We have studied the segregation of P and B impurities during oxidation of the Si(100) surface by means of combined static and dynamical first-principles simulations based on density functional theory. In the bare surface, dopants segregate to chemically stable surface sites or to locally compressed subsurface sites. Surface oxidation is accompanied by development of tensile surface stress up to 2.9 N/m at a coverage of 1.5 monolayers of oxygen and by formation of oxidised Si species with charges increasing approximately linearly with the number of neighbouring oxygen atoms. Substitutional P and B defects are energetically unstable within the native oxide layer, and are preferentially located at or beneath the Si/SiOx interface. Consistently, first-principles molecular dynamics simulations of native oxide formation on doped surfaces reveal that dopants avoid the formation of P-O and B-O bonds, suggesting a surface oxidation mechanism whereby impurities remain trapped at the Si/SiOx interface. This seems to preclude a direct influence of impurities on the surface electrostatics and, hence, on the interactions with an external environment.
The local environment and oxidation state of Mn impurity in strontium titanate doped with 3% Mn were studied by X-ray absorption fine structure spectroscopy. The influence of the preparation conditions on the incorporation of the impurity into the $A$ and $B$ sites was studied. It was established that Mn ions substituting for Ti are in the Mn$^{4+}$ oxidation state and on-center. Mn ions substituting for Sr are in the Mn$^{2+}$ oxidation state, are off-center and displaced from the lattice sites by $\sim$0.32 Å. It was demonstrated that studies of the X-ray absorption near-edge structure can b…
A 2D layer of spherical, crystalline Ge nanodots embedded in SiO2 was formed by low pressure chemical vapour deposition combined with furnace oxidation and rapid thermal annealing. The samples were characterized structurally by using transmission electron microscopy and rutherford back scattering spectrometry, as well as electrically by measuring CV and IV characteristics. It was found that formation of a high density Ge dots took place due to oxidation induced Ge segregation. The dots are situated in the SiO2 on the average distance 5 nm from the substrate. Strong evidence of charge storage e…
The oxidation of α-Ti(0001) surface was studied using density functional theory. In order to enhance the oxidation resistance, we substituted Ti atoms with Si atoms in Ti(0001) surface. We observe that Si prefers to segregate at the surface layer of Ti(0001) compared to other sub-surface layers. The Si solubility on the Ti(0001) surface was estimated to be around 75 at.%. While Si segregation was found to reduce the binding between the oxygen atom and the Ti(0001) surface, the barrier for oxygen to diffuse into the slab increases. The dissociation of the oxygen molecule on the clean and Si seg…
Atomic scale understanding of the role of hydrogen and oxygen segregation in the embrittlement of grain boundaries in a twinning induced plasticity steel
Behaviour of a rare earth impurity of Eu in the PbTe single crystals grown by the Bridgman method from the melt with different initial concentrations of impurity N about 1*10+19 cm-3 and less is investigated with X-ray fluorescent element analysis, Secondary Neutral Mass Spectroscopy (SNMS), and magnetic measurements. The impurity distributions along and across the doped ingots are established. It is revealed that doping impurity enters into the bulk of doped crystals only if its initial concentration in the melt is high enough, approximately 1*10+20 cm-3. If this concentration is lower, about…
The macroscopic behavior of polycrystalline materials is influenced by the local variation of properties caused by the presence of impurities and defects. The effect of these impurities at the atomic scale can either embrittle or strengthen grain boundaries within. Thus, it is imperative to understand the energetics associated with segregation to design materials with desirable properties. Here, molecular statics simulations were employed to analyze the energetics associated with the segregation of various elements (He, H, C, P, and V) to four <100> (Sigma 5 and 13 GBs) and six <110> (Sigma 3,…
The electronic properties of low-dimensional materials can be engineered by doping, but in the case of graphene nanoribbons (GNR) the proximity of two symmetry-breaking edges introduces an additional dependence on the location of an impurity across the width of the ribbon. This introduces energetically favorable locations for impurities, leading to a degree of spatial segregation in the impurity concentration. We develop a simple model to calculate the change in energy of a GNR system with an arbitrary impurity as that impurity is moved across the ribbon and validate its findings by comparison…
Heterogeneous ferroelectricity and conductivity of oxidized BaTiO$_3$ crystals: the role of nanoscale phase segregation in the surface region
Institute of Energy Technologies (IET-4), Forschungszentrum Jülich GmbH（尤利希研究中心能源技术研究所）；Peter Grünberg Institute (PGI-1), Forschungszentrum Jülich GmbH（尤利希研究中心彼得·格林贝格研究所）；Department of Materials Science and Engineering and Materials Research Institute, The Pennsylvania State University（宾夕法尼亚州立大学材料科学与工程系及材料研究所）；Institute of Physics, University of Silesia（西里西亚大学物理研究所）；Marian Smoluchowski Institute of Physics, Jagiellonian University（雅盖隆大学马里安·斯莫卢霍夫斯基物理研究所）
