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Polycrystalline and epitaxial iron silicide (FeSi) films with thicknesses of 3.2–20.35 nm are grown on a Si(111) substrate using solid-phase and molecular-beam epitaxy at 350°C, as confirmed by X-ray diffraction data. Morphological studies reveal that the films grown by solid-phase epitaxy are continuous and smooth with a root-mean-square roughness ranging from 0.4 to 1.1 nm, while those grown by molecular-beam epitaxy exhibit increased roughness and consist of coalesced grains with sizes up to 1 μm and a pit density of up to 1 × 107 cm–2. In the case of solid-phase epitaxy, an increase in the thickness leads to incomplete silicide formation and the emergence of a layer of disordered iron silicide with a thickness ranging from 10 to 20 nm, possibly with an excess of iron. This is confirmed by a change in the nature of the temperature dependence of the resistivity ρ from semiconductor to semimetallic, which leads to a decrease in the resistivity by 1.5–2 times. The nonmonotonic character of the temperature dependence of the resistivity ρ(T) for an ultrathin FeSi film with a thickness of 3.2 nm is identified, exhibiting a maximum around 230–240 K, a rising segment from 160 to 65 K with Eg = 14.8 meV, and further unsaturated growth down to 1.5 K. As the thickness of the FeSi films grown by molecular-beam epitaxy increases, the minimum and maximum are not observed, but the nonmonotonic trend of ρ(T) with decreasing temperature and the opening of the band gap Eg = 23 meV are preserved. The possible reasons for the observed effects in the ρ(T) dependences are considered. An anomalous Hall effect is detected in ultrathin and thin FeSi films grown by solid-phase and molecular-beam epitaxy, respectively, which confirms the weak ferromagnetic properties of the films. The results demonstrate the feasibility of growing and controlling the properties of ultrathin and thin FeSi films on silicon using methods of solid-phase and molecular-beam epitaxy, providing them with unique transport and magnetic properties not present in single crystals. [ABSTRACT FROM AUTHOR]

Mg2Si film 350–600 nm thick was formed on Si(111) substrate by ultrafast reactive deposition of Mg. Then 10–15 nm thick films of FeSi or CrSi2 were grown as cover layers on Mg2Si by codeposition of Fe/Si or Cr/Si. In comparison with bare Mg2Si/Si film, the measured transparency losses in 0.1–1.1 eV range are ∼17–25 and 10–35% for the samples with the FeSi and CrSi2 layers, respectively. The measured at room temperature resistivity is ∼0.24 mΩ cm for FeSi and 1.7 mΩ cm for CrSi2. The photoresponse of structures FeSi/Mg2Si/Si and CrSi2/Mg2Si/Si illuminated with the light filtered by a silicon plate demonstrate the spectral sensitivity in the range 980–1200 nm and peak intensity ∼255 and ∼620 μA/W respectively. [ABSTRACT FROM AUTHOR]

Epitaxial and polycrystalline films of iron (Fe), chromium (Cr), and calcium (Ca) silicides of various thicknesses (from 3.2 to 380 nm) were grown on silicon and sapphire substrates in ultrahigh vacuum by three methods (solid-phase epitaxy, reactive epitaxy and molecular beam epitaxy). The crystal structure and matching with the silicon lattice were determined for them using the X-ray diffraction method. A comparative analysis of Raman scattering spectra and far-IR spectroscopy spectra showed that films of semiconductor silicides have the strongest Raman peaks, and the detected shifts in their positions are caused by distortions in the silicide lattices. It has been established that in films of iron and chromium monosilicides at a fixed laser excitation wavelength ( nm) and a power of 3.4 mW, the strength of the Raman peaks decreases with decreasing film thickness and they disappear completely at a thickness below 10 nm. Chromium trisilicide films were grown on single-crystal sapphire, which made it possible for the first time to detect for it active Raman phonons at nm and a power of 0.42 mW at wave numbers 214.3 and 273.1 cm . The studied films of transition metal monosilicides are of significant interest from the perspective of their possible use as materials for thermoelectronics and spintronics, and systematized information on active Raman phonons and IR active phonons will make it possible to quickly determine the type of phase formed immediately after film growth. [ABSTRACT FROM AUTHOR]

