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Magneto-transport and magneto-optical properties of Cr-alloyed SnO2 thin films: A correlation between structural and magnetic behaviors
Solid State Communications 298 (2019) 113641
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Communication
Magneto-transport and magneto-optical properties of Cr-alloyed SnO2 thin films: A correlation between structural and magnetic behaviors
T
M. Mousavia, Sh. Tabatabai Yazdib,∗, M.M. Bagheri Mohagheghic a
Department of Physics, University of Bojnord, Bojnord, Iran Department of Physics, Payame Noor University (PNU), 19395-3697 Tehran, Iran c School of Physics, Damghan University, Damghan, Iran b
A R T I C LE I N FO
A B S T R A C T
Communicated by E.Y. Andrei
In this work, the tin oxide thin films alloyed with Cr up to 50 at.% were prepared via a spray pyrolysis method on the glass substrates, and their structural, magneto-optical and magneto-transport properties were studied. The results show that all the films are polycrystalline grown in a tetragonal rutile structure. A critical Cr concentration of about 15% was found concerning the lattice volume variations in SnO2:Cr films, as well as their magnetoresistance and magneto-optical properties suggesting a correlation between the structural and magnetic behaviors and indicating to two different mechanisms for Cr addition. For low Cr concentrations (less than 15%), the substitutional doping is the involving mechanism, while for more Cr concentrations, the interstitial one is predominant in the involved films.
Keywords: Tin oxide Magnetoresistance Faraday effect Spray pyrolysis Thin film
1. Introduction SnO2 is an oxide semiconductor widely studied in its thin film form as an important transparent conducting oxide (TCO) in view of its technologically considerable properties: It has a wide direct band gap of about 3.6 eV resulting in high optical transparency for visible light, a metal-like conductivity, easy doping, nontoxicity, thermal stability, mechanical hardness, low cost and chemical stability in the basic and acidic solutions. SnO2 crystallizes in tetragonal symmetry with P42/ mnm space group (rutile structure) with lattice parameters of a = 4.7373 Å and c = 3.1864 Å [1,2]. SnO2 film has been doped with a variety of ions to improve its physical and chemical properties needed for several applications (e.g. Refs. [3–5]). Many results have shown that several dopants can lead to an increase in the surface area by reducing the grain size and crystallinity [6]. In addition, recently there have been numerous reports on ferromagnetism above room temperature in the thin films of wide bandgap semiconductors doped with a small amount of transition metal (TM) ions in some cases as low as 1–2% (e.g. Ref. [7]). High temperature ferromagnetism was completely unexpected in these diluted magnetic semiconductors (DMS) since considering the super-exchange or double-exchange interactions and Curie temperature scaling theoretically as x1/2 in the dilute ferromagnetic materials [8,9], the magnetic doping level (x) lies far below the percolation threshold needed for a long-range order [10]. However, SnO2 is an attractive host ∗
material for DMS and following the report on high temperature ferromagnetism in Co-doped SnO2 films [11], numerous studies have been carried out on the SnO2 films doped with each of the transition metal ions for use in the spintronics functions. However, because of the high sensitivity of the material on the preparation method and deposition conditions, there is often little agreement between the results obtained from various studies by different groups. Zuo et al. reported on the ferromagnetic properties of SnO2:Cr films deposited onto Si (111) substrates by a sol-gel method due to the anti-ferromagnetic super-exchange interactions among the nearest neighbor Cr ions along with a decrease in the magnetic moment per Cr with the doping level [12]. Zhang et al. prepared SnO2:Cr films by a chemical vapor deposition and suggested ferromagnetism is due to oxygen vacancies as well as Cr doping [13]. Considering all the aspects of the reported studies, in this work we have undertaken another comprehensive investigation on SnO2:Cr films over a wide range of composition (with Cr concentration up to 50 at%) and studied the magneto-transport and magneto-optical properties of these DMS SnO2:Cr films in order to investigate magnetism at room temperature. (Their electrical and optical characteristics have been reported previously [14]). The studied Cr-alloyed SnO2 thin films were prepared by a typical spray pyrolysis technique.
Corresponding author. E-mail address: [email protected] (S. Tabatabai Yazdi).
https://doi.org/10.1016/j.ssc.2019.05.012 Received 24 May 2018; Received in revised form 10 January 2019; Accepted 19 May 2019 Available online 22 May 2019 0038-1098/ © 2019 Elsevier Ltd. All rights reserved.
