increase in free electron concentration (B–M shift) [18,19] in the films as a result of Fe doping [20].
Fig. 5. Transmittance spectra of Fe doped BSO films for various Fe concentrations deposited on fused silica substrate at 630 ◦ C. (Inset (a) shows the plots of (˛h)2 vs h for BSO films and b shows variation of band gap with Fe concentration).
Fig. 6 shows the room temperature Raman spectra of asprepared undoped and Fe doped BaSnO3 powder. Barium stannate ¯ structure do not have Raman active powder samples with Pm3m modes to give first order Raman spectrum because of the centrosymmetric crystal structure [21]. Raman shift observed in the undoped samples may be attributed to defects induced in the samples at high temperatures and formation of ␣-Fe2 O3 phase. The inclusion of Fe ions and formation of oxygen vacancies also contribute to the structural changes. Raman spectroscopy measurements exhibited sensitiveness to Fe dopant in the bulk samples. The differences among Raman spectra of BSO, 5BFS and 10BFS powder samples could be easily distinguished from Fig. 6. Raman shift obtained in the present study can be analysed on the basis of contributions from both BaSnO3 and Fe2 O3 . Undoped barium stannate have shown Raman active modes at 138, 224, 425, 551, 829, and 1122 cm−1 . The Raman peaks are assigned on the basis of the six fundamental vibrations of SnO6 octahedron which has Oh symmetry, in the distorted perovskite structure [22]. The Raman active modes v2 Eg , v4 F1u and v5 F2g were observed at 551, 224 and 138 cm−1 , respectively. The Raman peaks at 551 and 1055 cm−1 were suppressed in the Fe doped samples, showing the incorporation of Fe ions in the host lattice. In addition to some of the Raman active modes obtained for undoped BaSnO3 , Fe doped samples exhibited Raman peaks at 248, 324, 413, 492, 595, 665 and 1310 cm−1 . Raman spectrum of ␣-Fe2 O3 evolves from seven optical modes (2Ag + 5Eg ) [23]. The obtained Raman peaks of Fe doped BaSnO3 corresponds to Ag : 218, 492 cm−1 , Eg : 286, 413, 595 cm−1 . The peak at 665 cm−1 is assigned to disorder effects and/or the presence of Fe3 O4 . The presence of other two forms, namely FeO and ␥-Fe2 O3 are not considered, as these compounds are unstable at room temperature [24]. The Raman mode observed at 1310 cm−1 could be assigned to a secondary phonon–photon interaction [25,26]. The peaks which were not identified may be attributed to the structural disorder. Peak at 1055 cm−1 in the undoped sample corresponds to BaCO3 [27]. In the doped samples, BaCO3 has been completely eliminated as indicated by the absence of Raman peak corresponding to BaCO3 . A broad peak starting from 1000 cm−1 and extending up to 1760 cm−1 was obtained for doped samples. 10 at% Fe doped powder samples show strong Raman lines compared to samples with lower doping concentration. No evidence of phase segregation was recorded in the X-ray diffraction pattern of Fe doped BaSnO3 bulk and thin film samples. Atomic force microscopy (AFM) measurements of undoped and Fe doped BaSnO3 thin films deposited on fused silica substrate were
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Fig. 7. AFM images of Fe doped BaSnO3 films on quartz substrate. (a) 10BFS (RMS – 2.71 nm), (b) 5BFS (RMS – 0.48 nm), and (c) BSO (RMS – 1.29 nm).
