Currently, two-dimensional graphene-like crystals based on group IV elements are considered as one of the most promising materials for the development of next-generation devices. Materials such as silicene and germanene attract the attention of researchers because of their unique properties arising from similarity to graphene. Direct compatibility with existing silicon technology makes the synthesis of these materials an actual problem. Unlike graphene, the direct exfoliation method is not suitable for producing silicene and germanene, so the molecular beam epitaxy method is currently considered as the main method for fabricating these materials [1]. Numerous experimental works on the growth of two-dimensional layers of group IV elements have been already carried out; the main positive results were obtained on metal substrates [2–5]. Silicene was grown on an Ag(111) substrate, and germanene was grown on Au(111) and Pt(111) substrates [6]. However, growth on non-conducting substrates is more suitable for practical use in electronics. There are theoretical premises that silicene is well compatible with a CaF2 substrate [7]. This is also supported by experiments on the formation of CaSi2 films on CaF2/Si(111) substrates [8–10] because CaSi2 is essentially a material consisting of silicene layers intercalated with calcium. Free-standing silicene was obtained in some works using special chemical treatment of CaSi2. In [11], multilayer silicene oxide and multisilicene structures were prepared by removing Ca atoms from CaSi2 crystals in concentrated hydrochloric acid. To fabricate separate silicene and silicene oxide layers, a sonication process followed by centrifugation was applied. In the same work, Raman spectra were also measured for free-standing silicene with different degrees of oxidation. It was found that the main Raman peak for unoxidized silicene is located at \( \sim {\kern 1pt} 495\) cm–1 [11]. In [12], bisilicene layers embedded between CaF2 layers were fabricated by treating CaSi2 crystals in a BF4-based ionic liquid. These results indicate that CaF2 and CaSi2 materials can be used as a basis for the production of silicene. We recently published results on the molecular beam epitaxy growth of extended two-dimensional silicon islands on CaF2/Si(111) substrates [13]. The possibility of producing areas of two-dimensional Si layers on CaF2 was demonstrated. It was shown that the growth of silicon layers occurs from steps where the probability of forming bonds between silicon and calcium is high. Raman data show a peak at 418 cm–1 corresponding to vibrations in the two-dimensional plane of silicon atoms intercalated with calcium that indicates the initial growth of CaSi\(_{2}\) layers rather than the formation of free-standing silicene. For silicene growth, the surface of the substrate should have a hexagonal lattice. Such a lattice is obtained, e.g., by depositing a silver m-onolayer on Si(111) [14]. The surface of the substrate is modified, turning into Si(111)\(\sqrt 3 \times \sqrt 3 {\kern 1pt} R\)30°–Ag, which ensures the growth of silicene during silicon deposition. In this work, modified CaF2/Si(111) substrates with hexagonal surface packing resulting from electron irradiation are proposed for silicene growth. Silicon was deposited on such substrates and the formation of silicene islands was confirmed by atomic force microscopy (AFM) and Raman scattering m-ethods.

The developed method is directly related to the synthesis of CaSi2 films using electron irradiation during the molecular beam epitaxy growth of CaF2 layers on Si(111) substrates [8, 9]. The method is based on the phenomenon of radiolysis, the electron-stimulated decomposition of CaF2 into Ca and F [15] in the surface layers of the film. Fluorine is desorbed from the surface, and the remaining calcium atoms are chemically bonded to silicon atoms coming from the Si substrate at sufficiently high temperatures (>300°С) under electron irradiation [10]. Calcium silicide, formed under an electron beam during the deposition of CaF2, is a three-dimensional inhomogeneous material in the form of a triangular network of elongated crystallites protruding from the surface of the CaF2 film by tens of nanometers [9], which is not suitable for the growth of silicene. According to [16–18], one of the reasons for the formation of a nonplanar CaSi2 film is the lack of silicon atoms. At a sufficient amount of silicon, a film grows via the formation of two-dimensional CaSi2 islands lying in the Si(111) growth plane. To improve the film planarity, we used two approaches: the deposition of additional Si during the CaF2 growth with simultaneous electron irradiation and post-growth electron irradiation [13, 19]. The latter should also lead to the growth of a more uniform and planar film since it solves problems with residual CaF2 inclusions, decomposing them into Ca and F and allowing already formed two-dimensional CaSi2 islands to expand in the plane.

