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.
KeywordsIron silicides Reactive deposition epitaxy Thin films Scanning tunneling spectroscopy X-ray photoelectron spectroscopy
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.
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