Carbon Sublattice Research Articles

V8C7–δ superstructure in nonstoichiometric vanadium carbide powders

Neutron and X-ray diffraction methods have been applied jointly to study the crystal structure of the ordered phase of V8C7–δ in macro- and nanocrystalline vanadium carbide powders. The deviation of the composition of the ordered V8C7–δ phase from the stoichiometric composition V8C7 has been established. The magnitudes and directions of the displacements of C and V atoms have been determined. It has been shown that vanadium atoms are displaced towards a vacancy, increasing the electron density of the vacancy position of the carbon sublattice.

Model for conductivity compensation of moderately doped n- and p-4H-SiC by high-energy electron bombardment

The model of conductivity compensation in SiC under irradiation with high-energy electrons is presented. The following processes are considered to cause a decrease in the free carrier concentration: (i) formation of deep traps by intrinsic point defects, Frenkel pairs produced by irradiation; (ii) "deactivation" of the dopant via formation of neutral complexes including a dopant atom and a radiation-induced point defect; and (iii) formation of deep compensating traps via generation of charged complexes constituted by a dopant atom and a radiation-induced point defect. To determine the compensation mechanism, dose dependences of the deep compensation of moderately doped SiC (CVD) under electron irradiation have been experimentally studied. It is demonstrated that, in contrast to n-FZ-Si, moderately doped SiC (CVD) exhibits linear dependences (with a strongly nonlinear dependence observed for Si). Therefore, the conductivity compensation in silicon carbide under electron irradiation occurs due to deep traps formed by primary radiation defects (vacancies and interstitial atoms) in the silicon and carbon sublattices. It is known that the compensation in silicon is due to the formation of secondary radiation defects that include a dopant atom. It is shown that, in contrast to n-SiC (CVD), primary defects in only the carbon sublattice of moderately doped p-SiC (CVD) cannot account for the compensation process. In p-SiC, either primary defects in the silicon sublattice or defects in both sublattices are responsible for the conductivity compensation.

First-principles study of stability and properties on β-SiC/TiC(111) interface

The interfacial properties of β-SiC/TiC(111), such as work of adhesion, interface energy, fracture toughness, bonding nature, were investigated using first-principles calculations. Twenty four interface models with different terminations, carbon sublattice, and stacking sites were investigated. The thermodynamic stability of SiC/TiC(111) decreases as the order of C/C, Si/Ti, C/Ti, and Si/C terminations. The C/C-terminated top-site-stacked models (CCU3, CCT3) are most stable with the largest work of adhesion, smallest interface energy, and largest interfacial fracture toughness. The interfacial fracture toughness is predicted as 3.6 ∼ 4.3 MPa·m1/2. The valence electron density and partial density of states indicate that the interfacial bonding is mainly contributed from covalent C-C interactions caused by the hybridization of C-2p. The interfacial Si-C and Ti-C bonds are less covalent and much weaker than the interior ones, and the interfacial bonds are more inclined to decompose. The carbon layer is likely to form on the interface due to the decomposition. Our calculation results are compared and in line with previous investigations.

Measurement of Ionization Energies of Nitrogen in 4H-SiC by Traveling-Wave Method

The impurity bands and corresponding ionization energies of nitrogen atoms in a 4H-SiC crystal with a concentration of 1×1019 cm-3 are measured by a nondestructive and noncontact traveling-wave method. When a SiC sample was placed near the surface of a surface acoustic wave device, its conductivity can be obtained by measuring the attenuation of the piezo-potential traveling-wave grazing along the surface of the sample. Temperature-dependent conductivities corresponding to a freeze-out process of free carriers excited from nitrogen atoms were observed, and the corresponding ionization energies of the nitrogen atoms were estimated by the Arrhenius plot method. The ionization energies in the impurity bands originating from splits of the doping atoms at cubic and hexagonal sites in the carbon sublattice are 72.89 and 47.89 meV, respectively, at room temperature. The ionization energies are in good agreement with the results reported in other theoretical and experimental studies. We also found that the skin depth of the traveling wave in the sample is below 1 mm and that the mobility of the carriers is strongly affected by both ionized dopants and charged surface defects in the depletion region near the surface of the sample. The effects of the sample and traveling wave such as the polarization effects of the crystal and the frequency effects of the traveling wave are discussed.