Solid State Communications 298 (2019) 113641
M. Mousavi, et al.
2. Experimental methods
such as the lattice constants and the shape parameters were taken as the free parameters during the refinement process. In the first step, the global parameters such as the background and scale factors were refined, and then the structural parameters such as the lattice constants, profile shape and preferred orientation ones were refined in sequence. The background was fitted with a sixth order polynomial and the peak shapes were described by pseudo-voigt profiles. Fig. 1b presents the XRD data of the x = 0 film, for instance, along with its Rietveld refined profile. As seen, the observed and calculated profiles match to each other and all the experimental peaks are the allowed Bragg ones. The observed diffraction peaks corresponding to reflection planes (110), (101), (200), (211), (220), (310) and (301) provide a clear evidence for the formation of a single phase tetragonal rutile structure with P42/ mnm space group for the deposited films. The fitting quality of the experimental data is assessed by the “goodness of fit” parameter, χ2, and various reliability factors (R-factors) such as Rp (the profile factor), Rwp (weighted profile factor), Rexp (the expected weighted profile factor), RB (Bragg factor) and RF (crystallographic factor) [15]. The computed conventional agreement factors of Rietveld analysis are presented in Table 1. The obtained Rexp and Rwp values are found to be slightly large. This is due to the lower ratio of the diffraction peak intensity to the background in the nanocrystalline samples. Similar high values of R-factors have been reported for nanocrystalline materials by others, as well (e.g Refs. [17,18]). However, the low values obtained for χ2 justify the goodness of refinement. Cr-addition does not alter the crystal structure of the SnO2 film, but does affect the crystal growth (see Fig. 1a): In the pristine SnO2 film, the crystallites are grown preferably along (200) and (110) directions, while by Cr-addition, there exists a competition between the intensities of these peaks (others are not much affected). On increasing the Cr concentration, first (x < 0.20), (200) orientation becomes predominant and then on more Cr-alloying, the crystallites are oriented preferably along (110) direction. This means that there are preferred planes for Cr ions occupancy. The observed decrease in the intensity of the diffraction peaks is mainly due to the substitution of Cr ions driving Sn ions into the interstitial sites [19] and hence resulting in a less ordered state. Another effect of Cr-alloying is revealed as broadening of the peaks, i.e. a decrease in the crystallite size (except for the film of x = 0.50), i.e. Cr-addition inhibits the grain growth; Similar observation has been reported for the Cr-doped SnO2 nanoparticles with x up to 0.20, as well [12]. The mean crystallite (grain) size of the films calculated based on (110) reflection via Scherrer's formula lie in the range of 35–62 nm (Table 1). Cr-alloying results also in a slight shift in the predominant peaks' positions. The shift in XRD peaks’ position reflects a change in the unit cell volume of the films. The refined crystallographic parameters including the rutile lattice parameters of the SnO2:Cr films are summarized in Table 1. The evolution of lattice parameters a and c as well as the unit cell volume with Cr content in the films is shown in Fig. 2. As
The unalloyed and Cr-alloyed Sn1-xCrxO2 films with x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.40 and 0.50 were deposited by a spray pyrolysis technique as discussed in details in our previous paper [14]. The precursor solution was prepared using the appropriate amounts of stannic chloride (SnCl4:5H2O), chromium nitride (Cr(NO3)3:9H2O) and ethanol (CH3CH2OH) in distilled water. To enhance the solubility of the mixture, a few drops of hydrochloric acid (HCl) was added and stirred. The raw materials used were of purity of better than 99% (from MERCK). The deposited films were characterized for structural, compositional, morphological, magnetotransport and magneto-optical properties. The structural characterization of the films were carried out by the X-ray diffraction (XRD) patterns recorded using a D8 Advance Bruker system with Cu-Kα radiation (λ = 1.5406 Å) in the 2θ range of 20–70°. The XRD profiles were analyzed employing Rietveld refinement method, through the Fullprof software [15]. The average crystallite size 0.9 λ of the films was estimated using Scherrer's formula D = β cos θ where λ = 1.5406 Å, the X-ray wavelength, θ is the diffraction angle of a main reflection and β is the full width at half maximum (FWHM) of that peak [16]. The morphological and elemental analysis of surface of the films were performed via scanning electron microscopy (SEM) by a LEO 1450 VP system equipped with an energy dispersive X-rays spectrometer (EDX). To study the magnetotransport properties of the SnO2:Cr films, two electrodes were coated on the two ends of the samples by resistance thermal evaporation of aluminum in vacuum using an Edwards E306 coating system. The magnetoresistance (MR = (RH – RH=0)/RH=0, where R0 and RH are the resistances before and after applying a magnetic field, respectively) of the unalloyed and Cr-alloyed films was measured at room temperature under a magnetic field of 0.5 T. The magneto-optical Faraday effect in the deposited films was studied using a setup consisting of a polarized He–Ne laser light (λ = 632 nm) propagating along the film under a magnetic field of 0.8 T (being sufficient for the saturation magnetization to be achieved). The incident beam was parallel to the field direction. The magnitude of rotation of the polarization plane was detected with the accuracy of 0.003° by an oscilloscope. 3. Results and discussion 3.1. Structural and morphological analysis The XRD patterns of the Sn1-xCrxO2 films with x = 0–0.50 are shown in Fig. 1a. As seen, at room temperature all the films are polycrystalline and the crystal structure of the SnO2 film has not been changed by introducing Cr ions and increasing its content even up to 50 at%. The patterns were analyzed by Rietveld refinement using Fullprof program. To do so, the ions occupancies were fixed while the other parameters
Fig. 1. (a) XRD patterns of Sn1-xCrxO2 thin films with different Cr concentrations from 0 to 50 at% taken at room temperature by Cu-Kα radiation. (b) Observed XRD pattern (the dots) and Reitveld-based calculated profile (the solid lines) of the pristine film, along with their difference curve (observed minus calculated) at the bottom of the panel. The tick marks below the profile indicate the position of the allowed Bragg reflections.