Double ring formation on Co2MnSi (CMS) films is observed at electrical breakdown voltage during local anodic oxidation (LAO) using atomic force microscope (AFM). Corona effect and segregation of cobalt in the vicinity of the rings is studied using magnetic force microscopy and energy dispersive spectroscopy. Double ring forma-tion is attributed to the interaction of ablated material with the induced magnetic field during LAO. Steepness of forward bias transport characteristics from the unperturbed region of the CMS film suggest a non equilibrium spin contribution. Such mesoscopic textures in m…
We use new equations for the interstitial impurity diffusion fluxes under strain to study impurity atom redistribution in the vicinity of dislocations. Two levels of simulation are applied. The first one is evaluation of coefficients that determine the influence of strain tensor components on interstitial diffusion fluxes in FCC structures. For this purpose we have developed a model into the framework of a molecular static method taking into account an atom environment both near the interstitial site and for the saddle-point configuration. The second level is modeling the interstitial segregat…
The local structure and oxidation state of the cobalt impurity in SrTiO$_3$ is studied by X-ray absorption fine structure (XAFS) spectroscopy. The synthesis conditions, under which cobalt predominantly (up to 76%) substitutes the atoms at the $A$ site of the perovskite structure, is found for SrTiO$_3$(Co) samples. By varying the synthesis conditions, it is possible to appreciably change the ratio between the concentrations of cobalt atoms incorporated into the $A$ and $B$ sites. It is established that the oxidation state of cobalt is +2 at the $A$ site and +3 at the $B$ site. It is revealed t…
The interfacial structure of a silicon grain boundary (Si-GB) plays a decisive role on its chemical functionalization and has implications in diverse physical-chemical properties of the material. Therefore, GB interface is particularly relevant when the material is employed in high performance technological applications. Here, we studied from first principles the role of GB interface by providing an atomistic understanding of two different $Σ$3\{112\} Si-GB models. These models are (1$\times$1) and (1$\times$2) $Σ$3\{112\} Si-GBs which lead to different structural reconstruction. Starting from…
Segregated impurities at grain boundaries can dramatically change the mechanical behavior of metals, while the mechanism is still obscure in some cases. Here, we suggest an unified approach to investigate segregation and its effects on the mechanical properties of polycrystalline alloys using the example of 3$sp$ impurities (Mg, Al, Si, P, or S) at a special type $Σ5(310)[001]$ tilt grain boundary in Cu. We show that for these impurities segregating to the grain boundary the strain contribution to the work of grain boundary decohesion is small and that the chemical contribution correlates with…
XAFS studies of Ni-doped Ba$_{1-x}$Sr$_x$TiO$_3$ solid solution reveal that the Ni oxidation state changes from 4 in SrTiO$_3$ to 2.5 in BaTiO$_3$ when varying $x$. This change is accompanied by a noticeable change in the interatomic Ni-O distances in the first shell. The first-principles calculations show that nickel creates an impurity band in the forbidden band gap of BaTiO$_3$ and SrTiO$_3$, which explains the appearance of intense absorption of Ni-doped samples in the visible region. The analysis of the electronic structure of doped crystals and calculations of the oxygen vacancy formatio…
Segregation of impurities to grain boundaries plays an important role in both the stability and macroscopic behavior of polycrystalline materials. The research objective in this work is to better characterize the energetics and length scales involved with the process of solute and impurity segregation to grain boundaries. Molecular dynamics simulations are used to calculate the segregation energies for carbon within multiple grain boundary sites over a database of 125 symmetric tilt grain boundaries in Fe. The simulation results show that the majority of atomic sites near the grain boundary ha…
Recent experimental measurements of Ag impurity diffusion in the Σ5 (310) grain boundary (GB) in Cu revealed an unusual non-Arrhenius behavior suggestive of a possible structural transformation [Divinski et al., Phys. Rev. B 85, 144104 (2012)]. On the other hand, atomistic computer simulations have recently discovered phase transformations in high-angle GBs in metals [Frolov et al., arXiv:1211.1756v2 (2013)]. In this paper we report on atomistic simulations of Ag diffusion and segregation in two different structural phases of the Cu Σ5 (310) GB which transform to each other with temperature. T…
The energy of segregation of Cr from the bulk to the topmost surface layer in dilute Fe-Cr alloys is endothermic but small. On the other hand, the segregation energy from the bulk to the subsurface layer is not only endothermic but relatively large. Thus, the subsurface layer presents a huge barrier, not only for the segregation of Cr from the bulk to the surface, but also for its diffusion from the surface into the bulk. This means that the topmost layer alone cannot be used to determine the barrier for Cr segregation (the segregation energy), which is determined in these alloys by the subsur…
We study the kinetics of the redistribution of impurity atoms in the elastic fields of dislocations by computer simulation methods. A work consists of several stages. The first is the simulation of a dislocation core structure with a Burgers vector along [100] direction by using the modified molecular static method. The second is obtaining the coefficients that determine the influence of the components of the strain tensor on the diagonal elements of the matrix of diffusion coefficients and in the equations for fluxes of carbon atoms in bcc iron. The third stage is associated with modeling the…