Ab initio techniques have been used to investigate structural changes in semiconducting Mg2Si and Ca2Si thin films (from 17 nm down to 0.2 nm corresponding to the 2D structure) along with band-gap variations due to quantum confinement. Cubic Mg2Si(111) thin films being dynamically stable at thicknesses (d) larger than 0.3 nm displayed an indirect band gap, the reduction of which with increasing d could be reasonably well described by the simple effective mass approximation. Only 2D Mg2Si has a unique structure because of the orthorhombic distortion and the direct band gap. Since the surface energy of cubic Ca2Si(111) films was lower with respect to any surface of the orthorhombic phase, which is the ground state for the Ca2Si bulk, the metastable in-bulk cubic phase in the form of thin films turned out to be preferable in total energy than any orthorhombic Ca2Si thin film for d < 3 nm. Sizable structural distortion and the appearance of surface states in the gap region of Ca2Si thin films with d < 3 nm could be the reason for an odd dependence of the band-gap variation on d. [ABSTRACT FROM AUTHOR]

A nanocrystalline Ca2Si film on Al2O3(0001) is formed for the first time by converting a sacrificial 2D Mg2Si template into a Ca2Si seed layer with the preliminary formation of an amorphous 2D silicon layer. A Ca2Si seed layer on a sapphire substrate facilitates the growth of oriented Ca2Si films by molecular-beam epitaxy (MBE) at a temperature of 250°C, for which one epitaxial ratio is observed, Ca2Si(211)/Al2O3(0001). Studies of the optical properties and parameters of the band structure of Ca2Si on sapphire reveal the nature of the fundamental direct transition with an energy of 0.88 ± 0.01 eV. Direct interband transitions are observed in the band structure of Ca2Si: at 0.88, 1.16, 1.49, and 1.61 eV with increasing oscillator strength. At photon energies from 0.78 to 0.88 eV, the Urbach tail on defects in Ca2Si nanocrystals makes the main contribution to absorption. Absorption at grain boundaries is observed at photon energies from 0.6 to 0.78 eV, and at free carriers, at energies below 0.6 eV. The results obtained are important for optoelectronics in the infrared (IR) region of the spectrum. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

The magnetic properties of nanoobjects of nonmagnetic substances based on silicon and transition metal (iron) are considered. It is found that a system of -FeSi nanorods with a mean height of 6.6 nm and lengths from 50 to 200 nm is formed by deposition of iron with 5.22 monolayers in ultrahigh vacuum on a vicinal silicon surface Si(111)-4 followed by annealing at a temperature of C. Soft ferromagnetic loops with a coercive force from 90 to 180 Oe, which persist up to a temperature of 300 K, are observed in nanorods. It is found that easy magnetization axis is perpendicular to the nanorods, while the hard magnetization axis is parallel to them. This type of nanoobjects is promising for developing spintronic devices within the planar silicon technology. [ABSTRACT FROM AUTHOR]

Photonic crystals based on amorphous SiO2 nanospheres have been obtained in four-component H2O–Si(OC2H5)4–NH3–EtOH systems at a constant initial volume (100 mL), a constant molar ratio of NH3:Si(OC2H5)4 = 10 : 1, and varied molar ratio of H2O–Si(OC2H5)4 (x1) and H2O–EtOH (x2) within the range of 30–110 and 0.4–2.8, respectively. The increase in water concentration and simultaneous decrease in the alcohol concentration in the initial mixture have resulted in the reduction of mean diameter of the SiO2 spheres from 440 to 270 nm. The curves of the correlation curves between effective diameter of the nanospheres– and the H2O–Si(OC2H5)4 and H2O–EtOH molar ratio have shown two regions with different slopes: for the samples obtained at low H2O–Si(OC2H5)4 and H2O–EtOH molar ratios for these obtained at [H2O] : [Si(OC2H5)4] > 50, [H2O] : [EtOH] > 1. Correlations between the size of the nanospheres of oligomerized SiO2, the initial rate of the process, and the dielectric constant of the initial mixture have been found. The spectral parameters of the photonic crystals obtained on the basis of the amorphous SiO2 spheres have been affected by the H2O–Si(OC2H5)4 and H2O–EtOH molar ratios in the initial mixtures. [ABSTRACT FROM AUTHOR]

