The Experts below are selected from a list of 294 Experts worldwide ranked by ideXlab platform
Jack M. Blakely - One of the best experts on this subject based on the ideXlab platform.
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Surface and Interfacial Topography of Oxides on Si(111) With Ultra-Low Atomic Step Density
MRS Proceedings, 2011Co-Authors: Antonio C. Oliver, Jack M. BlakelyAbstract:ABSTRACTAtomic force microscopy has been used to study the morphology of the oxide surface and the Si-SiO2 interface after oxidation of Si(111) surfaces that are either totally free of atomic Steps or have well characterized low Step Density. The Step-free areas were formed by thermally processing a patterned Si surface in which flat areas are enclosed by a square array of ridges; flow of the atomic Steps into the surrounding ridge barriers produces a regular array of Step-free areas each of which can be up to ∼50µm×50µm. Arrays of widely spaced Steps (e.g. 5µm) can also be produced in the Step-free areas. AFM scans of the same areas were taken prior to (dry) oxidation, after oxidation, and after chemical removal of the oxide. It was found that at an oxide thickness in the 5-13nm range, the initial Step structure of the underlying Si substrates is translated through the oxide to the surface after oxidation with the oxide surface being somewhat rougher than the initial substrate. Furthermore, the initial Step morphology of the substrate remains at the Si-SiO2 interface after etching away the oxide by HF. The interface roughness is less than that of the oxide surface. The results suggest that the initial oxidation of silicon proceeds in a ‘layer by layer’ manner and not through a preferential Step-flow oxide growth mode.
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Thin SiO2 layers on Si(111) with ultralow atomic Step Density
Journal of Vacuum Science & Technology B, 2000Co-Authors: Antonio C. Oliver, Jack M. BlakelyAbstract:The morphologies of the oxide surface and of the Si–SiO2 interface that form on special Si(111) substrates have been studied by atomic force microscopy (AFM). The substrates are totally free of atomic Steps or have very low Step Density. Step-free regions are formed on patterned Si(111) by thermal processing. AFM scans of the same areas prior to oxidation, after oxidation, and after chemical removal of the oxide allow the relative roughnesses to be compared. The Step structure of the Si(111) substrate is translated to the oxide surface even for SiO2 layers in the 10 nm range. The lack of significant displacement of the atomic Steps at the Si–SiO2 interface indicates that the oxide grows by a layer-by-layer mechanism.
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Scanning tunneling microscopy studies of phase separation on Si(001) surfaces with periodic Step Density
Journal of Vacuum Science & Technology B, 1991Co-Authors: C. C. Umbach, M. E. Keeffe, Jack M. BlakelyAbstract:Using photolithography and ultrahigh vacuum annealing, we have produced atomic Step arrays on Si(001), where the number of Steps per unit length normal to the [110] direction changes periodically. These arrays afford the opportunity to study Step phenomena that depend on average Step Density. A scanning tunneling microscope was used to image the detailed structure of the arrays. A transition in Step height—from primarily monatomic to primarily biatomic Steps—as a function of surface orientation was observed. Sharp changes in local slope associated with this transition were also observed. The connection with the equilibrium crystal shape is discussed.
S. Stoyanov - One of the best experts on this subject based on the ideXlab platform.
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Step Density waves on growing vicinal crystal surfaces - Theory and experiment
Journal of Crystal Growth, 2016Co-Authors: Bogdan Ranguelov, Pierre Muller, J J Metois, S. StoyanovAbstract:Abstract The Burton, Cabrera and Frank (BCF) theory plays a key conceptual role in understanding and modeling the crystal growth of vicinal surfaces. In BCF theory the adatom concentration on a vicinal surface obeys to a diffusion equation, generally solved within quasi-static approximation where the adatom concentration at a given distance x from a Step has a steady state value n ( x ) . Recently, we show that going beyond this approximation (Ranguelov and Stoyanov, 2007) [6], for fast surface diffusion and slow attachment/detachment kinetics of adatoms at the Steps, a train of fast-moving Steps is unstable against the formation of Steps Density waves. More precisely, the Step Density waves are generated if the Step velocity exceeds a critical value related to the strength of the Step–Step repulsion. This theoretical treatment corresponds to the case when the time to reach a steady state concentration of adatoms on a given terrace is comparable to the time for a non-negligible change of the Step configuration leading to a terrace adatom concentration n ( x , t ) that depends not only on the terrace width, but also on its “past width”. This formation of Step Density waves originates from the high velocity of Step motion and has nothing to do with usual kinetic instabilities of Step bunching induced by Ehrlich–Schwoebel effect, surface electromigration and/or the impact of impurities on the Step rate. The so-predicted formation of Step Density waves is illustrated by numerical integration of the equations for Step motion. In order to complete our previous theoretical treatment of the non-stationary BCF problem, we perform an in-situ reflection electron microscopy experiment at specific temperature interval and direction of the heating current, in which, for the first time, the Step Density waves instability is evidenced on Si(111) surface during highest possible Si adatoms deposition rates.
