Self-organization in the formation of a nanoporous carbon material

A new mechanism of nanopore formation in carbon materials produced by the interaction of car-bides with chlorine is proposed. In essence, this method is the following. A series of nonlinear chemical reactions proceed in the course of a chemical interaction between chlorine and a carbide. If the external parameters, the component fluxes, and the diffusion rates satisfy certain relations, the self-organization process can occur. This process results in the creation of a periodic nanoporous structure in the carbon material formed. A mathematical model is proposed, the main characteristics of the process are calculated, and the restrictions on the parameters at which the formation of the porous structure becomes possible are found.     

Study of the microporosity in nanoporous carbon films by small-angle X-ray scattering

The structural difference in the microporous structures of nanoporous carbon films is revealed by small-angle X-ray scattering; it consists in a higher porosity of the layers formed from the titanium carbide. The pore shape is shown to be equiaxed. Pores 20 Å in diameter mainly contribute to the porosity of the nanoporous carbon films. The characteristics of the porous structure of the nanoporous carbon layers synthesized from the titanium or silicon carbide are found using small-angle X-ray scattering. The porous structure is shown to consist of two size fractions of equiaxed pores 10 and 40 Å in radius. The porosity of the films is mainly contributed by the pores of the small size fraction; their fraction is 46 or 10% for the layers synthesized from the titanium or silicon carbide, respectively.     

Features of the evolution of the structure and morphology of the surface of icosahedral copper particles in the annealing process

The effect of annealing on icosahedral small copper particles has been experimentally studied. The nanoporous structure and developed surface consisting of nanopores and nanowhiskers formed in the process of annealing of icosahedral small copper particles have been examined. Mechanisms of their formation have been proposed.     

Effect of surface plasmons at the Al/alumina interface on the formation of nanopores in Al anodization

We investigated the formation mechanism of nanoporous alumina in the electrochemical reaction of aluminium with oxalic acid solution in terms of the pinning effect of Fermi level at the metal–oxide interface. On the Al metal surface, the image potential state pins the standing mode of collectively excited surface electrons and the evanescent wave forms the plasmon mode, which can be reflected in oxide barrier formation. A nanoporous alumina thin film with an amorphous phase oxide layer on the Al surface can enhance light absorption at a shorter wavelength than 382 nm and provide strong Fabry–Perot oscillation in photoreflectance. The cathodoluminescence spectra show the surface plasmon mode as a consequence of the self-ordered oxide nanopores. The Kretschmann configuration of an attenuated total reflection set-up for prism/oxalic solution/Al interface, which also provides a prism-coupled surface plasmon and forms oxide nanopores on the Al surface via a laser-enhanced anodization process.     

硅纳米孔柱阵列的结构和光学特性研究

采用水热腐蚀技术在单晶硅衬底上制备出一种新的硅微米／纳米结构复合体系——硅纳米孔柱阵列(Si-NPA)，并对其表面形貌、结构及光学特性进行研究.Si-NPA的结构复合性体现为 在微米和纳米两个尺度上形成了三个分明的结构层次，即微米尺度的硅柱阵列结构、硅柱上 的纳米多孔结构以及组成孔壁的硅纳米晶粒.积分光反射谱和荧光光谱测试表明，Si-NPA具 有良好的光吸收和光致发光特性.依据Si－NPA积分反射谱的实验数据，采用Kramers-Kronig 变换关系计算得到了Si-NPA的复折射率和复介电函数、吸收系数等光学常数，并由此讨论了 Si-NPA相对于单晶硅的光学特性发生显著变化的原因.最后，通过分析Si-NPA的光吸收系数 与入射光子能量之间的关系，揭示出Si-NPA具有直接带隙半导体的电子结构特征，而且理论 计算得到的Si-NPA的带隙能与其光致发光谱的峰位能很好符合. 关键词： 硅纳米孔柱阵列 光学特性 电子结构 水热腐蚀     

Features and mechanisms of growth of cubic silicon carbide films on silicon

The mechanisms and specific features of the growth of silicon carbide layers through vacuum chemical epitaxy in the range of growth temperatures from 1000 to 700°C have been considered. The structure of the heterojunction formed has been studied using the results of the performed investigations of photoluminescence spectra in the near-infrared wavelength range and the data obtained from the mass spectrometric analysis. It has been found that, in the silicon layer adjacent to the 3C-SiC/Si heterojunction, the concentration of point defects significantly increases and the dislocation structure is not pronounced. According to the morphological examinations of the surface of the growing film, by analogy with the theory of thermal oxidation of silicon, the theory of carbidization of surface silicon layers has been constructed. A distinctive feature of the model under consideration is the inclusion of the counter diffusion fluxes of silicon atoms from the substrate to the surface of the structure. The growth rate of films and the activation energy of diffusion processes have been estimated. The performed experiments in combination with the developed growth model have explained the aggregates of voids observed in practice under the silicon carbide layer formed in the silicon matrix and the possibility of forming a developed surface morphology (the island growth of films) even under conditions using only one flow of hydrocarbons in the reactor.     

