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The surface of solids or their interface with the gas phase is often modeled by a slab, periodic in two dimensions and repeated artificially in the third. When studying charged systems, a compensating background charge is required to avoid the divergence of the Coulomb energy. However, the interactions between the periodic images of the localized charge and between the localized charge and its neutralizing background can cause significant errors in the total energy. We have implemented the correction scheme proposed by Komsa and Pasquarello (2013), which estimates the error in the total energy by modeling the distribution of the localized extra charge with Gaussian functions at different sites, and comparing its energy in the periodic and in the isolated case. The program is user-friendly and robust, it is automated for simple cases while keeping the flexibility for the advanced users to handle non-trivial ones. Program Summary Program title: SLABCC Program Files doi: http://dx.doi.org/10.17632/42zd5p8gxc.1 Licensing provisions: BSD 2-Clause Programming language: C++ Nature of problem: The error in the total energy of charged slab models under 3D periodic boundary condition Solution method: Reading the total charge density and total local potential including the ionic, and Hartree potential for the neutral and charged system and approximating the extra charge with several Gaussians embedded in a dielectric medium. Calculating the difference in the energy of the model between the isolated and periodic cases, and using it as correction of the total energy in the original system. Current version works with the Vienna Ab initio Simulation Package (VASP) file format .

The possibility of splitting water by UV-light on a TiO 2 electrode has created great interest in the material, however, it has been later questioned whether rutile can do the job at all without external bias. Anatase was suggested instead, but its efficiency is still a subject of debate. The problem is related to the position of the band edges, that is, of the Fermi-level ( E F ), with respect to the redox potentials of water. Here we present hybrid-functional calculations to align the band structures with the vacuum level, assuming the rutile (110) and anatase (101) surface being exposed to water. Our results show that both are capable of water splitting if no adsorbates other than molecular water are present. On a fully hydroxylated surface (i.e., H + and OH − adsorption on undercoordinated surface oxygen and titanium atoms, respectively), E F is only ∼0.5 eV above the H + /H 2 potential in case of anatase, and – depending on the level of reduction – roughly at, or below it for rutile. We also show, that the band edges (and E F ) shift up if OH + groups dominate the surface, increasing the driving force for water splitting. This is in line with the experience on titania reduced in hydrogen. Our results are further confirmed by calculating E F without the presence of water, and comparing it to work function measurements by photoelectron spectroscopy.

How small nanodiamonds can be is a crucial question for biomedical applications. To answer this question, we present here molecular dynamic simulations of the annealing of very small diamond clusters (diameter between 0.3 and 1.3 nm) of various shape in vacuum and in the presence of oxygen. Isothermal cycles of 30 ps were carried out at 500, 1000, 1500, and 2000 K with 10 ps ramps between them. Predominantly {100} faceted diamond clusters as small as 1 nm (~ 250 atoms) survive these short anneals up to 1500 K. Longer anneals at 1500 K, as well “accelerated” MD at very high temperatures, indicate that the diamond core is still preserved when thermal equilibration is reached. The primary effect of oxygen seems to be the saturation of threefold-coordinated surface carbon atoms and the etching of lower coordinated ones. Oxygen accelerates the graphitization somewhat but does not affect the critical size. Our result means that nanodiamonds with a core of only 0.8 nm can be kinetically stable up to 1500 K. This is significantly less than the lower limit of the thermodynamic stability (~ 1.9 nm).

First-principles spin-polarized density functional theory computations in generalized gradient approximation to investigate the adsorption of N 2 O on the neutral and negative surfaces of the anatase TiO 2 (1 0 1) and on the neutral and positive surfaces of CO pre-adsorbed TiO 2 were carried out. The adsorption energies for N 2 O adsorbed on the TiO 2 and the CO pre-adsorbed surfaces were obtained. Mechanisms for redox reaction of CO + N 2 O conversion to CO 2 + N 2 on the neutral and positive TiO 2 surfaces have been proposed and their photocatalytic involvement is described.

Supercell calculations using density functional theory with local or semi-local exchange functionals have proved to be a very successful tool in defect engineering, apparently leading to some degree of overconfidence lately. With a case study on selected defects, we demonstrate that the approximations involved in these functionals lead not only to an underestimation of the gap but also to related errors in the total energy difference of two defect configurations (be these in the same or different charge states). Sometimes the error is so serious, that even the common expectation: “DFT provides the correct ground state” is refuted. We also demonstrate, that semi-empirical hybrid exchange functionals, which reproduce the correct band gap, give good total energy differences even in these cases. In such calculations, the position of Kohn-Sham gap levels with respect to the band edges gives a good estimation of the vertical ionization energy. Based on that, we propose a simple correction scheme for checking the necessity of extended total energy calculations with a hybrid functional.

