Stability and mobility of mono- and di-interstitials in alpha-Fe

We report a detailed ab initio study of the stability and migration of self-interstitial atoms (SIAs) and di-interstitials (di-SIAs) in alpha-Fe. The <110> dumbbell is confirmed to be the most stable SIA configuration, 0.7 eV below the <111> dumbbell. The lowest-energy migration path corresponds to a nearest-neighbor translation-rotation jump with a barrier of 0.34 eV. The most stable configuration for di-SIAs consists of <110> parallel dumbbells. Their migration mechanism is similar to that for SIAs, with an activation energy of 0.42 eV. These results are at variance with predictions from existing empirical potentials and allow one to reconcile theory with experiments.     

SIA energetic and structural morphology of α-iron

In this paper, the stability of various atomic configurations containing a self-interstitial atom (SIA) in a model representing α-iron is investigated. From the energy panorama maps of the SIA located in possible non-equivalent interstitial sites, six relatively stable self-interstitial sites are found, whose structures and formation energies have been described and calculated using the modified analytical embedded atom method and molecular dynamics. The simulation results indicate that the [110] dumbbell interstitial is the energetically most favorable configuration, which is in good agreement with the experimental and ab initio results, and the distances between two displaced atoms that compose the [100], [110] and [111] direction dumbbells have been computed to be 0.68a, 0.65a and 0.29a, respectively, not all being about 0.75a apart. The relaxed displacements up to the fifth-nearest-neighbor atoms around the SIA in O interstitial position are also calculated.     

First-principles investigation of diffusion behaviours of H isotopes:From W(110) surface into bulk and in bulk W

<正>The diffusion behaviours of hydrogen（H）,deuterium（D）,and tritium（T） from W（110） surface into bulk and in bulk W are investigated using first-principles calculations combined with simplified models.The diffusion energy barrier is shown to be 1.87 eV from W（110） surface to the subsurface,along with a much reduced barrier of 0.06 eV for the reverse diffusion process.After H enters into the bulk,its diffusion energy barrier with quantum correction is 0.19 eV. In terms of the diffusion theory presented by Wert and Zener,the diffusion pre-exponential factor of H is calculated to be 1.57×10^-7 m²·s^-1,and it is quantitatively in agreement with the experimental value of 4.1×10^-7 m²·s^-1. Subsequently,according to mass dependence(（1/m）^1/2) of H isotope effect,the diffusion pre-exponential factors of D and T are estimated to be 1.11×10^-7 m²·s^-1 and 0.91×10^-7 m²·s^-1,respectively.     

Atomic adsorption of oxygen on Cu(111) and Cu(110)

We have studied angle-resolved inverse photoemission ( = 9.7 eV) after room temperature adsorption of oxygen on Cu(111) and Cu(110). On Cu(111) exposure to 500 L induces a band (3.0 eV aboveE _F at) which shows clear dispersion (1.0 eV) to higher energies for off normal incidence. Since no LEED superstructure is seen for that system, our results present strong evidence for the presence of short-range surface order. Two adsorbate bands are identified (2.8 eV and 6.3 eV at) on Cu(110)p(2×1)-O. Our results are in good agreement with a long-bridge adsorption site.     

Self-interstitial configuration in molybdenum studied by modified analytical embedded atom method

The stability of various atomic configurations containing a self-interstitial atom (SIA) in a model representing Mo has been investigated using the modified analytical embedded atom method (MAEAM). The lattice relaxations are treated with the molecular dynamics (MD) simulation at absolute zero of temperature. Six relatively stable self-interstitial configurations and formation energies have been described and calculated. The results indicate that the [111] dumbbell interstitial S₁₁₁ has the lowest formation energy, and in ascending order, the sequence of the configurations is predicted to be S₁₁₁, C, S₁₁₀, T, S₀₀₁ and O. From relaxed displacement field up to the fifth-NN atoms of six configurations, we know that the relaxed displacements depend not only on separation distances of the NN atoms from the defect centre but also strongly on the direction of the connected line between the NN atoms and the defect centre. The equilibrium distances between two nearest atoms in the core of the S₁₁₁, C, S₁₁₀, T, S₀₀₁ and O configurations are 0.72a, 0.72a, 0.71a, 0.72a, 0.70a and 0.70a, respectively.      