Fabrication of nanoporous manganese by laser cladding and selective electrochemical de-alloying

Fabrication of nanoporous Cu-Mn alloy coatings was investigated by a two-step process involving high power laser cladding of a homogeneous Cu40Mn60 alloy coatings followed by selectively electrochemical de-alloying. Auger mapping results indicate that nanoporous manganese was obtained by selective electrochemical etching of the less active Cu component owing to the passivation of the more active manganese in potassium nitrate solution. The surface morphology of the porous Mn was a ribbon-like structure, different from interconnected bicontinuous nanopores that are usually obtained by de-alloying. The influence of de-alloying time, electric potential and temperature on the formation of nanoprosity is systematically investigated. Nanopore sizes can be tailored to be less than 100 nm. Under optimal etching conditions the nanopore size was below 25 nm. The surface area of the nanoporous manganese layer was enhanced by up to 990 times compared with that of a polished sample.     

Initial stages of the growth of barium strontium titanate films on a semi-isolating silicon carbide substrate

The initial stages of the growth of ferroelectric barium strontium titanate films on single-crystal silicon carbide substrates have been studied for the first time. The choice of a substrate with high thermal conductivity has been due to the possibility of applying these structures in powerful microwave devices. The temperature ranges separating the mechanism of the surface diffusion of deposited atoms from the diffusion via a gaseous phase during the growth of multicomponent films have been determined. The studies show that the mass transfer by means of surface diffusion leads to the formation of small-height nuclei that cover a large area of the substrate, whereas the mass transfer via a gaseous phase leads to the formation of a “columnar” islandtype structure with small percentage of covering the substrate and larger island heights.     

Physical model for the evolution of the defect system of silicon carbide with allowance for the internal elastic stress fields during implantation of Al+ and N+ and subsequent annealing

A theoretical analysis is offered for the formation and development of defects in silicon carbide implanted with nitrogen and aluminum ions and then annealed. The diffusion of defects, the formation of complexes, and the influence of the internal elastic stress fields produced by the implanted ions and the created complexes on the migration of interstials are taken into account. The computed distributions of defects agree satisfactorily with the experimental data. Certain kinetic parameters of silicon carbide are estimated numerically. Zh. Tekh. Fiz. 69, 43–50 (October 1999)     

Birefringence and anisotropic optical absorption in porous silicon

The refractive indices and the coefficients of optical absorption by free charge carriers and local vibrations in porous silicon (por-Si) films, comprising nanometer-sized silicon residues (nanocrystals) separated by nanometer-sized pores (nanopores) formed in the course of electrochemical etching of the initial single crystal silicon, have been studied by polarization-resolved IR absorption spectroscopy techniques. It is shown that the birefringence observed in por-Si is related to the anisotropic shapes of nanocrystals and nanopores, while the anisotropy (dichroism) of absorption by the local vibrational modes is determined predominantly by the microrelief of the surface of nanocrystals. It is demonstrated that silicon-hydrogen surface bonds in nanocrystals can be restored by means of selective hydrogen thermodesorption with the formation of a considerable number of H-terminated surface Si-Si dimers.     

Simulation of porous silicon formation and silicon epitaxy on its surface

Processes of porous silicon formation and silicon epitaxy on its surface are studied using the Monte Carlo method. The model for porous silicon formation under anode etching allows for non-uniformity of charge distribution over the silicon-electrolyte interface. Processes of diffusion, generation and recombination of holes, as well as dimensional quantization, are also considered. Gilmer's model, extended to the case of a rough surface, is used to study epitaxy. The structures obtained by simulations at different levels of doping of the crystal substrate and for various parameters (temperature, HF concentration, and anode current density) are presented. Analysis of nanoporous structures showed that the porosity changes with depth, and fractal dimensionality exists below 10 nm. It has been shown that epitaxy, developing by formation of metastable nuclei at the edges of pores, by their subsequent growth along the perimenter and by formation of a thin continuous overhanging layer, may be described within the framework of this model. Three-dimensional images of near-surface layers formed at different stages of epitaxy have been obtained. The dependence of the epitaxy kinetics on the amount of deposited silicon for different structure porosities has been revealed. Institute of Physics of Semiconductors, Siberian Branch, Russian Academy of Science. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 49–56, March, 1999.     