carried out in non contact mode using Agilent 5500 series AFM (Fig. 7). From the surface morphology, temporal changes of the surface were investigated using root mean square roughness (RMS). The surface roughness of the films analysed over a 6 m × 6 m area revealed low surface roughness for all films investigated. RMS value for film doped with 5 at% Fe was 0.48 nm. The low value of FWHM (Fig. 3) implies good crytstallinity for this film. Phase purity as revealed by the XRD pattern, optimal deposition conditions and uniform thickness contribute to the surface morphology of the films. The uniformity in surface morphology exposes the reliability of growth parameters. 3.4. Magnetic properties Ferromagnetism has been reported in degenerate and non degenerate semiconductors and insulators with 3d dopant concentration below the percolation threshold [13]. BaSnO3 is a diamagnetic material in the undoped state which shows room temperature ferromagnetism on doping with Fe. Magnetization (M) vs applied magnetic field (H) at room temperature with magnetic hysteresis loops of Fe doped BaSnO3 bulk samples are shown in Fig. 8. In the present study, we have observed diamagnetic character of BaSnO3 in powder samples below 3 at% Fe concentration (inset a of Fig. 8). Both diamagnetic and ferromagnetic behaviours were presented by sample doped with 3 at% Fe. Inset b of Fig. 8 shows the magnified M–H curve for 3BFS powder sample in the range −3kOe–+3kOe. Above this doping concentration, the material showed typical ferromagnetic character which is evident from the increase in saturation magnetization obtained for 5BFS (0.28 emu/g) and 10BFS (0.9 emu/g) bulk samples. A coercivity of 1kOe and 2kOe and a remanence ratio (Mr /Mmax ) of 30 and 28% were obtained respectively for 10 and 5 at% Fe doped BaSnO3
Fig. 8. Magnetic field (M–H) curve of Fe doped BaSnO3 powder. Inset (a) shows M–H curve for 3BFS and (b) exhibits expanded view in the range −3kOe–+3kOe.
samples. No impurity phases were detected in the XRD pattern of the bulk and thin film samples. Micro-Raman analysis (Fig. 6) has shown the presence of ␣-Fe2 O3 and defect oriented Raman shift for 10 at% Fe doped BaSnO3 bulk samples. Though phase segregation of ␣-Fe2 O3 was detected in the bulk by Micro-Raman analysis, no such evidence of phase segregation was observed in the Fe doped thin films. Hence the origin of ferromagnetic ordering in the bulk material could be considered as a combination of phase segregation due to Fe doping and defect oriented characteristic of the material. 5 and 10 at% Fe doped BaSnO3 with maximum saturation magnetization was selected for the growth of thin films using PLD technique on commercially available Si/SiO2 /TiO2 /Pt (PtSi) wafers. The thickness of the deposited film was nearly 320 nm. Field dependent magnetization (M–H) curve at room temperature after subtracting the diamagnetic property of the substrate is shown in Fig. 9. Inset a of Fig. 9 exhibits the diamagnetic character of undoped barium stannate film. Expanded view of the 5BFS and 10BFS films in the range of −2kOe–+2kOe is exhibited as the inset b of Fig. 9. Fe doped (10 at %) samples showed saturation magnetization (Ms ) of 24.5 emu/cc and coercivity 600 Oe whereas 5 at% Fe doped samples have saturation magnetization of 4 emu/cc and a large coercive field is required to demagnetize the sample. Coey et al. have proposed that the ferromagnetic ordering of Fe ions is due to F-centre exchange (FCE) mechanism [12]. F centres are oxygen vacancies with trapped carriers. The substitution of Sn4+ ions with Fe2+ ions produces oxygen vacancies. Usually thin films deposited using PLD at high temperatures are oxygen deficient [28]. These oxygen vacancies can provide enough room for carriers to get trapped whereby increasing the number of F-centres. The magnetic moment of two magnetic dopant ions (Fe) are coupled ferromagnetically by the antiferromagnetic interaction between the spin of
Fig. 9. Magnetic field (M–H) curve of Fe doped BaSnO3 films. Inset (a) shows M–H curve of BSO film and (b) exhibits expanded view of 5 and 10 BFS films in the range −2kOe–+2kOe.
