Kinetic Tight-binding Ising Model Research Articles

Structure and composition of the segregated Cu in V 2O 5/Cu system

We have investigated segregation of copper at the surface of V 2O 5 films deposited onto Cu substrate by employing surface analysis techniques. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) confirmed that the Cu is segregated at the surface and its chemical state is Cu 2O. According to secondary ion mass spectroscopy (SIMS) and glow discharge spectroscopy (GDS), the Cu concentration inside the deposited V 2O 5 layer is low. Ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS) revealed the segregation alters the surface local density of states. Surface analysis of deposited samples in ultra high vacuum (UHV) condition verified that the segregation occurs during the deposition. We have extended kinetic tight binding Ising model (KTBIM) to explain the surface segregation during the deposition. Simulation data approve the possibility of surface segregation during room temperature deposition. These results point out that on pure Cu substrate, oxidation occurs during the segregation and low surface energy of Cu 2O is the original cause of the segregation.

Importance of proper choice of transition rates in kinetic simulations of dynamic processes

Proper transition rates in kinetic mean-field and Monte Carlo simulations of dynamic processes are very important to obtain realistic results. We show that by proper choice of the transition rates one can unify the advantages and eliminate the disadvantages of different models used in the literature [see, e.g., Senhaji et al., Surf. Sci. 274, 297 (1992 )( kinetic tight binding Ising model), Martin, Phys. Rev. B 41, 2279 (1990) and Cserhati et al., Surf. Sci. 290, 345 (1993)]. Furthermore, we also show that this choice cannot be considered simply as an extension of the previous ones. It contains an additional parameter controlling the ratio of the transition rates in the bulk and close to the surface sGsurf / Gbulkd, which influences the kinetics of surface segregation. We show that some results obtained previously need reconsideration. We illustrate, e.g., how the composition dependence of the transition rates (diffusion coefficient) influences the ‘surfactant formation and dissolution mode’. For example, the dissolution kinetics can deviate from the parabolic law on the nanoscale in accordance with our recent results [Erdelyi et al., Phys. Rev. B 69, 113407 (2004)]. Furthermore, we also present how the sGsurf / Gbulkd ratio affects the kinetic segregation isotherm.

Linear time dependence of the surfactant effect: A local equilibrium under flux

We have investigated the dissolution kinetics of an A metal thin film deposited on a (100) B metal under thermal annealing for a system with a strong B segregation (e.g., Ni/Ag). The interdiffusion is described using both Monte Carlo and mean-field simulations with an effective exchange mechanism derived from the kinetic tight-binding Ising model. Surprisingly, the surface enrichment in substrate B element is found to increase linearly with time instead of following the usual $\sqrt{t}$ law. We demonstrate that this t law occurs when the diffusion coefficient of B atoms in the A deposit is high or when the deposit thickness decreases. Moreover it is shown that, in this surfactant regime, the usual local equilibrium rule which drives the kinetics has to be generalized under the flux of B atoms coming from the deposit/substrate interface. Consequences of this new formulation on nonequilibrium dynamical behaviors during thin film dissolutions are discussed.

Layer-by-layer versus surfactant dissolution modes in heteroepitaxy

~Received 18 May 1999! Dissolution of a thick A film into aB substrate during metal heteroepitaxy ( A/B) obeys general trends which are described here in the case where the corresponding AB alloy presents a tendency to bulk phase separation and to B surface segregation ~e.g., A5Ni, B5Ag). Using the kinetic tight-binding Ising model, either in the mean-field or in the Monte Carlo framework, we findlayer-by-layer dissolution modes which, depending on the annealing temperature T, are preceded ( T.Tk) or not ( T,Tk) by the rising of capping B monolayers burying an almost intact A film ~surfactantlike effect!. Tk is found to decrease as the tendency to surface segregation of the substrate element increases, which can be understood in terms of local equilibrium at both the surface and the interface of the deposit. The model gives access to the main kinetic laws which are obeyed in the two dissolution modes, i.e., the total quantity of deposit matter decreases linearly with the square root of time. @S0163-1829~99!00243-X#

Theoretical prediction of new dissolution modes during metal heteroepitaxy

By means of the kinetic tight-binding Ising model we investigate the Ni/Ag dissolution modes resulting from the thermal interdiffusion between the two elements. For a thin deposit (1 ML) we show that the dissolution proceeds by the formation of encapsulated Ni precipitates covered by two pure Ag monolayers. For a thick deposit (10 ML) we find two distinct layer-by-layer modes depending on the annealing temperature. At high temperature the kinetics start with the apparition of capping Ag monolayers on a buried almost intact Ni film ( surfactant-layer-by-layer mode). Below a critical temperature T c, the dissolution proceeds without any Ag surface enrichment ( layer-by-layer mode). We find that T c decreases when the surface driving forces which favour the segregation of the substrate elements increase.

