Effects of Sn doping on the morphology and properties of Fe-doped In2O3 epitaxial films
© Zhou et al.; licensee Springer. 2012
Received: 1 November 2012
Accepted: 19 November 2012
Published: 30 November 2012
(Sn, Fe)-codoped In2O3 epitaxial films were deposited on (111)-oriented Y-stabilized ZrO2 substrates by pulsed laser deposition with constant Fe concentration and different Sn concentrations. The influence of Sn concentration on the crystal structure and properties of Fe-doped In2O3 ferromagnetic semiconductor films has been investigated systematically. Experimental results indicate that Sn doping can effectively reduce the surface roughness and suppresses breakup of the films into separated islands. At the same time, the optical band gap increases and the electrical properties improve correspondingly. However, although the carrier density increases dramatically with the Sn doping, no obvious change of the ferromagnetism is observed. This is explained by a modified bounded magnetic polaron model.
KeywordsMagnetic semiconductor Indium oxide Morphology Epitaxial film
In the past two decades, diluted magnetic semiconductors (DMSs) have attracted considerable interests due to their novel physical properties and potential applications in spin-based devices. Many material systems of DMSs, such as ZnO, TiO2, SnO2, In2O3, GaAs, and also GeMn[2–5], have been widely studied. Among various kinds of oxide DMSs, transition metal-doped In2O3 has attracted great attention because of its excellent optical and electric properties, and its room-temperature ferromagnetism has been observed in Fe-, Co-, Ni-, and Cr-doped In2O3[6–9]. Among these elements, Fe doping is particularly interesting and has attracted lots of attention because of the high solubility (as high as 20%) of Fe ions into In2O3 lattice and the high magnetic moment of the Fe3+ ion, which makes Fe-doped In2O3 a fascinating DMS. Many research works have been conducted on Fe-doped In2O3 films, and high-temperature ferromagnetism was reported by several groups[10–12]. Spin-polarized carriers were also revealed in this material by the anomalous Hall effect (AHE). These results indicate that Fe-doped In2O3 may be a promising ferromagnetic semiconductor for future spintronic devices. However, for most device applications, smooth surfaces, high crystalline quality, and controllable optical and electrical properties are necessary. Although epitaxial Fe-doped In2O3 films with room-temperature ferromagnetism and AHE have been grown and studied extensively by now, most works were focused on studying their physical properties and very little effort has been directed toward the growth of high-quality Fe-doped In2O3 thin films with controlled surface morphology. In our previous work, a very rough surface with square-shaped columnar structures was observed in Fe-doped In2O3 epitaxial films grown on Y-stabilized ZrO2 (YSZ) (100) substrates. Similar rough island-like morphology has been observed in undoped In2O3 epitaxial films grown on YSZ (100) substrates, which is attributed to the thermodynamically preferred island (Volmer-Weber) growth mode. As we know, In2O3 is not only used as the host of DMSs, but also is the basis of the most important transparent and conductive materials in industry application. Sn-doped In2O3, so-called indium tin oxide (ITO), is widely applied in optoelectronic devices due to its high optical transparency in the visible range and high electric conductivity. Many works focused on the growth mechanism of ITO films have been reported, which showed that the surface morphology and properties of In2O3 films can be effectively improved by Sn doping[15–17]. Similarly, it should be practicable to improve the corresponding properties of Fe-doped In2O3 ferromagnetic semiconductors by doping appropriate Sn in the same way as ITO. Moreover, the carrier density of Fe-doped In2O3 films will increase with Sn doping accordingly[18, 19]. It is generally accepted that the magnetic coupling in DMS is closely related to the carrier concentration, so the Sn doping is desired to modulate the magnetic property of Fe-doped In2O3 films also. In this paper, (Sn, Fe)-codoped In2O3 epitaxial films were deposited on YSZ substrates. The effects of Sn doping on the surface morphology and the optical, electrical, and magnetic properties of Fe-doped In2O3 ferromagnetic films were studied systematically. An apparent improvement of surface morphology was observed as the Sn concentration increased. At the same time, a corresponding increase of optical band gap and carrier density was found. However, no obvious relation between the carrier density and the ferromagnetism of the films was observed, which was explained by a modified bounded magnetic polaron (BMP) model.
