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.

The presented paper investigates the formation of nanodiamond structures within multilayer graphene through irradiation with fast heavy ions. The study demonstrates that Xe26+ ions with energies ranging from 26 to 167 MeV can create diamond regions in graphene with lateral sizes ranging from 5 to 20 nm. The density of nanodiamonds formed in the few -layer graphene films is estimated to be approximately (5-30)% of the ion fluence. We show that the final structure of the diamond structures is influenced by factors such as surface orientation, number of graphene layers, and lateral size. Atomistic simulations predicted unusual mechanical properties of the formed 2D composite: its Young's modulus obtained by indentation can significantly exceed the stiffness of the original graphene film.

This work is devoted to the study of the influence of controlled Si(111) surface nitridation on the epitaxial growth of AlN-on-Si nucleation layers with reduced tensile stress on ordered crystalline silicon nitride phase. The Si(111) surface nitridation process was performed at low ammonia flux and substrate temperatures in the range of 700-900 degrees C and was studied using RHEED and STM techniques. A universal criterion, namely the stage of the nitridation process completion is introduced, taking into account the influence of substrate temperature, ammonia flux and nitridation time. The 100 nm AlN nucleation layer on silicon substrates grown by ammonia molecular beam epitaxy is studied using AFM, XRD, HR-TEM and Raman spectroscopy techniques. The Raman data show that reducing the nitridation temperature from 900 degrees C to 700 degrees C not only deteriorates the crystalline quality of the subsequent AlN nucleation layers, but also reduces the residual tensile stress by almost 30%. In the present contribution, micro-Raman spectroscopy is used to determine the nature of the defects formed during the high temperature growth of the AlN nucleation layers and confirms them to be inversion domains. The HR-TEM technique was used to study the AlN/Si interface in AlN-on-Si nucleation layers grown on a nitridated silicon surface at 700 degrees C and 900 degrees C at the optimum stage of the nitridation process completion. HR-TEM images of AlN nucleation layers revealed regions with different AlN/Si(111) interfaces: 1) AlN/amorph-Si3N4/Si, 2) AlN/SiN(8 x 8)/Si, and 3) AlN/Si with a sharp interface boundary. Using fast Fourier transform image analysis, it is shown that the presence of amorphous Si3N4 phase inclusions in the AlN/Si interface boundary introduces tensile stresses in the AlN nucleation layer which can be reduced by lowering the nitridation temperature. The results obtained clearly show that one of the causes of cracks in III-nitride layers grown on silicon substrates is the formation of tensile AlN layers with a high content of the amorphous Si3N4 phase at the AlN/Si interface, which is characteristic of silicon nitridation at elevated temperatures (> 700 degrees C).

The effect of annealing time on the InSb nanocrystal formation in a silicon-on-insulator structure, containing, near the Si/SiO2 interface, Si and SiO2 regions implanted with Sb+ and In+ ions, respectively, was studied. The annealing temperature was 1100 degrees C. A change in the nanocrystal growth direction was obtained as the annealing time increased from 1 to 90 min. After the 1 min annealing, the InSb nanocrystals grew within the Si matrix and were faceted. As the annealing time increased to 90 min, the nanocrystals grew from the Si/SiO2 interface into the SiO2 matrix; they had a half-spherical shape. A respective change in the phonon mode was observed, too. The origin of the obtained effect is discussed.

Using in situ reflection electron microscopy we have presented latest studies of Si(111) and Bi2Se3(0001) surface processes during sublimation, homo- and heteroepitaxial growth of layered metal chalcogenides. A structural kinetic diagram of interaction between a selenium molecular beam and Si(111) surface has been demonstrated. We have shown congruent sublimation caused by annealing of Bi2Se3(0001) substrates in a selenium flux, visualized and described layer by layer homoepitaxial Bi2Se3 growth. We have presented techniques of layered SnSe2 and In2Se3 growth on Si(111) and Bi2Se3(0001) surfaces and shown that heteroepitaxial growth starts with the 2D islands formation. The nucleation and growth of an impurity-induced phase in the form of 0.4 nm high 2D islands during high-temperature submonolayer indium deposition on Bi2Se3(0001) under exposure to Se molecular beam has been demonstrated.