Point Defects in Chemical-Vapor Deposited Diamond, High-Purity Semi-Insulating SiC, and Epitaxial GaN

The native and impurity induced point defects in undoped chemi-cal-vapor deposited diamond, high-purity semi-insulating SiC and grown by molecular beam epitaxy GaN were investigated by isothermal charge-based deep level transient spectroscopy (Q-DLTS). It was found that deep levels in undoped diamond have a continuous energy spectrum with two maximums at energies 0.71 and 0.54 eV. Several discrete deep levels were found in SiC. Defect level E1 is attributed to atom displacement on the carbon sublattice in SiC. Levels E2, E4 are related to (Vc - Vsi) complex and silicon vacancy Vsi, respectively. Obtained values of active-tion energy and capture cross-section allow expecting that deep level E3 associated with the carbon vacancy Vc, while acceptor level En is vanadium-induced level. Eight deep levels were observed in GaN. Levels E1, E2, E7, and E8 have been observed in low doped n-GaN are characterized for the first time in our studies.

Ab initio Green's function calculation of hyperfine interactions for shallow defects in semiconductors

AbstractThe structural identification of shallow defects and the control of their concentration is one of the key ingredients in semiconductor technology. Electron paramagnetic resonance (EPR) is the most sensitive experimental technique to address this problem, but requires comparative theoretical simulation of the microscopic structures. Whereas the standard supercell approach provides an accurate first principles analysis of localized deep defects, the wave function of shallow dopants often extends over several thousands of atoms and has to be treated in an empirical one‐particle approach, the effective mass approximation (EMA). We report that modeling the shallow defects via a Green's functions approach allows an ab inito description of the so‐called central‐cell correction to EMA that accounts for the specific local part of the potential. The spatial distribution of the donor wave functions comes out within an accuracy that allows a prediction of hyperfine (hf) interactions including the Kohn–Luttinger oscillations within the ligand hf structure – an effect that in quantum computing plays a key role in the control of the exchange coupling of qubits in semiconductors. For phosphorus in silicon, we show that in strained material these Kohn–Luttinger oscillations are partially suppressed. Our results indicate that in contrast to the prediction of the EMA, there exists no high‐stress limit for the reduction of the central P‐related hf splitting. The application of the approach onto donors in SiC confirms an incorporation of the P atoms onto both, the silicon as well as the carbon sublattice, if the donors are incorporated far away from thermal equilibrium.

Composition of cementite in the dependence on the temperature. In situ neutron diffraction study and Ab initio calculations

The structure state of carbon eutectoid steel has been studied by the in situ neutron diffraction method in the temperature range from room temperature to 800°C. It has been shown that an increase in the temperature is accompanied by a decrease in the concentration of carbon in cementite, whereas its weight fraction and the parameters of the orthorhombic lattice change slightly. The ab initio calculations of the nonstoichiometric carbides Fe3Cx (0 < x < 1) indicate that the structure of cementite remains stable upon the appearance of vacancies in the carbon sublattice with a relatively low formation energy. Thus, cementite should be considered as an interstitial phase Fe3Cx with a wide homogeneity range.