Growth of (Sn, Fe)-codoped In2O3 epitaxial films
Since the lattice parameter of YSZ (cubic structure, lattice parameter 2aYSZ = 10.26 Å) is similar to that of In2O3 (cubic bixbyite structure, aIn2O3 = 10.118 Å) with the lattice mismatch smaller than 1.6%, epitaxial growth of Fe-doped In2O3 films on YSZ substrates is expected. In this letter, a (111)-oriented single-crystal YSZ substrate was chosen due to the fact that the (111) surface of In2O3 has the lowest energy amongst the low-index surfaces, which is beneficial to the epitaxial growth. (Sn, Fe)-codoped In2O3 thin films were deposited by pulsed laser deposition (PLD) at a substrate temperature of 600°C. The stoichiometric targets were prepared from high-purity (99.99%) In2O3, Fe2O3, and SnO2 powders. For all the targets, the atom ratio Fe/(In + Fe + Sn) is fixed at 5%, and Sn/(In + Fe + Sn) is 0%, 1%, 3%, and 5%, respectively. The powders were mixed in a mechanical ball mill for 5 h, pressed into a 4-cm-diameter pellet, and then sintered at 1,350°C for 10 h in the air. The targets were ablated using a KrF excimer laser (COMPexPro 201, Coherent Inc., Santa Clara, CA, USA) with a pulse repetition rate of 1 Hz and an energy of 400 mJ/pulse for 5,000 pulses, which produces a film with a thickness of about 100 nm. During deposition, the pressure in the PLD chamber was maintained at high vacuum (about 4.0 × 10−5 Pa). After the deposition, the samples were cooled down naturally with the system at the same pressure.
Characterization of the (Sn, Fe)-codoped epitaxial films
The crystal structure of the (Sn, Fe)-codoped films was analyzed by X-ray diffraction (XRD; XD-3, PG Instruments Ltd., Beijing, China) and high-resolution X-ray diffraction (HRXRD; D8-Discover, Bruker Corp., Karlsruhe, Germany) with Cu Kα radiation (λ = 0.15406 nm). The surface morphology was characterized by atomic force microscopy (AFM; Solver P47 PRO, NT-MDT Co., Moscow, Russia) under contact mode. The optical transmittance was measured using an UV-visible dual-beam spectrophotometer (TU-1900, PG Instruments, Ltd., Beijing, China). The transport properties of the films were determined by Hall effect measurement in the van der Pauw four-point configuration using a SQUID magnetometer (MPMS XL-7, Quantum Design, San Diego, CA, USA). The magnetic measurements were performed using an alternating gradient magnetometer (MicroMag 2900, Princeton Measurements Corp., Princeton, NJ, USA) at room temperature.
Results and discussion
Crystal structure and surface morphology
Electronic transport properties
To explain the ferromagnetism in In1−xMn x As, Ga1−xMn x As, and Ge1−xMn x DMSs, Kaminski and Das Sarma proposed that the spontaneous magnetization arose from a percolation of BMPs. Recently, this model was also used to explain the magnetism in oxide DMSs, such as Cu- and Ag-doped ZnO films[26, 27]. In this model, the carriers localized around the oxygen defect strongly couple with the doped magnetic ions and form a BMP sphere. The distance between BMP spheres is determined by the concentration of oxygen defects. When adjacent BMP spheres are sufficiently close to each other, the spin-polarized variable-range hopping between nearby BMP spheres will happen, thus leading to a magnetic coupling between the two BMP spheres. Ferromagnetism phase will form when this sort of BMP coupling percolates throughout the entire film. Within the certain range of defect concentration, the magnetic coupling of BMP spheres increases as the distance between BMP spheres decreases, which is caused by the increase in defect concentration. It was reported that the BMP theory cannot describe the electric property accurately and the concentration of bound polarized carriers derived from the theory is much smaller than the result of the experimental Hall effect measurement. A modified BMP model has been suggested by Chou et al. to interpret and explain both the electric and magnetic properties of the oxide DMS[28, 29]. According to the modified BMP model, only carriers in localized states contribute to the magnetic coupling, while other carriers in the conduction band have no discernable effect on ferromagnetism of the samples. In our present case, although Sn doping increases carrier density significantly, the concentration of oxygen defect as the center of the BMP sphere in the films does not change a lot. In addition, the change in the radius of the BMP sphere as a result of the increase in carrier density is so small that it does not vary the number of magnetic irons which are included in the BMP sphere. Consequently, the ferromagnetism of the Fe-doped In2O3 films does not strikingly change with the increase in carrier density by Sn doping.
Epitaxial (Sn, Fe)-codoped In2O3 films with different Sn concentrations were deposited on YSZ (111) substrates by PLD. The crystal structure and surface morphology of Fe-doped In2O3 films show significant improvement by Sn doping, which is important for future spintronic device application. At the same time, the optical and electric transport properties show disciplinary changes with the Sn concentration. However, contrary to the widely accepted carrier-induced mechanism in oxide DMSs, no significant relation between the ferromagnetism of the films and the carrier density by Sn doping is observed. This result is well consistent with the modified BMP model which suggests that the magnetic coupling in oxide DMSs is mediated by the localized carriers, not the conductive carriers.
