Multimodal size distribution of Si nanoclusters in SiO₂ as manifestation of interaction in the space of sizes

This paper summarizes the results of experimental studies revealing a multimodal character of the function of size distribution of Si nanoclusters in light-emitting SiO₂:Si structures. To explain the nature of this multimodality, a “liquid” approach to the description of the coalescence stage based on taking into account the correlation effects caused by particle-particle interactions in the space of sizes is proposed. This approach is justified by a high concentration of solutions where the nuclei of the new phase are described by a multimodal function of size distribution.Узагальнені результати експериментальних досліджень, які засвідчують полімодальність функції розмірного розподілу кремнієвих наночасток в матриці SiO₂ в випадку сильного перенасичення розчину SiO2:Si кремнійовою компонентою. Для пояснення природи полімодальності запропоновано «рідинний» підхід в описі стадії коалесценції, який оснований на врахуванні процесів міжчастинкової взаємодії в просторі розмірів. Запропонований підхід може виявитись перспективним для розвитку фізичних основ керування спектральним складом і інтенсивністю випромінювання кремнійових випромінювачів світла.Обобщены результаты экспериментальных исследований, свидетельствующие о полимодальности функции размерного распределения кремниевых наночастиц в матрице SiO₂ в случае сильного пересыщения раствора SiO2:Si кремниевой составляющей. Для объяснения природы полимодальности предложен «жидкостный» подход в описании стадии коалесценции, основанный на учете процессов межчастичного взаимодействия в пространстве размеров. Предложенный подход перспективен для развития физических принципов управления спектральным составом и интенсивностью излучения кремниевых излучателей света

Drift correction of the analyzed area during the study of the lateral elemental composition distribution in single semiconductor nanostructures by scanning Auger microscopy

The main difficulty in obtaining the lateral elemental composition distribution maps of the semiconductor nanostructures by Scanning Auger Microscopy is the thermal drift of the analyzed area, arising from its local heating with the electron probe and subsequent shift. Therefore, the main goal of the study was the development of the effective thermal drift correction procedure. The measurements were carried out on GeSi/Si nanoislands obtained with molecular beam epitaxy by means of Ge deposition on Si(100) substrate. Use of the thermal drift correction procedure made it possible to get the lateral elemental composition distribution maps of Si and Ge for various types of GeSi/Si nanoislands. The presence of the germanium core and silicon shell in both the dome GeSi/Si nanoislands and pyramid ones was established. In the authors’ opinion, this type of elemental distribution is a result of the completeness of the interdiffusion processes course in the island/wetting layer/substrate system, which play the key role in the nucleation, evolution and growth of GeSi/Si nanoislands. The proposed procedure of the thermal drift correction of the analyzed area allows direct determination of the lateral composition distribution of the GeSi/Si nanoislands with the size of the structural elements down to 10 nm

Investigation of structural perfection of SiC ingots grown by a sublimation method

Monocrystalline SiC ingots were grown by a modified Lely method using 6H-SiC seed crystals with (0001) base plane. The crystal growth was carried out in the temperature range 2200-2500 ⁰C at Ar pressure from 2 to 40 mbar. The rate of growth varied between 0.3 and 1.5 mm/hour in the C-axis direction. At growth time of about 15 hours we obtained the ingots with 35 mm useful diameter. To determine the polytype composition of SiC ingots the Raman scattering technique was used. The structural defects were investigated by means of reflection and transmission light microscopy and by selective etching. In the best ingots the dislocation density did not exceed 102 cm⁻², the micropipe density - 10-20 cm⁻², and blocks were absent

Biomorphous SiC ceramics prepared from cork oak as precursor

Porous ceramic materials of SiC were synthesized from carbon matrices obtained via pyrolysis of natural cork as precursor. We propose a method for the fabrication of complex-shaped porous ceramic hardware consisting of separate parts prepared from natural cork. It is demonstrated that the thickness of the carbon-matrix walls can be increased through their impregnation with Bakelite phenolic glue solution followed by pyrolysis. This decreases the material's porosity and can be used as a way to modify its mechanical and thermal characteristics. Both the carbon matrices (resulted from the pyrolysis step) and the resultant SiC ceramics are shown to be pseudomorphous to the structure of initial cork. Depending on the synthesis temperature, 3C-SiC, 6H-SiC, or a mixture of these polytypes, could be obtained. By varying the mass ratio of initial carbon and silicon components, stoichiometric SiC or SiC:C:Si, SiC:C, and SiC:Si ceramics could be produced. The structure, as well as chemical and phase composition of the prepared materials were studied by means of Raman spectroscopy and scanning electron microscopy

Efficient core-SiO₂/shell-Au nanostructures for surface enhanced Raman scattering

The efficient SERS (surface enhanced Raman scattering) substrates that are films of nanoparticles (NP) of the “core–shell” type, where the core of SiO2, and the shell of gold nanoparticles, were developed in this work. Application of scanning electron microscopy and optical absorption enabled to find correlation between surface morphology of nanostructures and position of the plasmon absorption band. It helped to adjust the latter to the wavelength of exciting laser radiation. It has been shown that the designed nanostructures are able to enhance electric field of an emitting dipole not only due to adjustment of the band frequency for plasmon absorption to the wavelength of exciting laser radiation but also due to contribution of the so-called “hot spots” to enhancement of electric field scattering. Analysis of characteristics inherent to SERS substrates with nanostructures of the soil core – Au shell type has shown that they enhance the Raman signal by 5 orders higher as compared with the substrates based on SiO2 nanospheres not covered with gold nanoparticles