High-resolution normal emission UV-photoelectron spectra and band structure of Cu

The energy distribution curves of photoelectrons emitted along the normal direction from Cu (111), (001) and (110) surfaces using unpolarized HeI (hv=21.2 eV) and HeII (hv=40.8 eV, 48.4 eV) radiation incident at 45° to the surface were measured. For the interpretation the band structure as calculated by Janak et al. and by the combined interpolation scheme was used. The high resolution spectra along the [111] and [001] directions show dominant features from direct transitions, indicating strong residualk _z conservation, and density of states features, too. The band gap emission along the [110] direction using HeI radiation can be explained with direct transitions into evanescent states with nearly free electron bands. From the width of the peaks electron escape depths were calculated.     

A theoretical study of hydrogen adsorption and diffusion on a W(1 1 0) surface

We have used density functional theory to investigate hydrogen adsorption and diffusion on a W(1 1 0) surface. Hydrogen adsorption structures were examined from low coverage to one monolayer, and a threefold hollow site was found to be the most stable site at all coverages. In contrast to previous assertions, the work function decrease is not due to electron transfer from the hydrogen atoms to the W surface, but due to electron depletion at the vacuum region above the hydrogen atoms. Hydrogen atoms can diffuse via short-bridge sites and long-bridge sites at a coverage of θ = 1.0. Although the calculated activation energy for hydrogen diffusion via a short-bridge site is as small as 0.05 eV, field emission microscope experiments have shown that the activation energy for hydrogen diffusion is about 0.20 eV, which agrees fairly well with our calculated value of the activation energy via a long-bridge site. This discrepancy can be related to hydrogen delocalization on the W(1 1 0) surface, which has been suggested by electron energy loss spectroscopy experiments.     

First-principles investigation on diffusion behaviours of H isotopes: From W(110) surface into bulk and in bulk W

The diffusion behaviours of hydrogen (H), deuterium (D), and tritium (T) from W(110) surface into bulk and in bulk W are investigated using a first-principles calculations combined with simplified models. The diffusion energy barrier is shown to be 1.87 eV from W(110) surface to the subsurface, along with a much reduced barrier of 0.06 eV for the reverse diffusion process. After H enters into the bulk, its diffusion energy barrier with quantum correction is 0.19 eV. In terms of the diffusion theory presented by Wert and Zener, the diffusion pre-exponential factor of H is calculated to be 1.57×10^-7 m²·s^-1, and it is quantitatively in agreement with experimental value of 4.1×10^-7 m²·s^-1. Subsequently, according to mass dependence (√1/m ) of H isotope effect, the diffusion pre-exponential factors of D and T are estimated to be 1.11×10^-7 m²·s^-1 and 0.91×10^-7 m²·s^-1, respectively.     

金红石TiO_2中本征缺陷扩散性质的第一性原理计算

基于密度泛函理论的爬坡弹性带方法,对金红石相二氧化钛晶体中钛间隙、钛空位、氧间隙、氧空位4种本征缺陷的扩散特征进行了研究.对比4种本征缺陷在晶格内部沿不同扩散路径的过渡态势垒后发现,缺陷扩散过程呈现出明显的各向异性.其中,钛间隙和氧间隙沿[001]方向具有最小的扩散势垒路径,激活能分别为0.505 eV和0.859 eV;氧空位和钛空位的势垒最小的扩散路径分别沿[110]方向和[111]方向,激活能分别为0.735 eV和2.375 eV.     

Desorption kinetics and directional dependence of the surface diffusion of potassium on stepped W(100) surfaces

Using a surface ionisation ion microscope the desorption parameters and the diffusion constant of potassium were measured on stepped W(100) surfaces. The activation energy of ionic desorption as well as the corresponding prefactor do not depend on the step density; the mean adsorption lifetime τ can be expressed as τ=1.6×10^?14s exp(2.44 eV/kT).Whereas the surface diffusion of potassium on “flat” W(100) and on W(S)-[9(100)×(110)] was found to be isotropic, on W(S)- [5(100)×(110)] and W(S)-[3(100)×(110)] it occurs preferentially parallel to the step direction. The diffusion constant D_∥ for this direction has roughly the same value for all investigated surfaces: D_∥=7.8×10^?2 cm²s^?1 exp(?0.42 eV/kT). For the direction perpendicular to the steps D⊥ depends on the step density, whereby the activation energy as well as the prefactor increase with increasing step density.     