K.K. James et al. / Applied Surface Science 282 (2013) 121–125
the F-centre. The orbital radius of the F-centre depends on Bohr radius and dielectric constant of the host material. The decrease in dielectric constant decreases the magnetic moment per Fe ion. The reduction in magnetic moment was compensated by the increase in number of oxygen vacancies created by the substitution of Sn ions with Fe ions. The antiferromagnetic super exchange interaction which is mediated by the overlapping 2p-orbital of O2− between two Fe ions reduces the ferromagnetic ordering in Fe–O systems. This antiferromagnetic ordering has not affected the ferromagnetic behaviour, which may become effective at higher doping concentrations. Low oxygen pressure and low laser energies favour the onset of magnetism and ferromagnetic ordering in Fe doped thin films. Reports on ferromagnetic character of Fe doped BaSnO3 has confirmed this theory and assumed the material to be a typical DMS system [11]. The ferromagnetic character of both bulk polycrystalline and thin films of BaSn1−x Fex O3d was found to increase with increase in the dopant (Fe) concentration. One cannot totally neglect the possibility of formation of clusters or strain induced structural changes, since there are reports in the literature about the magnetic properties observed due to defective growth. A detailed and in depth investigation is required to confirm the nature and origin of magnetism in thin films of Fe doped BaSnO3 . 4. Conclusion In the present study, structural, optical and magnetic properties of Fe-doped BaSnO3 in bulk and thin films grown by PLD have been investigated. The absence of secondary phases in the XRD pattern indicates that all films have cubic symmetry. 5 at% Fe doped barium stannate thin films exhibit maximum crystallinity and transparency over 85% in the visible region. BaSnO3 can be used for making transparent electrodes, if one is able to increase the transport behaviour. This material is a promising candidate for the future optoelectronic device technology. The presence of oxygen vacancies, structural deformation and surface morphology improve the ferromagnetic character of both bulk and thin films of Fe doped BaSnO3 . Raman spectra showed the presence of phase separation due to Fe dopant in the bulk samples. However, the absence of impurity phases exhibited by the XRD and Raman data of the thin films clearly indicates the suitability of PLD technique for the growth of ferromagnetic films of Fe doped barium stannate. Room temperature ferromagnetism exhibited by 5 and 10 at% Fe doped barium stannate bulk and thin film samples is a clear indication of the multiferroic character of this material. Experiments are underway to investigate the possibility of multiferroic behaviour.
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Acknowledgements One of the authors (K. K. James) thanks UGC for providing FDP grant and DST for providing equipments and other facilities under Nano mission. The authors thank SAIF, IIT Madras for VSM measurements. References [1] B. Ostrick, M. Fleischer, H. Meixner, Journal of the American Ceramic Society 80 (1997) 2153. [2] C.H. Ahn, J.M. Triscone, J. Mannhart, Nature 424 (2003) 1015. [3] O. Auciello, J.F. Scott, R. Ramesh, Physics Today 51 (1998) 22. [4] K. Balamurugan, N.H. Kumar, B. Ramachandran, M.S.R. Rao, J.A. Chelvan, P.N. Santhosh, Solid State Communications 149 (2009) 884. [5] H.F. Wang, Q.Z. Liu, F. Chen, G.Y. Gao, W. Wu, X.H. Chen, Journal of Applied Physiology 101 (2007) 105. [6] Q. Liu, J. Dai, Z. Liu, X. Zhang, G. Zhu, G. Ding, Journal of Physics D: Applied Physics 43 (2010) 401. [7] U. Lampe, J. Gebringer, H. Meixner, Sensors and Actuators 24 (1995) 657. [8] C.V. Gopal Reddy, S.V. Manorama, V.J. Rao, A. Lobo, S.K. Kulkarni, Thin Solid Films 348 (1999) 261. [9] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. Von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488. [10] R. Kumar, A.P. Singh, P. Thakur, K.H. Chae, W.K. Choi, B. Angadi, S.D. Kaushik, S. Patnaik, Journal of Physics D: Applied Physics 41 (2008) 155002. [11] K. Balamurugan, N. Harish Kumar, J.A. Chelvane, P.N. Santhosh, Journal of Alloys and Compounds 472 (2009) 