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Evaporation and growth of crystals: Propagation of Step-Density compression waves at vicinal surfaces
Physical Review B, 2007Co-Authors: Bogdan Ranguelov, S. StoyanovAbstract:We studied the Step dynamics during crystal sublimation and growth in the limit of fast surface diffusion and slow kinetics of atom attachment-detachment at the Steps. For this limit we formulate a model free of the quasi-static approximation in the calculation of the adatom concentration on the terraces at the crystal surface. Such a model provides a relatively simple way to study the linear stability of a Step train in a presence of Step-Step repulsion and an absence of destabilizing factors (as Schwoebel effect, surface electromigration etc.). The central result is that a critical velocity of the Steps in the train exists which separates the stability and instability regimes. When the Step velocity exceeds its critical value the plot of these trajectories manifests clear space and time periodicity (Step Density compression waves propagate on the vicinal surface). This ordered motion of the Steps is preceded by a relatively short transition period of disordered Step dynamics.
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properties and dynamic interaction of Step Density waves at a crystal surface during electromigration affected sublimation
Physical Review B, 1998Co-Authors: S. Stoyanov, Vesselin TonchevAbstract:Electromigration affected sublimation is a complicated phenomenon, involving surface transport coupled to a process of atom exchange between the two-dimensional gas of adatoms and the crystal phase. The case of intensive exchange is theoretically treated and equations of Step motion are derived for the case of ``nontransparent'' Steps (kinetics with local conservation of adatoms). The numerical integration of these equations manifests Step bunching (a formation of Step Density waves) at Step-down direction of the electromigration of adatoms. We studied some properties of the Step Density waves: the amplitude (the maximum slope of the bunch) and its dependence on the number of Steps in the bunch, the kinematic wave velocity and the dynamic interaction of waves of different amplitudes. The central result of this work is the dependence of the minimum interStep distance (in the steady state shape of the bunch) on the model parameters. This dependence, extracted from numerical study, is presented in terms of scaling laws ${l}_{\mathrm{min}}\ensuremath{\sim}{N}^{\ensuremath{-}r}{(A/F)}^{q}$, where $N$ is the number of Steps in the bunch, $A$ is the magnitude of Step-Step repulsion, and $F$ is the force, inducing electromigration of the adatoms. Both scaling exponents $r$ and $q$ depend on the power $n$ in the Step-Step repulsion dependence on the interStep distance ${(U=A/l}^{n})$ and, therefore, they are a key to the problem of experimental evaluation of $n$. A striking result of this model is the constant value of ${l}_{\mathrm{min}}$ in a wide range of values of the average diffusion distance ${\ensuremath{\lambda}}_{s}.$ Thus one cannot relate the temperature dependence of ${l}_{\mathrm{min}}$ to the temperature dependence of ${\ensuremath{\lambda}}_{s}.$ Numerical analysis of the dynamics of Steps at a crystal surface of small misorientation angle reveals two types of dynamic interaction of bunches of Steps: ``bunch size exchange'' and ``effective coalescence.'' The former type of interaction is rather interesting \char22{} a smaller (and faster) bunch approaches a larger one and they travel together until the initially larger bunch achieves (by losing Steps) a size, smaller than the size of its partner, and runs away of it.
Olivier Pierrelouis - One of the best experts on this subject based on the ideXlab platform.
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vicinal silicon surfaces from Step Density wave to faceting
Physical Review B, 2007Co-Authors: Frederic Leroy, Pierre Muller, J J Metois, Olivier PierrelouisAbstract:This paper investigates faceting mechanisms induced by electromigration in the regime where atomic Steps are transparent. For this purpose we study several vicinal orientations by means of in-situ (optical diffraction, electronic microscopy) as well as ex-situ (AFM, microprofilometry) visualization techniques. The data show that faceting proceeds in two stages. The first stage is short and leads to the appearance of a Step Density wave, with a wavelength roughly independent of the surface orientation. The second stage is much slower, and leads to the formation of a hill-and-valley structure, the period of which depends on the initial surface orientation. A simple continuum model enables us to point out why the wavelength of the Step Density wave does not depend on the microscale details of the surface. The final wavelength is controlled by the competition between elastic Step-Step interaction and facet edge energy cost. Finally, the surface stress angular dependence is shown to emerge as a coarsed-grained picture from the Step model.
Marc T M Koper - One of the best experts on this subject based on the ideXlab platform.