SURFACE ALLOY FORMATION IN THE Cu–Pd(111) SYSTEM: A KTBIM APPROACH

We study the kinetics of formation and dissolution of surface alloy obtained by depositing one monolayer of Pd on Cu(111) at room temperature. This is performed within the kinetic tight-binding Ising model. We also present the equilibrium segregation isotherm for Pd c Cu 1-c(111) and show the relation between kinetics (surface alloy) and equilibrium (alloy surface) via the local equilibrium concept.

Microstructure of the surfactantlike effect in Ni/Ag(100) and (111)

Recent experiments on Ni/Ag systems have shown that the growth mode can be a floating monosubstrate or bisubstrate layer burying the deposited film. We study this surfactant effect by both mean-field and Monte Carlo kinetic approaches. The kinetic tight-binding Ising model in mean-field approximation gives quasisteady profiles consistent with local equilibrium concepts. In this framework, we find two and one capping Ag monolayers for Ag(100) and Ag(111), respectively, on a mostly pure Ni monolayer. Monte Carlo simulations reveal that this Ni ``sandwich'' structure is in fact made of embedded Ni precipitates with low interface energy regular polyhedra shapes. The cluster distribution is found to be larger for the less compact surface [i.e., (100)] and to obey the ``Ostwald ripening'' law.

An experimental and theoretical study of surface segregation in a Fe-6at.%Si bicrystal

The analysis of segregation phenomena in bicrystals is an important step for the understanding of combined effects of the elementary diffusion processes involved in the segregation in polycristalline systems. The segregation of Si and P in a Fe-6at.%Si bicrystal with a (100) and (110) surface has been investigated by means of AES (Auger electron spectroscopy). For these experiments the technique of a linearly increased temperature has been applied. Significant differences between the segregation kinetics at the two surfaces of the sample have been found on the one hand for the maximum coverage of P and on the other hand for the high temperature behaviour of Si. Additionally, model calculations based on the KTBIM (kinetic tight binding Ising model) have been performed to qualitatively describe the experimental results. It is shown, that the striking differences between the segregation behaviour at the two differently oriented surfaces can be explained by different segregation energies of P, whereas Si plays a minor role due to its relatively small segregation energy.

Theoretical study of surface alloy formation through generation and annihilation of vacancies

Recent STM experiments suggest that the formation of a surface alloy, when Mn is deposited on Cu(100), could be attributed to a kink-induced vacancy formation, followed by Mn incorporation through vacancy annihilation in the step vicinity. We present an extension of the kinetic tight-binding Ising model, which takes into account the preferential formation of vacancies at the kink, their diffusion towards the centre of the terrace and their annihilation by Mn adatoms. The jump frequencies are obtained using molecular dynamics within the second moment approximation of the tight-binding scheme. The spatial distribution of incorporated Mn and its asymmetry between the upper and lower terrace are discussed in view of the STM experiments.

Flux dependence of the surfactant effect in [formula omitted]: a theoretical study

Recent experiments on systems such as Rh Ag or Ni Ag have shown that, in an intermediate range of temperature, competition between the flux of the deposited adatoms and the interdiffusion between the deposit and the substrate can lead to an original growth mode in which a floating mono- or bi-substrate layer buries the deposited film. This surfactant effect has been previously modelled, in the simplified situation of successive flashes of a complete monolayer, in the framework of the kinetic tight-binding Ising model. We show here that the extension of this model to the case of a continuous incoming flux leads to a wide variety of behaviours concerning the depth and spatial extension of the buried film as a function of the flux.

Segregation Kinetic Simulation and Segregation Energy Determination on a CuAg(111) Alloy

AbstractUsing a method based on the kinetic tight binding Ising model (KTBIM), the segregation energy on the (111) face of a CuAg 0.45 at% alloy, is derived as a function of surface coverage, from segregation kinetics measured at 763 K. Nearly the same segregation energy value as from equilibrium experiments is obtained. Besides, it is shown that a surface size effect term, proportional to the square of the surface concentration, has to be introduced in the segregation energy, in order to take into account the surface strain energy which occurs in the first layer when the silver atoms segregate at the surface.

