Synthesis of silicon carbide hexagonal nanoprisms

SiC hexagonal nanoprisms have been prepared by a reaction of multiwall carbon nanotubes and Si vapor in an Astro furnace at 1450 °C for 3 h. The polytype, morphology, crystal structure of the nanoprisms were studied by X-ray powder diffraction, scanning electron microscopy and high resolution transmission electron microscopy, showing their hexagonal nanoprism shapes with a 3C-SiC single crystal structure with a diameter of about 100 nm and 2 μm in length. The photoluminescence spectrum of the nanoprisms exhibits a significant blue-shift relative to bulk 3C-SiC and other nanostructured SiC. The possible growth mechanism that controls the nanostructure formation is also analysed. PACS 42.70.Nq; 68.37.-d; 78.55.-m; 78.67.Bf; 81.07.Vb     

Recrystallization of hexagonal silicon carbide after gold ion irradiation and thermal annealing

Silicon carbide (SiC) single crystals with the 6H polytype structure were irradiated with 4.0-MeV Au ions at room temperature (RT) for increasing fluences ranging from 1?×?10¹² to 2?×?10¹⁵ cm^?2, corresponding to irradiation doses from ~0.03 to 5.3 displacements per atom (dpa). The damage build-up was studied by micro-Raman spectroscopy that shows a progressive amorphization by the decrease and broadening of 6H-SiC lattice phonon peaks and the related growth of bands assigned to Si–Si and C–C homonuclear bonds. A saturation of the lattice damage fraction deduced from Raman spectra is found for ~0.8?dpa (i.e. ion fluence of 3?×?10¹⁴ cm^?2). This process is accompanied by an increase and saturation of the out-of-plane expansion (also for ~0.8?dpa), deduced from the step height at the sample surface, as measured by phase-shift interferometry. Isochronal thermal annealing experiments were then performed on partially amorphous (from 30 to 90%) and fully amorphous samples for temperatures from 200 °C up to 1500 °C under vacuum. Damage recovery and densification take place at the same annealing stage with an onset temperature of ~200 °C. Almost complete 6H polytype regrowth is found for partially amorphous samples (for doses lower than 0.8 dpa) at 1000 °C, whereas a residual damage and swelling remain for larger doses. In the latter case, these unrelaxed internal stresses give rise to an exfoliation process for higher annealing temperatures.     

Monte Carlo study of the hetero-polytypical growth of cubic on hexagonal silicon carbide polytypes

In this article we use three dimensional kinetic Monte Carlo simulations on super-lattices to study the hetero-polytypical growth of cubic silicon carbide polytype (3C-SiC) on misoriented hexagonal (4H and 6H) substrates. We analyze the quality of the 3C-SiC film varying the polytype, the miscut angle and the initial surface morphology of the substrate. We find that the use of 6H misoriented (4°–10° off) substrates, with step bunched surfaces, can strongly improve the quality of the cubic epitaxial film whereas the 3C/4H growth is affected by the generation of dislocations, due to the incommensurable periodicity of the 3C (3) and the 4H (4) polytypes. For these reasons, a proper pre-growth treatment of 6H misoriented substrates can be the key for the growth of high quality, twin free, 3C-SiC films.     

Trap-recharging waves versus damped,forced charge-density oscillations in hexagonal silicon carbide

The structural parameters and hydrostatic pressure coefficients of CdS_xTe_1-x in the two phases, namely zinc-blende and NaCl as well as the transition pressures from zinc-blende to NaCl structures at various S concentrations are presented. The calculations are performed using the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT) in the local density approximation (LDA), and two developed refinements, namely the generalized gradient approximation (GGA) of Perdew et al. for the structural properties and Engel-Vosko for the band structure calculations. Detailed comparisons are made with published experimental and theoretical data and show generally good agreement. The present results regarding the studied quantities for compositions x in the 0–1 range (0 < x < 1) and for the NaCl phase are predictions and may serve as a reference for experimental work.     

Phonon focusing effect on thermal conductivity of hexagonal group III-nitrides and silicon carbide crystals

Thermal conductivity κ of 4H-, 6H-SiC and wurtzite GaN, InN, AlN crystals is calculated accounting phonon focusing effect at low temperatures with only diffusive phonon boundary scattering. The orientation dependence of thermal conductivity is similar in these materials. Thermal conductivity is enhanced in the direction of approximately 45^∘ to the c axis up to 27% for GaN, 24% for 6H-SiC, 32% for InN, and 9% for AlN compared to the isotropic case for the circular cross-section and finite-length samples. Contributions of transverse T1 and T2 modes are nearly the same, about 40–45%, while the focusing of T2 mode contributes essentially to the angular dependence of κ. We find that the phonon focusing does not change κ value (within 5%) in the direction of 60^∘ to the c axis in all hexagonal crystals studied so far.     

