Homogeneous crystalline FeSi2 films of c (4 × 8) phase grown on Si (111) by reactive deposition epitaxy
© Zou et al.; licensee Springer. 2013
Received: 11 November 2013
Accepted: 26 November 2013
Published: 5 December 2013
The growth of iron silicides on Si (111) using reactive deposition epitaxy method was studied by scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). Instead of the mixture of different silicide phases, a homogeneous crystalline film of c (4 × 8) phase was formed on the Si (111) surface at approximately 750°C. Scanning tunneling spectra show that the film exhibits a semiconducting character with a band gap of approximately 0.85 eV. Compared with elemental Fe, the Fe 2p peaks of the film exhibit a lower spin-orbit splitting (−0.3 eV) and the Fe 2p3/2 level has a smaller full-width at half maximum (−0.6 eV) and a higher binding energy (+0.3 eV). Quantitative XPS analysis shows that the c (4 × 8) phase is in the FeSi2 stoichiometry regime. The c (4 × 8) pattern could result from the ordered arrangement of defects of Fe vacancies in the buried Fe layers.
Iron silicides grown on silicon surfaces have attracted much attention in the last decade because of their possible applications in different technological areas [1–4]. The equilibrium Fe-Si phase diagram shows that there exist four stable bulk compounds: Fe3Si crystallizing in cubic D 03 structure, simple cubic ϵ-FeSi, tetragonal α-FeSi2, and orthorhombic β-FeSi2.These iron silicides exhibit metallic, semiconductor, or insulating behavior depending on their structures. For example, Fe3Si is ferromagnetic and is a promising candidate as spin injectors in future spintronic devices such as magnetic tunnel junctions . β-FeSi2 is semiconducting with a direct band gap of approximately 0.85 eV, which fits into the window of maximum transmission of optical fibers and is expected to be a suitable material for optoelectronic devices such as light detectors or near-infrared sources [2, 7]. In addition to the above stable compounds, metastable iron silicide phases which do not exist in the bulk phase diagram can also be grown and stabilized on silicon surfaces by epitaxy due to the enhanced surface energy of thin films. It has been reported that very thin films of metastable γ-FeSi2 phase with a cubic CaF2 structure [8, 9], FeSi1+x (0 ≤ x ≤ 1) phase with a defect CsCl structure [10, 11] and a new silicide phase with a c (4 × 8) surface periodicity [2, 12, 13] can be grown on Si (111) substrate by solid-phase epitaxy (SPE), which was realized by depositing iron on the silicon substrate at room temperature and then annealing the film at an elevated temperature.
Despite the interesting properties and potential applications, it is challenging to control the silicide reaction at the Fe/Si interface and grow a flat and single-phase thin film of iron silicide with the demanded structure. Due to the variety of existing compounds and the complexity of growth kinetics, the iron silicide thin films usually grow into a mixture of different phases with heterogeneous morphology [2, 5, 13]. Different from the silicide reaction in SPE, which is realized under iron-rich condition, reactive deposition epitaxy (RDE) (deposition of iron on the silicon substrate heated to a determined temperature) most probably involves diffusion of monomers on the surface, which may lead to the formation of unusual silicide structures. It has been reported that RDE favors the production of Si-rich phases and single crystalline epitaxial structures [14, 15]. In this paper, we performed a scanning tunneling microscope (STM) study on the reactive epitaxial growth of iron silicides on Si (111)-(7 × 7) surface at different temperatures. We found that a thicker homogeneous and crystalline c (4 × 8) iron silicide thin film can be formed on the Si (111) surface with an extremely low deposition rate. The thickness of the film can be up to approximately 6.3 Å, which is significantly larger than that obtained previously by RDE method. This film could be used in the optoelectronic devices or serve as a precursor surface applicable in magnetic technological fields.
Iron silicide thin films were grown on Si (111) substrates by using an ultrahigh vacuum (UHV) molecular beam epitaxy-STM system (Multiprobe XP, Omicron, Taunusstein, Germany) with a base pressure of less than 5.0 × 10−11 mbar. P-doped, n-type Si (111) substrates with resistivity of approximately 1 Ω cm were cleaned in UHV by the well-established annealing and flashing procedures . Iron was deposited on the clean substrates by heating iron lumps (purity 99.998%) in a Mo crucible with electron bombardment. The iron flux was monitored by an internal ion collector mounted near the evaporation source. During deposition, the substrates were heated by direct current and the temperatures were measured using an infrared pyrometer. The deposition rate was controlled from approximately 0.01 to 0.07 ML min−1 (1 ML = 1 iron atom per 1 × 1 surface mesh = 7.8 × 1014 atoms cm−2) . An electrochemically etched tungsten tip was used for scanning. All STM images were recorded at room temperature with a bias voltage (Vs) of approximately 2.0 V and a tunneling current (I) of 0.1 to 0.25 nA.