2
Solid State Communications 298 (2019) 113641
M. Mousavi, et al.
Table 1 Crystallite size (D) and Rietveld refined crystallographic parameters of the Sn1-xCrxO2 thin films with different Cr concentrations from 0 to 50 at%, along with their thickness (t), resistivity (ρ), carrier type and its concentration (n/p). sample (x)
D (nm)
a (Å)
c (Å)
V (Å3)
RWP (%)
Rexp (%)
χ2
t (nm)
ρ (Ω cm)
n/p (cm−3)
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.40 0.50
54 48 – 45 46 45 35 – 62
4.725 4.718 – 4.694 4.709 4.724 4.730 – 4.747
3.229 3.233 – 3.221 3.219 3.202 3.211 – 3.223
72.089 71.965 – 70.970 71.380 71.517 71.839 – 72.627
15.3 13.8 – 20.1 15.4 17.7 12.6 – 14.3
13.26 12.29 – 13.84 11.51 12.42 10.39 – 11.60
1.33 1.26 – 2.11 1.79 2.03 1.47 – 1.52
382 397 381 370 366 375 372 380 371
1.31 × 10−2 1.29 × 10−2 1.50 × 10−1 3.10 × 10−2 1.20 3.81 × 10−1 4.70 × 10−1 6.11 × 10−1 60
n = 9.1 × 1019 p = 3.7 × 1019 p = 4.4 × 1018 p = 9.2 × 1019 n = 5.3 × 1017 n = 6.7 × 1018 n = 1.6 × 1018 n = 1.1 × 1019 n = 1.1 × 1015
films, their Faraday effect and magnetoresistance have been investigated. The optical properties of the involved films have been previously studied [14]. The high transmittance of the films in visible region makes the magneto-optical Faraday effect suitable for evaluating their magnetic properties [21]. Fig. 4 shows the saturated Faraday rotation of the SnO2:Cr transparent thin films under a 0.8 T field measured at 632 nm at room temperature. It exhibits an increasing trend on Cr-addition up to x ≈ 0.05–0.10 (the Faraday rotation for the films of 0.05 ≤ x ≤ 0.15 was found to be roughly the same, considering the measurements error) followed by a significant reduction for the samples with 0.15 < x < 0.3. For more Cr concentrations, the angle of Faraday rotation increases. The initial enhancement in the magnetic properties of the films on Cr-doping up to x ≈ 0.10 reflects the increased localized magnetic moments, i.e. the enhanced magnetization. While, the subsequent reduction above x = 0.15 is associated to weakening of the magnetic ordering on Cr-alloying due to a decrease in the carrier concentration. (The results of the electrical measurements of the involved films, reported elsewhere [14], are summarized in Table 1.) The presence of the maximum saturation magnetization in a critical Cr concentration has been reported by others for the Sn1-xCrxO2 nanoparticles, however at a different concentration of x = 0.025, approximately where the lattice volume of the samples reveals a minimum value [20]. Fig. 5 shows magnetoresistance (MR = (RH – RH=0)/RH=0) of the SnO2:Cr films measured at room temperature under a magnetic field of 0.5 T. At first glance, one can see that the MR is negative for all the films. Dauzhenka et al. have studied the MR of the SnO2 films at low temperatures and proposed the weak localization and electron-electron interactions as the origin of the observed negative values [22]. As the negative MR is believed to be suppressed with the temperature decrease in both three- and two-dimensional semiconducting materials [23], the negative MR values observed for the involved SnO2:Cr films at room temperature can be due to the interaction between the weakly localized carriers, as well. However, another possible cause for the observed negative MR concerns the suppression of the spin-flip scattering by the applied magnetic field [24]. As seen in Fig. 5, the MR magnitude increases first with Cr-addition reaching a maximum at x ≈ 0.15 and then decreases. The presence of the critical Cr concentration of x = 0.15 being roughly observed in the Faraday rotation results as well, as discussed is due to the increase in the saturation magnetization of the films with x up to 0.15 followed by its reduction caused by destroying the magnetic moments' ordering. It is most notable that the critical Cr concentration for the magnetic properties corresponding to the maximum saturation magnetization (xM ≈ 0.10–0.15) coincides approximately with the Cr concentration obtained from the structural results corresponding to the minimum unit cell volume (xL = 0.15). This may suggest a direct correlation of structural-magnetic properties. As mentioned in the structural analysis section, for low Cr-doping (x < 0.15) the substitutional doping is the involving mechanism accompanying oxygen vacancies. This explains the observed lattice contraction as well as the enhanced magnetic
Fig. 2. Variation of the lattice parameters a and c as well as the unit cell volume with the Cr concentration in the Sn1-xCrxO2 thin films. In this and the following figures, the lines connecting the data points are only guides for the eye.
seen by Cr-addition, a and V decrease gradually reaching a minimum at x ≈ 0.15 and then increase; c shows less pronounced variations. This behavior has been also observed in the Sn1-xCrxO2 nanoparticles, but with a much lower critical Cr content (xL ≈ 2.5%) [20]. This observation implies that two different mechanisms should work for Cr addition. As discussed previously in Ref. [14], the substitutional doping can be assumed as the dominant mechanism for low Cr-doping (x < 0.15): Sn4+ ions are substituted by smaller ions of Cr3+. Furthermore, Cr3+ having less charge leads to the removal of some oxygen ions for the charge compensation. Since the ionic radius of Cr3+ (0.62 Å) is smaller than both Sn4+ (0.69 Å) and O2− (1.35–1.42 Å), a lattice contraction is reasonable on low Cr-doping in SnO2. Whereas heavily Cr-alloying (x ≥ 0.15) is governed by a different mechanism: The small lattice expansion may be due to the possibility of the interstitial accommodation of Cr ions (being sufficiently small to occupy the interstitial sites of SnO2 lattice). On the other hand, oxygen vacancies being created as a result of Cr incorporation lead to the structural disorder and may cause a higher coordination number. In this way, lattice expansion is a possible result of the high Cr-alloying. The SEM images of all the involved SnO2:Cr films showed a crackand pore-free appearance surface containing the nano-grains with a uniform distribution. The SEM images of Sn1-xCrxO2 films with x = 0.05 and 0.30, for instances, are presented in Fig. 3. The SEM micrographs confirmed the result obtained on the basis of broadening of the XRD peaks: decrease in grain size of the films on Cr-addition. The EDX spectra of the films, revealing the peaks belonging to Sn, O and Cr, confirmed existence of the Cr-alloyed tin oxide phase in the deposited films. 3.2. Magneto-optical and magneto-transport properties In order to examine the magnetic behavior expected in our DMS 3
Solid State Communications 298 (2019) 113641
M. Mousavi, et al.
Fig. 3. SEM images of the Sn1-xCrxO2 thin films with (a) x = 0.05 and (b) x = 0.30.
