The growth of a single-phase epitaxial Ca5Si3 film of 40 nm by MBE on a Si(111) substrate at 500 degrees C was performed for the first time. Structural features of the Ca5Si3 film were investigated by HRTEM and GIXRD. Calculations of the absorption coefficient of Ca5Si3 films from optical reflectance and transmittance spectra revealed noticeable transparency (up to 63 %) in the mid-IR region and free-carrier absorption in the far-IR region, that was confirmed by metallic conductivity. The phonon structure of Ca5Si3 with nine Raman peaks (101, 110, 125, 162, 192, 220, 241, 342 and 379 cm(-1)) was identified for the first time. The obtained experimental results are also supplemented by data of ab initio calculations, which point out Ca5Si3 to be a promising material for the transparent contact in photonic devices.

In this paper we present the results of ab initio calculations on the ground state of the CrSi bulk and experimental studies on the structure and morphology of thin and ultrathin CrSi films grown on Si(111) substrates. Even though CrSi is expected to crystallize in the simple cubic B20 structure (space group #198, P213), we show that the ground state of CrSi is monoclinic (space group #4, P21). According to the calculated band structures monoclinic CrSi is a gapless semiconductor with holes as the main charge carriers and antiferromagnetic ordering, while cubic CrSi is a half-metal with ferromagnetic ordering. Ultra-thin CrSi films (3.19-4.31 nm) grown by the solid-phase epitaxy are indeed characterized by the monoclinic structure. For thin CrSi films (32-83 nm) fabricated by the molecular beam epitaxy, the coexistence of grains in the monoclinic and cubic phases is revealed in the films whereas the volume of these grains depends on the silicon to chromium flow ratio during the growth.

By means of ab initio techniques with the hybrid functional we show the existence of a new phase of FeSi with the monoclinic symmetry (space group P2(1)) originated from the B20 cubic structure (space group P2(1)3) due to slight orthorhombic distortion, which is turned out to be the ground state. The monoclinic FeSi not only displays the minimum in the total energy, but it is also characterized by a phonon spectrum without imaginary frequencies and by conducting properties (contrary to semiconducting properties of cubic FeSi) with antiferromagnetic ordering and the magnetic moment of 2.3 mu(B) for each Fe atom. These findings are supported by data of X-ray diffraction and high-resolution transmission electron microscopy of ultrathin FeSi films (similar to 3nm in thickness) grown on Si(1 1 1) by solid-phase epitaxy indicating the monoclinic symmetry to fit better the film structure as compared to the cubic symmetry, as well as by resistivity versus temperature measurements within a wide temperature range (2-300 K) pointing out bad metal properties. The analysis of field and temperature dependences of the magnetic moment of ultrathin FeSi films shows the presence of a ferromagnetic-antiferromagnetic two-phase state. We also discuss how our findings of the new phase of FeSi can interpret its known experimental data on electronic, transport and optical properties without involving the metal-insulator transition and Kondo-like effects.

Using solid-phase and molecular-beam epitaxy methods at 350°C, polycrystalline and epitaxial films of iron monosilicide (FeSi) with a thickness of 3.2 to 20.35 nm were grown on a Si(111) substrate, which was confirmed by X-ray diffraction data. Morphological studies have shown that the films are continuous and smooth with a root-mean-square roughness of 0.4–1.1 nm when grown by solid-phase epitaxy, and in the case of molecular beam epitaxy, they have an increased roughness and consist of coalesced grains with sizes up to 1 μm and a puncture density up to 1 × 107 cm–2. In solid-phase epitaxy, an increase in thickness leads to incomplete silicide formation and the appearance of a layer of disordered iron monosilicide with a thickness of 10 to 20 nm. This is confirmed by a change in the temperature dependence of resistivity ρ from semiconductor to semi-metallic and a decrease in resistivity by one and a half to two times. The nonmonotonic nature of the temperature dependence of the resistivity ρ ultrathin FeSi film with a thickness of 3.2 nm has been established, in which a maximum at 230–240 K, a region of growth from 160 to 65 K with Eg = 14.8 meV and further growth without saturation to a temperature of 1.5 K are observed. With increasing thickness of FeSi films grown by molecular-beam epitaxy, the minimum and maximum are not observed, but the tendency of nonmonotonic growth of ρ(T) with decreasing temperature and the opening of the band gap Eg = 23 meV remains. The probable reasons for the occurrence of effects in the dependences ρ(T) are considered. In ultrathin and thin FeSi films grown by solid-phase and molecular-beam epitaxy, respectively, an anomalous Hall effect was found, which was confirmed by the weak ferromagnetic properties of the films. The results obtained proved the possibility of growing and controlling the properties of ultrathin and thin FeSi films on silicon obtained by solid-phase and molecular-beam epitaxy, which ensured the appearance of their unique transport and magnetic properties that are absent in single crystals.