X-ray photoelectron spectroscopy (XPS) spectra were acquired with a Kratos Axis Ultra DLD spectrometer using a monochromatic Al Kα source (1,486.6 eV). A detailed description of the experimental apparatus and the measurement conditions can be found in . The XPS peak areas and peak decompositions (i.e., curve fitting) were determined using software XPSPEAK 4.1 . Prior to fitting, Shirley background was subtracted and then peaks were fitted with mixed Lorentzian-Gaussian functions. The spectra were deconvoluted into components consisting of spin-orbit split Voigt functions [the intensity of the (Fe, Si) 2p1/2 is half that of the (Fe, Si) 2p3/2, and the full-width at half maximum (FWHM) is the same for both the splitting peaks]. The smallest number of components, with which a good fitting can be achieved for the experimental data, was adopted for the chemical state analysis.
Results and discussion
Previous studies showed that several metastable silicides [1 × 1, 2 × 2, and c (4 × 8) phases] that do not exist in the bulk phase diagram can be grown epitaxially on the Si (111) substrate under the strain from the substrate. The 1 × 1 phase can be assigned to the FeSi with a CsCl structure, while the 2 × 2 phase can be assigned to the γ-FeSi2 with a CaF2 structure and the FeSi1 + x (0 ≤ x ≤1) with a defect CsCl structure . The FeSi1 + x (0 ≤ x ≤1) can be derived from the CsCl structure by introducing Fe vacancies distributed in a random fashion. The heights observed for the type A islands prove that the 2 × 2 phase is FeSi1 + x (0 ≤ x ≤1) because the corresponding crystal structure has a spacing of 1.57 Å between equivalent atomic planes. If the 2 × 2 phase is γ-FeSi2 in the CaF2 structure, the heights in multiples of 3.14 Å should be observed [8, 10]. Furthermore, the tunneling current–voltage (I-V) curve measured on top of the type A islands (Figure 2c) exhibits a semiconducting character with a band gap of approximately 0.9 eV, verifying that the 2 × 2 phase is not γ-FeSi2 because γ-FeSi2 is metallic [5, 9]. The c (4 × 8) pattern could result from the formation of periodic defects of vacancies and/or Si substitution on the Fe sites in the buried Fe layers. These defects modify the local density of states above the Si atoms of the topmost layer, resulting in the different brightness of the protrusions [2, 13]. Similar to the 2 × 2 phase, the I-V curve measured on top of the c (4 × 8) structure also shows a semiconducting character with a band gap of approximately 0.85 eV, as shown in Figure 2d.
In summary, using RDE method, we have shown that a homogeneous crystalline iron silicide thin film of c (4 × 8) phase can be grown on the Si (111) surface at a temperature above approximately 750°C. The thickness of the c (4 × 8) film can be up to approximately 6.3 Å. This result is quite different from the previous results obtained using the SPE method, where the c (4 × 8) film has a definite thickness in the range of 1.4 to 1.9 Å. We attribute the larger thickness of the c (4 × 8) film obtained by the RDE method to the supply of sufficient free Si atoms during the silicide reaction. Scanning tunneling spectroscopy measurements show that the c (4 × 8) thin film exhibits a semiconducting character with a band gap of approximately 0.85 eV. Quantitative XPS analysis shows that the c (4 × 8) phase is in the FeSi2 stoichiometry regime. This homogeneous c (4 × 8) thin film could be used in the optoelectronic devices or serve as a precursor surface applicable in magnetic technological fields.
This work was supported by the National Natural Science Foundation of China under grant no. 61176017 and the Innovation Program of Shanghai Municipal Education Commission under grant no. 12ZZ025.