dominant mechanism, lack of adequate oxygen vacancies or free electrons result in the lattice expansion as well as the magnetism weakening. Therefore, the room temperature magnetism observed in the Cralloyed SnO2 films should be due to oxygen vacancies, since Cr is known to be paramagnetic at high temperatures and anti-ferromagnetic below 308 K. 4. Conclusion In this research, the effect of Cr-addition with different concentrations up to 50 at.% on the structural, magneto-optical and magnetotransport properties of the tin oxide thin films prepared by a spray pyrolysis technique has been investigated. The results revealed that Craddition in the tin oxide thin films strongly affects their structural and magnetization-related properties. The XRD patterns show that Cr-alloying does not affect the crystal structure of the SnO2 film and all the films are polycrystalline grown in a tetragonal rutile structure, but does affect their crystal growth as there are preferred planes for the Cr ions occupancy. The lattice volume of Sn1-xCrxO2 films was found to be minimum at a critical Cr concentration of x = 0.15, exactly where their magnetoresistance and roughly where their magneto-optical properties exhibit an extremum as well. This suggests a correlation between the structural and magnetic behaviors in the involved SnO2:Cr films and indicates to two different mechanisms for Cr addition: For low Cr concentrations (x < 0.15), the substitutional doping is the involving mechanism, while for more Cr concentrations, the interstitial one is dominant in the studied films. Resuming, the SnO2:Cr films exhibit room-temperature magnetism and the saturation magnetization reaches a maximum value at the Cr concentration of about 15%. This work has not only presented promising candidates for the spintronics applications, but has also contributed to a better understanding of this transparent magnetic semiconducting system.
Fig. 4. Saturated Faraday rotation of the Sn1-xCrxO2 thin films measured at 632 nm.
Declarations of interest None. References [1] A. Fakhim Lamrani, M. Belaiche, A. Benyoussef, El Kenz, Electronic structures and ferromagnetism of SnO2 (rutile) doped with double-impurities: first-principles calculations, J. Appl. Phys. 115 (2015) 013910 (6). [2] Y. Ziat, A. Benyoussef, A. El Kenz, Magnetic and electronic properties of Cr- and Mndoped SnO2: ab initio calculations, J. Phys. Chem. Solids 75 (2014) 701–709. [3] SkF. Ahmed, S. Khan, P.K. Ghosh, M.K. Mitra, K.K. Chattopadhyay, Effect of Al doping on the conductivity type inversion and electro-optical properties of SnO2 thin films synthesized by sol-gel technique, J. Sol. Gel Sci. Technol. 39 (2006) 241–247. [4] D. Burgard, C. Goebbert, R. Nass, Synthesis of nanocrystalline, redispersable antimony-doped SnO2 particles for the preparation of conductive, transparent coatings, J. Sol. Gel Sci. Technol. 13 (1998) 789–792.
Fig. 5. Magnetoresistance of the Sn1-xCrxO2 thin films under a magnetic field of 0.5 T at room temperature.
ordering: The oxygen vacancies contributing electrons assist the ferromagnetic interactions in close proximity of Cr ions [20]. While for more Cr concentration (x > 0.15), where the interstitial doping is the 4
Solid State Communications 298 (2019) 113641
M. Mousavi, et al.
[5] M.-M. Bagheri-Mohagheghi, M. Shokooh-Saremi, The influence of Al doping on the electrical, optical and structural properties of SnO2 transparent conducting films deposited by the spray pyrolysis technique, J. Phys. D Appl. Phys. 37 (8) (2004) 1248–1253. [6] A.S. Ahmed, S.M. Muhamed, M.L. Singla, S. Tabassum, A.H. Naqvi, A. Azam, Band gap narrowing and fluorescent properties of nickel doped SnO2 nanoparticles, J. Lumin. 131 (2011) 1–6. [7] S.-J. Liu, L.Y. Chen, C.Y. Liu, H.W. Fang, JaH. Hsieh, J.Y. Juang, Physical properties of polycrystalline Cr-doped SnO2 films grown on glasses using reactive dc magnetron co-sputtering technique, Appl. Sur. Sci. 257 (6) (2011) 2254–2258. [8] A. Kaminski, S. Das Sarma, Polaron percolation in diluted magnetic semiconductors, Phys. Rev. Lett. 88 (2002) 247202 (4). [9] J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Donor impurity band exchange in dilute ferromagnetic oxides, Nat. Mater. 4 (2005) 173–179. [10] C.B. Fitzgerald, M. Venkatesan, L.S. Dorneles, R. Gunning, P. Stamenov, J.M.D. Coey, P.A. Stampe, R.J. Kennedy, E.C. Moreira, U.S. Sias, Magnetism in dilute magnetic oxide thin films based on SnO2, Phys. Rev. B 74 (2006) 115307 (10). [11] S.B. Ogale, R.J. Choudhary, J.P. Buban, S.E. Lofland, S.R. Shinde, S.N. Kale, V.N. Kulkarni, J. Higgins, C. Lanci, J.R. Simpson, N.D. Browning, S. Das Sarma, H.D. Drew, R.L. Greene, T. Venkatesan, High temperature ferromagnetism with a giant magnetic moment in transparent Co-doped SnO2−δ, Phys. Rev. Lett. 91 (2003) 077205 (4). [12] Y. Zuo, S. Ge, Li Zhang, S. Yan, X. Zhou, Y. Xiao, Room temperature ferromagnetism of Sn1−xCrxO2 films fabricated by sol–gel method, J. Alloy. Compd. 475 (2009) 60–63. [13] L. Zhang, S. Ge, Y. Zuo, J. Wang, J. Qi, Ferromagnetic properties in undoped and Crdoped SnO2 nanowires, Scripta Mater. 63 (2010) 953–956. [14] M.M. Bagheri Mohagheghi, Sh Tabatabai Yazdi, M. Mousavi, Transport, structural and optical properties of SnO2 transparent semiconductor thin films alloyed with