Cu在Pt表面扩散激活能和扩散系数的ASED-MO计算

本文应用ASED-MO方法,计算研究了Cu在Pt(111),(100),(110)表面的扩散问题。Cu原子在上述三个表面上的扩散激活能的计算结果分别为0.167eV,0.162eV和0.668eV;300K时的扩散系数分别为3.04×10¹⁰m²/s,3.69×10^-10m²/s和2.42×10^-18m²/s。计算结果表明,Cu原子在Pt(111)和(100)面上,扩散激活能很小,极易迁移,而在(110)面上,激活能较大,扩散系数甚小。     

Ab initio pseudopotential study of vacancies and self-interstitials in hcp titanium

By means of an ab initio plane-wave pseudopotential method, monovacancy, divacancy and self-interstitials in hcp titanium are investigated. The calculated monovacancy formation energy is 1.97 eV, which is in excellent agreement with other theoretical calculations, and agrees qualitatively with published experimental results. The relaxation of the atoms around a single vacancy is observed to be small. Two divacancy configurations, the in-plane and the off-plane, have also been shown to be equally stable. With regards to the interstitials, of the eight configurations studied, two (octahedral and basal octahedral) have relatively lower formation energies and are, thus, the most likely stable configurations. We find small energy differences between them, suggesting their possible co-existence. It is also observed that the tetrahedral configuration decays to a split dumbbell configuration, whereas both the basal tetrahedral and the basal pseudocrowdion interstitials decay to the basal octahedral configuration. Using the nudged elastic band method (NEB), we determine a possible minimum energy path (MEP) for the diffusion of self-interstitial titanium atoms from an octahedral site to the nearest octahedral site. The energy barrier for this migration mechanism is shown to be about 0.20 eV.     

Diffusion processes of trimers on missing row surfaces: \mathrm{Cu }_{3}/\mathrm{Ag }(110) and \mathrm{Ag }_{3}/\mathrm{Cu }(110)

A semi-empirical potential according to the embedded atom, has been applied to investigate the diffusion of trimers by computing the energy barriers for different mechanisms. Our attention was more focused on the leapfrog process which is likely to occur on missing row surfaces. The activation barriers of this mechanism are calculated using drag method at 0K. These barriers are found to be 0.64 and 0.68 eV for hopping out the channel for $\mathrm{Cu }_{3}/\mathrm{Ag }(110) \mathrm{and Ag }_{3}/\mathrm{Cu }$ (110) respectively. While for hopping down at the other side they are about 0.42 and 0.32 eV. Moreover, a deep metastable position is observed during leapfrog diffusion leading to some spectacular trimer motion. At high temperature and essentially for $\mathrm{Cu }_{3}/\mathrm{Ag }$ (110), we also observed a competition between leapfrog process and concerted jump mechanism with a deformation of trimer geometry. Implications of these findings are briefly discussed.     

Diffusion of the diboron pair in silicon

We propose a novel mechanism for the diffusion of a diboron pair in Si, based on first principles density functional theory. We find a reaction pathway along which the boron pair diffuses from one lowest energy configuration of [B-B](s)-< 001> to an equivalent structure at an adjacent equivalent site through three local minimum states denoted as [B-B](s)-< 111>, B(s)-B (i), and B(s)-B (s)-Si (i). The activation energy for the diffusion is estimated to be 1.81 eV in the generalized gradient approximation. A kinetic model suggests that the diboron diffusion plays an important role in determining diffusion profiles during ultrashallow junction processing (which requires high boron-dopant concentration as well as high annealing temperature).     

Non-nearest-neighbor jumps in 2D diffusion: Pd on W(110)

A variety of jumps has in the past been identified in diffusion of atoms on 1D channeled surfaces. To establish the jump processes important in diffusion on a 2D surface, the movement of individual Pd atoms has been examined on W(110). From the distribution of displacements of Pd at high temperatures, double jumps are found along the close-packed <111>. For the first time, sizable differences are also observed between the mean-square displacements along x and y, which demonstrate unexpected contributions from jumps along <110>, but not along <001>. These jumps proceed over activation barriers higher than for single jumps, under conditions predicted from previous work with Pd on the channeled W(211).     