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effect of Step Density and orientation on the apparent ph dependence of hydrogen and hydroxide adsorption on Stepped platinum surfaces
Journal of Physical Chemistry C, 2018Co-Authors: Ian T Mccrum, Xiaoting Chen, Kathleen A Schwarz, Michael J Janik, Marc T M KoperAbstract:The effect of the alkali-metal cation (Li+, Na+, K+, and Cs+) on the non-Nernstian pH shift of the Pt(554) and Pt(533) Step-associated voltammetric peak is elucidated over a wide pH window (1-13), through computation and experiment. In conjunction with our previously reported study on Pt(553), the non-Nernstian pH shift of the Step-induced peak is found to be independent of the Step Density and the Step orientation. In our prior work, we explained the sharp peak as due to the exchange between adsorbed hydrogen and hydroxyl along the Step and the non-Nernstian shift as a result of the adsorption of an alkali-metal cation and its subsequent weakening of hydroxyl adsorption. Our Density functional theory results support this same mechanism on Pt(533) and capture the effect of alkali-metal cation identity and alkali cation coverage well, where increasing electrolyte pH and cation concentration leads to increased cation coverage and a greater weakening effect on hydroxide adsorption. This work paints a consistent picture for the mechanism of these effects, expanding our fundamental understanding of the electrode/electrolyte interface and practical ability to control hydrogen and hydroxyl adsorption thermodynamics via the electrolyte composition, important for improving fuel cell and electrolyzer performance.
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Electro-oxidation of ethanol and acetaldehyde on platinum single-crystal electrodes.
Faraday Discussions, 2008Co-Authors: Marc T M KoperAbstract:The electrochemical oxidation of ethanol and acetaldehyde in sulfuric acid and perchloric acid were studied at Pt (111), Pt (110), and a number of Pt [n(111)×(111)] single-crystal electrodes. The oxidation of ethanol shows a marked dependence on the surface structure, roughly increasing as the surface Step Density increases. The oxidation of acetaldehyde shows a reversed correlation, the activity decreasing with increasing Step Density. Based on the results obtained here and reported earlier in electrochemical and ultrahigh vacuum literature, a detailed reaction scheme for the ethanol oxidation mechanism is suggested.
B.a. Joyce - One of the best experts on this subject based on the ideXlab platform.
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Direct evidence for the Step Density model in the initial stages of the layer-by-layer homoepitaxial growth of GaAs(111)A
Surface Science, 1998Co-Authors: D. M. Holmes, J. L. Sudijono, C. F. Mcconville, Tim Jones, B.a. JoyceAbstract:Scanning tunnelling microscopy (STM) has been used to investigate the morphological basis of the specular beam intensity oscillations observed in reflection high-energy electron diffraction (RHEED) studies during the initial stages of GaAs(111)A homoepitaxy. Analysis of STM images after the deposition of controlled amounts of GaAs up to a coverage of 2 monolayers show a strong relationship between the Step Density and the RHEED specular beam intensity. It is shown that the RHEED oscillations observed during the initial stages of growth reflect the temporal variation in surface Step Density.
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Step Density variations and reflection high energy electron diffraction intensity oscillations during epitaxial growth on vicinal gaas 001
Physical Review B, 1992Co-Authors: T Shitara, M. R. Wilby, D D Vvedensky, J Zhang, J H Neave, B.a. JoyceAbstract:Systematic measurements and simulations are reported for the transition from growth by nucleation and accretion of two-dimensional islands to Step advancement on misoriented GaAs(001) surfaces during epitaxial growth. The growth conditions have been chosen in order to satisfy as much as possible the underlying assumptions of a solid-on-solid model of epitaxial growth, namely, that the adatom mobility is isotropic (by using a surface misoriented toward the [010] direction), the effect of the As is not rate determining (by using a As/Ga ratio of 2.5), and that the presence of any surface reconstruction can be subsumed in effective migration parameters, i.e., that no explicit account of surface reconstruction is required if the reconstruction does not change (by maintaining the 2\ifmmode\times\else\texttimes\fi{}4 reconstruction). The diffraction conditions for reflection high-energy electron-diffraction (RHEED) measurements were chosen to eliminate as much as possible the contribution of well-known incoherent features to the specular intensity on GaAs(001). Using these growth and diffraction conditions, the parameters of a solid-on-solid model have been optimized by performing extensive simulations to quantitatively reproduce the measured misorientation-angle dependence and the Ga flux dependence of the growth-mode transition temperature. Since at the chosen diffraction conditions, the kinematic diffraction is insensitive to surface morphologies, we have modeled the growth-induced loss of intensity from the specular beam as being due to the Steps on the surface, with each Step acting as an individual source of scattering. Direct comparisons between the time-dependent Density of surface Steps of the simulated surfaces and the RHEED specular intensity profiles measured during growth reveal several qualitative and quantitative similarities. The most striking of these is that the two quantities show the same relative change of magnitude with time and temperature for a given misorientation and Ga flux. The implications of these comparisons are discussed for the growth dynamics of GaAs(001), for the scattering processes in RHEED, and the morphological sensitivity of RHEED for these diffraction conditions.