Composition of the microwave plasma in chemical vapor deposition (CVD) of diamond in the low pressure range (25 mbar) is reported, using in situ mass spectrometry (MS) in a novel way, to reduce the possibility of recombination for radicals during the sampling process. While the general trends, observed earlier for increasing methane content of the gas feed, are reproduced, it is shown that the relative amount of radicals (most of all CH3 and C2H) is substantially higher than reported before. In fact their total concentration is at least equal to that of the most abundant stable species, C2H2.

The secondary emission coefficient, δ, of a given solid changes upon bombardment of the surface by an electron beam of high current density. Secondary emission and work function changes were measured simultaneously by work function spectroscopy during electron irradiation. The decrease of secondary emission and a successive increase of work function occurred on contaminated Si and W plates. The dependence of the work function- and the secondary emission change on primary energy was observed to be qualitatively the same. The change of δ as a function of primary electron energy has a maximum in the energy region where the mean free path of primaries and secondaries lies in the same range. From the results it can be assumed that the low energy secondaries have a great influence on Δδ.

The D I low-temperature photoluminescence center is a well-known defect stable up to 1700°C annealing in SiC, still its structure is not known after decades of study. Combining experimental and theoretical studies in this paper we will show that the properties of an anti-site pair can reproduce the measured one-electron level position and local vibration modes of the D I center and the model is consistent with other experimental findings as well. We give theoretical values of the hyperfine constants of the anti-site pair in its paramagnetic state as a means to confirm our model.

Reactions of oxygen molecules with an initially perfect 4H-SiC/SiO2 interface are studied by first principle model calculations. We show that the most likely route of reactions in the oxidation process produces carbon interstitials at the interface, which can easily be emitted into the oxide. As a result, a stationary concentration of carbon interstitials is present at the interface with a gradient decreasing toward the oxide. The electronic structure of these defects may explain many of the observed features in the distribution of interface states.

We investigate the validity of the Makov–Payne correction if applied to supercell calculations for charged defects by comparing the results of a supercell calculations with those obtained using a Green's function approach. For several defects in 3C–SiC the observed energy differences between both calculations are not bridged by the corrections of Makov–Payne. Whereas a correction proportional to the defect charge squared is predicted, the observed differences between supercell and Green's function approaches depend almost linearly on the charge. Possible reasons for this discrepancy are discussed.

The low pressure high temperature oxidation behavior of Ti–Al intermetallics are of interest to power technology aiming to synthesize this material by sintering of powders. This paper presents in situ surface analytical studies of the composition of a two-phase TiAl/Ti 3 Al bulk microcrystalline system after oxidizing the same (sputtered) reference surface for 30 min at various oxygen partial pressures and temperatures varying between room temperature (RT) and 1000 °C. The results show that oxidation already begins at very low ( −10 mbar) oxygen pressure, producing Al 2 O 3 and the lower oxidation states of Ti. As the oxygen pressure and oxidation temperature increases, TiO 2 becomes dominant up to 900 °C. No phase transition of Al 2 O 3 has been observed in this range. No sign of a blocking behavior of the oxide layer is seen. At 1000 °C a new oxide phase, Al 2 TiO 5 appears, changing the composition and behavior of the surface drastically. The observed results can be explained by qualitative thermodynamic arguments. The thickness and composition of the oxide overlayer is, however, primarily determined by the oxygen supply.

We present the results of the first molecular dynamics simulations under realistic conditions pertinent to the incubation period of the bias enhanced nucleation process of diamond, based on the experimentally obtained average energies of CH3 and C2H2, at different bias voltages. It was found that the energies measured at a bias where the nucleation enhancement sets in are critical for the subplantation of both types of ions. In the case of the typically applied bias voltages, acetylene plays the dominant role. The molecule breaks up and the average final penetration depth of the carbon atoms is ∼5 A. The bombardment causes a significant increase of the mass density and the sp3 content in the range 4–8 A below the surface despite the elevated temperature. The depth obtained for the main structural changes agrees well with the width of the amorphous carbon layer observed at the end of the incubation period for epitaxial nucleation.