Electronic structures and bonding of oxygen on plutonium layers

Oxygen adsorptions on -Pu (100) and (111) surfaces have been studied at both non-spin-polarized and spin-polarized levels using the generalized gradient approximation of density functional theory (GGA-DFT) with Perdew and Wang (PW) functionals. The center position of the (100) surface is found to be the most favorable site with chemisorption energies of 7.386 eV and 7.080 eV at the two levels of theory. The distances of the oxygen adatom from the Pu surface are found to be 0.92 Å and 1.02 Å, respectively. For the (111) surface non-spin-polarized calculations, the center position is also the preferred site with a chemisorption energy of 7.070 eV and the distance of the adatom being 1.31 Å, but for spin-polarized calculations the bridge and the center sites are found to be basically degenerate, the difference in chemisorption energies being only 0.021 eV. In general, due to the adsorption of oxygen, plutonium 5f orbitals are pushed further below the Fermi energy, compared to the bare plutonium layers. The work function, in general, increases due to oxygen adsorption on plutonium surfaces.Received: 20 July 2004, Published online: 9 September 2004PACS: 71.15.-m Methods of electronic structure calculations - 71.15.Mb Density functional theory, local density approximation, gradient and other corrections - 71.15.Nc Total energy and cohesive energy calculations 71.27. + a Strongly correlated electron systems; heavy fermions     

Low energy ion impact phenomena on single crystal surfaces

The dynamics of a solid bombarded by a 600 eV Ar⁺ ion have been studied classically by computer simulation. The model uses a crystallite of about 250 atoms described by pair potentials derived from elastic constants and which reproduce the surface binding energy of the solid. The relative calculated yield of secondary atom emission from the three low index faces of Cu follow the previously determined experimental order (111) > (100) > (110). We find major differences in the sputtering mechanisms for these faces. On (110), the impacted atom is ejected most frequently, while on (111) and (100) it almost never leaves the solid. We report the energy distribution of the sputtered particles for each face. The simulation successfully predicts the shape of the curve including the low energy maximum which is observed experimentally near 2 eV. In addition our model shows that many low energy atoms attempt to leave the crystal but are subsequently trapped to the solid at large distances from their original sites. This mechanism of radiation enhanced diffusion inevitably occurs in conjunction with sputtering or any other heavy secondary particle emission or scattering process.     

Surface mass transport of Ag and In on vicinal Si(111)

Mass transport of Ag and In on vicinal Si(111) has been investigated by scanning Auger microscopy (SAM). Highly anisotropic surface diffusion and surface electromigration due to direct current were observed for Ag and In adatoms on 0°−, 0.5°−, 3°− and 6°−off vicinal Si(111) surfaces. The diffusion on the intermediate layer is strongly enhanced in the direction parallel to the step edge for Ag adatoms, while it is remarkably suppressed in the direction perpendicular to the step edge for In adatoms. The activation energy of the diffusion for the Ag adatoms ranged between 0.81 and 1.3 eV, while that for In adatoms increased from 0.31 to 0.66 eV with increasing the vicinal angle. The anisotropic diffusion transport is explained in terms of the step structure and the difference in the binding energy at the step site and the terrace site.     

The emission of neutral clusters in sputtering

The mass, angle, and energy resolved emission of neutral clusters in sputtering was studied for a variety of metals and semiconductors. The main phenomena and results are the following: (i) Cluster emission from a series of transition metals reveals a prominent contribution of clusters to the total flux of ejected particles but there is no simple scaling of cluster intensities with the average sputtering yields. With increasing number of constituents, relative intensities of neutral clusters decrease much faster than those of secondary-ion clusters. (ii) The relative intensities of clusters emitted from amorphous and crystalline semiconductors are identical, but the energy spectra of Ge _n -clusters (n = 1–4) sputtered from Ge (111) peak at a slightly higher energy (1 eV) as compared to spectra taken from amorphous Ge. The intensities of all Ge _n -clusters exhibit the same dependence on emission angle; this holds for both the amorphous and crystalline Ge-sample. (iii) The flux of neutral monomers, dimers, and trimers sputtered from Cu(111), Ni(111), and Ag(111) crystals shows a pronouncedly anisotropic emission along the 110 lattice directions which is ascribed to a momentum alignment in the anisotropic part of the collision cascade. Energy spectra taken along 110 peak at higher energies than those obtained from a random emission angle.     

Field-emission flicker noise from potassium adsorbed on W(111) bounded and unbounded surface diffusion by spectral analysis

The spectral density functionsW(f) of the field-emission flicker noise of potassium adsorbed on the tungsten (111) region were determined. The analysis in terms of the Gesley-Swanson or the Timm and Van der Ziel model gives surface diffusion energies between 0.54 and 0.31 eV for (average) coverages from 0.15 to 0.38 and diffusion prefactors between 3×10^–1 and 10^–4 cm²/s. The fitting procedures are successful for both bounded and unbounded diffusion though in opposite order of temperature, as expected.On leave from Wrocaw University