First-principles study of uranium carbide: Accommodation of point defects and of helium, xenon, and oxygen impurities

Point defects and volatile impurities (helium, xenon, oxygen) in uranium monocarbide UC are studied by first-principles calculations. Preliminarily, bulk properties of UC and of two other uranium carbide phases, ${\text{UC}}_{2}$ and ${\text{U}}_{2}{\text{C}}_{3}$, are calculated in order to compare them to experimental data and to get confidence in the use of the generalized gradient approximation for this class of compounds. The subsequent study of different types of point defects shows that the carbon sublattice best accommodates the defects. The perturbation of the crystal structure induced by the defects is weak and the interaction between defects is found short range. Interstitial carbon dumbbells possibly play an important role in the diffusion of carbon atoms. The most favorable location of diluted helium, xenon, and oxygen impurities in the UC crystal lattice is then determined. The rare-gas atoms occupy preferably a uranium substitution site or a uranium site in a U-C bivacancy. But their incorporation in UC is, however, not energetically favorable, especially for xenon, suggesting their propensity to diffuse in the material and/or form bubbles. On the other hand, oxygen atoms are very favorably incorporated as diluted atoms in the UC lattice, confirming the easy oxidation of UC. The oxygen atoms preferably occupy a carbon substitution site or the carbon site of a U-C bivacancy. Our results are compared to available experimental data on UC and to similar studies by first-principles calculations for other carbides and nitrides with the rock-salt structure.

The influence of palladium impurities on vacancy diffusion in cubic silicon carbide

AbstractThe basic properties of palladium impurities in silicon carbide, such as solubility or diffusion mechanisms, are far from being well understood. In a recent paper I presented a systematic study of stability and kinetic properties of Pd in cubic silicon carbide using first principles calculations. In this paper I focus on the effect of the presence of palladium in silicon carbide, even in very low concentrations, on the kinetic properties of carbon vacancies. I apply a odel of Pd diffusion through a vacancy mechanism on the carbon sublattice and extract the correlation factors leading to an enhancement of vacancy migration, due to the coupling of iffusion fluxes between vacancies and palladium impurities.

Thermal stability of irradiation-induced point defects in cubic silicon carbide

This work aims specifically at studying the evolution of point defects induced by electron irradiation in the cubic polytype of SiC (3C-SiC) at temperatures ranging from 10 to 1450 K by means of photoluminescence (PL) spectroscopy. We identified a first annealing stage between 200 and 245 K, which probably results from migration of interstitials in the carbon sublattice. Moreover, we confirmed the high thermal stability of defect-related PL signals up to about 1100 K and calculated the activation energies associated with their annihilation. Finally, we studied the effect of a high temperature treatment at 1400 K on the DI center PL intensity in a single-crystal sample irradiated by electrons below the threshold displacement energy of the silicon sublattice. This allows checking the relevance of recent defect models based upon the migration of atoms in the carbon sublattice during the irradiation process. We conclude that the DI center does not involve the silicon vacancy and could be assigned to an isolated silicon antisite SiC.

Effect of carbon vacancies on the electric resistivity of nonstoichiometric VC y vanadium carbide

The influence of the temperature, concentration, and distribution of structure vacancies of the carbon sublattice on the electric resistivity ρ of nonstoichiometric VCy vanadium carbide (0.66 ≤ y ≤ 0.875) has been studied in the temperature range of 300–1200 K. The symmetry and structure characteristics of the ordered V6C5 and V8C7 phases formed owing to low-temperature annealing on various sections of the homogeneity region of the VCy carbide. The dependence of the residual electric resistivity on the content of the disordered vanadium carbide is explained by the atom-vacancy interaction and the change in the carrier concentration in the homogeneity region of VCy.

Effect of heat release conditions on the phase composition of the combustion products of a Fe2O3/TiO2/Al/C thermite mixture

An X-ray method was used to study the phase composition of the combustion products of a Fe2O3/TiO2/Al/C thermite composite mixture, in particular, during combustion of samples under cooling by water. The data of the experiments are compared with the results of thermodynamic calculations. A probable explanation of the reasons of the angular displacement of the line of iron in the X-ray spectrum of the combustion products and a mechanism for the crystallization of an eutectic TiC + α-Fe mixture from the melt of the Ti/Fe/C ternary system. The composition of the combustion products was found to be different from the equilibrium one, which is manifested in the presence of defects in the carbon sublattice of titanium carbide. A probable explanation of the deficiency of carbon is proposed.

