Raman scattering in mosaic silicon carbide films

The Raman spectra of mosaic silicon carbide films grown on silicon substrates by solid-state epitaxy have been studied. The main polytypes forming the film material have been determined. It has been experimentally revealed that the properties of the silicon carbide film are changed after an aluminum nitride film is deposited on the former film. This has been interpreted as a manifestation of good damping properties of the SiC film when layers of other semiconductors are grown on it.     

Specific features and mechanisms of photoluminescence of nanostructured silicon carbide films grown on silicon in vacuum

The light-emitting properties of cubic silicon carbide films grown by vacuum vapor phase epitaxy on Si(100) and Si(111) substrates under conditions of decreased growth temperatures (T _gr ∼ 900–700°C) have been discussed. Structural investigations have revealed a nanocrystalline structure and, simultaneously, a homogeneity of the phase composition of the grown 3C-SiC films. Photoluminescence spectra of these structures under excitation of the electronic subsystem by a helium-cadmium laser (λ_excit = 325 nm) are characterized by a rather intense luminescence band with the maximum shifted toward the ultraviolet (∼3 eV) region of the spectral range. It has been found that the integral curve of photoluminescence at low temperatures of measurements is split into a set of Lorentzian components. The correlation between these components and the specific features of the crystal structure of the grown silicon carbide layers has been analyzed.     

七种新的SiC六方多型体

本文提出了用X射线劳埃法鉴定SiC六方多型体类型的方法。在详细地研究了xH类型与基本类型6H,15R和4H的倒易阵点间相互配置关系的基础上,推导出了这些具体关系,这种关系对于xH—6H来说仅有十二种,对于xH—15R仅有三十种,对于xH—4H仅有八种。上述的点间关系表和推求xH类型单位晶胞密堆积层数的公式同样可以普遍适用于以其他X射线照相法鉴定SiC六方多型体类型的工作中。用本文所提出的方法研究了许多实验室升华法制备的SiC单晶体以及部分工业SiC晶体。发现了七种六方SiC新多型体141H,80H,58H,55H,15H,9H和7H。新类型的定间羣为C_3v¹(C3m),六方晶胞c轴参数分别为:355.26?,201.57?,146.14?,138.58?,37.794?,22.676?和17.637?。     

SiC polytypes process affected by Ge predeposition on Si(111) substrates

Structural and optical measurements were performed on silicon carbide (SiC) samples containing several polytypes. The SiC samples investigated were grown on (111) Si substrates by solid source molecular beam epitaxy (SSMBE). Several quantities of Ge were predeposited before the growth procedure. The influence of Ge on the SiC polytypes formation was studied by X-Ray, FTIR and μ-Raman characterizations methods. The spectra of the samples with less than one Ge monolayer exhibit a mixture of 2H, 15R and 3C–SiC polytypes. This mixture is due to the mismatch between the heterostructure layers. We propose that the Ge predeposition in the heterostructure can be used to stabilize and unify the polytypes formation.     

New method for growing silicon carbide on silicon by solid-phase epitaxy: Model and experiment

A new method of solid-state epitaxy of silicon carbide (SiC) on silicon (Si) is proposed theoretically and realized experimentally. Films of various polytypes of SiC on Si(111) grow through a chemical reaction (at T = 1100–1400°C) between single-crystal silicon and gaseous carbon oxide CO (at p = 10–300 Pa). Some silicon atoms transform into gaseous silicon oxide SiO and escape from the system, which brings about the formation of vacancies and pores in the silicon near the interface between the silicon and the silicon carbide. These pores provide significant relaxation of the elastic stresses caused by the lattice misfit between Si and SiC. X-ray diffraction, electron diffraction, and electron microscopy studies and luminescence analysis showed that the silicon carbide layers are epitaxial, homogeneous over the thickness, and can contain various polytypes and a mixture of them, depending on the growth conditions. The typical pore size is 1 to 5 μm at film thicknesses of ～20 to 100 nm. Thermodynamic nucleation theory is generalized to the case where a chemical reaction occurs. Kinetic and thermodynamic theories of this growth mechanism are constructed, and the time dependences of the number of new-phase nuclei, the concentrations of chemical components, and the film thickness are calculated. A model is proposed for relaxation of elastic stresses in a film favored by vacancies and pores in the substrate.     

Synthesis of α -SiC nanocrystals by carbothermal reduction of spherical nanoparticles of amorphous silicon dioxide

The method for carbothermal reduction of spherical particles of amorphous silicon dioxide is developed, and hexagonal α-SiC polytype nanocrystals are synthesized. The prepared samples are characterized by X-ray diffraction, Raman spectroscopy, photoluminescence spectroscopy, and electron microscopy. The silicon carbide nanocrystals prepared have sizes in the range 5–50 nm depending on the diameter of initial silicon dioxide particles. A detailed analysis of the positions of the lines in the Raman spectra, their broadening, and shift makes it possible to reliably establish that the samples under investigation predominantly contain the 6H and 4H silicon carbide polytypes and insignificant amounts of the 2H and 3C phases. The 15R and 21R polytypes in the samples are absent. It is noted that the samples are characterized by a substantial size effect: the luminescence intensity of small silicon carbide nanocrystals is more than three times higher than that of large SiC nanocrystals.     