This article describes the development of an Si-based light-emitting diode with β-FeSi2 nanocrystals embedded in the active layer. Favorable epitaxial conditions allow us to obtain a direct band gap type-I band alignment Si/β-FeSi2 nanocrystals/Si heterostructure with optical transition at a wavelength range of 1500-1550 nm at room temperature. Transmission electron microscopy data reveal strained, defect-free β-FeSi2 nanocrystals of diameter 6 and 25 nm embedded in the Si matrix. Intense electroluminescence was observed at a pumping current density as low as 0.7A/cm². The device reached an optical emission power of up to 25 µW at 9A/cm² with an external quantum efficiency of 0.009%. Watt-Ampere characteristic linearity suggests that the optical power margin of the light-emitting diode has not been exhausted. Band structure calculations explain the luminescence as being mainly due to radiative recombination in the large β-FeSi2 nanocrystals resulting from the realization of an indirect-to-direct band gap electronic configuration transformation arising from a favorable deformation of nanocrystals. The direct band gap structure and the measured short decay time of the luminescence of several tens of ns give rise to a fast operation speed for the device. Thus a method for developing a silicon-based photonic integrated circuit, combining complementary metal-oxide-semiconductor technology functionality and near-infrared light emission, is reported here. [ABSTRACT FROM AUTHOR]

Real-time thermal analysis is used for the synthesis of silica globules. The rate of the reaction of sol synthesis is determined. The reaction order is found, depending on the concentration of water, and the kinetic and diffusion regions are identified. Dried (not annealed) samples are studied via DSC. The dynamics of ethanol and water evolution from the samples is studied in analyzing the heating and cooling curves. Structural phase transformations of the formation of β-cristobalite and its transition to the α-form are observed. The dependence of the amorphousness of the sample on the amount of water in the solid phase is determined. It is found that when the water–TEOS molar ratio is less than fifty, the process proceeds slowly and hydrolysis is incomplete. An explanation of this pattern is proposed. A reaction equation for the kinetic region is determined for the case when the indicated molar ratio is more than 100. A general chemical formula for the main structure of a globule's core is proposed, with allowance for the presence of cristobalite fragments in all samples. Based on an analysis of this formula, a condition is found for the case when the hexagonal ring cannot be the basis of the structure, and the amount of chemically retained water can be determined. A stoichiometric formula for the product that forms in the diffusion region is obtained. [ABSTRACT FROM AUTHOR]

The electrical properties of thin iron films deposited at room temperature (RT) on clean Si(100) and on the prefabricated Si(100)-c(4×12)-Al surface phase (SP) have been studied by means of in situ Hall effect and conductance measurements for iron coverage (0–2.3) nm. It is shown that the Si(100)-c(4×12)-Al SP blocks intermixing of iron and substrate atoms. The conductance and mobility of the majority carriers in this SP are higher than those in Si(100)2×1, within the temperature range from RT to 460 K. For iron coverage below 2 nm, the sample conductance is significantly lower than that for the bare Si(100). The iron film grown on the Si(100)-c(4×12)-Al becomes continuous at a coverage approximately half as thick as for the film grown on the Si(100)2×1 surface. [ABSTRACT FROM AUTHOR]

A quantum mechanical polarizable force field (QMPFF) has been applied to the noncovalent interactions of molecular hydrogen as well as closed-shell monoatomic species (CSMS): rare gases, alkali cations, and halide anions. The importance of all the main energy components is demonstrated: electrostatics (including penetration effect), exchange repulsion, dispersion, and induction. As the MP2 level of quantum mechanics, which is used to parametrize QMPFF, significantly underestimates the H2–H2 dimer binding energy, the force field was refined using state-of-the-art CCSD(T) data. The approach demonstrates excellent transferability, which is confirmed by accurate reproduction of mixed H2-CSMS dimers and the second virial coefficient of hydrogen vapor. [ABSTRACT FROM AUTHOR]

The authors have recently introduced a general, polarizable force field QMPFF fitted solely to high-level quantum mechanical data for simulations of biomolecular systems. Here the authors demonstrate using an advanced version QMPFF3 how the problem of insufficient accuracy of the MP2-based training set for the aromatic carbon atom type can be effectively solved by a simple model correction using state-of-the-art CCSD(T) data. The approach demonstrates excellent transferability, which is confirmed for three phases of matter by accurate calculations of the second virial coefficient for benzene vapor and various properties of liquid benzene and polyaromatic hydrocarbon crystals. [ABSTRACT FROM AUTHOR]

