Papers by Keyword: Surface Diffusion

Abstract: This paper presents an investigation into the surface morphology control of 4H-SiC (0001) wafers cut to 4º off during thermal processing, aiming to suppress the propagation of basal plane dislocations (BPD) into the epitaxial growth layer. Developing methods for debunching rough surfaces with macro step bunching (MSB) using thermal processes removes many of the limitations of the conventional epitaxial growth process. This study presents a surface morphology control method that includes debunching of steps by thermal sublimation etching/growth using the Dynamic AGE-ing^® (DA) method. By controlling the surface morphology before and after growth using this method, the dependence of the BPD-threading edge dislocation (TED) conversion ratio on surface morphology was systematically revealed. By selecting the optimal pre- and post-growth surface morphology, a 100 % BPD-TED conversion ratio was obtained for the 10 mm × 25 mm area. It was indicated that an innovative and stable surface morphology control technique using the DA sublimation process could solve numerous technological challenges in various fields.

Abstract: Impurity incorporation during vapor-phase epitaxy on stepped surfaces was modeled by classifying rate-limiting processes into i) surface diffusion, ii) step kinetics, and iii) segregation. Examples were shown for i) desorption-limited Al incorporation during chemical vapor deposition (CVD) of (0001) SiC, ii) preferential desorption of C atoms from kinks during CVD of Al-doped (000-1) SiC, and iii) segregation-limited C incorporation during metalorganic vapor-phase epitaxy of (0001), (000-1), and (10-10) GaN.

Abstract: The structure of electrolytically deposited nanocrystalline alloys of the CoP-CoNiP systems under low-temperature heating was investigated by means of high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM), and analytical methods such as energy dispersive x-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Structural relaxation and crystallization were investigated at temperatures from 150°C to 300°C. Structural and compositional inhomogeneities were found in the CoP-CoNiP alloys, while the local changes in composition were found to reach 15 at.%. Nanocrystals in the alloys grew most intensely in the presence of a free surface. It was determined that the local diffusion coefficient ranged from 1.2 to 2.4 10⁻¹⁸ m²/s, which could be explained by the surface diffusion prevalence. The data gathered in these investigations can be further used to predict the thermal stability of CoP-CoNiP alloys.

Abstract: In W.W. Mullins' classical 1957 paper on thermal grooving, motion by surface diffusion was proposed to describe the development of a thermal groove separating two grains in a simple semi-infinite planar geometry. After making a small slope approximation which is often realistic, Mullins' sought self-similar solutions, and obtained an explicit time series solution for the groove depth. In the years since, Mullins' grooving solution has become a standard tool; however it has yet to be rigorously demonstrated that self-similar solutions exist when the small slope approximation is not applicable. Here we demonstrate that reformulation of Mullins' nonlinear problem in arc-length variables yields a particularly simple fully nonlinear formulation, which is useful for verifying large slope grooving properties and which should aid in proving existence.

Abstract: The principles of pulse electrolysis modes selection and their effect on the size distribution of the powder electrolytic copper in electrolyte with additions of water-soluble polymers, polyvinylpyrrolidone (PVP) and polyacrylamide (PAA) are considered in the paper. The research conducted with use of chronopotentiometry and potential transients as well as a simulation model of the electrocrystallisation process showed that the values of pulse duration were greater than the transient time of electroactive species and the pause duration equal to the linear part of potential transient was optimum to obtain a powder with a predominant fraction of nanoscale particles . PVP and PAA additives help to reduce the average particle size in the nanoscale fraction copper powder particles to 25-40 nm. Selected current pulse mode effect corresponds to the reduction of the emergence of new centers of crystallization probability and the adatoms on the surface of the cathode movement probability.