Effect of plasmon-phonon excitations on the coefficient of reflection from the surface of hexagonal silicon carbide

The coefficient R(ν) of reflection from the surface of 6H-SiC single crystals is studied in the case in which the long-wavelength optical vibrations of the lattice are coupled with the electron plasma. It is shown for the first time that the anisotropy of the properties of electrons and phonons in 6H-SiC gives rise to special features in the spectrum of the coupled vibrations and the transparency regions. It is found, in particular, that if the axis of the crystal lies in the polarization plane of the incident radiation (0<θ<90°), for 30 cm⁻¹⩽ν _p⊥<320 cm⁻¹ the spectrum of R(ν) acquires three regions of transparency and opacity, and for ν _p⊥⩾320 cm⁻¹ four such regions, which are absent in an isotropic medium. The width of these regions is found to depend on the electron concentration in the conduction band and on the angle θ. Zh. éksp. Teor. Fiz. 116, 646–654 (August 1999)     

Photoluminescent properties of silicon carbide and porous silicon carbide after annealing

Photoluminescent (PL) p-type 6H porous silicon carbides (PSCs), which showed a strong blue-green photoluminescence band centered at approximately 490 nm, were annealed in Ar and vacuum conditions. The morphological, optical, and chemical states after annealing are reported on electrochemically etched SiC semiconductors.The thermal treatments in the Ar and vacuum environments showed different trends in the PL spectra of the PSC. In particular, in the case of annealing in a vacuum, the PL spectra showed both a weak red PL peak near 630 nm and a relatively intense PL peak at around 430 nm in the violet region. SEM images showed that the etched surface had spherical nanostructures, mesostructures, and islands. With increasing annealing temperature it changes all spherical nanostructures. The average pore size observed at the surface of the PSC before annealing was of the order of approximately 10 nm.In order to investigate the surface of a series of samples in detail, both the detection of a particular chemical species and the electronic environments at the surface are examined using X-ray photoelectron spectroscopy (XPS). The chemical states from each XPS spectrum depend differently before and after annealing the surface at various temperatures. From these results, the PL spectra could be attributed not only to the quantum size effects but also to the oxide state.     

Luminescence spectra of hexagonal forms of silicon carbide in mosaic films grown by solid-state epitaxy

The luminescence spectra of silicon carbide films grown on silicon by solid-state epitaxy have been studied. It has been shown that, depending on the growth conditions, one can obtain films of different SiC polytypes, including the cubic and hexagonal ones. In many cases, the films thus grown display a mixture of various polytypes, but it is possible to prepare films of predominantly hexagonal symmetry (the coexistence of the 4H and 2H hexagonal phases, which are close in properties, is also possible). It thus has been demonstrated that the silicon carbide films grown on silicon by solid-state epitaxy are promising for application as damping layers in fabrication of wide-band-gap hexagonal semiconductors on silicon substrates.     

On the existence and stability of double-walled armchair silicon carbide nanotubes

A systematic study of type 1 armchair double-walled SiC nanotubes (DWNTs) (n,n)@(m,m) (3≤n≤6;7≤m≤12) using the finite cluster approximation is presented. The geometries of the tubes have been spin optimized using the hybrid functional B3LYP (Becke’s three-parameter exchange functional and the Lee-Yang-Parr correlation functional) and the all-electron 3-21G* basis set. The study indicates that the stabilities of the double-walled SiC nanotubes are of the same order as those of single-walled SiC nanotubes suggesting the possibilities of experimental synthesis of both single-walled and double-walled SiC nanotubes. The binding energy per atom or the cohesive energy of the double-walled nanotubes depends not only on the number of atoms but also on the coupling of the constituent single-walled nanotubes. The formation energy of the DWNTs is found to be maximum when the interlayer separation is about 3.5 Å. The DWNTs (n,n)@(n+4,n+4) are found to have large formation energies. In particular, (5,5)@(9,9) DWNT is the most stable tube in our study with a binding energy per atom of 5.07 eV, the largest formation energy of 12.39 eV, an interlayer separation of 3.58 Å and a “band gap” of 1.97 eV. All double-walled SiC nanotubes are found to be semiconductors, with the band gaps decreasing from single-walled nanotubes to double-walled nanotubes.     