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chromium: carrier type conversion, J. Mater. Sci. Mater. Electron. 28 (2017) 13328–13335. J. Rodriguez-Carvajal, Introduction to the Program FULLPROF: Refinement of Crystal and Magnetic Structures from Powder and Single Crystal Data, Léon Brillouin Lab. (CEA-CNRS), CEA/Saclay, 2015. S.L. Panahi, D. Dastan, N.B. Chaure, Characterization of zirconia nanoparticles grown by sol–gel method, Adv. Sci. Lett. 22 (4) (2016) 941–944. V. Kumar, S. Kumari, P. Kumar, M. Kar, L. Kumar, Structural analysis by rietveld method and its correlation with optical properties of nanocrystalline zinc oxide, Adv. Mater. Lett. 6 (2) (2015) 139–147. J.-H. Im, J. Chung, S.-J. Kim, N.-G. Par, Synthesis, structure, and photovoltaic property of a nanocrystalline 2H perovskite-type novel sensitizer (CH3CH2NH3) PbI3, Nanoscale. Res. Lett. 7 (7) (2012) 353. J. Joseph, V. Mathew, K.E. Abraham, Studies on Cu, Fe, and Mn doped SnO2 semiconducting transparent films prepared by a vapour deposition technique, Chin. J. Phys. 45 (1) (2007) 84–97. C. Van Komen, A. Thurber, K.M. Reddy, J. Hays, A. Punnoose, Structure–magnetic property relationship in transition metal (M=V,Cr,Mn,Fe,Co,Ni) doped SnO2 nanoparticles, J. Appl. Phys. 103 (2008) 07D141 (3). S.R. Woodford, A. Bringer, S. Blügel, Interpreting magnetization from Faraday rotation in birefringent, magnetic media, J. Appl. Phys. 101 (2007) 053912 (7). T.A. Dauzhenka, V.K. Ksenevich, I.A. Bashmakov, J. Galibert, Origin of negative magnetoresistance in polycrystalline SnO2 films, Phys. Rev. B 83 (2011) 165309 (9). N.V. Agrinskaya, V.I. Kozub, A.V. Shumilin, E. Sobko, Magnetoresistance in semiconductor structures with hopping conductivity: effects of random potential and generalization for the case of acceptor states, Phys. Rev. B 82 (2010) 075201 (8). M. Zenger, J. Moser, W. Wegscheider, D. Weiss, T. Dietl, High-field magnetoresistance of Fe∕GaAs∕Fe tunnel junctions, J. Appl. Phys. 96 (2004) 2400–2402.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Communication
Magneto-transport and magneto-optical properties of Cr-alloyed SnO2 thin films: A correlation between structural and magnetic behaviors
T
M. Mousavia, Sh. Tabatabai Yazdib,∗, M.M. Bagheri Mohagheghic a
Department of Physics, University of Bojnord, Bojnord, Iran Department of Physics, Payame Noor University (PNU), 19395-3697 Tehran, Iran c School of Physics, Damghan University, Damghan, Iran b
A R T I C LE I N FO
A B S T R A C T
Communicated by E.Y. Andrei
In this work, the tin oxide thin films alloyed with Cr up to 50 at.% were prepared via a spray pyrolysis method on the glass substrates, and their structural, magneto-optical and magneto-transport properties were studied. The results show that all the films are polycrystalline grown in a tetragonal rutile structure. A critical Cr concentration of about 15% was found concerning the lattice volume variations in SnO2:Cr films, as well as their magnetoresistance and magneto-optical properties suggesting a correlation between the structural and magnetic behaviors and indicating to two different mechanisms for Cr addition. For low Cr concentrations (less than 15%), the substitutional doping is the involving mechanism, while for more Cr concentrations, the interstitial one is predominant in the involved films.
Keywords: Tin oxide Magnetoresistance Faraday effect Spray pyrolysis Thin film
1. Introduction SnO2 is an oxide semiconductor widely studied in its thin film form as an important transparent conducting oxide (TCO) in view of its technologically considerable properties: It has a wide direct band gap of about 3.6 eV resulting in high optical transparency for visible light, a metal-like conductivity, easy doping, nontoxicity, thermal stability, mechanical hardness, low cost and chemical stability in the basic and acidic solutions. SnO2 crystallizes in tetragonal symmetry with P42/ mnm space group (rutile structure) with lattice parameters of a = 4.7373 Å and c = 3.1864 Å [1,2]. SnO2 film has been doped with a variety of ions to improve its physical and chemical properties needed for several applications (e.g. Refs. [3–5]). Many results have shown that several dopants can lead to an increase in the surface area by reducing the grain size and crystallinity [6]. In addition, recently there have been numerous reports on ferromagnetism above room temperature in the thin films of wide bandgap semiconductors doped with a small amount of transition metal (TM) ions in some cases as low as 1–2% (e.g. Ref. [7]). High temperature ferromagnetism was completely unexpected in these diluted magnetic semiconductors (DMS) since considering the super-exchange or double-exchange interactions and Curie temperature scaling theoretically as x1/2 in the dilute ferromagnetic materials [8,9], the magnetic doping level (x) lies far below the percolation threshold needed for a long-range order [10]. However, SnO2 is an attractive host ∗
material for DMS and following the report on high temperature ferromagnetism in Co-doped SnO2 films [11], numerous studies have been carried out on the SnO2 films doped with each of the transition metal ions for use in the spintronics functions. However, because of the high sensitivity of the material on the preparation method and deposition conditions, there is often little agreement between the results obtained from various studies by different groups. Zuo et al. reported on the ferromagnetic properties of SnO2:Cr films deposited onto Si (111) substrates by a sol-gel method due to the anti-ferromagnetic super-exchange interactions among the nearest neighbor Cr ions along with a decrease in the magnetic moment per Cr with the doping level [12]. Zhang et al. prepared SnO2:Cr films by a chemical vapor deposition and suggested ferromagnetism is due to oxygen vacancies as well as Cr doping [13]. Considering all the aspects of the reported studies, in this work we have undertaken another comprehensive investigation on SnO2:Cr films over a wide range of composition (with Cr concentration up to 50 at%) and studied the magneto-transport and magneto-optical properties of these DMS SnO2:Cr films in order to investigate magnetism at room temperature. (Their electrical and optical characteristics have been reported previously [14]). The studied Cr-alloyed SnO2 thin films were prepared by a typical spray pyrolysis technique.