Schottky contacts on p-type silicon, with embedded arrays of β-FeSi2 nanocrystallites, were studied by current-voltage (I-V), deep level transient spectroscopy (DLTS), and low-frequency noise measurements. Forward I-V characteristics on logarithmic scale indicate that space-charge limited current (SCLC) dominates the carrier transport in these diodes. From an analysis of the SCLC characteristics, we found that two arrays of β-FeSi2 nanocrystallites induce a trap level at 320 meV above the valence band edge with concentration of 5×1014 cm-3, which is in relatively good agreement with the trap detected by DLTS. By inserting ten arrays of β-FeSi2 nanocrystallites, two trap levels at 300 and 340 meV with concentrations of 2.6×1014 and 1.1×1015 cm-3, respectively, were found from the SCLC analysis. By increasing the number of the inserted arrays of β-FeSi2 nanocrystallites from 2 to 10, a trap at 402 meV with concentration of 6.5×1015 cm-3 has been detected by DLTS, failing to detect the second trap observed by SCLC measurements. The normalized power spectral density SI/I2 vs V exhibits a peak at the transition voltage from the trap-filling regime to the SCLC regime, corresponding to a deep trap level. It is demonstrated that the noise technique is more sensitive than the SCLC and DLTS techniques in extracting information about the deep trap parameters. [ABSTRACT FROM AUTHOR]

The p + – p − – n diode structures with a variable number of β -FeSi2 and CrSi2 nanocrystal (NC) bilayer were studied at room and liquid nitrogen temperatures. The high crystal quality of Si and embedded NCs was determined by HRTEM cross-sectional data. This diode demonstrated a photoresponse in the IR region (1.02–0.8 eV) which was associated with a decrease in recombination losses with the main contribution coming from CrSi2 NCs. Calculations of photoyield and its extrapolation to zero resulted in band gap values of 0.35 eV at 300 K and 0.46 eV at 90 K. The spectral sensitivity of 1. 0 6 × 1 0 − 4 A/W and specific detectivity of 1. 3 8 × 1 0 6 (cm Hz 1 ∕ 2 /W) were determined at λ = 1. 5 μ m and T=300 K. [ABSTRACT FROM AUTHOR]

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 K. The RDE-grown CaSi2 film has displayed a semiconducting type of the magnetoresistance. [ABSTRACT FROM AUTHOR]

The interaction of porous silicon (PS) with aqueous solutions of Fe(NO3)3 with different molar (M) concentrations causes introduction of iron ions into silicon pores (PS-Fe), formation of adsorbed iron coatings with different thicknesses, and an increase in the stability of PS layers, which is important for development of device structures based of PS. To treat PS layers with solutions by the immersion method, it is necessary to determine how this affects the spectral composition and intensity of photoluminescence (PL), as well as the kinetics of PL changes during long storage under atmospheric conditions. Upon treatment of freshly prepared PS by immersion into in a Fe(NO3)3 aqueous solution, it was found that, after short-term storage (up to 5 days) of the PS samples, the PL intensity increases by 7.5 and 3-3.6 times at low (0.2 M) and high (0.7-0.8 M) concentrations of Fe(NO3)3, respectively, compared to the PL intensity of an untreated PS layer. After long-term storage (4 months), the PL intensity of PS-Fe samples with concentrations of 0.1-0.2 and 0.7-0.8 M was observed to considerably increase (by 8-18 times) with unchanged position of the PL peak with respect to untreated PS. However, at the Fe(NO3)3 concentration of 0.3 М, the PL intensity decreases and the PL peak shifts to the blue, which is explained by incomplete coverage of the PS surface by an adsorbed iron layer. The kinetics of PL spectra during long-term storage is analyzed, and a model is proposed to explain the PL intensity and spectral composition. [ABSTRACT FROM AUTHOR]

The main mechanisms of the visible electroluminescence (EL) of porous silicon are reviewed. Characteristics of photoluminescence and EL of diode structures based on porous silicon are compared. Metals having a smaller value of the electron work function (3.6 eV, Mg) than do Al and Au are proposed as the material for making contacts in such diode structures to increase the efficiency of their EL in the visible region of the spectrum. The main problems and prospects of light-emitting devices based on porous silicon are formulated. [ABSTRACT FROM AUTHOR]

A Si n-i-p avalanche photodetector with embedded β-FeSi2 nanocrystals was developed. The device showed an ultrabroadband photoresponse from the visible (400 nm) to short-wavelength infrared (1800 nm) ranges. Specific detectivity at zero bias conditions reaches 2×109 cmHz1/2/W at 1300 nm and 2×108 cmHz1/2/W above 1400 nm at room temperature. Observed quantumconfined Stark effect together with avalanche multiplication resulted in a simultaneous two orders of magnitude increase in the photoresponse and spectral sensitivity expanding to 1800 nm when the device is operated in avalanche mode. The application fields of the proposed photodetector potentially include integrated Si photonics and multicolor photodetection; the quantum-confined Stark effect gives grounds for the development of fast-operated electro-optical modulators. [ABSTRACT FROM AUTHOR]