The experiments were carried out on a Katun’-100 molecular beam epitaxy facility equipped with a CaF2 effusion source with a graphite crucible under ultrahigh vacuum conditions. The films were grown on Si(111) substrates. Two samples were grown: a test sample to control the state of the surface before top silicon layer deposition and the second sample on which the same growth stages were repeated and then an additional top silicon layer was deposited. Before growth, all samples were subjected to a standard double surface cleaning procedure [20]. An electron gun, which is a part of a high-energy electron diffraction unit used to monitor the state of the surface during molecular beam epitaxy, served as an electron source. Electron beam irradiation was carried out at an accelerating voltage of 20 keV and a current density of 50 A/cm2. The beam incidence angle was 4°. The same electron beam was used to modify the properties of the growing film. The CaF2 epitaxial film was grown at a deposition rate of 0.3 Å/s. Our previous studies have shown that a strip with a characteristic metallic luster (1–2 mm wide) is formed on the surface of the growing film during CaF2 deposition under the electron beam. This strip represents various CaSi2 polymorphs depending on the conditions of CaF2 deposition [8–10]. In this work, solid-phase epitaxy followed by annealing was used at the initial stage of growth to obtain a planar CaSi2 film. At a temperature of 200°С, a 2-nm-thick CaF2 layer was deposited, after which annealing was carried out at a temperature of 600°С until crystalline reflections appeared in the high-energy electron diffraction pattern. Consequently, a thin calcium-enriched layer can be formed under the electron beam. Such a surface is more suitable for the formation of two-dimensional silicon-based structures since it has a higher surface energy compared to CaF2 [21] and partially solves the problem of the unfavorable ratio of the surface energies of silicon and CaF2 [22]. As our experiments showed [16], when silicon is deposited on this surface under the electron beam, two-dimensional CaSi2 islands are formed in the (111) plane.

At the next stage, eight paired CaF2 and Si layers with a thickness of 2 and 0.6 nm (two bilayers), respectively, were deposited sequentially. Each time after silicon deposition, growth was stopped for 2 min, which provided additional irradiation of the surface with electrons in the region under the electron beam. After the deposition of the last silicon layer, the irradiation time was increased to 15 min. In this case, the electron beam did not shift; i.e., the same surface area was irradiated throughout the entire growth. This was done in accordance with our previous results showing that post-growth electron irradiation for ~15 min leads to an increase in the number and size of two-dimensional CaSi2 islands [19]. All growth procedures (except the initial one) and electron irradiation were carried out at a temperature of 550°С. This growth temperature was chosen according to [23], where the optimal temperature range for obtaining a planar surface of CaF2/Si(111) films with a thickness of about 10 nm was found.

After 15 min of irradiation, the test sample was taken out of the vacuum chamber, and AFM and Raman studies of the area modified by the electron beam were carried out. The AFM study confirmed the formation of two-dimensional hexagonal islands on the surface, which is consistent with the previous results [19]. Thus, the test sample, more precisely its electron beam modified area, can be considered as a template for growing silicene. Raman examination of the test sample demonstrated a standard set of three Raman peaks, corresponding to CaSi2 (3R polymorph [8]).

After the test experiment, the second sample was grown, on which all procedures for fabricating a template surface were implemented. At the last step, after 15 min of irradiation, the electron beam was turned off, and ten silicon monolayers were deposited on the surface. Immediately after growth, Raman and AFM studies were carried out on the area previously modified by the electron beam. The experimental methods are described in the supplementary material.