A comprehensive study of the annealing effect (300-400 degrees C) on the electrical properties, morphology and chemical composition of the Au/Pt/Ti/n-InAlAs interface (Schottky contact) is carried out. It is shown that the Schottky contact pre-annealing during the formation or primary short (-1 min) annealing significantly increases the barrier height to the standard 0.68-0.7 eV with an ideality factor close to 1.1 due to the formation of a homogeneous amorphous TiAs layer with a small metallic (elemental) indium content. A further annealing at temperatures 300-350 degrees C for up to 20 min does not lead to significant changes in the morphology and electrical parameters of the Schottky contact. The annealing at the temperature of 400 degrees C (-10 min) leads to an increase in the barrier height and the ideality factor to the values of 0.73 and 1.3, respectively. In this case, the formation of an about 20 nm thick TiAs layer and indium clusters shaped as a pyramids at the Ti/InAlAs interface is also observed. Analysis of the temperature dependences of Schottky barrier parameters within the Tung model showed that only structural changes at the interface after the 400 degrees C annealing lead to a significant increase in the Ti/InAlAs Schottky contact homogeneity, reducing the density of local regions with a lowered barrier height.

Magnetic nanoparticles embedded into semiconductors have current perspectives for use in semiconducting spintronics. In this work, 40 keV Fe+ ions were implanted in high fluences of (0.5 ÷ 1.5) × 1017 ion/cm2 into an oxide semiconductor and single-crystalline TiO2 plates of rutile structure with (100) or (001) face orientations. Microstructure, elemental-phase composition, and magnetic properties of the Fe-ion-implanted TiO2 were studied by scanning and transmission electron microscopies (SEM and TEM), X-ray photoelectron (XPS) and Rutherford backscattering (RBS) spectroscopies, as well as vibrating-sample magnetometry (VSM). The high-fluence ion implantation results in the formation of magnetic nanoparticles of metallic iron beneath the irradiated surface of rutile. The induced ferromagnetism and observed two- or four-fold magnetic anisotropy are associated with the endotaxial growth of Fe nanoparticles oriented along the crystallographic axes of TiO2.

In the present study we investigated the nanostructuring processes in locally suspended few-layer graphene (FLG) films by irradiation with high energy ions (Xe, 26–167 MeV). For such an energy range, the main channel of energy transfer to FLG is local, short-term excitation of the electronic subsystem. The irradiation doses used in this study are 1 × 1011–5 × 1012 ion/cm2. The structural transformations in the films were identified by Raman spectroscopy and transmission electron microscopy. Two types of nanostructures formed in the FLG films as a result of irradiation were revealed. At low irradiation doses the nanostructures were formed preferably at a certain distance from the ion track and had the form of 15–35 nm “bunches”. We assumed that the internal mechanical stress that arises due to the excited atoms ejection from the central track part creates conditions for the nanodiamond formation near the track periphery. Depending on the energy of the irradiating ions, the local restructuring of films at the periphery of the ion tracks can lead either to the formation of nanodiamonds (ND) or to the formation of AA’ (or ABC) stacking. The compressive strain value and pressure at the periphery of the ion track were estimated as ~0.15–0.22% and ~0.8–1.2 GPa, respectively. The main novel results are the first visualization of ion tracks in graphene in the form of diamond or diamond-like rings, the determination of the main condition for the diamond formation (the absence of a substrate in combination with high ion energy), and estimates of the local strain at the track periphery. Generally, we have developed a novel material and have found how to control the film properties by introducing regions similar to quantum dots with the diamond interface in FLG films.

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.