Corresponding author. E-mail address: [email protected] (S. Tabatabai Yazdi).
https://doi.org/10.1016/j.ssc.2019.05.012 Received 24 May 2018; Received in revised form 10 January 2019; Accepted 19 May 2019 Available online 22 May 2019 0038-1098/ © 2019 Elsevier Ltd. All rights reserved.
Solid State Communications 298 (2019) 113641
M. Mousavi, et al.
2. Experimental methods
such as the lattice constants and the shape parameters were taken as the free parameters during the refinement process. In the first step, the global parameters such as the background and scale factors were refined, and then the structural parameters such as the lattice constants, profile shape and preferred orientation ones were refined in sequence. The background was fitted with a sixth order polynomial and the peak shapes were described by pseudo-voigt profiles. Fig. 1b presents the XRD data of the x = 0 film, for instance, along with its Rietveld refined profile. As seen, the observed and calculated profiles match to each other and all the experimental peaks are the allowed Bragg ones. The observed diffraction peaks corresponding to reflection planes (110), (101), (200), (211), (220), (310) and (301) provide a clear evidence for the formation of a single phase tetragonal rutile structure with P42/ mnm space group for the deposited films. The fitting quality of the experimental data is assessed by the “goodness of fit” parameter, χ2, and various reliability factors (R-factors) such as Rp (the profile factor), Rwp (weighted profile factor), Rexp (the expected weighted profile factor), RB (Bragg factor) and RF (crystallographic factor) [15]. The computed conventional agreement factors of Rietveld analysis are presented in Table 1. The obtained Rexp and Rwp values are found to be slightly large. This is due to the lower ratio of the diffraction peak intensity to the background in the nanocrystalline samples. Similar high values of R-factors have been reported for nanocrystalline materials by others, as well (e.g Refs. [17,18]). However, the low values obtained for χ2 justify the goodness of refinement. Cr-addition does not alter the crystal structure of the SnO2 film, but does affect the crystal growth (see Fig. 1a): In the pristine SnO2 film, the crystallites are grown preferably along (200) and (110) directions, while by Cr-addition, there exists a competition between the intensities of these peaks (others are not much affected). On increasing the Cr concentration, first (x < 0.20), (200) orientation becomes predominant and then on more Cr-alloying, the crystallites are oriented preferably along (110) direction. This means that there are preferred planes for Cr ions occupancy. The observed decrease in the intensity of the diffraction peaks is mainly due to the substitution of Cr ions driving Sn ions into the interstitial sites [19] and hence resulting in a less ordered state. Another effect of Cr-alloying is revealed as broadening of the peaks, i.e. a decrease in the crystallite size (except for the film of x = 0.50), i.e. Cr-addition inhibits the grain growth; Similar observation has been reported for the Cr-doped SnO2 nanoparticles with x up to 0.20, as well [12]. The mean crystallite (grain) size of the films calculated based on (110) reflection via Scherrer's formula lie in the range of 35–62 nm (Table 1). Cr-alloying results also in a slight shift in the predominant peaks' positions. The shift in XRD peaks’ position reflects a change in the unit cell volume of the films. The refined crystallographic parameters including the rutile lattice parameters of the SnO2:Cr films are summarized in Table 1. The evolution of lattice parameters a and c as well as the unit cell volume with Cr content in the films is shown in Fig. 2. As
The unalloyed and Cr-alloyed Sn1-xCrxO2 films with x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.40 and 0.50 were deposited by a spray pyrolysis technique as discussed in details in our previous paper [14]. The precursor solution was prepared using the appropriate amounts of stannic chloride (SnCl4:5H2O), chromium nitride (Cr(NO3)3:9H2O) and ethanol (CH3CH2OH) in distilled water. To enhance the solubility of the mixture, a few drops of hydrochloric acid (HCl) was added and stirred. The raw materials used were of purity of better than 99% (from MERCK). The deposited films were characterized for structural, compositional, morphological, magnetotransport and magneto-optical properties. The structural characterization of the films were carried out by the X-ray diffraction (XRD) patterns recorded using a D8 Advance Bruker system with Cu-Kα radiation (λ = 1.5406 Å) in the 2θ range of 20–70°. The XRD profiles were analyzed employing Rietveld refinement method, through the Fullprof software [15]. The average crystallite size 0.9 λ of the films was estimated using Scherrer's formula D = β cos θ where λ = 1.5406 Å, the X-ray wavelength, θ is the diffraction angle of a main reflection and β is the full width at half maximum (FWHM) of that peak [16]. The morphological and elemental analysis of surface of the films were performed via scanning electron microscopy (SEM) by a LEO 1450 VP system equipped with an energy dispersive X-rays spectrometer (EDX). To study the magnetotransport properties of the SnO2:Cr films, two electrodes were coated on the two ends of the samples by resistance thermal evaporation of aluminum in vacuum using an Edwards E306 coating system. The magnetoresistance (MR = (RH – RH=0)/RH=0, where R0 and RH are the resistances before and after applying a magnetic field, respectively) of the unalloyed and Cr-alloyed films was measured at room temperature under a magnetic field of 0.5 T. The magneto-optical Faraday effect in the deposited films was studied using a setup consisting of a polarized He–Ne laser light (λ = 632 nm) propagating along the film under a magnetic field of 0.8 T (being sufficient for the saturation magnetization to be achieved). The incident beam was parallel to the field direction. The magnitude of rotation of the polarization plane was detected with the accuracy of 0.003° by an oscilloscope. 3. Results and discussion 3.1. Structural and morphological analysis The XRD patterns of the Sn1-xCrxO2 films with x = 0–0.50 are shown in Fig. 1a. As seen, at room temperature all the films are polycrystalline and the crystal structure of the SnO2 film has not been changed by introducing Cr ions and increasing its content even up to 50 at%. The patterns were analyzed by Rietveld refinement using Fullprof program. To do so, the ions occupancies were fixed while the other parameters
Fig. 1. (a) XRD patterns of Sn1-xCrxO2 thin films with different Cr concentrations from 0 to 50 at% taken at room temperature by Cu-Kα radiation. (b) Observed XRD pattern (the dots) and Reitveld-based calculated profile (the solid lines) of the pristine film, along with their difference curve (observed minus calculated) at the bottom of the panel. The tick marks below the profile indicate the position of the allowed Bragg reflections.