The paper compares the formation conditions, crystal structure, conductivity, and thermoelectric properties of thin CrSi and CoSi films on silicon substrates with (111) and (100) orientations. It is found that, despite the insufficiently good conductivity, the films have a noticeable Seebeck coefficient (up to 70 μV/K) in the temperature range of 200-450 K: positive for CrSi and negative for CoSi. The maximum power factor is estimated at 2 to 3 mW/(m2×K). For CoSi, first-principles calculations of the phonon structure and lattice thermal conductivity are performed, including in the form of nanowires. Estimates are made for the achievable dimensionless thermoelectric figure of merit of CoSi thin films: ZT=0.40–0.50 T=200-450 K.

The formation of thin films of indium antimonide on Si(111) from a stoichiometric mixture with a thickness of 32–48 nm was performed by solid-phase epitaxy (SPE) at a temperature of 320–380°C under ultrahigh vacuum conditions. It is shown that the use of an array of high-density InSb seed islands makes it possible to form a large-block epitaxial InSb film, while a solid-phase epitaxy from a mixture deposited on a clean surface produces a granular polycrystalline film. Based on the analysis of low energy electron diffraction patterns, X-ray diffraction data and Raman spectra, the stresses in the resulting films were determined: in the out of plane direction the films are weakly compressed by 0.1–0.14

In this paper, we report on optimizing the conditions for subsequently growing single-phase films of calcium monosilicide (CaSi) and calcium disilicide (CaSi2) on single -crys-tal silicon by reactive deposition epitaxy (RDE) and molecular beam epitaxy (MBE). The tem-perature range for the growth of CaSi films (400-500 degrees C) was determined, as well as the tem-perature range (600-680 degrees C) for the growth of CaSi2 films on silicon with three orientations: (111), (100) and (110). The minimum temperatures for the epitaxial growth of CaSi films by the RDE method and CaSi2films by the MBE method were determined, amounting to, respec-tively, T = 475 degrees C and T = 640 degrees C. An increase in the ratio of Ca to Si deposition rates to 26 made it possible to grow a large-block CaSi2 epitaxial film with the hR6 structure by the MBE method at T = 680 degrees C. Raman spectra and reflection spectra from single-phase epitaxial CaSi and CaSi2 films on silicon were recorded and identified for the first time. The correspondence between the experimental reflection spectra and the theoretically calculated reflection spectra in terms of amplitude and peak positions at photon energies of 0.1-6.5 eV has been established. Single-phase CaSi and CaSi2 films retain transparency in the photon energy range 0.4-1.2 eV.

The solid-state formation of gallium antimonide on Si(111) from a stoichiometric mixture of GaSb in the temperature range of 300 °C–500 °C and thicknesses of 12–40 nm was studied under ultrahigh vacuum conditions. The influence of the preformed GaSb seed islands on the morphology, composition, and structure was studied. It has been found that at 300 °C a strained continuous polycrystalline film is formed, which rupture at 350 °C. It has been shown that a continuous single-crystal GaSb film grows at 400 °C–500 °C if the sample is annealed at a weak antimony flow. This is also facilitated by the preliminary formation of a high density of nanosize GaSb seed islands. As a result, a continuous relaxed film with epitaxial relations GaSb(111)∣∣Si(111) and GaSb[1–10]∣∣Si[1–10] was obtained from a GaSb mixture 40 nm thick at 500 °C. We demonstrate a possibility of direct formation of GaSb on Si(111) without buffer layers of other chemical elements.

In this study, the appearance of magnetic moments and ferromagnetism in nanostructures of non-magnetic materials based on silicon and transition metals (such as iron) was considered experimentally and theoretically. An analysis of the related literature shows that for a monolayer iron coating on a vicinal silicon surface with (111) orientation after solid-phase annealing at 450–550 °C, self-ordered two-dimensional islands of α-FeSi2 displaying superparamagnetic properties are formed. We studied the transition to ferromagnetic properties in a system of α-FeSi2 nanorods (NRs) in the temperature range of 2–300 K with an increase in the iron coverage to 5.22 monolayers. The structure of the NRs was verified along with distortions in their lattice parameters due to heteroepitaxial growth. The formation of single-domain grains in α-FeSi2 NRs with a cross-section of 6.6 × 30 nm2 was confirmed by low-temperature and field studies and FORC (first-order magnetization reversal curves) diagrams. A mechanism for maintaining ferromagnetic properties is proposed. Ab initio calculations in freestanding α-FeSi2 nanowires revealed the formation of magnetic moments for some surface Fe atoms only at specific facets. The difference in the averaged magnetic moments between theory and experiments can confirm the presence of possible contributions from defects on the surface of the NRs and in the bulk of the α-FeSi2 NR crystal lattice. The formed α-FeSi2 NRs with ferromagnetic properties up to 300 K are crucial for spintronic device development within planar silicon technology.