Wannier-Stark resonances under strong localization conditions in natural silicon-carbide superlattices

A detailed experimental investigation is made of the electronic transport under conditions of Wannier-Stark localization of carriers in a natural superlattice of hexagonal polytypes of silicon carbide. The 4H and 6H polytypes, which possess different superlattice and miniband spectrum parameters, are employed. Direct measurements of the electronic current versus the average electric field in the active region of the sample revealed a series of regions of negative differential conductivity in fields ranging from 500 to 2100 kV/cm. Analysis of the results shows that the observed current resonances are associated with the development of the Wannier-Stark quantization process and are due to conduction mechanisms such as hopping conduction, induced between the levels of a Wannier-Stark ladder by a resonant electron-phonon interaction, and the resonant interminiband tunneling from the first into the second miniband. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 2, 105–109 (25 July 1996)     

Electronic structure and spatial distribution of the spin density of shallow nitrogen donors in the SiC lattice

The discovery of unique magnetooptical properties of paramagnetic centers in silicon carbide, which make it possible to control spins of small arrays of centers of atomic sizes to single centers at room temperatures, using the techniques of optical detection of the magnetic resonance, posed a number of problems, among which one of the main ones is the creation of conditions under which spin relaxation effects are minimized. As studies of properties of spin nitrogen-vacancy centers in diamond showed, the main contribution to spin relaxation is made by the interaction with nitrogen donors, being a major impurity in diamond. A similar problem exists for silicon carbide, since nitrogen donors are also basic background impurities. The objective of this work is to study the spatial distribution of the spin density of nitrogen donors in two basic silicon carbide polytypes, i.e., 4H-SiC and 6H-SiC, to use this information for minimizing the interaction of nitrogen donors with paramagnetic centers in silicon carbide. The results of the study are analyzed by magnetic resonance methods; the spin density distribution on the nearest coordination spheres of nitrogen donors occupying carbon sites in silicon carbide is determined. It is concluded that paramagnetic centers in the 4H-SiC polytype, including silicon vacancies, can be more stable to the interactions with unpaired donor electrons, since electrons are not localized on the coordination sphere closest to the paramagnetic center in this case.     

Convergent-beam electron diffraction analysis of GaSe crystals grown from the melt by different doping elements

Summary A systematic investigation of the structural modifications of GaSe crystals, grown from the melt by different doping elements, has been performed by convergent-beam electron diffraction technique, in order to analyse the dependence of the structure on the doping atoms. Iodine-doped crystals have shown the ɛ-2H hexagonal and γ-3R rhombohedral polytypes. The structure of crystals doped either by silver, or copper, or cadmium, or zinc, or arsenic has been proved a mixture of the ɛ-2H hexagonal and of γ, 9 R, 12 R, 15 R rhombohedral phases. Ingots doped either by zinc or arsenic have shown the ɛ polytype prevailing in some zones and the γ structure in the other ones. The ɛ modification is dominant in ingots doped by the remaining atoms.     

Lattice dynamics and the incommensurability of the structures of hexagonal polytypes of diamond

In the density functional theory approximation, we have calculated the phonon dispersions and the densities of vibrational states of the 2H, 3C, 4H, 6H, and 8H hexagonal polytypes of diamond. We have found that a one-dimensional incommensurate modulation of the structure arises along the hexagonal axis, the parameter of which not only exceeds the parameters of basic translations of the crystal lattices of polytypes, but also is not a multiple of them. Based on the estimation of interactions between bilayers of carbon atoms in the structure of polytypes, we have assumed that competing interactions between bilayers are the main mechanism by which incommensurability arises. We have shown that optical measurements of vibrational frequencies in the center of the Brillouin zone of polytypes make it possible to retrieve the dispersion of acoustic branches of cubic diamond.     

研究SiC多种多型体连生方式的X射线方法

本文提出了一种研究碳化硅多种类型连生的晶体中连生次序的实验方法。用超声波研磨法将极硬的薄片状碳化硅单晶逐层磨薄并逐次摄取双套劳埃照片。对劳埃法得出的SiC多型类型数据的分析可以确定各种连生多型体的次序和比例。用一晶体的实验结果对Mitchell的碳化硅多型性机理作了讨论。     

Computer simulation of the structure and raman spectra of GaAs polytypes

The structure, energy of formation, and Raman spectra of several polytypes (3C, 2H, 4H, and 8H) of gallium arsenide GaAs have been investigated using quantum mechanical calculations based on the local density functional theory. It has been found that the energy of the formation of hexagonal polytypes increases with an increase in the length of the periodicity and approaches the value corresponding to the ground state, i.e., to the structure of the 3C polytype. It has been shown that the calculated frequencies of normal vibrations of different polytypes are consistent, with good accuracy (±6 cm^?1), with the scheme of folding of the phonon branches. In the calculated Raman spectra of the polytypes, there are new lines (forbidden in the spectrum of the 3C polytype) which can serve as characteristic lines of other polytypes. Similar lines can be found in the Raman spectra of GaAs nanowhiskers. This result has opened up new prospects for the application of Raman spectroscopy to the characterization of the structure of these nano-objects.     