Imperfection of carbon sublattice: Effect on the properties of refractory carbides of transition metals

The paper outlines comprehensive research of the physical and technical properties possessed by monocarbides of transition metals from groups IV and V of the Periodic Table within their homogeneity region. Experimental results are interpreted.

Different roles of carbon and silicon interstitials in the interstitial-mediated boron diffusion inSiC

The interstitial and vacancy mediated boron diffusion in silicon carbide is investigated with an ab initio method. The boron interstitials in $p$-type and $n$-type materials are found to be far more mobile than the boron-vacancy complexes. A kick-out mechanism and an interstitialcy mechanism govern the diffusion in $p$-type/intrinsic and $n$-type material, respectively. A comparison of activation energies demonstrates that the equilibrium diffusion is dominated by the positive hexagonal interstitial for typical experimental conditions. The activation energy and the charge state is in agreement with experimental findings. The analysis of the kick-out reaction demonstrates that silicon and carbon interstitials have different effects on boron acceptors on the silicon and carbon sublattice (${\mathrm{B}}_{\mathrm{Si}}$ and ${\mathrm{B}}_{\mathrm{C}}$). While silicon interstitials mediate the diffusion of both acceptors, carbon interstitials are only relevant for ${\mathrm{B}}_{\mathrm{C}}$. A larger kick-in barrier into silicon sites is found than into carbon sites. This implies a dominant formation of ${\mathrm{B}}_{\mathrm{C}}$ in the extended diffusion tails of boron profiles. The stable boron complexes with carbon or boron interstitials are found that potentially reduce the boron diffusion. This and the characteristics of the kick-out mechanism facilitate an explanation of recent co-implantation experiments.

Reassignment of phosphorus-related donors in SiC

Combining efficient density-functional based tight-binding molecular dynamics with ab initio calculations, we show that despite higher formation energies the incorporation of phosphorus at the carbon sublattice is favored by kinetic effects during the annealing processes. Based on the calculated hyperfine parameters the shallow ${\mathrm{P}}_{1}$,${\mathrm{P}}_{2}$ donors can be attributed to this kind of incorporation (${\mathrm{P}}_{\mathrm{C}}$ or ${\mathrm{P}}_{\mathrm{C}}{\mathrm{C}}_{\mathrm{Si}}$) only. The former controversially discussed spectra with small splittings are most probably due to isolated P at the silicon sublattice $({\mathrm{P}}_{\mathrm{Si}})$. Our theoretical results are supported by new electron paramagnetic resonance measurements additionally presented in this work: in 6H-SiC samples, in situ P-doped during growth, only the so-called $\mathrm{P}+\mathrm{V}$ spectrum occurs at temperatures between $8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, strongly indicating that at least three classes of different phosphorus-related defects are involved.

Effect of ion current density on damage in Al ion implanted SiC

The damage created by implantation of Al ions into single crystalline 4H-SiC has been analyzed using a combination of ion beam techniques and spectroscopic ellipsometry. The samples were implanted at room temperature with 150 keV Al + ions in the fluence range of 4 × 10 14 cm −2 to 2 × 10 15 cm −2 with current densities of 0.4 and 2.5 μA cm −2. In order to study simultaneously the depth distribution of the disorder produced in both the carbon and silicon sublattice, we used 3.5 MeV He beam in channeling geometry. In this condition the cross-section for carbon is enhanced by a factor of ∼6. The structural recovery of the samples after a further high temperature annealing has been studied. These results have been compared with the optical properties of the samples measured by spectroscopic ellipsometry.