2
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Table 1 Crystallite size (D) and Rietveld refined crystallographic parameters of the Sn1-xCrxO2 thin films with different Cr concentrations from 0 to 50 at%, along with their thickness (t), resistivity (ρ), carrier type and its concentration (n/p). sample (x)
D (nm)
a (Å)
c (Å)
V (Å3)
RWP (%)
Rexp (%)
χ2
t (nm)
ρ (Ω cm)
n/p (cm−3)
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.40 0.50
54 48 – 45 46 45 35 – 62
4.725 4.718 – 4.694 4.709 4.724 4.730 – 4.747
3.229 3.233 – 3.221 3.219 3.202 3.211 – 3.223
72.089 71.965 – 70.970 71.380 71.517 71.839 – 72.627
15.3 13.8 – 20.1 15.4 17.7 12.6 – 14.3
13.26 12.29 – 13.84 11.51 12.42 10.39 – 11.60
1.33 1.26 – 2.11 1.79 2.03 1.47 – 1.52
382 397 381 370 366 375 372 380 371
1.31 × 10−2 1.29 × 10−2 1.50 × 10−1 3.10 × 10−2 1.20 3.81 × 10−1 4.70 × 10−1 6.11 × 10−1 60
n = 9.1 × 1019 p = 3.7 × 1019 p = 4.4 × 1018 p = 9.2 × 1019 n = 5.3 × 1017 n = 6.7 × 1018 n = 1.6 × 1018 n = 1.1 × 1019 n = 1.1 × 1015
films, their Faraday effect and magnetoresistance have been investigated. The optical properties of the involved films have been previously studied [14]. The high transmittance of the films in visible region makes the magneto-optical Faraday effect suitable for evaluating their magnetic properties [21]. Fig. 4 shows the saturated Faraday rotation of the SnO2:Cr transparent thin films under a 0.8 T field measured at 632 nm at room temperature. It exhibits an increasing trend on Cr-addition up to x ≈ 0.05–0.10 (the Faraday rotation for the films of 0.05 ≤ x ≤ 0.15 was found to be roughly the same, considering the measurements error) followed by a significant reduction for the samples with 0.15 < x < 0.3. For more Cr concentrations, the angle of Faraday rotation increases. The initial enhancement in the magnetic properties of the films on Cr-doping up to x ≈ 0.10 reflects the increased localized magnetic moments, i.e. the enhanced magnetization. While, the subsequent reduction above x = 0.15 is associated to weakening of the magnetic ordering on Cr-alloying due to a decrease in the carrier concentration. (The results of the electrical measurements of the involved films, reported elsewhere [14], are summarized in Table 1.) The presence of the maximum saturation magnetization in a critical Cr concentration has been reported by others for the Sn1-xCrxO2 nanoparticles, however at a different concentration of x = 0.025, approximately where the lattice volume of the samples reveals a minimum value [20]. Fig. 5 shows magnetoresistance (MR = (RH – RH=0)/RH=0) of the SnO2:Cr films measured at room temperature under a magnetic field of 0.5 T. At first glance, one can see that the MR is negative for all the films. Dauzhenka et al. have studied the MR of the SnO2 films at low temperatures and proposed the weak localization and electron-electron interactions as the origin of the observed negative values [22]. As the negative MR is believed to be suppressed with the temperature decrease in both three- and two-dimensional semiconducting materials [23], the negative MR values observed for the involved SnO2:Cr films at room temperature can be due to the interaction between the weakly localized carriers, as well. However, another possible cause for the observed negative MR concerns the suppression of the spin-flip scattering by the applied magnetic field [24]. As seen in Fig. 5, the MR magnitude increases first with Cr-addition reaching a maximum at x ≈ 0.15 and then decreases. The presence of the critical Cr concentration of x = 0.15 being roughly observed in the Faraday rotation results as well, as discussed is due to the increase in the saturation magnetization of the films with x up to 0.15 followed by its reduction caused by destroying the magnetic moments' ordering. It is most notable that the critical Cr concentration for the magnetic properties corresponding to the maximum saturation magnetization (xM ≈ 0.10–0.15) coincides approximately with the Cr concentration obtained from the structural results corresponding to the minimum unit cell volume (xL = 0.15). This may suggest a direct correlation of structural-magnetic properties. As mentioned in the structural analysis section, for low Cr-doping (x < 0.15) the substitutional doping is the involving mechanism accompanying oxygen vacancies. This explains the observed lattice contraction as well as the enhanced magnetic
Fig. 2. Variation of the lattice parameters a and c as well as the unit cell volume with the Cr concentration in the Sn1-xCrxO2 thin films. In this and the following figures, the lines connecting the data points are only guides for the eye.