Anisotropy of elastic deformations in multilayer (In,Ga)As/GaAs structures with quantum wires: X-ray diffractometry study

Using the method of high-resolution X-ray diffraction (HRXRD), we have studied 17-period In₀.₃Ga₀.₇As/GaAs multilayer structure with self-assembled quantum wires (QWRs) grown by the MBE and subjected to postgrowth rapid thermal annealing (RTA) at temperatures (Tann) from 550 to 850 °C for 30 s. It has been shown that the spatial arrangement of QWRs (lateral and vertical) causes the quasi-periodical strain distribution, the strains being essentially anisotropic relatively to crystallographic directions of 〈011〉 type. At Tann ≤ 750 °С, the driving mechanism of structural transformations is relaxation of residual strains due to thermally-activated and strain-enhanced processes of In/Ga atom interdiffusion at the interface QWRs-2D layer, which does not result in considerable changes of the In concentration in (In,Ga)As QWRs. The presence of two superlattice vertical periods in the samples under study and their changes during RTA we explained by an anisotropic character of elastic strain distribution and lowered structure symmetry. The revealed increase in the (In,Ga)As QWRs lateral period caused by RTA is a direct evidence of running lateral mass-transfer processes and can be explained using the model “nucleation plus strain-enhanced In/Ga atom lateral interdiffusion”. At low annealing temperatures, there takes place dissolution of intermediate QWRs as a result of interdiffusion enhanced by residual anisotropic strains. At high RTA temperatures, the interdiffusion process is mainly determined by the composition gradient existing between QWRs and 2D layer

Properties of SiGe/Si heterostructures fabricated by ion implantation technique

A comprehensive electrical characterisation of the SiGe/Si heterostructures has been performed in the wide temperature range (10270 K). Four structures fabricated by the Ge⁺ ion implantation technique at different substrate temperatures (room temperature, 150°C, 450°C and 600°C) have been studied. The diode I-V characteristics, thermally stimulated capacitance and currents were measured and the presence and parameters of shallow trap levels were determined in dependence on the substrate temperature. The sample implanted at 450°C shows the best diode operation reflecting the higher quality of the surface silicon layer as compared to RT- and 150°C-implanted samples. Implantation-induced mechanical stresses have been investigated by Raman spectroscopy. For the first time the cryogenic TSCR technique has been applied to this system which makes it possible to investigate strain in the silicon layer due to SiGe layer formation

Investigation of electron-phonon interaction in bulk and nanostructured semiconductors

In this paper, the problem of electron-phonon interaction (EPI) innsemiconductor crystals and quantum dots (QDs) is considered. It is shown that the model of strong EPI developed for organic molecular crystals can be successfully applied to bulk and nanosized semiconductors. The idea of the approach proposed here is to describe the experimental Raman (or absorption) spectra containing the phonon replicas theoretically by varying the EPI constant. The main parameter of the theoretical expression describing the experimental spectrum is the ratio of EPI constant to the frequency of the corresponding phonon mode. Based on the experimental and theoretical results, we have found that decreasing the size of CdSxSe₁₋x QDs embedded in borosilicate glass matrix results in some enhancement of electron-phonon interaction

Physical properties of nanocrystaline PbS synthesized by electrolytic method

The possibility of obtaining nanocrystaline lead sulfide by an electrolytic method using lead electrodes is demonstrated, and the influence of temperature on the synthesis process is investigated. Based on the results of X-ray diffraction studies, the chemical and phase composition of the obtained samples is determined, as well as the parameters of the unit cell of the crystals lattice. The size of the nanocrystallites and the magnitude of residual mechanical strain in them is determined using the methods of Debye-Scherrer and Williamson-Hall. The results of X-ray diffraction are in agreement with the results of the Raman scattering on phonons

Optical characterization of pseudomorphic AlGaAs/InGaAs/GaAs heterostructures

Pseudomorphic strained-layer AlxGa₁₋xAs/InyGa₁₋yAs/GaAs heterostructures have been studied by means of photoluminescence and Raman scattering. It is established the correlation between the photoluminescence line shape changes and the Raman spectra modification when the quantum well width is below the critical layer thickness estimated to be of 25 nm for y = 0.1. The photoluminescence feature observed for the InGaAs quantum well width equal to 20 nm as extremely narrow exciton-like peak with the full width at half of maximum equal to 1.5 meV at low temperature (T = 6 K) transforms into broad band of the full width at half of maximum equal to 16 meV when the quantum well width reaches the value about of 12 nm. The photoluminescence line shape broadening is accompanied by the modifications of Raman spectra. A new line arising at the spectral position ν = 160 cm⁻¹ is assigned to impurity-induced longitudinal acoustic mode of InyGa₁₋yAs. The changes observed in optical spectra are related to the generation of defects in the under-critical layer thickness region