9. [12] J.M.D. Coey, A.P. Douvalis, C.B. Fitzgerald, M. Venkatesan, Applied Physics Letters 84 (2004) 1332. [13] S. Upadhyay, O. Prakash, D. Kumar, Materials Letters 49 (2001) 251. [14] E. Bevillon, A. Chesnaud, Y. Wang, G. Dezanneau, Journal of Physics: Condensed Matter 20 (2008) 145217. [15] T. Maekawa, K. Kurosaki, S. Yamanaka, Journal of Alloys and Compounds 416 (2006) 214. [16] P. Kubelka, Journal of the Optical Society of America 38 (1948) 448. [17] P. Kubelka, F. Munk, Zhurnal Tekhnicheskoi Fiziki 12 (1931) 593. [18] E. Burstein, Physiological Reviews 93 (1954) 632. [19] T.S. Moss, Proceedings of the Physical Society, London, Section B 67 (1954) 775. [20] H. Kima, U.A. Piqueb, J.S. Horwitzb, H. Muratab, Z.H. Kafafib, C.M. Gilmorea, D.B. Chrisey, Thin Solid Films 377 (2000) 798. [21] W.G. Fateley, F.R. Dollish, N.T. McDevitt, F.F. Bentley, Infrared and Raman Selection Rules for Molecules and Lattice Vibrations: The Correlation Method, 3rd ed., Wiley Interscience, New York, 1972. [22] A.S. Deepa, S. Vidya, P.C. Manu, S. Solomon, A. John, J.K. Thomas, Journal of Alloys and Compounds 509 (2011) 1830. [23] P. Lottici, C. Baratto, D. Bersani, G. Antonioli, A. Montenero, M. Guarneri, Optical Materials 9 (1998) 368. [24] P. Kumar, P. Sharma, R. Shrivastav, S. Dass, V.R. Satsangi, International Journal of Hydrogen Energy 36 (2011) 2777. [25] K.F. McCarty, Solid State Communications 68 (1988) 799. [26] M.J. Massey, U. Baier, R. Merlin, W.H. Weber, Physical Review B: Condensed Matter 41 (1990) 7822. [27] R.A. Nyquist, C.L. Putzig, M.A. Leugers, Infrared and Raman Spectral Atlas of Inorganic Compounds and Organic Salts, Academic Press, New York, 1997. [28] S.T. Lee, N. Fujimura, T. Ito, Japanese Journal of Applied Physics 34 (1995) 5168.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structural, optical and magnetic properties of Fe-doped barium stannate thin films grown by PLD K.K. James, Arun Aravind, M.K. Jayaraj ∗ Nanophotonic and Optoelectronic Devices Laboratory, Department of Physics, Cochin University of Science and Technology, Kerala 682022, India
a r t i c l e
i n f o
Article history: Received 18 August 2012 Received in revised form 19 April 2013 Accepted 16 May 2013 Available online 23 May 2013 Keywords: PLD XRD AFM Raman spectroscopy Ferromagnetism
a b s t r a c t Barium stannate is a wide band gap semiconductor with cubic perovskite structure. Polycrystalline bulk samples of BaSn1−x Fex O3d (BFS), with x = 0.00, 0.02, 0.03, 0.05 and 0.10 were prepared by solid-state reaction. In this paper, we report the growth of undoped and Fe doped barium stannate thin films on fused silica substrate using pulsed laser deposition (PLD) technique at a relatively high substrate temperature and low oxygen pressure. The deposited films have wide bandgap and are transparent in the visible region. The X-ray diffraction analysis of the films confirmed the cubic structure. Microstructural studies were carried out using micro-Raman spectroscopy and AFM analysis. Defect induced Raman shifts were observed in the samples. Magnetic studies revealed an increase in magnetic properties for films doped with 10 at% Fe doped samples. © 2013 Elsevier B.V. All rights reserved.
1. Introduction BaSnO3 is an n-type semiconductor material with perovskite structure [1]. Perovskite materials are well known for their superconducting, magnetic and electro optic properties [2,3]. The optical bandgap reported in the literature for undoped BaSnO3 is 3.1 eV [4]. Thin films of La-doped BaSnO3 with optical transparency in the visible region deposited on MgO (0 0 1) substrates have a bandgap 4.02 eV [5]. Electrical resistivity of Sb-doped barium stannate is of the order of 2.43 m cm [6]. The gas sensing properties of barium stannate have also been explored to a great extent [7,8]. In addition to the physical properties induced by charge carriers, the focus of researchers has now turned towards spin based electronic devices [9]. Materials exhibiting intrinsic ferromagnetism at room temperature attract the attention of the electronic industry. The Fe ion implanted ZnO thin films [10] and Fe doped BaSnO3 films [11] exhibit ferromagnetic properties at room temperature with Curie temperature well below 500 K. Coey et al. [12] have proposed that the ferromagnetic ordering of Fe ions is due to F-centre exchange (FCE) mechanism. BaSnO3 has a cubic perovskite structure. The difference in the ionic radii of the Sn4+ and Fe2+ ions [11] produces an increase in lattice constant. A combination of electrical conductivity and ferromagnetism in the doped state together with optical properties make this material a possible candidate for the future