The high-resistance silicon layers were formed by the CO+ ion implantation of silicon substrates and a subsequent annealing at 1100 degrees C. The structural and electric properties of the ion-implanted layers were studied. The formation of 3C-SiC and 6H-SiC nanocrystals was obtained in the ion implanted layers. It was shown that the formation of SiC precipitates results in the positive charge compensation in the buried SiO2 layer of a silicon-on insulator structure. The origin of the obtained results is discussed.

The use of low-temperature (LT) GaAs layers as dislocation filters in GaAs/Si heterostructures (HSs) was investigated in this study. The effects of intermediate LT-GaAs layers and of the post-growth and cyclic in situ annealing on the structural properties of GaAs/LT-GaAs/GaAs/Si(001) HSs were studied. It was found that the introduction of LT-GaAs layers, in combination with post-growth cyclic annealing, reduced the threading dislocation density down to 5 × 106 cm−2, the root-mean-square roughness of the GaAs surface down to 1.1 nm, and the concentration of non-radiative recombination centers in the near-surface GaAs/Si regions down to the homoepitaxial GaAs level. Possible reasons for the improvement in the quality of near-surface GaAs layers are discussed. On the one hand, the presence of elastic deformations in the GaAs/LT-GaAs system led to dislocation line bending. On the other hand, gallium vacancies, formed in the LT-GaAs layers, diffused into the overlying GaAs layers and led to an increase in the dislocation glide rate. It was demonstrated that the GaAs/Si HSs obtained with these techniques are suitable for growing high-quality light-emitting HSs with self-assembled quantum dots.

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.

Структура и морфология массивов вертикально ориентированных УНТ (ВОУНТ), выращенных методом CVD на подложках Fe—Al2O3/Si(001), исследованы с помощью сканирующей и высокоразрешающей просвечивающей электронной микроскопии (ВРЭМ) и комбинационного рассеяния света (КРС). Найдено, что для воспроизводимого роста сплошных массивов ВОУНТ толщина напыленного слоя Fe должна составлять не менее 2 нм, при этом размер частиц катализатора, сформированного отжигом при 700 °C, варьируется в диапазоне 2—10 нм, а массив состоит преимущественно из одно- и двустенных УНТ диаметром 1—6 нм. Спектр КРС характеризуется наличием радиальной дышащей моды в интервале 95—232 см–1 и интенсивной G моды с расщеплением на пики 1594 см–1 и 1568 см–1 при возбуждении лазером с длиной волны λ = 785 нм. Обе моды и наличие дышащих мод в спектре КРС, согласно опубликованным данным, указывают на доминирующий полупроводниковый характер трубок в массиве. Измерение поверхностного сопротивления массива ВОУНТ дает значение 320±20 Ом/□.

The structural and optical characteristics of heterostructures with InGaAlAs/InAlAs quantum wells, in which a quaternary alloy is obtained by alternating monolayer growth of InAlAs and InGaAs layers by molecular beam epitaxy, have been investigated. It has been shown that obtained heterostructures are promising for creation of electro-absorption modulators designed for a wavelength of 1.55 μm with the extinction coefficient of more than 20 dB at a voltage of less than 4 V. Keywords: Electro-absorption modulator, molecular beam epitaxy, quantum wells, Stark effect.

The condition of the same distribution of free carriers in thin films is necessary for comparing the mobility and analyzing the scattering mechanisms of carriers near semiconductor film/insulator interfaces. In thin film/insulator systems with different design parameters, it is difficult to ensure the same distribution of free carriers due to physical phenomenon such as the coupling effect. In this study, TCAD simulations of thin-film transistors, which have been used to monitor Si film properties, were applied to find parameters that allow tuning the potential distribution and, accordingly, the distribution of free carriers in films. It was found that such parameters are the film regime, the density of induced carriers, the gate voltage or threshold voltage of transistors. The conditions for the selection of parameters were found that ensure the same distribution of free carriers in thin-film structures for the cases of different thicknesses of films and the surrounding dielectrics. It was shown that the proposed approach can be used for a comparative analysis of the mobility in thin films and makes it possible to eliminate errors associated with different distributions of carriers in the films due to the coupling effect.