Theoretical study of the thermodynamic stability and electronic structure of thin films of 3C, 2H,and 2D silicon carbides

Silicon carbide is among the most common materials used in semiconductor engineering. Silicon carbide thin films are attractive from the standpoint of designing devices based on heterojunctions. This is due to a characteristic feature of this compound, such as polytypism, leading to the difference in the physical properties and also hampering the preparation of high-quality material samples. In this work, the thermodynamic stability and electronic structure of thin films based on the polytypes 3C, 2H, and 2D with a thickness of a few nanometers have been studied.     

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.     

Nonlinear-optical and structural properties of nanocrystalline silicon carbide films

The aim of this study is to investigate the nonlinearity of refraction in nanostructured silicon carbide films depending on their structural features (synthesis conditions for such films, substrate temperature during their deposition, concentration of the crystalline phase in the film, Si/C ratio of atomic concentrations in the film, and size of SiC nanocrystals formed in the film). The corresponding dependences are obtained, as well as the values of nonlinear-optical third-order susceptibility χ⁽³⁾(ω; ω, −ω, ω) for various silicon polytypes (3C, 21R, and 27R) which exceed the value of χ⁽³⁾ in bulk silicon carbide single crystals by four orders of magnitude.     

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.     

Heteroepitaxial GaN films on silicon substrates with porous buffer layers

New complex buffer layers based on a porous material have been developed for epitaxial growth of GaN films on Si substrates. The characteristics of gallium nitride heteroepitaxial layers grown on silicon substrates with new buffer layers by metal-organic vapor phase epitaxy are investigated. It is shown that the porous buffer layers improve the electric homogeneity and increase the photoluminescence intensity of epitaxial GaN films on Si substrates to the values comparable with those for reference GaN films on Al₂O₃ substrates. It is found that a fianite layer in a complex buffer is a barrier for silicon diffusion from the substrate into a GaN film.     

Effect of thickness on the microstructure of GaN films on Al2O3 (0001) by laser molecular beam epitaxy

Heteroepitaxial GaN films are grown on sapphire (0001) substrates using laser molecular beam epitaxy. The growth processes are in-situ monitored by reflection high energy electron diffraction. It is revealed that the growth mode of GaN transformed from three-dimensional (3D) island mode to two-dimensional (2D) layer-by-layer mode with the increase of thickness. This paper investigates the interfacial strain relaxation of GaN films by analysing their diffraction patterns. Calculation shows that the strain is completely relaxed when the thickness reaches 15 nm. The surface morphology evolution indicates that island merging and reduction of the island-edge barrier provide an effective way to make GaN films follow a 2D layer-by-layer growth mode. The 110-nm GaN films with a 2D growth mode have smooth regular hexagonal shapes. The X-ray diffraction indicates that thickness has a significant effect on the crystallized quality of GaN thin films.     

The properties of boron carbide/silicon heterojunction diodes fabricated by Plasma-Enhanced Chemical Vapor Deposition

p-n heterojunction diodes have been fabricated from boron carbide (B_1–x C _x ) and n-type Si(111). Boron carbide thin films were deposited on Si(111) using Plasma-Enhanced Chemical Vapor Deposition (PECVD) from nido-pentaborane (B₅H₉) and methane (CH₄). Composition of boron carbide thin films was controlled by changing the relative partial pressure ratio between nido-pentaborane and methane. The properties of the diodes were strongly affected by the composition and thickness of boron carbide layer and operation temperatures. Boron carbide/silicon heterojunction diodes show rectifying properties at temperatures below 300° C. The temperature dependence of reverse current is strongly dependent upon the energy of the band gap of the boron carbide films.     

Photoluminescence of nanostructured cubic-lattice silicon carbide films grown on a silicon wafer

The light-emitting properties of cubic-lattice silicon carbide SiC films grown on Si(100) and Si(111) substrates with VPE at low temperatures (T _gr ∼ 700°C) are discussed. Investigations of the grown films reveal a homogeneous nanocrystalline structure involving only the 3C-SiC phase. When the electron subsystem of the structure is excited by a He-Cd laser emitting at λ_exit = 325 nm, the photoluminescence (PL) spectra contain a rather strong emission band shifted by about 3 eV toward a short-wave spectral region. At low temperatures, the PL integral curve is split into a set of Lorentz components. The relation between these components and the peculiarities of the energy spectrum of electrons in the nanocrystalline grains of the silicon carbide layers is discussed.