∗ Corresponding author. Tel.: +91 4842577404; fax: +91 4842577595. E-mail addresses: [email protected] (K.K. James), [email protected] (M.K. Jayaraj). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.05.076
optoelectronic industry. In the present study, the possibility of depositing thin films of Fe doped BaSnO3 on commercially available transparent fused silica substrate which is inexpensive compared with other single crystalline substrates has been explored. A systematic study on the optical properties of thin films in the doped and undoped state has been carried out. A brief analysis of the ferromagnetic behaviour of bulk and thin film samples are incorporated in this work to invoke the potential of Fe doped BaSnO3 as a magnetic material. 2. Experimental details Fe doped BaSnO3 powders were prepared by solid state reaction using high purity BaCO3 , SnO2 and Fe2 O3 . The Fe content in the BaSnO3 powder was made to vary from 0, 2, 3, 5 and 10 at% and they are named as BSO, 2BFS, 3BFS, 5BFS, and 10BFS respectively. Stoichiometric amounts of the starting materials were mixed using agate mortar and pestle with acetone as mixing agent. Calcination of the mixed powder was done for 12 h at 800 ◦ C. The calcined powders were reground and kept for 18 h at 850 ◦ C. The powder was mixed with poly vinyl alcohol as binder and cold pressed into cylindrical pellets of diameter 13 mm and thickness 1–1.5 mm by applying a pressure of 370 MPa. Pellets were sintered in air at a temperature 900 ◦ C for 24 h. The colour changed from white to coral as the Fe concentration was increased. The prepared pellets were polished with fine emery paper and dried at a temperature 200 ◦ C for 1 h. The % porosity was calculated using the relation: % porosity =
Dth − Db × 100 Dth
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K.K. James et al. / Applied Surface Science 282 (2013) 121–125
Fig. 2. XRD pattern of undoped and Fe doped BSO films deposited on fused silica substrate. Fig. 1. XRD pattern of undoped and Fe doped BSO powder samples.
where Dth is the theoretical density and Db is the measured density. The theoretical density of the undoped BaSnO3 is 7.2 g/cm3 [13]. For samples sintered at 1300 ◦ C, the density reported is 5.6 g/cm3 . In the present study, the estimated density of undoped sample is 5.4 g/cm3 . Porosity of the sample was 21%. The Fe doped samples showed an increase in porosity as the doping concentration increased. Porosity of 5 and 10 at% Fe doped BaSnO3 was 22 and 25% respectively. These pellets were used as the target for pulsed laser deposition. The deposition was carried out using fourth harmonic (266 nm) of Nd-YAG laser with repetition rate of 10 Hz and pulse width 6–7 ns. The ablation was carried out at laser energy density of 1.1 J cm−2 . The target was kept rotating during the ablation for uniform ablation and to avoid pitting of the target surface. The target to substrate distance was fixed at 4.5 cm and duration of deposition was 1 h for all samples. Before depositing the film, the base pressure of the chamber was brought down to 5 × 10−4 Pa. The deposition temperature and oxygen pressure inside the chamber was optimized and maintained at 630 ◦ C and 5 × 10−2 Pa respectively. Deposition of BaSnO3 on a substrate like fused silica is more economical than other single crystalline substrates currently being used. The thicknesses of the films were measured using stylus profiler (Dektak6M). The average thickness of deposited films was 200 nm. For VSM measurements, Fe doped BaSnO3 films were deposited on Si/SiO2 /TiO2 /Pt (PtSi) wafers. The X-ray diffractometer (PANalytical X’Pert Pro) with Cu K␣ ˚ radiation was used for recording the diffraction pat( = 1.5418 A) tern. Microstructural studies were carried out using Horiba Jobin Yvon LabRam HR Raman spectrometer and Agilent 5500 series AFM.