This work is supported by the State Key Research Development Program of China (2010CB833103), the National Natural Science Foundation of China (60976073, 11274201), and the Foundation for Outstanding Young Scientist in Shandong Province (BS2010CL036).
- Wolf SA, Awschalom DD, Buhrman RA, Daughton JM, von Molnár S, Roukes ML, Chtchelkanova AY, Treger DM: Spintronics: a spin-based electronics vision for the future. Science 2001, 294: 1488–1495. 10.1126/science.1065389View ArticleGoogle Scholar
- Li WW, Yu WL, Jiang YJ, Jing CB, Zhu JY, Zhu M, Hu ZG, Tang XD, Chu JH: Structure‚ optical‚ and room-temperature ferromagnetic properties of pure and transition-metal-(Cr‚ Mn‚ and Ni)-doped ZnO nanocrystalline films grown by the sol–gel method. J Phys Chem C 2010, 114: 11951. 10.1021/jp103183vView ArticleGoogle Scholar
- Prellier W, Fouchet A, Mercey B: Oxide-diluted magnetic semiconductors: a review of the experimental status. J Phys Condens Matter 2003, 15: R1583. 10.1088/0953-8984/15/37/R01View ArticleGoogle Scholar
- Wang Y, Xiu F, Wang Y, Zou J, Beyermann WP, Zhou Y, Wang KL: Coherent magnetic semiconductor nanodot arrays. Nanoscale Res Lett 2011, 6: 134.View ArticleGoogle Scholar
- Wang Y, Liao Z, Xu H, Xiu F, Kou X, Wang Y, Wang KL, Drennan J, Zou J: Structural evolution of GeMn/Ge superlattices grown by molecular beam epitaxy under different growth conditions. Nanoscale Res Lett 2011, 6: 624. 10.1186/1556-276X-6-624View ArticleGoogle Scholar
- Xing PF, Chen YX, Yan SS, Liu GL, Mei LM, Wang K, Han XD, Zhang Z: High temperature ferromagnetism and perpendicular magnetic anisotropy in Fe-doped In2O3 films. Appl Phys Lett 2008, 92: 022513. 10.1063/1.2834369View ArticleGoogle Scholar
- Hakimi AMHR, Schoofs F, Bali R, Stelmashenko NA, Blamire MG: Origin of magnetism in cobalt-doped indium tin oxide thin films. Phys Rev B 2010, 82: 144429.View ArticleGoogle Scholar
- Peleckis G, Wang XL, Dou SX: T High temperature ferromagnetism in Ni-doped In2O3 and indium-tin oxide. Appl Phys Lett 2006, 89: 022501. 10.1063/1.2220529View ArticleGoogle Scholar
- Xing GZ, Yi JB, Wang DD, Liao L, Yu T, Shen ZX, Huan CHA, Sum TC, Ding J, Wu T: Strong correlation between ferromagnetism and oxygen deficiency in Cr-doped In2O3-δ nanostructures. Phys Rev B 2009, 79: 174406.View ArticleGoogle Scholar
- Xu XH, Jiang FX, Zhang J, Fan XC, Wu HS, Gehring GA: Magnetic and transport properties of n-type Fe-doped In2O3 ferromagnetic thin films. Appl Phys Lett 2009, 94: 212510. 10.1063/1.3147190View ArticleGoogle Scholar
- Jiang FX, Xu XH, Zhang J, Fan XC, Wu HS, Alshammari M, Feng Q, Blythe HJ, Score DS, Addison K, Qahtani MA, Gehring GA: Room temperature ferromagnetism in metallic and insulating (In1-xFex)2O3 thin films. J Appl Phys 2011, 109: 053907. 10.1063/1.3559298View ArticleGoogle Scholar
- Kim H, Osofsky M, Miller MM, Qadri SB, Auyeung RCY, Piqué A: Room temperature ferromagnetism in transparent Fe-doped In2O3 films. Appl Phys Lett 2012, 100: 032404. 10.1063/1.3678038View ArticleGoogle Scholar
- Xing PF, Chen YX, Tang MJ, Yan SS, Liu GL, Mei LM, Jiao J: Room-temperature anisotropic ferromagnetism in Fe-doped In2O3 heteroepitaxial films. Chin Phys Lett 2009, 26: 117503. 10.1088/0256-307X/26/11/117503View ArticleGoogle Scholar
- Bourlange A, Payne DJ, Palgrave RG, Foord JS, Egdell RG, Jacobs RMJ, Schertel A, Hutchison JL, Dobson PJ: Investigation of the growth of In2O3 on Y-stabilized ZrO2(100) by oxygen plasma assisted molecular beam epitaxy. Thin Solid Films 2009, 517: 4286–4294. 10.1016/j.tsf.2008.11.134View ArticleGoogle Scholar