- Walter S, Bandorf R, Weiss W, Heinz K, Starke U, Strass M, Bockstedte M, Pankratov O: Chemical termination of the CsCl-structure FeSi/Si (111) film surface and its multilayer relaxation. Phys Rev B 2003, 67: 085413.View ArticleGoogle Scholar
- Krause M, Blobner F, Hammer L, Heinz K, Starke U: Homogeneous surface iron silicide formation on Si (111): The c (8 × 4) phase. Phys Rev B 2003, 68: 125306.View ArticleGoogle Scholar
- Garreau G, Hajjar S, Gewinner G, Pirri C: High resolution scanning tunneling spectroscopy of ultrathin iron silicide grown on Si (111): origin of the c (4 × 8) long range order. Phys Rev B 2005, 71: 193308.View ArticleGoogle Scholar
- Kataoka K, Hattori K, Miyatake Y, Daimon H: Iron silicides grown by solid phase epitaxy on a Si (111) surface: schematic phase diagram. Phys Rev B 2006, 74: 155406.View ArticleGoogle Scholar
- Wawro A, Suto S, Czajka R, Kasuya A: Thermal reaction of iron with a Si (111) vicinal surface: surface ordering and growth of CsCl-type iron silicide. Phys Rev B 2003, 67: 195401.View ArticleGoogle Scholar
- Dahal N, Chikan V: Phase-controlled synthesis of iron silicide (Fe3Si and FeSi2) nanoparticles in solution. Chem Mater 2010, 22: 2892. 10.1021/cm100224bView ArticleGoogle Scholar
- González JC, Miquita DR, da Silva MIN, Magalhães-Paniago R, Moreira MVB, de Oliveira AG: Phase formation in iron silicide nanodots grown by reactive deposition epitaxy on Si (111). Phys Rev B 2010, 81: 113403.View ArticleGoogle Scholar
- Weiß W, Kutschera M, Starke U, Mozaffari M, Reshöft K, Köhler U, Heinz K: Development of structural phases of iron silicide films on Si(111) studied by LEED, AES and STM. AES and STM. Surf Sci 1997, 377: 861.View ArticleGoogle Scholar
- Wallart X, Nys JP, Tételin C: Growth of ultrathin iron silicide films: observation of the γ-FeSi2phase by electron spectroscopies. Phys Rev B 1994, 49: 5714. 10.1103/PhysRevB.49.5714View ArticleGoogle Scholar
- Raunau W, Niehus H, Schilling T, Comsa G: Scanning tunneling microscopy and spectroscopy of iron silicide epitaxially grown on Si (111). Surf Sci 1993, 286: 203. 10.1016/0039-6028(93)90406-AView ArticleGoogle Scholar
- von Känel H, Mäder KA, Müller E, Onda N, Sirringhaus H: Structural and electronic properties of metastable epitaxial FeSi1+xfilms on Si (111). Phys Rev B 1992, 45: 13807. 10.1103/PhysRevB.45.13807View ArticleGoogle Scholar
- Sugimoto Y, Abe M, Konoshita S, Morita S: Direct observation of the vacancy site of the iron silicide c (4 × 8) phase using frequency modulation atomic force microscopy. Nanotechnology 2007, 18: 084012. 10.1088/0957-4484/18/8/084012View ArticleGoogle Scholar
- Hajjar S, Garreau G, Pelletier S, Bolmont D: Pirri C: p (1 × 1) to c (4 × 8) periodicity change in ultrathin iron silicide on Si (111). Phys Rev B 2003, 68: 033302.View ArticleGoogle Scholar
- He Z, Stevens M, Smith DJ, Bennett PA: Epitaxial titanium silicide islands and nanowires. Surf Sci 2003, 524: 148. 10.1016/S0039-6028(02)02506-2View ArticleGoogle Scholar
- Bennett PA, Ashcroft B, He Z, Tromp RM: Growth dynamics of titanium silicide nanowires observed with low-energy electron microscopy. J Vac Sci Technol B 2002, 20: 2500. 10.1116/1.1525006View ArticleGoogle Scholar
- Zou ZQ, Li WC, Liu XY, Shi GM: Self-assembled growth of MnSi~1.7nanowires with a single orientation and a large aspect ratio on Si (110) surfaces. Nanoscale Res Lett 2013, 8: 45. 10.1186/1556-276X-8-45View ArticleGoogle Scholar
- Zou ZQ, Shi GM, Sun LM, Liu XY: Manganese nanoclusters and MnSi 1.7nanowires formed on Si (110): a comparative X-ray photoelectron spectroscopy study. J Appl Phys 2013, 113: 024305. 10.1063/1.4774098View ArticleGoogle Scholar
- Gaowei M, Muller EM, Rumaiz AK, Weiland C, Cockayne E, Jordan-Sweet J, Smedley J, Woicik JC: Annealing dependence of diamond-metal Schottky barrier heights probed by hard X-ray photoelectron spectroscopy. Appl Phys Lett 2012, 100: 201606. 10.1063/1.4718028View ArticleGoogle Scholar
- Michel EG: Epitaxial iron silicides: geometry, electronic structure and applications. Appl Surf Sci 1997, 117/118: 294.View ArticleGoogle Scholar
- Ohtsu N, Oku M, Nomura A, Sugawara T, Shishido T, Wagatsuma K: X-ray photoelectron spectroscopic studies on initial oxidation of iron and manganese mono-silicides. Appl Surf Sci 2008, 254: 3288. 10.1016/j.apsusc.2007.11.005View ArticleGoogle Scholar
- Egert B, Panzner G: Bonding state of silicon segregated to α-iron surfaces and on iron silicide surfaces studied by electron spectroscopy. Phys Rev B 1984, 2091: 29.Google Scholar
- Rührnschopf K, Borgmann D, Wedler G: Growth of Fe on Si (100) at room temperature and formation of iron silicide. Thin Solid Films 1996, 280: 171. 10.1016/0040-6090(95)08248-4View ArticleGoogle Scholar
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