Abstract: The aim of this study is to resolve the phenomenon of formation of mesoscopic structures on the surface of heteroepitaxial thin film system due to surface diffusion by considering the effects of both surface and interface stresses. Elastic stress field caused by curved surface is solved by using the constitutive equations of linear elasticity for the bulk and surface phases. Based on the method of superposition, a boundary perturbation technique, Goursat-Kolosov complex potentials and Muskhelishvili representations, the boundary value problem is reduced to the successive solution of a system of singular and hypersingular integral equations for any order of approximation. This solution and thermodynamic approach allows us to derive a governing equation which gives the amplitude changing of a surface roughness with time.

Abstract: Polycrystalline materials typically contain a very large number of grains whose surrounding grain boundaries evolve over time to reducethe overall energy of the microstructure. The evolution of the microstructure is influencedby the motion of the exterior surface since the grain boundaries couple to the exterior surface of the specimen; these effects can be appreciable especially in thin specimens. We model these effects using the classical framework of Mullins, in whichgrain boundaries move by mean curvature motion, V_n =A κ, and the exterior surface evolves by surface diffusion, V_n = -BΔ_s κ. Here V_n and κ denote the normal velocity and the mean curvature of the respective evolving surfaces, and Δ_s is the surface Laplacian. A classical way to determine A, the ``reduced mobility," is to make measurements based on the half-loop bicrystalline geometry. In this geometry one of the two grains, which embedded within the other, recedes at a roughly constant rate which can provide an estimate for A. In this note, we report on findings concerning the effects of the exterior surface on grain boundary motion and mobility measurements in the context of the half-loop bicrystalline geometry. We assume that the ratio of grain boundary energy to the exterior surface energy is small, and suitable assumptions are made of the specimen aspect ratio.

Abstract: Fine nanoporous copper was fabricated from the amorphous Ti-Cu alloys with a minor addition of silver in 10 mM HF solutions. The pore sizes decreased from 100 nm to 12 nm with the increase of the Ag contents in comparison of Ti₆₀Cu₄₀ ribbons free of Ag. With increasing of the dealloying time, the sizes of the nanopores and ligaments increased for the nanostrucutres on Ti₆₀Cu₃₈Ag₂ ribbons since the segregation of the Ag phase which triggered the galvanic dissolution of the adjacent Cu matrix in form of micro-couplings to further coarsen the nanoporous Cu. On the contrary, the trace formation of the Ag phase on the Ti₆₀Cu₃₉Ag₁ ribbons had a weak ability to motivate the galvanic dissolution, indicating by the constant pore sizes and slight decrease in the ligament sizes with the increase in the dealloying time. The refinement of the nanoporous structures was ascribed to the drastic decrease in the surface diffusivity. The decrease in the surface diffusivity due to the involvement of Ag with a lower surface diffusivity in comparison of Cu was more than one order of magnitude. The involvement of Ag adatoms restricted the diffusion of Cu adatoms in the interface regions in the inward and outward directions.

Abstract: In this article we present a different view on the results of experimental investigation of the self - diffusion on Pd (111) published in „Surface Science“ [1]. Our consideration is based on the band model of diffusion. This model is able to explain the Meyer-Neldel rule (MNR) and to clarify “puzzles” mentioned in [1]. The aim of this article is also to familiarize the readers with this model, to the band model of diffusion.

Abstract: The geometry structure and adsorption energy of Cu cluster adsorbed on Au (010) surface were affected by both the height of adsorption site and temperature. The height of adsorption site has major impact on the geometry structure and adsorption energy of the cluster when the temperature is low; while it has minor impact on the geometry structure and adsorption energy of the cluster at higher temperature. The adsorption energy is relevant to the atom number of the first layer on the surface vector of the clusters.The structural of Cu clusters on Au surface and its diffusion properties were studied in this paper by molecular dynamics simulation method and computer graphics techniques. The interaction potential between atoms is adopted by EAM form proposed by Johnson, simulation adopted by canonical ensemble of molecular dynamics method, and computer simulation was adopted to simulate the atomic structure of copper clusters of different amounts of atomic layers at different temperature during heating process. Moreover the analysis of the distribution function and the mean square displacement curve were performed by two representativ kinds of copper clusters Cu₅₅, Cu₂₀₁.