The possibility of fabricating two-dimensional Si layers on a CaF 2 /Si(111) film by molecular beam epitaxy is studied. The growth conditions, under which the regions of two-dimensional Si layers are formed, are found. Raman spectroscopy, transmission electron microscopy, photoluminescence, and electron paramagnetic resonance (EPR) studies have shown that regions of two-dimensional Si layers are formed in epitaxial structures prepared by the deposition of one to three biatomic Si layers on the CaF 2 /Si(111) film surface at a temperature of 550°С. The Raman spectra of these structures exhibit a narrow peak at 418 cm –1 , which is due to light scattering on vibrations of Si atoms in the plane of a two-dimensional Si layer intercalated with calcium. The EPR spectra of multilayer structures with regions of two-dimensional Si layers embedded in CaF 2 demonstrate an isotropic signal with an asymmetric Dyson shape and the g -factor $$g = 1.9992$$ under illumination. Consequently, this signal can be attributed to photoinduced conduction electrons in extended two-dimensional Si islands. These results may be useful for understanding the mechanisms of the formation of two-dimensional materials on CaF 2 /Si(111) substrates.

The structure and morphology of vertically aligned CNT (VACNT) arrays grown by CVD on Fe–Al2O3/Si(001) substrates are studied using scanning and high-resolution transmission electron microscopy (HRTEM) methods and Raman scattering. It is established that reproducible growth of continuous VACNT arrays is achieved only if the deposited Fe layer is at least 2 nm thick, while the particle size of the catalyst formed by annealing at 700 °C varies in a range of 2-10 nm and the array consists mainly of single- and double-walled CNTs with a diameter of 1-6 nm. The Raman spectrum is characterized by the presence of a radial breathing mode in the region 95-232 cm–1 and an intense G mode that is split into peaks at 1594 cm–1 and 1568 cm–1 upon laser excitation at the wavelength λ = 785 nm. According to the literature data, both modes indicate predominantly semiconductor nature of tubes in the array. The measured surface resistance of the VACNT array is 320±20 Ω/□.

The morphology, chemical composition, and electronic properties of Al2O3/InGaAs interfaces with and without anodic oxide layers, formed in DC plasma (O2, Ar) with different contents of the fluorinating component (CF4), were studied. It is shown that thin fluorinated anodic oxide layers, in combination with annealing at 300 °C, reduce the density of interface states by a factor of 3–4 over the entire bandgap. The minimum state density values near the midgap determined by the Terman method are about 2 × 1012 eV−1 cm−2. However, it is demonstrated that, in contrast to the Al2O3/InGaAs interface, the interface with a fluorinated oxide is not stable and degrades when heated above 300 °C.

Textured silicon surface structures, in particular black silicon (b-Si), open up possibilities for Si-based solar cells and photodetectors to be extremely thin and highly sensitive owing to perfect light-trapping and antireflection properties. However, near-infrared (NIR) performance of bare b-Si is limited by Si band gap of 1.12 eV. This work reports a simple method to increase NIR absorption of b-Si by in vacuo silicidation with magnesium. Obtained Mg2Si/b-Si heterostructure has a complex geometry where b-Si nanocones are covered by Mg2Si shells and crowned with flake-like Mg2Si hexagons. After growth optimization, Mg2Si cover atop b-Si resulted in 5-fold lower total reflectivity (3.7%) and optical absorption to be no lower than 88% over 200-1800 nm spectral range. More importantly, Mg2Si/b-Si heterostructure is more adjusted to match AM-1.5 solar spectrum with theoretically higher photogenerated current density. The maximal advantage is demonstrated in the NIR region compared to bare b-Si in full accordance with one's expectations about NIR-sensitive narrow band gap (0.75 eV) semiconductor with high absorption coefficient, which is Mg2Si. Results of optical simulation confirmed the superiority of Mg2Si/b-Si NIR performance. Therefore, this new wide-band optical absorber called black silicide proved rather competitive alongside state-of-the-art approaches to extend b-Si spectral blackness.