On the calculation of dielectric properties from computer simulations

We report a reaction field calculation for Stockmayer particles at reduced density $\text{?}{}^1\text{= 0.8}$, reduced temperature $\text{T}{}^1\text{= 1.35}$ and reduced dipole moment $\text{μ}{}^1\text{= 1.64}$ using a relatively large cutoff distance for the dipolar interaction. The dipole-dipole correlation function is compared with corresponding simulations using truly periodic boundary conditions. The mean square fluctuation of the total dipole moment are shown to exhibit a long period oscillation, precluding a determination of the dielectric constant with an accuracy better than 15%, in a Monte-Carlo calculation of the order of 5 × 10⁶ configurations.     

Excitation of stable transverse wavepackets with quadratic and cubic susceptibilities

We have identified a family of (2+1)D spatial solitary waves which can stably propagate in bulk media in the presence of coexisting diffraction, self-focusing Kerr and quadratic nonlinearities. In a conspicuous range of excitation conditions close to the stationary solutions, the emerging wavepackets are immune to the detrimental occurrence of filamentation and collapse, typical of pure Kerr media. The presence of a second-order contribution to the cubic nonlinear response is, therefore, able to prevent optical damage in applications relying on self-guidance. We show that the cross-phase modulation plays an important effect on stability. Our estimate shows that the effects of the cubic susceptibility cannot be neglected below a certain beam size in realistic crystals (e.g. KTP or similar).     

The nonlinear refractive indices and nonlinear third-order susceptibilities of quadratic crystals

The third-order nonlinear susceptibilities and nonlinear refractive indices of KDP, BBO, and LiNbO₃ crystals at wavelengths of 1064 and 532 nm are measured using the Z-scan technique. The measurements are made for different energies of incident radiation and different focusing conditions and crystal lengths. It is found that, as the angle between the light beam and the principal optic axis of a KDP crystal increases, the nonlinearity drops, with its magnitude at a wavelength of 1064 nm being higher than at 532 nm. For a BBO crystal, the nonlinearity dispersion is normal. The mechanisms of nonlinear losses in KDP, BBO, and LiNbO₃ crystals are identified. The values of the nonlinear susceptibilities thus obtained are analyzed and compared with the results of calculations based on an empirical model and with the data of other authors.     

Positron annihilation in diamond,silicon and silicon carbide

Recent positron lifetime and doppler broadening results on silicon, diamond and silicon carbide are presented in this contribution. In as-grown Czochralski Si ingols vacancies are found to be retained after growth at concentrations typically around 3×10¹⁶/cm³. 10 MeV eleciron irradiation of variously doped Si wafers shows that only high doping concentrations well in excess of the interstitial oxygen concentration causes an increase in the amount of monovacancies retained.In porous silicon very long-lived positronium lifetimes in the range 40–90 ns are found. Polycrystalline diamond films contain various types of vacancy agglomerates but these are found to be inhomogeneously distributed from crystallite to crystallite. Electron irradiation of silicon carbide results in two vacancy-related lifetimes which are interpreted as resulting from carbon and silicon vacancies.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Thermopower of biomorphic silicon carbide

The thermopower coefficients of cubic bio-SiC, a high-porosity semiconductor with cellular pores prepared from the biocarbon template of white eucalyptus wood, and single-crystal β-SiC taken as a reference are measured in the temperature range 5–280 K. It is revealed that, in the low-temperature range, the samples are characterized by a thermopower contribution associated with the electron drag by phonons. The thermopower of the bio-SiC samples is measured both along and across the empty pore channels and is found to be anisotropic. Two models are proposed to account for the anisotropy of the thermopower in cubic bio-SiC.     

Negative crystals of silicon carbide

Systems of negative silicon carbide crystals are classified and studied by experimental methods. The crystal structure and morphology forming during growth, etching, and erosion are discussed.     

Magnetism of hydrogen-irradiated silicon carbide

Spin-polarized density functional theory is used to study two-hydrogen defect complexes in silicon carbide. We find that the magnetism depends on the distances of the two hydrogen atoms. Magnetism appears when the two hydrogen defects are distant from each other, and magnetism cancels out if they are close to each other. The critical distance between the two hydrogen defects is determined.