Fig. 2 shows the X-ray diffraction pattern of Fe doped BaSnO3 films deposited on fused silica substrate. Reflections from secondary phases, Fe or its oxides were absent in the XRD patterns of bulk and thin film samples. XRD reflections (2 values) are shifted to lower angle for 5BFS sample. Lattice constant was similar to that obtained for La doped samples except for 5BFS. Minimum FWHM of 5BFS film indicates better crystallinity of the deposited film (Fig. 3). ˚ which decreased Lattice constant of the undoped film was 4.189 A, as the Fe doping concentration increased and the minimum was for 5BFS. 10 at% Fe doped film showed an increase in lattice constant and FWHM with respect to 5BFS. The ionic radii of Sn4+ and Fe2+ ions are 0.69 A˚ and 0.78 A˚ respectively. As a result of Fe incorporation, the lattice of BaSnO3 expands. The increase in volume of the unit cell contributes to the shift in 2 values towards lower angle. The grain size of the powder sample from the X-ray diffraction data was calculated using the Debye–Scherrer formula D = 0.9/ˇ cos, where D is the grain size of the crystallite, is the wavelength of the X-rays used, ˇ is the broadening of diffraction line measured at the half of its maximum intensity in radians and is the angle of diffraction. The grain size is nearly 21 nm for all bulk samples. The variation of grain size and FWHM with Fe doping concentration was found to be very small. The deposited films have an average grain size 17 nm. 3.2. Optical properties Diffuse reflectance spectra (DRS) spectra of undoped barium stannate and Fe-doped BaSnO3 powders using UV Vis NIR spectroscopy are shown in Fig. 4. BaSO4 powder was used as reference
3. Results 3.1. Crystal structure The phase purity of the prepared powders was examined using X-ray diffraction (XRD) measurements (Fig. 1). The reflections from the (1 1 0), (2 0 0), (2 1 1) planes of cubic BaSnO3 powder were identified in comparison with jcpds 15-0780. The lattice parameter reported by Bevillon et al., obtained from the density functional cal˚ culations [14] is 4.156 A˚ and the experimental value [15] is 4.116 A. The lattice constant for the undoped powder sample in the present ˚ which is in very good agreement study was found to be 4.121 A, with the theoretical and the previous experimental results. Lattice constant slightly increased with Fe concentration. 5 and 10 at% Fe doped samples have lattice constant 4.122 and 4.124 A˚ respectively. Minimum value for full width at half maximum (FWHM) was obtained for 5 at% Fe doped sample. Reflections from impurity or Fe2 O3 phases were absent in the recorded XRD pattern.
Fig. 3. Variation of lattice constant and FWHM of undoped and Fe doped films with Fe concentration.
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Fig. 4. Plot of [(k/s)h]2 vs energy of undoped and Fe doped BaSnO3 . (Inset shows the variation of bandgap for Fe doped BSO powder for various Fe concentrations.)
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Fig. 6. Micro-Raman spectra of undoped and Fe doped BaSnO3 powder samples.
3.3. Microstructural analysis for measuring relative diffuse reflectance. From the reflectance, the absorbance was calculated using Kubelka–Munk equation [16,17]. The bandgap estimated for undoped BaSnO3 from the plot of [(k/s) h]2 vs h is 3.3 ± 0.04 eV, where k and s are the absorption and scattering coefficients, and h the photon energy. As doping concentration of Fe increased, the bandgap exhibits decreasing tendency (inset of Fig. 4). The sample doped with 10 at% Fe has a bandgap 3.1 ± 0.04 eV. Fe doping produces defect states and tail states extending into the bandgap. As a result, the sharp band edge disappears and the bandgap decreases. A similar behaviour of splitting of energy levels of Mn 3d orbitals was observed in Mn-doped BaSnO3 [4]. All the films have a transmission more than 85% in the visible region. The transmittance spectra of Fe doped BSO film for various Fe concentrations deposited on fused silica substrate at 630 ◦ C are given in Fig. 5. (˛h)2 vs h plots indicate that the material is of direct bandgap (inset a of Fig. 5). The bandgap for undoped and Fe doped BaSnO3 thin films was 3.7 ± 0.03 eV. Inset b of Fig. 5 shows the variation of bandgap of BSO thin films with Fe doping. It was found that bandgap of Fe doped BaSnO3 thin films was larger than bulk samples. A slight increase in bandgap of Fe doped BSO thin films was also observed with respect to undoped film. This may be attributed to the shifting of absorption edge towards higher energy with increase in free electron concentration (B–M shift) [18,19] in the films as a result of Fe doping [20].
Fig. 5. Transmittance spectra of Fe doped BSO films for various Fe concentrations deposited on fused silica substrate at 630 ◦ C. (Inset (a) shows the plots of (˛h)2 vs h for BSO films and b shows variation of band gap with Fe concentration).