It is well known that a secondary plasma is created during the late period of the bias enhanced nucleation (BEN) process, which ultimately affects the quality (texture, morphology) of the deposited layer in an unfavourable manner. Our earlier ion beam mass spectrometry (IBMS) measurements have shown a significant increase in the kinetic energy of the incoming ions to the surface in the presence of this secondary plasma. Reducing the bias, the kinetic energy of the ions is correspondingly reduced, so the detrimental effect to the freshly generated surface is substantially diminished. The technique can also be used to ensure a homogeneous coating of three-dimensional structures. We describe a multi-step nucleation/deposition microwave plasma CVD process. Based on computer simulations of the electric field, two nucleation periods were used. In the first one, a constant 200-V DC bias was applied, while in the second nucleation period, the biasing voltage was controlled in time in a stepwise-like manner. By this technique, a pinhole-free protective coating could be deposited onto the silicon surface of a micromachined capacitive pressure sensor chip.

Experimental investigations showed passivation of the p-type dopants B and Al in 4H-SiC by the formation of B+H and Al+H complexes. The dissociation energies of these complexes differed by 0.9 eV. Ab initio supercell calculations have been performed to investigate the interaction of H with B and Al in hexagonal 4H-SiC. The total energy, geometry and electronic structure of the possible complexes have been determined. Site dependencies have also been investigated. The most stable configurations were found with H at a bond center site next to B at the Si site, and with H at the antibonding site of a carbon atom which is first neighbor to Al at a Si site. Both the BSi+HBC and the AlSi+HAB(C) complexes turned out to be electrically inactive. The different structure of the passivated complexes explains the observed difference in their dissociation energy: the calculated difference of the binding energies of these complexes is 0.9 eV, which agrees well with the experimental finding. © 2001 Elsevier Science B.V. All rights reserved.

In the present work the impedance spectroscopy and secondary ion mass spectrometry (SIMS) were used to study the oxygen/β-Ga2O3 interaction between 578 and 835°C. The overall resistance of the β-Ga2O3 is composed of the bulk resistance and the grain boundary resistance. If oxygen is present, dominantly the characteristics of the space charge region — formed at the grain boundaries — change. The mechanism of the bulk conduction is unchanged between 578 and 835°C, however, the processes determining the grain boundary resistance are different above and below ∼700°C. At low temperatures the formation and adsorption of O2− ions is probable, while at higher temperatures the presence of O− ions is dominant on the grain boundaries.

The nitrogen point defect in diamond and cubic SiC was studied using an ab-initio plane wave pseudopotential approach that highlights the differences in the conduction bands of the two materials. Each N defect energy level is located relative to the various energy bands. The N defect level in diamond shows very little dispersion through the bands, which is indicative of its localized character. On the contrary, in cubic SiC the defect is far less localized, with the level essentially depending on the occupancy of either the C or Si site. Both sites may give n-type behavior, but N on the C site is preferred, with a defect level substantially pinned to the conduction band.

Ion beam mass spectrometry was used for in situ, real-time, mass-selective energy analysis of the incoming ions during bias-enhanced nucleation in microwave plasma chemical vapor deposition of diamond. H + y and C x H + y fluxes versus energy have been measured in the incubation period and after the onset of nucleation enhancement. Hydrocarbons with x=2 are dominant among the ions that strike the surface, and species with low hydrogen content (y=3) are most abundant. At 200 V bias, in the incubation period, the energy distributions of the main hydrocarbon ions have a broad peak with a small high-energy tail. The average energies are between 50 and 70 eV. H+ shows a wide distribution between 10 and 150 eV. On lowering the bias to a value below which nucleation enhancement becomes negligible the average ion energies decrease to about 30–35 eV. After the sharp increase of the sample current, which accompanies the onset of nucleation enhancement, the flux and average ion energies increase significantly. C 2 H + 2 becomes the dominant species with average energy of about 100 eV. A considerable flux of H+ and H + 3 ions reaches the surface with energy as high as 160–180 eV.