High-Sensitivity Ion Beam Analytical Method for Studying Ion-Implanted SiC

Abstract. The combination of Backscattering Spectrometry and channeling with the 12 C(,) 12 C nuclear resonance at 4.26 MeV to study radiation damage in the carbon sublattice of 6H-SiC was applied. 6H SiC samples were implanted with 200 keV Al ions in a dose range of 410 14 cm -2 - 110 16 cm -2 with different current densities at room temperatures. The implanted samples were annealed at 1200oC in argon for 1 hour. In the case of low dose, low current density implantation damage recovery seems to be complete. Introduction Silicon carbide is a wide bandgap semiconductor material with many advantages for high temperature, high power and high frequency applications. Its local doping can only be performed by ion implantation, due to the very low diffusivity of the dopants in SiC, even at high temperatures. However, an equivalent ion beam irradiation creates more damage in the crystal lattice of SiC than it would be in Si. Moreover in the case of SiC the damage creation depends not only the dose of implanted atoms, but on the ion current during implantation [1]. The damage process and the structural recovery after a further high temperature annealing are not yet fully understood. Ion beam analytical methods are suitable for observing ion implantation damage. Widely used Rutherford Backscattering method with channeling (BS/C) provides information on the damage only in the silicon sublattice since the small signals from carbon are situated on the high background of silicon. In our present work the 4.26 MeV

Investigation of ion implantation-induced damage in the carbon and silicon sublattices of 6H-SiC

Single crystal 6H-SiC samples were irradiated at room temperature with 200 keV Al+ ions at fluences ranging from 3.5×1013 to 2.8×1014 ion/cm2. Depth profiles of crystal defects both in the C and Si sublattices were measured by Backscattering Spectrometry combined with channeling technique (BS/C) at 3550 keV 4He+ ion beam energy along the 〈0001〉 axial channeling direction. Damage in the carbon sublattice was found to be higher than in the silicon one. Moreover, the C/Si damage ratio decreased with increasing fluence. The crystal defect profiles can be well simulated both by full-cascade SRIM and Crystal–TRIM programs. Effective displacement energies for carbon and silicon sublattices in the applied fluence range of Al implantation were determined by comparing SRIM simulations to BS/C results.

Disorder-induced phonon self-energy of semiconductors with binary isotopic composition

Self-energy effects of Raman phonons in isotopically disordered semiconductors are deduced by perturbation theory and compared to experimental data. In contrast to the acoustic frequency region, higher-order terms contribute significantly to the self-energy at optical phonon frequencies. The asymmetric dependence of the self-energy of a binary isotope system $m_{1-x} M_x$ on the concentration of the heavier isotope mass x can be explained by taking into account second- and third-order perturbation terms. For elemental semiconductors, the maximum of the self-energy occurs at concentrations with $0.5<x<0.7$, depending on the strength of the third-order term. Reasonable approximations are imposed that allow us to derive explicit expressions for the ratio of successive perturbation terms of the real and the imaginary part of the self-energy. This basic theoretical approach is compatible with Raman spectroscopic results on diamond and silicon, with calculations based on the coherent potential approximation, and with theoretical results obtained using {\it ab initio} electronic theory. The extension of the formalism to binary compounds, by taking into account the eigenvectors at the individual sublattices, is straightforward. In this manner, we interpret recent experimental results on the disorder-induced broadening of the TO (folded) modes of SiC with a $^{13}{\rm C}$-enriched carbon sublattice. \cite{Rohmfeld00,Rohmfeld01}

Boron Centers in 4H-SiC

The origin of the deep boron related acceptor in SIC is subject to a lot of controversy. Based on ENDOR investigations, a B-Si+V-C model was suggested, while PL studies indicated the acceptor on the carbon sublattice. Our former ab initio LDA molecular cluster calculation showed that in the B-Si+V-C complex the carbon vacancy acts as the acceptor. Now, ah initio LDA supercell calculations have been carried out for boron-related complexes to calculate the occupation levels in 4H-SiC. It has been found that the 0/- level for the B-Si+V-C complex lies in the upper half of the gap, therefore it can be disregarded as the origin of the deep boron-related acceptor level. Investigating other feasible boron-related complexes, B-Si+Si-C appears to be the best candidate.