We simulate the conjugation of crystal lattices of two-dimensional Mg2Si layers with atomically clean Si(001)2×1 and Si(110)"16×2" surfaces. Thick films are grown by molecular-beam epitaxy (MBE) through the formation of Ca2Si seed layers. For a Si(001) substrate, three different silicides are formed in a 140-nm thick film at a temperature of T = 300°C at a Ca-to-Si deposition rate ratio of 4.7: Ca2Si, CaSi, and hR3-CaSi2 with comparable contributions. With a decrease in the MBE-growth temperature to 250°C and a Ca-to-Si deposition-rate ratio of 8.4, a polycrystalline Ca2Si film is formed on Si(110) with the minimal contribution from CaSi. Polycrystalline and epitaxial Ca2Si films with thicknesses from 22 to 114 nm are grown on Si(111) substrates by the sacrificial-template method and MBE growth at a temperature of 250°C and different ratios of Ca and Si deposition rates (4.0–20.0). The minimum deposition-rate ratio ensures single-phase growth with the Ca2Si(100)/Si(111) epitaxial ratio; as it increases from 7.3, polycrystalline films grow with three orientations on Si(111): Ca2Si(100), Ca2Si(110), and Ca2Si(111). In Ca2Si films, regardless of their structure, the fundamental transition is masked by the Urbach edge in the photon-energy range of 0.78–1.0 eV, and then the second direct interband transition is identified (E2d = 1.095 ± 0.1 eV).

The processes involved in the Mg2Si film growth on the Si(111) surface at different temperatures in UHV were studied in a single experiment with a sample whose surface had the wedge-shaped temperature distribution within the range of 360-510 degrees C. The Mg2Si film was grown by a pulsed deposition of Mg at the rate of up to -104 nm/s. The Raman spectroscopy data show that Mg2Si formed only on the colder sample area, but for T > 484 degrees C it was not observed. The mean residence time of a Mg atom on Si(111) at 480 degrees C is estimated as - 10-6 s. The paradigm of experiments with non-uniformly heated samples is briefly considered. While studying various temperature-dependent processes, the necessary number of experiments can be radically reduced by creating temperature distribution on the sample surface.

Single-phase films of semiconductor and semimetallic calcium silicides (Ca2Si, CaSi, and CaSi2), as well as films with a significant contribution of Ca5Si3 and Ca14Si19 silicides, were grown on single-crystal silicon and sapphire substrates. The analysis of the crystal structure of the grown films was carried out and the criterion of their matching with silicon and sapphire substrates was determined. Some lattice-matching models were proposed, and the subsequent deformations of the silicide lattices were estimated. Film’s optical functions, including the optical transparency, were calculated from the optical spectroscopy data and an extended comparison was performed with the results of ab initio calculations. The real limits of the optical transparency for the films on sapphire substrates were established. The maximum transparency limit (3.9 eV) was observed for the CaSi film. Based on an analysis of the photoelectric properties of Ca2Si/Si diodes on n- and p-type silicon substrates, a perspective of their applications in silicon optoelectronics was discussed.

The possibility of ordered embedding of CrSi2 and beta-FeSi2 nanocrystals (NCs) into a Si(111) substrate of n-type conductivity has been demonstrated. The types of NCs and silicon matching were determined, and the types of the crystal lattice deformations of CrSi2 and beta-FeSi2 NCs were established. For six-layer heterostructures with embedded CrSi2 and CrSi2 + beta-FeSi2 NCs, a change in the Seebeck coefficient sign from negative to positive was observed for the first time in a 250-400 K temperature range, which indicates a nanocrystals to silicon hole injection. An estimate was made of the minimum number of holes, injected from a single nanocrystal, which are necessary to compensate for the thermo-emf from an n-type substrate. Band diagrams were constructed for the beta-FeSi2-p NC/Si-n and CrSi2-p NC/Si-n heterojunctions, and the discontinuities in the valence and conduction bands were determined. The minimum barrier (0.065 eV) for holes in the Si-p/NC beta-FeSi2-p/Si-n system indicates their thermal generation as the main mechanism. The main mechanism of hole injection from Si-p/NC CrSi2-p/Si-n system is tunneling through the surface states in the Si band gap due to the large discontinuity (1.075 eV) in the Si valence band.