Mg2Si thin film was embedded in amorphous silicon matrix by solid phase epitaxy. The structure and optical properties were investigated by electron energy loss, X-ray photoelectron, Raman, and photo thermal deflection spectroscopy measurements. It was found that in the photon energy range of 0.8-1.7 eV, the light absorption of the structure with magnesium silicide (Mg2Si) film embedded in a-Si(i) matrix is 1.5 times higher than that for the same structure without Mg2Si. [ABSTRACT FROM AUTHOR]

Nowadays, scientists investigate new more efficient materials for creating solar cells. One of such materials is thin films of barium disilicide. In this paper, we describe formation of BaSi2 films on Si(111) by layer-by-layer solid phase epitaxy and study their optical properties in visible and infrared range. [ABSTRACT FROM AUTHOR]

A strong nonlinear increase in the photoluminescence intensity under laser excitation at room temperature is found for porous silicon obtained by anodic oxidation. It is shown that the maximum photoluminescence intensity corresponds to samples of anodically oxidized porous silicon in the intermediate oxidation state. Laser excitation is found to increase the intensity of vibrational modes in the О-SiH, Si-O-Si, and Si-O-H configurations with respect to the Si-H mode intensity in IR absorption spectra. It is experimentally confirmed that the oxide structure on the surface of silicon crystallites and the structure of the Si/SiO interface determine to a great extent the photoluminescence characteristics. [ABSTRACT FROM AUTHOR]

By using solid phase epitaxy of thin Fe films and molecular beam epitaxy of Si, a p+ -Si/p-Si/β-FeSi2 nanocrystallites/n-Si(111) diode structure was fabricated. Transmission electron microscopy data confirmed a well-defined multilayered structure with embedded nanocrystallites of two typical sizes: 3-4 and 15-20 nm, and almost coherent epitaxy of the nanocrystallites with the Si matrix. The diode at zero bias conditions exhibited a current responsivity of 1.7 mA/W, an external quantum efficiency of about 0.2%, and a specific detectivity of 1.2 x 109 cm x Hz1/2/W at a wavelength of 1300 nm at room temperature. In the avalanche mode, the responsivity reached up to 20 mA/W (2% in terms of efficiency) with a value of avalanche gain equal to 5. The data obtained indicate that embedding of β-FeSi2 nanocrystallites into the depletion region of the Si p-n junction results in expansion of the spectral sensitivity up to 1600 nm and an increase of the photoresponse by more than two orders of magnitude in comparison with a conventional Si p-n junction. Thereby, fabricated structure combines advantage of the silicon photodiode functionality and simplicity with near infrared light detection capability of β-FeSi2. [ABSTRACT FROM AUTHOR]

The structural and optical properties of thin Ge films deposited onto semiconducting and insulating substrates and modified by pulsed laser radiation are studied. The films are deposited by the sputtering of a Ge target with a low-energy Xe ion beam. Crystallization of the films is conducted by their exposure to nanosecond ruby laser radiation pulses (λ = 0.694 μm) with the energy density W = 0.2−1.4 J cm. During pulsed laser treatment, the irradiated area is probed with quasi-cw (quasi-continuous-wave) laser radiation (λ = 0.532 and 1.064 μm), with the reflectance recorded R( t). Experimental data on the lifetime of the Ge melt are compared with the results of calculation, and good agreement between them is demonstrated. Through the use of a number of techniques, the dependences of the composition of the films, their crystal structure, the level of strains, and the reflectance and transmittance on the conditions of deposition and annealing are established. [ABSTRACT FROM AUTHOR]

The electroluminescence efficiency of silicon light-emitting diode structures with several layers of β-FeSi nanocrystallites embedded in the p-n junction is investigated. The nanocrystallites were formed by either solid-phase epitaxy or a combination of reactive and solid-phase epitaxy. For the structures in which the nanocrystallites were formed by the combined method, electroluminescence is observed only at low temperatures (below 70K). This is indicative of a high concentration of defects acting as nonradiative-recombination centers. For the structures with nanocrystallites formed by solid-phase epitaxy, intense electroluminescence is observed up to room temperature. The dependence of the electroluminescence intensity on the size of the nanocrystallites is studied. [ABSTRACT FROM AUTHOR]