Ion beam mass spectrometer (IBMS) has been developed for in situ, real-time monitoring of the ion composition of a plasma in the vicinity of the substrate surface up to a pressure of 100 mbar. IBMS is a quadruple mass spectrometer-based plasma sampling system installed into a separately pumped small analysis chamber. The top part of the IBMS is specially designed to serve as the sample holder of the reactor chamber. A technique containing wet etching and laser treatment has been developed to form an orifice in various substrates, that matches the pressure range to be studied. This orifice samples the particles arriving onto the surface from the plasma, but is small enough to maintain effusive gas flow into the analysis chamber. Behind the orifice, a specially designed ion optics directs the ions coming from the process chamber into the QMS for mass and energy separation. Since particles are not altered after sampling, non-stable species can also be detected. The ion optics ensures that a high rate of ions reaches the QMS. The intensity ratios in the spectra correspond to the concentration ratios of the ions arriving to the substrate surface. The QMS is also equipped with a conventional cross beam ion source allowing measurements of neutral species with post-ionization. The performace of the system is demonstrated in a microwave plasma enhanced diamond CVD system.

Density-functional based tight-binding molecular-dynamics and semiempirical molecular orbital calculations are applied to identify the 1.9 eV luminescence center in SiOx. The identification is made possible by an 890 cm−1 vibrational line that is correlated to the luminescence peak. Two possible models are examined: (i) Si6 rings in substituted siloxenes are known to show 1.8 eV emission. A model SiO crystal is built based upon Si rings interconnected by bridging O atoms. The structure is found to be stable up to 1000 K, but the correlated vibration is not observed. (ii) a-SiO is exposed to simulated annealing and the quenched amorphous structure is analysed for occurance of Si rings and the previously proposed non-bridging oxygen hole centers (NBOHCs). Localized stretching vibrations of the NBOHCs are found to be in the experimentally detected frequency range, thus explaining the correlation with the observed and calculated photoluminescence peak.

The most favourite candidate to explain visible photoluminescence (PL) of porous silicon (PS) is quantum confinement. It has been shown theoretically that decreasing size of silicon particles leads to the widening of the gap (physical confinement). According to calculations, red luminescence requires particle sizes that are too small to be directly observed in PS. On the other hand, chemical effects, other than etching, seem to affect the PL frequency. These effects indicate similarities to the PL of siloxene which is due to isolation of Si6 rings by oxygen atoms (chemical confinement). Using self-consistent semi-empirical calculations, we have shown that the observed correlation between shift of the PL and Raman bands in PS can only be explained by chemical effects. Therefore, we suggest a surface recombination model of PL in PS, where the emission is determined primarily by physical confinement in features with nanometer dimensions on the surface of microcrystallites, while chemical substitutions tune the PL into the red. Empirical tight binding calculations on such particles show quantitative agreement with observed PL energies.

The time evolution of diamond film nucleation and growth have been investigated by spectroscopic ellipsometry (SE). Rutherford backscattering spectrometry (RBS) combined with elastic recoil detection (ERD) techniques. Diamond films were prepared using the microwave plasma CVD (MW-CVD) method on Si. In the plasma chamber, bias voltage of −200 V DC was applied to the Si substrate for 4–12 min in H2-CH4 (10%) to nucleate diamond. The substrate modification by H2 plasma cleaning and the DC bias nucleation of carbon layer followed by diamond film growth in H2-CH4 (0.5%) gas were studied. After H2 plasma cleaning the native 2.7 nm thick SiO2 is partly removed and the bias nucleation resulted in a mixture of SiO2 and SiC. With longer time, a very slow increase in film thickness up to 4.6 nm and a fast enrichment of the SiC contents have been observed. Thicker diamond films were found to consist of a 2–40 nm thick porous SiC-H layer, a bulk microcrystalline diamond layer and an amorphous capping layer 2–6 nm thick. The refractive index and the density of the bulk layers were identical to those of the single crystal diamond mixed with about 2% graphite and hydrogen. Porous layered structures were found in films reaching up to 2–40% porosities.

A very important question for modeling diamond growth is whether long-range surface migration should be taken into account as a limiting step in the growth on differently oriented surfaces. The migration of active surface sites as well as that of layerbuilding adsorbants (CH 3 , bridging CH 2 , etc.) is of interest. Calculation of migration barriers for polyatomic units is an extremely demanding computational task, especially since the surface has to be simulated appropriately. The PM3 semi-empirical method has already proven its usefulness in growth modeling studies. This method was used to calculate migration barriers in a static approximation ( T = 0 K ) on thin hydrogenated diamond slabs. Results are presented for C(111):H as well as for 2 × 1 reconstructed C(001):H surfaces.

