Theoretical study of the nonpolar surfaces and their oxygen passivation in 4H- and 6H-SiC

Structure and stability of nonpolar surfaces in 4H- and 6H-SiC have been investigated within the framework of a self-consistent charge density functional based tight binding method. The lowest energy stoichiometric surface is corrugated for (10 (1) over bar0) but atomically smooth for (11 (2) over bar0). The most stable clean surfaces are Si rich. independent of the growth conditions. Unlike the polar surfaces both nonpolar surfaces can completely be passivated by a single SiO2 adlayer

Performance based seismic qualification of reinforced concrete nuclear materials processing facilities

A seismic qualification of a reinforced concrete nuclear materials processing facility using performance based acceptance criteria is presented. Performance goals are defined in terms of a minimum annual seismic failure frequency. Pushover analyses are used to determine the building`s ultimate capacity and relate the capacity to roof drift and joint rotation. Nonlinear dynamic analyses are used to quantify the building`s drift using a suite of ground motion intensities representing varying soil conditions and levels of seismic hazard. A correlation between joint rotation and building drift to damage state is developed from experimental data. The damage state and seismic hazard are convolved to determine annual seismic failure frequency. The results of this rigorous approach is compared to those using equivalent force methods and pushover techniques recommended by ATC-19 and FEMA-273

Structure of the silicon vacancy in 6H-SiC after annealing identified as the carbon vacancy–carbon antisite pair

We investigated radiation-induced defects in neutron-irradiated and subsequently annealed 6H-silicon carbide (SiC) with electron paramagnetic resonance (EPR), the magnetic circular dichroism of the absorption (MCDA), and MCDA-detected EPR (MCDA-EPR). In samples annealed beyond the annealing temperature of the isolated silicon vacancy we observed photoinduced EPR spectra of spin S=1 centers that occur in orientations expected for nearest neighbor pair defects. EPR spectra of the defect on the three inequivalent lattice sites were resolved and attributed to optical transitions between photon energies of 999 and 1075 meV by MCDA-EPR. The resolved hyperfine structure indicates the presence of one single carbon nucleus and several silicon ligand nuclei. These experimental findings are interpreted with help of total energy and spin density data obtained from the standard local-spin density approximation of the density-functional theory, using relaxed defect geometries obtained from the self-consistent charge density-functional theory based tight binding scheme. We have checked several defect models of which only the photoexcited spin triplet state of the carbon antisite–carbon vacancy pair (CSi-VC) in the doubly positive charge state can explain all experimental findings. We propose that the (CSi-VC) defect is formed from the isolated silicon vacancy as an annealing product by the movement of a carbon neighbor into the vacancy

Theoretical investigation of carbon defects and diffusion in α-quartz

The geometries, formation energies, and diffusion barriers of carbon point defects in silica (α-quartz) have been calculated using a charge-self-consistent density-functional based nonorthogonal tight-binding method. It is found that bonded interstitial carbon configurations have significantly lower formation energies (on the order of 5 eV) than substitutionals. The activation energy of atomic C diffusion via trapping and detrapping in interstitial positions is about 2.7 eV. Extraction of a CO molecule requires an activation energy <3.1 eV but the CO molecule can diffuse with an activation energy <0.4 eV. Retrapping in oxygen vacancies is hindered—unlike for O2—by a barrier of about 2 eV

Methane adsorption on graphene from first principles including dispersion interaction

The methane-graphene interaction is studied using density functional theory complemented with a semiempirical dispersion correction scheme (DFT-D), an ab initio van der Waals density functional (vdW-DF) ansatz as well as using Møller Plesset perturbation theory (MP2). The adsorption energy of 0.17 eV and the molecular distance of 3.28 Å obtained from the MP2 calculations are close to the experimental data, while the vdW-DF scheme results either in a realistic adsorption energy or a realistic adsorption geometry, depending on the underlying exchange-correlation functional. The present implementation of DFT-D overbinds about as much as bare DFT calculations underbind, but yields a meaningful adsorption height