Fig. 6 shows the room temperature Raman spectra of asprepared undoped and Fe doped BaSnO3 powder. Barium stannate ¯ structure do not have Raman active powder samples with Pm3m modes to give first order Raman spectrum because of the centrosymmetric crystal structure [21]. Raman shift observed in the undoped samples may be attributed to defects induced in the samples at high temperatures and formation of ␣-Fe2 O3 phase. The inclusion of Fe ions and formation of oxygen vacancies also contribute to the structural changes. Raman spectroscopy measurements exhibited sensitiveness to Fe dopant in the bulk samples. The differences among Raman spectra of BSO, 5BFS and 10BFS powder samples could be easily distinguished from Fig. 6. Raman shift obtained in the present study can be analysed on the basis of contributions from both BaSnO3 and Fe2 O3 . Undoped barium stannate have shown Raman active modes at 138, 224, 425, 551, 829, and 1122 cm−1 . The Raman peaks are assigned on the basis of the six fundamental vibrations of SnO6 octahedron which has Oh symmetry, in the distorted perovskite structure [22]. The Raman active modes v2 Eg , v4 F1u and v5 F2g were observed at 551, 224 and 138 cm−1 , respectively. The Raman peaks at 551 and 1055 cm−1 were suppressed in the Fe doped samples, showing the incorporation of Fe ions in the host lattice. In addition to some of the Raman active modes obtained for undoped BaSnO3 , Fe doped samples exhibited Raman peaks at 248, 324, 413, 492, 595, 665 and 1310 cm−1 . Raman spectrum of ␣-Fe2 O3 evolves from seven optical modes (2Ag + 5Eg ) [23]. The obtained Raman peaks of Fe doped BaSnO3 corresponds to Ag : 218, 492 cm−1 , Eg : 286, 413, 595 cm−1 . The peak at 665 cm−1 is assigned to disorder effects and/or the presence of Fe3 O4 . The presence of other two forms, namely FeO and ␥-Fe2 O3 are not considered, as these compounds are unstable at room temperature [24]. The Raman mode observed at 1310 cm−1 could be assigned to a secondary phonon–photon interaction [25,26]. The peaks which were not identified may be attributed to the structural disorder. Peak at 1055 cm−1 in the undoped sample corresponds to BaCO3 [27]. In the doped samples, BaCO3 has been completely eliminated as indicated by the absence of Raman peak corresponding to BaCO3 . A broad peak starting from 1000 cm−1 and extending up to 1760 cm−1 was obtained for doped samples. 10 at% Fe doped powder samples show strong Raman lines compared to samples with lower doping concentration. No evidence of phase segregation was recorded in the X-ray diffraction pattern of Fe doped BaSnO3 bulk and thin film samples. Atomic force microscopy (AFM) measurements of undoped and Fe doped BaSnO3 thin films deposited on fused silica substrate were
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Fig. 7. AFM images of Fe doped BaSnO3 films on quartz substrate. (a) 10BFS (RMS – 2.71 nm), (b) 5BFS (RMS – 0.48 nm), and (c) BSO (RMS – 1.29 nm).
carried out in non contact mode using Agilent 5500 series AFM (Fig. 7). From the surface morphology, temporal changes of the surface were investigated using root mean square roughness (RMS). The surface roughness of the films analysed over a 6 m × 6 m area revealed low surface roughness for all films investigated. RMS value for film doped with 5 at% Fe was 0.48 nm. The low value of FWHM (Fig. 3) implies good crytstallinity for this film. Phase purity as revealed by the XRD pattern, optimal deposition conditions and uniform thickness contribute to the surface morphology of the films. The uniformity in surface morphology exposes the reliability of growth parameters. 3.4. Magnetic properties Ferromagnetism has been reported in degenerate and non degenerate semiconductors and insulators with 3d dopant concentration below the percolation threshold [13]. BaSnO3 is a diamagnetic material in the undoped state which shows room temperature ferromagnetism on doping with Fe. Magnetization (M) vs applied magnetic field (H) at room temperature with magnetic hysteresis loops of Fe doped BaSnO3 bulk samples are shown in Fig. 8. In the present study, we have observed diamagnetic character of BaSnO3 in powder samples below 3 at% Fe concentration (inset a of Fig. 8). Both diamagnetic and ferromagnetic behaviours were presented by sample doped with 3 at% Fe. Inset b of Fig. 8 shows the magnified M–H curve for 3BFS powder sample in the range −3kOe–+3kOe. Above this doping concentration, the material showed typical ferromagnetic character which is evident from the increase in saturation magnetization obtained for 5BFS (0.28 emu/g) and 10BFS (0.9 emu/g) bulk samples. A coercivity of 1kOe and 2kOe and a remanence ratio (Mr /Mmax ) of 30 and 28% were obtained respectively for 10 and 5 at% Fe doped BaSnO3
Fig. 8. Magnetic field (M–H) curve of Fe doped BaSnO3 powder. Inset (a) shows M–H curve for 3BFS and (b) exhibits expanded view in the range −3kOe–+3kOe.