- Ohta H, Orita M, Hirano M, Tanji H, Kawazoe H, Hosono H: Highly electrically conductive indium–tin–oxide thin films epitaxially grown on yttria-stabilized zirconia (100) by pulsed-laser deposition. Appl Phys Lett 2000, 76: 2740–2742. 10.1063/1.126461View ArticleGoogle Scholar
- Ohta H, Orita M, Hirano M, Hosono H: Surface morphology and crystal quality of low resistive indium tin oxide grown on yittria-stabilized zirconia. J Appl Phys 2002, 91: 3547–3550. 10.1063/1.1448873View ArticleGoogle Scholar
- Dekkers JM, Rijnders G, Blank DHA: Role of Sn doping in In2O3 thin films on polymer substrates by pulsed-laser deposition at room temperature. Appl Phys Lett 2006, 88: 151908. 10.1063/1.2195096View ArticleGoogle Scholar
- Kim H, Gilmore CM, Piqué A, Horwitz JS, Mattoussi H, Murata H, Kafafi ZH, Chrisey DB: Electrical, optical, and structural properties of indium–tin–oxide thin films for organic light-emitting devices. J Appl Phys 1999, 86: 6451–6461. 10.1063/1.371708View ArticleGoogle Scholar
- Bourlange A, Payne DJ, Palgrave RG, Zhang HL, Foord JS, Egdell RG, Jacobs RMJ, Veal TD, King PDC, McConville CF: The influence of Sn doping on the growth of In2O3 on Y-stabilized ZrO2(100) by oxygen plasma assisted molecular beam epitaxy. J Appl Phys 2009, 106: 013703. 10.1063/1.3153966View ArticleGoogle Scholar
- Hamberg I, Granqvist CG, Berggren KF, Sernelius BE, Engström L: Band-gap widening in heavily Sn-doped In2O3. Phys Rev B 1984, 30: 3240–3249. 10.1103/PhysRevB.30.3240View ArticleGoogle Scholar
- Gao J, Chen R, Li DH, Jiang L, Ye JC, Ma XC, Chen XD, Xiong QH, Sun HD, Wu T: UV light emitting transparent conducting tin-doped indium oxide (ITO) nanowires. Nanotechnology 2011, 22: 195706. 10.1088/0957-4484/22/19/195706View ArticleGoogle Scholar
- Gupta RK, Ghosh K, Kahol PK: Room temperature ferromagnetic multilayer thin film based on indium oxide and iron oxide for transparent spintronic applications. Mater Lett 2010, 64: 2022–2024. 10.1016/j.matlet.2010.06.026View ArticleGoogle Scholar
- Burstein E: Anomalous optical absorption limit in InSb. Phys Rev 1954, 93: 632–633. 10.1103/PhysRev.93.632View ArticleGoogle Scholar
- Moss TS: The interpretation of the properties of indium antimonide. Proc Phys Soc London B 1954, 67: 775–782. 10.1088/0370-1301/67/10/306View ArticleGoogle Scholar
- Kaminski A: Das Sarma S: Polaron percolation in diluted magnetic semiconductors. Phys Rev Lett 2002, 88: 247202.View ArticleGoogle Scholar
- He M, Tian YF, Springer D, Putra IA, Xing GZ, Chia EEM, Cheong SA, Wu T: Polaronic transport and magnetism in Ag-doped ZnO. Appl Phys Lett 2011, 99: 222511. 10.1063/1.3665401View ArticleGoogle Scholar
- Tian YF, Li YF, He M, Putra IA, Peng HY, Yao B, Cheong SA, Wu T: Bound magnetic polarons and p-d exchange interaction in ferromagnetic insulating Cu-doped ZnO. Appl Phys Lett 2011, 98: 162503. 10.1063/1.3579544View ArticleGoogle Scholar
- Chou H, Lin CP, Huang JCA, Hsu HS: Magnetic coupling and electric conduction in oxide diluted magnetic semiconductors. Phys Rev B 2008, 77: 245210.View ArticleGoogle Scholar
- Chou H, Lin CP, Hsu HS, Sun SJ: The role of carriers in spin current and magnetic coupling for ZnO:Co diluted magnetic oxides. Appl Phys Lett 2010, 96: 092503. 10.1063/1.3309588View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.