Isolated oxygen defects in 3C- and 4H-SiC: A theoretical study

Ab initio calculations in the local-density approximation have been carried out in SiC to determine the possible configurations of the isolated oxygen impurity. Equilibrium geometry and occupation levels were calculated. Substitutional oxygen in 3C-SiC is a relatively shallow effective mass like double donor on the carbon site (O-C) and a hyperdeep double donor on the Si site (O-Si). In 4H-SiC O-C is still a double donor but with a more localized electron state. In 3C-SiC O-C is substantially more stable under any condition than O-Si or interstitial oxygen (O-i). In 4H-SiC O-C is also the most stable one except for heavy n-type doping. We propose that O-C is at the core of the electrically active oxygen-related defect family found by deep level transient spectroscopy in 4H-SiC. The consequences of the site preference of oxygen on the SiC/SiO2 interface are discussed

EPR, ESE and pulsed ENDOR study of nitrogen related centers in 4H-SiC wafers grown by different technologies

D-band electron paramagnetic resonance (EPR) measurements as well as X and Q-band field-swept Electron Spin Echo (ESE) and pulsed Electron Nuclear Double Resonance (ENDOR) studies were performed on a series of n-type 4H-SiC wafers grown by different techniques including sublimation sandwich method (SSM), physical vapor transport (PVT) and modified Lely method. Depending on the C/Si ratio and the growth temperature the n-type 4H-SiC wafers revealed, besides a triplet due to nitrogen residing on the cubic site (Nc), two nitrogen (N) related EPR spectra with g||=2.0055, g⊥=2.0010 and g||=2.0063, g⊥=2.0005 with different intensities. In the samples with low C/Si ratio the EPR spectrum with g||=2.0055, g⊥=2.0010 consists of a triplet with low intensity which is tentatively explained as a N-related complex, while in the samples with high C/Si ratio the triplet is transformed into one structureless line of high intensity, which is explained as being due to an exchange interaction between N donors. In the samples grown at low temperature with enhanced carbon concentration the EPR line with g||=2.0063, g⊥=2.0005 and a small hyperfine (hf) interaction dominates the EPR spectrum. It is attributed to N on the hexagonal lattice site. The interpretation of the EPR data is supported by activation energies and donor concentrations obtained from Hall effect measurements for three donor levels in this series of 4H-SiC samples

Paramagnetic signature of microcrystalline silicon carbide

Abstract. The most important challenge on the way to optimized solar cells is to make the thickness of the individual layers smaller than the diffusion length of the charge carriers, in order to keep the collection efficiency close to unity. Here, we propose ß-SiC microcrystals grown by a sol-gel based process as a promising acceptor material. The samples are characterized by optical spectroscopy and electron paramagnetic resonance (EPR). With the help of band structures for selected surface states calculated in the framework of density functional theory (DFT) a possible scenario for the observed acceptor process is discussed

Formation of Water in the Warm Atmospheres of Protoplanetary Disks

The gas-phase chemistry of water in protoplanetary disks is analyzed with a model based on X-ray heating and ionization of the disk atmosphere. Several uncertain processes appear to play critical roles in generating the column densities of warm water that are detected from disks at infrared wavelengths. The dominant factors are the reactions that form molecular hydrogen, including formation on warm grains, and the ionization and heating of the atmosphere. All of these can work together to produce a region of high water abundances in the molecular transition layer of the inner disk atmosphere, where atoms are transformed into molecules, the temperature drops from thousands to hundreds of Kelvins, and the ionization begins to be dominated by the heavy elements. Grain formation of molecular hydrogen and mechanical heating of the atmosphere can play important roles in this region and directly affect the amount of warm water in protoplanetary disk atmospheres. Thus it may be possible to account for the existing measurements of water emission from Tauri disks without invoking transport of water from cooler to warmer regions. The hydroxyl radical OH is under-abundant in this model of disk atmospheres and requires consideration of additional production and excitation processes