The AFM study of the sample after silicon deposition (see Fig. 1) showed that the surface generally retains a hexagonal structure. New smaller islands appear on the surface of the two-dimensional CaSi2 islands already formed at the previous stage (see scheme in Fig. 2). The AFM profile of a typical hexagonal island (see Fig. 3) shows the steps with a height that is a multiple of \( \approx {\kern 1pt} 0.3\) nm. According to [24], the height of the step between layers in multisilicene should be \( \approx {\kern 1pt} 0.3\) nm, and the obtained value is a strong reason for the conclusion about the formation of multisilicene islands. In Fig. 4, new formed islands with a typical lateral size of \( \approx {\kern 1pt} 25\) nm and height of \( \approx {\kern 1pt} 0.3\) nm are visible on the surface of a separate hexagonal island. The supplementary material provides a 3D image of this area (see Fig. S1). In general, the resulting surface relief is similar to that observed in [25], where multisilicene islands were fabricated on an Ag(111) substrate using the molecular beam epitaxy method. The comparison with the results of [24, 25] suggests that the growth of multisilicene at the initial stage (one deposited silicon monolayer) occurs via the formation of extended terraces, on the surface of which, with further deposition of silicon, smaller terraces are formed, etc. Most likely, Fig. 4 shows islands of the last upper multisilicene layer.

Fig. 1.
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(Color online) 1.5 × 1.5-µm atomic force microscopy image of the surface relief of the sample with silicon deposited after electron irradiation.

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Fig. 2.
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(Color online) Schematic image of the sample with silicon deposited after electron irradiation.

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Fig. 3.
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(Color online) Atomic force microscopy (a) image and (b) profile of the surface of the sample with silicon deposited after electron irradiation.

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Fig. 4.
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(Color online) (а) Atomic force microscopy image of the surface area with the two-dimensional hexagonal island after deposition of ten silicon monolayers. The selected relief height range is indicated on the scale to the right of the image. The height range was chosen so that silicene islands on the surface were visible. (b) Atomic force microscopy profile of the resulting silicene island.

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Raman spectra of samples with a deposited Si layer (see Fig. 5) include a shoulder near \( \approx {\kern 1pt} 495\) cm–1, which is absent in the spectra of the test samples (see Fig. S2 in the supplementary material). When subtracting the spectrum of the substrate, a Raman peak corresponding to free-standing silicene appears in this region. The remaining peaks, in particular the three peaks at 341, 386, and 413 cm–1, typical of CaSi2 [8] (3R polymorph) are also observed for test samples, and the peak at 443 cm–1 corresponds to the CaF2/Si(111) heterointerface [13]. It should be noted that the Raman signal for silicene grown on silver-modified Si(111)\(\sqrt 3 \times \sqrt 3 {\kern 1pt} R\)30°–Ag substrates is observed at \( \approx {\kern 1pt} 524\) cm–1, which is caused by the effect of the substrate [14]. In our case, the effect of the substrate on the upper silicene layers can be neglected because silicene grows on silicene since CaSi2 is a material composed of silicene layers intercalated with calcium [26]. The effect of calcium atoms on the first silicene layer cannot possibly be neglected, and the atoms of this layer contribute to the Raman peak at 413 cm–1, corresponding to two-dimensional vibrations of silicon atoms intercalated with calcium [27], while higher layers of silicene contribute to the signal at 495 cm–1.

Fig. 5.
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(Color online) Raman spectra (1) from the sample with silicon deposited after electron irradiation, (2) from the Si(111) substrate, and (3) their difference taken to highlight the Raman peak associated with silicene.

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To summarize, we have demonstrated that a CaSi2 surface with a hexagonal packing formed under the electron beam can be used as a natural template for the subsequent growth of silicene. The fabricated structures with silicene islands grown on CaF2/Si(111) substrates modified by electron irradiation are the first step towards the fabrication of two-dimensional graphene-like structures using substrates of this type. The results obtained can be used in the future to develop methods for obtaining not only silicene, but also other transgraphenes based on group IV elements.