Kinetics of surface segregation during linear programmed heating

An analysis of the experimentally investigated kinetics of surface segregation can be used to obtain information on the diffusion properties of the system near the surface. A promising way for an efficient experimental analysis of the temperature dependence of the effective diffusion coefficients is the investigation of surface segregation with linearly programmed heating (LPH), in which the temperature is increased linearly with time during the experiment. Model calculations of the segregation kinetics during LPH have been performed on the basis of a modified KTBIM (kinetic tight-binding Ising model) for dilute binary and ternary systems to investigate the general segregation kinetics and the applicability of this method in real systems. The results allow one to determine the conditions under which experimental data can be used to obtain the Arrhenius parameters of the effective diffusion coefficients of the segregating elements in real systems. Additional model calculations performed to describe the segregation kinetics of N and S in polycrystalline Fe and Sn at the Cu(111) surface show excellent agreement with experimental results and demonstrate the sensitivity of this model.

Surface segregation kinetics in binary substitutional systems: a comparison between MDE and KTBIM

Because of its great technological relevance the investigation of the kinetics of interface segregation is gaining increasing interest. During the last few years two successful models describing the kinetics of surface segregation in substitutional systems have been developed, the MDE (modified Darken equations) based on the macroscopic diffusion equation and the KTBIM (kinetic tight-binding Ising model) based on the microscopic jump probabilities of atoms of neighbouring atom planes. The resulting segregation kinetics obtained by these models are compared and discussed. Whereas the results for the long-time segregation behaviour are very similar especially in dilute systems, the predicted segregation kinetics in concentrated systems and during the initial phase of the segregation process strongly depends on the applied model. This discrepancy between the results obtained with the two models in part can be avoided by using the concept of an enhanced subsurface mobility of the segregand in the MDE.

Model calculations of temperature programmed surface segregation

The investigation of the kinetics of surface segregation can reveal important information on the diffusion properties of the system near the surface. A promising way for an efficient experimental analysis of the temperature dependence of the effective diffusion coefficients is the temperature programmed segregation (TPS), in which the temperature is increased linearly with time during the experiment. Model calculations of the segregation kinetics during TPS have been performed on the basis of the KTBIM (kinetic tight binding Ising model) for dilute binary and ternary systems to investigate the applicability of this method in real systems. The results give hints on the conditions under which experimental results can be used to obtain the Arrhenius parameters of the effective diffusion coefficients of the segregating elements in real systems.

Kinetics of segregation and dissolution in Cu 1− cAg c and surface phase transition: comparison between mean field and Monte Carlo calculations

In this work we study the relation between the equilibrium surface segregation and the kinetics of segregation and dissolution in the CuAg(111) system at T= 1000 K, which presents surface phase transition. This is performed with the recently developed kinetic tight binding Ising model (KTBIM), which allows us to prove the efficiency of the local equilibrium concept. For the kinetics of dissolution we obtain great differences between the mean field kinetics and the Monte Carlo one. This last one, which agrees very well with the experimental data, takes into account the strong chemical inhomogeneity of the surface in the domain of the phase transition, contrary to the homogeneous mean field approach.

On a “surfactant-like” behaviour of [formula omitted] deposit

Auger experiments performed during the annealing at 620 K of one monolayer of Ni, deposited at room temperature on a Ag(100) substrate, show a very rapid decrease of the Ni signal indicating an almost immediate disappearance of Ni atoms from the surface, whereas annealing at 770 K the reverse deposit Ni Ag(100) does not lead to any dissolution of Ag, even after a long time. This clearly non-Fickian behaviour can be understood in terms of local equilibrium near the surface, by means of a recently developed microscopic kinetic model (KTBIM: kinetic tight-binding Ising model) taking into account the driving forces of surface segregation. Moreover, the KTBIM simulation of the dissolution of additional Ni layers, successively deposited on Ag(100), reveals the existence of a “floating” Ag bi-layer which stays above the Ni deposit, analogous to a surfactant during semiconductor growth.

Alloy surfaces and surface alloys: from equilibrium to kinetics

We study the relation between the surface segregation at thermodynamical equilibrium in an alloy A c B 1− c and the kinetics of dissolution of A B or B A . This is possible within a new kinetic model based on the electronic structure: the kinetic tight-binding Ising model. In particular, the existence of a local equilibrium between the surface and the first underlayer is stressed. Some illustrations are given in the particular case of two systems, one presenting a tendency to bulk phase separation and surface layering transitions (CuAg) and the other a tendency to bulk ordering and surface concentration profile transitions (PtNi). The notion of “ surface alloy” is defined in the latter case.

Is the segregation-dissolution kinetics driven by a surface local equilibrium? An answer via the kinetic tight-binding Ising model

We study here the relation between the surface segregation at thermodynamical equilibrium in an alloy AcB1−c and the kinetics of either segregation in the system A-B or dissolution of AB or BA. This is possible within a new kinetic model based on the electronic structure: the kinetic tight-binding Ising model (KTBIM). In particular, the existence of a local equilibrium between the surface and the first underlayer is stressed. Some illustrations are given in the particular case of the W-Re system.