seen by Cr-addition, a and V decrease gradually reaching a minimum at x ≈ 0.15 and then increase; c shows less pronounced variations. This behavior has been also observed in the Sn1-xCrxO2 nanoparticles, but with a much lower critical Cr content (xL ≈ 2.5%) [20]. This observation implies that two different mechanisms should work for Cr addition. As discussed previously in Ref. [14], the substitutional doping can be assumed as the dominant mechanism for low Cr-doping (x < 0.15): Sn4+ ions are substituted by smaller ions of Cr3+. Furthermore, Cr3+ having less charge leads to the removal of some oxygen ions for the charge compensation. Since the ionic radius of Cr3+ (0.62 Å) is smaller than both Sn4+ (0.69 Å) and O2− (1.35–1.42 Å), a lattice contraction is reasonable on low Cr-doping in SnO2. Whereas heavily Cr-alloying (x ≥ 0.15) is governed by a different mechanism: The small lattice expansion may be due to the possibility of the interstitial accommodation of Cr ions (being sufficiently small to occupy the interstitial sites of SnO2 lattice). On the other hand, oxygen vacancies being created as a result of Cr incorporation lead to the structural disorder and may cause a higher coordination number. In this way, lattice expansion is a possible result of the high Cr-alloying. The SEM images of all the involved SnO2:Cr films showed a crackand pore-free appearance surface containing the nano-grains with a uniform distribution. The SEM images of Sn1-xCrxO2 films with x = 0.05 and 0.30, for instances, are presented in Fig. 3. The SEM micrographs confirmed the result obtained on the basis of broadening of the XRD peaks: decrease in grain size of the films on Cr-addition. The EDX spectra of the films, revealing the peaks belonging to Sn, O and Cr, confirmed existence of the Cr-alloyed tin oxide phase in the deposited films. 3.2. Magneto-optical and magneto-transport properties In order to examine the magnetic behavior expected in our DMS 3
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Fig. 3. SEM images of the Sn1-xCrxO2 thin films with (a) x = 0.05 and (b) x = 0.30.
dominant mechanism, lack of adequate oxygen vacancies or free electrons result in the lattice expansion as well as the magnetism weakening. Therefore, the room temperature magnetism observed in the Cralloyed SnO2 films should be due to oxygen vacancies, since Cr is known to be paramagnetic at high temperatures and anti-ferromagnetic below 308 K. 4. Conclusion In this research, the effect of Cr-addition with different concentrations up to 50 at.% on the structural, magneto-optical and magnetotransport properties of the tin oxide thin films prepared by a spray pyrolysis technique has been investigated. The results revealed that Craddition in the tin oxide thin films strongly affects their structural and magnetization-related properties. The XRD patterns show that Cr-alloying does not affect the crystal structure of the SnO2 film and all the films are polycrystalline grown in a tetragonal rutile structure, but does affect their crystal growth as there are preferred planes for the Cr ions occupancy. The lattice volume of Sn1-xCrxO2 films was found to be minimum at a critical Cr concentration of x = 0.15, exactly where their magnetoresistance and roughly where their magneto-optical properties exhibit an extremum as well. This suggests a correlation between the structural and magnetic behaviors in the involved SnO2:Cr films and indicates to two different mechanisms for Cr addition: For low Cr concentrations (x < 0.15), the substitutional doping is the involving mechanism, while for more Cr concentrations, the interstitial one is dominant in the studied films. Resuming, the SnO2:Cr films exhibit room-temperature magnetism and the saturation magnetization reaches a maximum value at the Cr concentration of about 15%. This work has not only presented promising candidates for the spintronics applications, but has also contributed to a better understanding of this transparent magnetic semiconducting system.
Fig. 4. Saturated Faraday rotation of the Sn1-xCrxO2 thin films measured at 632 nm.
Declarations of interest None. References [1] A. Fakhim Lamrani, M. Belaiche, A. Benyoussef, El Kenz, Electronic structures and ferromagnetism of SnO2 (rutile) doped with double-impurities: first-principles calculations, J. Appl. Phys. 115 (2015) 013910 (6). [2] Y. Ziat, A. Benyoussef, A. El Kenz, Magnetic and electronic properties of Cr- and Mndoped SnO2: ab initio calculations, J. Phys. Chem. Solids 75 (2014) 701–709. [3] SkF. Ahmed, S. Khan, P.K. Ghosh, M.K. Mitra, K.K. Chattopadhyay, Effect of Al doping on the conductivity type inversion and electro-optical properties of SnO2 thin films synthesized by sol-gel technique, J. Sol. Gel Sci. Technol. 39 (2006) 241–247. [4] D. Burgard, C. Goebbert, R. Nass, Synthesis of nanocrystalline, redispersable antimony-doped SnO2 particles for the preparation of conductive, transparent coatings, J. Sol. Gel Sci. Technol. 13 (1998) 789–792.
Fig. 5. Magnetoresistance of the Sn1-xCrxO2 thin films under a magnetic field of 0.5 T at room temperature.
ordering: The oxygen vacancies contributing electrons assist the ferromagnetic interactions in close proximity of Cr ions [20]. While for more Cr concentration (x > 0.15), where the interstitial doping is the 4
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