Multi-crystalline silicon is widely used for producing low-cost and high-efficiency solar cells. During crystal growth and device fabrication, silicon solar cells contain grain boundaries (GBs) which are preferential segregation sites for atomic impurities such as oxygen atoms. GBs can induce charge carriers recombination significantly reducing carrier lifetimes and therefore they can be detrimental for Si device performance. We studied the correlation between structural, energetic and electronic properties of Σ3{111} Si GB in the presence of vacancies, strain and multiple O segregation. The s…
Ferritic steels possibly strengthened by oxide dispersion are candidates as structural materials for generation IV and fusion nuclear reactors. Their use is limited by incomplete knowledge of the iron-chromium phase diagram at low temperatures and of the phenomena inducing preferential segregation of one element at grain boundaries or at surfaces. In this context, this work contributes to the multi-scale study of the model iron-chromium alloy and their free surfaces by numerical simulations. This study begins with ab initio calculations of properties related to the mixture of atoms of iron and…
Using ab initio methods based on the density functional theory, we investigate the segregation and formation energies for various dopants (Si, Be, Zn, Sn), commonly used to obtain p- or ntype conductivity in GaAs and InAs nanowires. The distribution of Au and O atoms, which may be unintentionally incorporated during the wire growth, is also studied. The calculations performed for nanowires of zinc blende and wurtzite structure show that the distribution of most of the impurities depends on the crystal structure of the wires. For example, it is shown that the same growth conditions can lead to …
Segregation, precipitation, and phase separation in Fe-Cr systems is investigated. Monte Carlo simulations using semiempirical interatomic potential, first-principles total energy calculations, and experimental spectroscopy are used. In order to obtain a general picture of the relation of the atomic interactions and properties of Fe-Cr alloys in bulk, surface, and interface regions several complementary methods has to be used. Using Exact Muffin-Tin Orbitals method the effective chemical potential as a function of Cr content (0-15 at.% Cr) is calculated for a surface, second atomic layer and b…
We have studied the critical temperature of Diluted Magnetic Semiconductors by means of Monte Carlo simulations and Coherent-Potential-Approximation (CPA) calculations. In our model for this syste m, the magnetic ions couple with the carriers through an antiferromagnetic exchange interaction, $J$, and an electrostatic interaction $W$. The effective impurity potential $J-W$ controls the hybridization between the magnetic impurities and the hole charge on the dopants. We find that the critical temperature depends substantially on the hole charge on the magnetic impurities. The CPA critical tempe…
The incorporation of oxygen atoms has been simulated into either nitrogen or uranium vacancy at the UN(001) surface, sub-surface or central layers. For calculations on the corresponding slab models both the relativistic pseudopotentials and the method of projector augmented-waves (PAW) as implemented in the VASP computer code have been used. The energies of O atom incorporation and solution within the defective UN surface have been calculated and discussed. For different configurations of oxygen ions at vacancies within the UN(001) slab, the calculated density of states and electronic charge r…
We have used density functional theory calculations based on the projector augmented wave method to investigate the electronic structure of Au-incorporated anatase TiO$_2$(001) surface. Due to the coordination with several level oxygens, Au atoms can be encapsulated inside TiO$_2$ slab. Au is adsorbed over the surface Ti--O bond, so called the bridge site on anatase TiO$_2$(001)--1$\times$1 surface. However, for 0.25 ML coverage, Au atoms energetically prefer to stay at 0.64 Å above the midpoint of the two surface oxygens which is significantly closer to the surface layer. When implanted insid…
Understanding how properties of materials change due to nuclear transmutations is a major challenge for the design of structural components for a fusion power plant. In this study, by combining a first-principles matrix Hamiltonian approach with thermodynamic integration we investigate quasi-steady state configurations of multi-component alloys, containing defects, over a broad range of temperature and composition. The model enables simulating transmutation-induced segregation effects in materials, including tungsten where the phenomenon is strongly pronounced. Finite-temperature analysis show…
First-principles density functional theory calculations were carried out to determine the low energy geometries of anatase TiO$_2$(001) with Pt implants in the sublayers as substitutional and interstitial impurities as well as on the surface in the form of adsorbates. We investigated the effect of such a systematic Pt incorporation in the electronic structure of this surface for isolated and interacting impurities with an emphasis on the reduction in the band gap to visible region. Comprehensive calculations, for 1x1 surface, showed that Pt ions at interstitial cavities result in local segrega…