PM3 semi-empirical calculations have been used to model the primary diamond nucleation in the graphite-like initial carbon layer usually produced on substrates other than diamond. The effect of quenched-in density increase was simulated by compressing the graphitic environment (represented by a small graphite cluster with cyclic boundary conditions) of a carbon atom with an additional methyl group in its vicinity. The critical (linear) compression for the “sp 2 → sp 3 phase transformation” of the fourfold coordinated carbon was estimated to be 3%.

Two of the main models for the explanation of strong visible-light emission in silicon, namely physical quantum confinement of electrons in nanometer-size “wires” or spherical crystallites, and chemical quantum confinement to subnanometer-size silicon particles due to the isolating effect of oxygen atoms, are considered with regard to a possible correlation between Raman shift and photoluminescence (PL) peak position. The physical confinement model predicts opposite shifts with changing size of the confinement. In the chemical confinement model, the shift of the gap is not a size effect; it occurs owing to chemical substitution of the bond terminators of the silicon atoms. This is accompanied by a parallel shift in the Raman frequency. The calculation of the vibration spectra of small size wires and spheres allows the correct assignment of experimentally observed Raman peaks. With the help of this assignment, the analysis of the observed spectra shows a parallel shift of Raman and PL peaks. The calculated frequencies for siloxene derivatives (a known manifestation of chemical quantum confinement) are lower than those observed in most porous silicon samples: still the parallel shift favors the idea of chemical as against physical quantum confinement.

We review the optical and luminescence properties of siloxene (Si 6 O 3 H 6 ). The preparation and basic structural properties of siloxene are described, and theoretical results concerning optical transitions in different modifications of siloxene are discussed. The dominant structural building blocks in as-prepared siloxene are two-dimensional Si planes which gave rise to a direct band gap in the visible energy range. The optical properties of annealed siloxene originate from isolated Si 6 rings which act as radiative recombination centers. The luminescence of annealed siloxene and porous silicon are compared in detail. In particular, new experimental results based on optically detected magnetic resonance provide microscopic information about radiative states in porous Si which is incompatible with the conventional quantum confinement model.

Geometry, relative stability, and vibration frequencies of hydrogen complexes — with and without the presence of vacancies and self-interstitials — are calculated. Assignments proposed by various experimental investigators for hydrogen related vibrational bands are discussed based on the relation between Si-H bond-character and stretching mode frequency.

The stability of hydrogen complexes aligned in a (1 1 1) plane of silicon is studied theoretically. Pairs of hydrogen atoms saturating broken bonds between adjacent planes are found to be the most stable arrangement with the heat of formation increasing as the platelet grows. The related stress is estimated and electronic effects are discussed.

Previously it has been shown that the correlation between shifts in luminescence and Raman peak positions in various porous silicon samples cannot be explained by the size effect of the silicon microstructures, as postulated by the (physical) quantum confinement model. It is also unlikely that siloxene occurs in significant quantities in porous Si. In this contribution, a possibility is presented for radiative recombination of carriers on the surface of a silicon microstructure, to explain strong visible luminescence in porous silicon. Nanometre-size structures on the surfaces of silicon microcrystallites introduce localized resonances into the (bulk-like) bands of the crystallites, giving rise to larger-than-bandgap transitions. These surface structures act as electron- and hole-traps and, due to localization of the carriers, recombination is possible neglecting momentum selection rules. Results of semi-empirical calculations on model structures are presented.

Calculations of the equilibrium geometries, binding energies and normal mode vibrations of a large number of defects containing hydrogen atoms, vacancies and self-interstitials are reported using the MINDO/3-CCM theoretical framework. On the basis of these results, a preliminary assignment of the infrared spectra of proton-implanted and as-grown silicon is presented.

Recent theoretical calculations are discussed which show that the minimum energy site for hydrogen in silicon is the bond-centered site, while a secondary minimum is at the anti-bonding site, it is noted that these results are strongly dependent on the relaxation experienced by the silicon atoms neighboring the hydrogen. Several experimental results are discussed, namely, the observed model of the hydrogen-passivated acceptor, etching of the silicon surface by hydrogen, the exponential depth dependence of the near-surface hydrogen diffusion profile, and related muonium results, and from these results it is argued that, in some instances at least, the BC-site is the lowest energy site for hydrogen.