samples. No impurity phases were detected in the XRD pattern of the bulk and thin film samples. Micro-Raman analysis (Fig. 6) has shown the presence of ␣-Fe2 O3 and defect oriented Raman shift for 10 at% Fe doped BaSnO3 bulk samples. Though phase segregation of ␣-Fe2 O3 was detected in the bulk by Micro-Raman analysis, no such evidence of phase segregation was observed in the Fe doped thin films. Hence the origin of ferromagnetic ordering in the bulk material could be considered as a combination of phase segregation due to Fe doping and defect oriented characteristic of the material. 5 and 10 at% Fe doped BaSnO3 with maximum saturation magnetization was selected for the growth of thin films using PLD technique on commercially available Si/SiO2 /TiO2 /Pt (PtSi) wafers. The thickness of the deposited film was nearly 320 nm. Field dependent magnetization (M–H) curve at room temperature after subtracting the diamagnetic property of the substrate is shown in Fig. 9. Inset a of Fig. 9 exhibits the diamagnetic character of undoped barium stannate film. Expanded view of the 5BFS and 10BFS films in the range of −2kOe–+2kOe is exhibited as the inset b of Fig. 9. Fe doped (10 at %) samples showed saturation magnetization (Ms ) of 24.5 emu/cc and coercivity 600 Oe whereas 5 at% Fe doped samples have saturation magnetization of 4 emu/cc and a large coercive field is required to demagnetize the sample. Coey et al. have proposed that the ferromagnetic ordering of Fe ions is due to F-centre exchange (FCE) mechanism [12]. F centres are oxygen vacancies with trapped carriers. The substitution of Sn4+ ions with Fe2+ ions produces oxygen vacancies. Usually thin films deposited using PLD at high temperatures are oxygen deficient [28]. These oxygen vacancies can provide enough room for carriers to get trapped whereby increasing the number of F-centres. The magnetic moment of two magnetic dopant ions (Fe) are coupled ferromagnetically by the antiferromagnetic interaction between the spin of
Fig. 9. Magnetic field (M–H) curve of Fe doped BaSnO3 films. Inset (a) shows M–H curve of BSO film and (b) exhibits expanded view of 5 and 10 BFS films in the range −2kOe–+2kOe.
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the F-centre. The orbital radius of the F-centre depends on Bohr radius and dielectric constant of the host material. The decrease in dielectric constant decreases the magnetic moment per Fe ion. The reduction in magnetic moment was compensated by the increase in number of oxygen vacancies created by the substitution of Sn ions with Fe ions. The antiferromagnetic super exchange interaction which is mediated by the overlapping 2p-orbital of O2− between two Fe ions reduces the ferromagnetic ordering in Fe–O systems. This antiferromagnetic ordering has not affected the ferromagnetic behaviour, which may become effective at higher doping concentrations. Low oxygen pressure and low laser energies favour the onset of magnetism and ferromagnetic ordering in Fe doped thin films. Reports on ferromagnetic character of Fe doped BaSnO3 has confirmed this theory and assumed the material to be a typical DMS system [11]. The ferromagnetic character of both bulk polycrystalline and thin films of BaSn1−x Fex O3d was found to increase with increase in the dopant (Fe) concentration. One cannot totally neglect the possibility of formation of clusters or strain induced structural changes, since there are reports in the literature about the magnetic properties observed due to defective growth. A detailed and in depth investigation is required to confirm the nature and origin of magnetism in thin films of Fe doped BaSnO3 . 4. Conclusion In the present study, structural, optical and magnetic properties of Fe-doped BaSnO3 in bulk and thin films grown by PLD have been investigated. The absence of secondary phases in the XRD pattern indicates that all films have cubic symmetry. 5 at% Fe doped barium stannate thin films exhibit maximum crystallinity and transparency over 85% in the visible region. BaSnO3 can be used for making transparent electrodes, if one is able to increase the transport behaviour. This material is a promising candidate for the future optoelectronic device technology. The presence of oxygen vacancies, structural deformation and surface morphology improve the ferromagnetic character of both bulk and thin films of Fe doped BaSnO3 . Raman spectra showed the presence of phase separation due to Fe dopant in the bulk samples. However, the absence of impurity phases exhibited by the XRD and Raman data of the thin films clearly indicates the suitability of PLD technique for the growth of ferromagnetic films of Fe doped barium stannate. Room temperature ferromagnetism exhibited by 5 and 10 at% Fe doped barium stannate bulk and thin film samples is a clear indication of the multiferroic character of this material. Experiments are underway to investigate the possibility of multiferroic behaviour.
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