A silicon heterostructure with four layers of embedded gallium antimonide nanocrystals was formed on a Si(111) substrate by co-deposition of Ga and Sb using solid-phase epitaxy. Si(111)2 × 1-Sb surface reconstruction formed prior to the GaSb nanocrystal growth, which kept the nanocrystals from decomposition during the silicon epitaxial layer formation. P- and n-type structures were obtained using silicon of different conductivity types. In the 200–550 K temperature range they showed increases in both electrical conductivity and Seebeck coefficient compared with the bulk samples, which was explained by the peculiarities of the heterointerface band alignment and density of states at the Fermi level. The results open the way for creating multilayer structures with enhanced thermoelectric performance.

This article investigates the growth of calcium (Ca) silicides on the Si(001)2x1 surface during molecular beam epitaxy (MBE) from two sources of Ca and Si with a variable flow ratio from three to 10 at a fixed substrate temperature of 500 degrees C. According to atomic force microscopy (AFM), the grown films consist of grains with a mutually perpendicular orientation and have pronounced grain boundaries with an inhomogeneous relief. The best crystalline qualities of the films, both CaSi and CaSi2, were confirmed for the minimum ratio (about 3.0) of Ca to Si deposition flows by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) data in both cross-sections and in planar sections. At the same time, according to XRD and HRTEM data, for all formed films over the CaSi(010) layer on Si(001), the growth of hR3-CaSi2 was observed with epitaxial relationships: hR3-CaSi2(001)//CaSi(010)//Si(001). An analysis of these studies showed that calcium monosilicide (CaSi) is the first silicide phase in the Ca-Si(001) system at 500 degrees C due to (1) the maximum modulus of its heat of formation, (2) a decrease in the Ca accommodation coefficient to silicon less than unity and the formation of a mixture close to CaSi, (3) a better matching between the parameters of the CaSi(010) plane lattice and the Si(001) plane lattice, and (4) lower CaSi (010)/Si (001) interface energy compared with the CaSi2(001)/Si(001) system. It is assumed that the formation of CaSi2 occurs in the subsurface region of the growing film at 500 degrees C due to increased diffusion of Ca atoms into the Si substrate and increased intergranular diffusion of Si atoms, which leads to the formation of a mixture with the CaSi2 composition. Both factors create conditions for the crystallization of CaSi2 on the surface of the CaSi layer at T = 500 degrees C. The epitaxial ordering of CaSi2 on the CaSi surface and the decrease in the energy of hR3-CaSi2(001)/CaSi(010) interface is associated with small distortions of their lattices during superstructural matching, which is proved by modeling their matching.

Thick epitaxial Ca2Si(100) films were first grown on Si(111) substrates by forming a sacrificial Mg2Si(111) template and converting it into the Ca2Si template. It was found that a temperature of 250 degrees C is sufficient to transfer it into the Ca2Si template with sufficient uniformity. During Ca and Si co-deposition at 250 degrees C, epitaxial Ca2Si(100) domains with two orientations are formed in a thin (100 nm) film, and increasing the substrate temperature to 300 degrees C leads to a deterioration in the Ca2Si crystalline quality due to a partial violation of its continuity and grain growth of the CaSi phase from Si substrate. An increase in the film thickness to 400 nm at 250 degrees C led to the appearance, in addition to the Ca2Si(100) epitaxial phase, of the second Ca2Si(010) epitaxial phase with both contributing to the LEED pattern. From the transmission and reflection spectra of the grown samples, it was found that Ca2Si film has a first direct interband transition at E-1d = 1.095 +/- 0.15 eV, strong defect adsorption lower 1.0 eV and dispersionless refractive index n(o) <= 3.8. Eight Raman peaks and 6 FIR peaks were first registered and identified, which are in good agreement with theoretical calculations. The absorption coefficients characteristic of FIR peaks was determined, which can be used further in the quick estimation of the thickness of Ca2Si films through an intensity of FIR absorption peaks.