Raman spectroscopic study of femtosecond laser-induced phase transformation associated with ripple formation on single-crystal SiC

Raman spectroscopy was performed to investigate microscopic structural changes associated with a ripple structure formation initiated by femtosecond laser irradiation on the surface of single-crystal silicon carbide. The amorphous phases of silicon carbide, silicon, and carbon were observed. The intensity ratio between amorphous silicon carbide and silicon changed discretely at the boundary between fine and coarse ripples. The physical processes responsible for the formation of the ripple structure are discussed.     

Photonic absorption bands in the spectra of nanoporous metallic films

The light absorption spectra of multilayer nanoporous metallic films containing spherical dielectric inclusions are calculated in the framework of the self-consistent electrodynamic approach (the Korringa-Kohn-Rostoker method). It is demonstrated that the coupling between Mie plasmons in different layers of nanopores leads to the formation of strong light absorption bands over a wide range of frequencies.     

Diffusion in porous silicon carbide

By the example of vanadium and erbium diffusion in porous silicon carbide, the semiconductor porous structure modification during thermal annealing has been simulated and the effect of this modification on impurity diffusion has been considered. A comparison of calculated and experimental profiles of the erbium and vanadium distributions in porous silicon carbide shows that the consideration of porous structure modification due to vacancy redistribution makes it possible to adequately describe diffusion in the porous semiconductor.     

Stable blue-green and ul traviolet photoluminescence from silicon carbide on porous silicon

β-Silicon carbide layers have been prepared by high temperature pyrolysis of polyimide Langmuir-Blodgett films on porous silicon substrate in vacuum. The formation of silicon carbide is confirmed by the IR and XRD spectra. It is found that photoluminescence still exists and appears in the blue-green and ultraviolet regions after thermal treatment at 900°C. These results indicate that the silicon carbide layers, which are formed, are responsible for the blue-green luminescence.     

Model of silicon carbide formation on porous substrates

The formation of thin silicon carbide layers as a result of solid-phase processes is related to the evolution of nanoscale porosity and chemical reactions on pore surfaces. Numerical experiments, which simulate blistering under the action of Xe+ ions in the metal-insulator (Mo/Si) bilayer make it possible to establish the relationship between the porosity parameters and layer stresses and the irradiation conditions. Similar patterns in the formation of defects (pores and cracks) in crystalline silicon characterize its interaction with carbon dioxide when silicon carbide is formed. The calculated characteristics of the nucleation in the Mo/Si bilayers are analyzed to optimize the solid-phase epitaxy of silicon carbide.     

Theoretical analysis of the formation of impurity precipitates in nanocavities. I. thermodynamic analysis

The formation of the impurity phase in materials containing nanopores is investigated theoretically. The formation of impurity clusters on the inner surface of pores is studied using the thermodynamic approach. The most advantageous states of metal impurities in silicon are determined, and comparison with available experimental data is carried out. The possibility of the formation of cobalt nanoclusters in subsurface cavities in copper is substantiated theoretically.     

Synthesis of epitaxial silicon carbide films through the substitution of atoms in the silicon crystal lattice: A review

A review of recent advances in the field of epitaxial growth of SiC films on Si by means of a new method of epitaxial substitution of film atoms for substrate atoms has been presented. The basic statements of the theory of the new method used for synthesizing SiC on Si have been considered and extensive experimental data have been reported. The elastic energy relaxation mechanism implemented during the growth of epitaxial SiC films on Si by means of the new method of substitution of atoms has been described. This method consists in substituting a part of carbon atoms for silicon matrix atoms with the formation of silicon carbide molecules. It has been found experimentally that the substitution for matrix atoms occurs gradually without destroying the crystalline structure of the matrix. The orientation of the film is determined by the “old” crystalline structure of the initial silicon matrix rather than by the silicon substrate surface only, as is the case where conventional methods are used for growing the films. The new growth method has been compared with the classical mechanisms of thin film growth. The structure and composition of the grown SiC layers have been described in detail. A new mechanism of first-order phase transformations in solids with a chemical reaction through an intermediate state promoting the formation of a new-phase nuclei has been discussed. The mechanism providing the occurrence of a wide class of heterogeneous chemical reactions between the gas phase and a solid has been elucidated using the example of the chemical interaction of the CO gas with the single-crystal Si matrix. It has been shown that this mechanism makes it possible to grow a new type of templates, i.e., substrates with buffer transition layers for growing wide-band-gap semiconductor films on silicon. A number of heteroepitaxial films of wide-band-gap semiconductors, such as SiC, AlN, GaN, and AlGaN on silicon, whose quality is sufficient for the fabrication of a wide class of micro- and optoelectronic devices, have been grown on the SiC/Si substrate grown by solid-phase epitaxy.