The methods of heteroepitaxial growth of Si/CaSi2/Si(111) double heterostructures (DHS) at 500 degrees C have been developed. Thin CaSi2 layers with the thicknesses of 14-40 nm have been successfully embedded in the silicon matrix. The hR6-CaSi2 (001)parallel to Si(111) with hR6-CaSi2 [100]parallel to Si[110] epitaxial relationship has been conserved for the embedded CaSi2 layer regardless of its thickness and the Si overgrowth mode (molecular beam epitaxy or solid phase epitaxy). The embedded CaSi2 layers are characterized by the lattice parameter distortion of about +/- 4% due to the difference in the thermal expansion coefficients of the silicide and silicon. Two types of Si overgrowth atop CaSi2 (001) planes have been observed: (i) {111}-twinned Si crystals were found onto the CaSi2 (001) surface in the DHS with CaSi2 thickness of 32-40 nm, which have preserved the {111} planes parallel to the Si(111) ones of the substrate; (ii) a polycrystalline twinned Si capping layer with a variable thickness has been formed in the samples with the smallest CaSi2 thickness (14-16 nm). Experimentally determined optical functions for the CaSi2 layer embedded in the silicon matrix have shown the presence of degenerate semiconducting properties with strong absorbance at the photon energies higher than 2.3 eV and small contribution from the free carrier absorption at 0.4-1.2 eV. Ab initio calculations within the generalized gradient approximation and screened hybrid functional of the hR-6 CaSi2 bulk with and without lattice distortion (by +/- 3%) have demonstrated the metal or gapless semiconductor energy band structure, because the Fermi level crosses several bands also assuming a huge free carrier concentration. The low-temperature Hall measurements and magnetoresistance measurements have proved that CaSi2 films on silicon are a gapless semiconductor with two types of carrier "pockets" (holes and electrons) that determine the resulting conductivity, concentration and mobility as a function of the Fermi level shift with the temperature increase. Mechanisms of the experimentally observed optical transparency of CaSi2 in the infra-red range are discussed. (C) 2018 Elsevier B.V. All rights reserved.

The morphology and structure of iron silicide nanorods formed on Si (111) vicinal surface by the SPE method at T = 630 °C were studied. Optimal Fe coverage and Fe deposition rate for the formation of a dense array of the nanorods (54-65% of the substrate area) on Si (111) surface with 3-4o miscut angles were established. The aspect ratio of the nanorods is 1.9 – 3.3. Cross-sectional images of a high-resolution transmission electron microscopy (HRTEM) have shown that the nanorods have α-FeSi2 crystal structure. They are strained along the “a” axis and stretched along the “c” axis, which increased the unit cell volume by 10.3%. According to HRTEM image analysis, the nanorods have the following epitaxial relationships: α-FeSi2[01]//Si [10] and α-FeSi2(112)//Si (111). All the data obtained have provided, for the first time, a direct evidence of α-FeSi2 nanorods formation on Si (111) vicinal surface without noticeable penetration of Fe atoms into the Si substrate.

The crystal structure, optical and electrical properties of Ca silicide films grown by MBE and RDE processes on Si(1 0 0) and Si(1 1 1) substrates have been compared. Optical spectroscopy studies, including Raman spectroscopy, have shown two silicide phases: CaSi2 and CaSi. The RDE-grown film on Si(1 1 1) substrate with the CaSi2 phase is characterized by higher transparency than MBE-grown films. Therefore, the CaSi2 RDE-grown phase is found to be predominant and has a lower absorption in the IR region of the spectrum. After 1.5-year storage of Ca silicide film oxidation, partial destruction of the CaSi2 phase and conservation of the CaSi layer were revealed by TEM and HRTEM analyses. The complex behavior of magnetoresistance versus magnetic induction, including positive and negative curves, has been found for the MBE-grown films in the temperature range of 1.3–300[Formula: see text]K. The RDE-grown CaSi2 film has displayed a semiconducting type of the magnetoresistance.

Cellulases are the major components of multienzyme systems applied in processes of bioconversion of renewable lignocellulosic feedstocks to various useful products. The hydrolytic efficiency of enzyme mixes based on recombinant wild-type endoglucanase II, cellobiohydrolases I and II from the Penicillium verruculosum fungus (in the presence of Aspergillus niger β-glucosidase) with mixes of mutant forms of these enzymes in the hydrolysis of cellulosic materials is compared, and the influence of temperature and substrate concentration on the glucose yield is studied. The mutant cellulases represented proteins, in which N-linked glycans were partially removed using site-directed mutagenesis. In the hydrolysis of microcrystalline cellulose and milled aspen wood by mixes of mutant cellulases, the yields of glucose after 24–72 h of an enzymatic reaction were higher by 31–38% and 11–27%, respectively, than those for the compositions based on the wild-type enzymes. The highest product concentrations, using mutant enzyme compositions, are achieved at 50°С when the hydrolysis temperature is varied in the range of 40 to 60°С. Increasing the substrate concentration in the reaction system from 5 to 50 g/L (while maintaining the enzyme dosage at the same level) led to a 2.6–2.8-fold increase in the glucose yield, accompanied by a decrease in the cellulose conversion degree.