Diffusion Processes Of Adatoms Research Articles

Nanocone formation on ion-bombarded InP surfaces

Surfaces of InP(100) were bombarded by 2.1keV Ar+ ions under normal incidence. The total accumulated ion fluence the samples were exposed to was varied roughly from 1×1017cm−2 to 3×1018cm−2 while ion fluxes in the range of (0.1−2)×1014cm−2s−1 were used. The surface morphology resulting from these ion irradiations was examined by scanning electron microscopy (SEM). For a specific range of ion fluxes and fluences, regular closely-spaced nanocone arrays are found to form, with the cones having typical base diameters of about 100nm. Around their outer peripheries, they commonly exhibit rather distinct protrusions, an observation not reported in previous investigations. The inspection of the initial stages of nanocone formation indicates that the growth of adatom islands, first on the pristine surface and later on previously formed islands, may lead to these 3D nanostructures. This finding would imply that surface diffusion processes of adatoms constitute an important mechanism in the evolution of the observed topography.

Long jumps contribution to the adatom diffusion process near the step edge: The case of Ag/Cu(110)

In this work, the diffusion of a single Ag adatom on a low-index Cu surface (110) in the presence of a step edge is studied using the embedded-atom method (EAM). Molecular static simulation is carried out in order to calculate the activation energy of different diffusion processes. Our findings are in a good agreement with results existing in the literature indicating that adatom diffusion via jump process is more favored than the other mechanisms. The activation energy corresponding to diffusing via hopping is found to be 0.25 eV (at 0 K). On the other hand, the activation barrier calculated by molecular dynamics (MD) simulation for a large range of temperature (310–500 K) is found to be around 0.25 eV for both upper and lower position leading to a good agreement between static and dynamic calculations. The prefactor for Ag adatom self-diffusion via hopping on Cu(110) surface near the step edge is examined. The results show that the prefactors are 2.7 and 3.6 × 104 cm2 s−1 for the upper and lower position, respectively. This is in line with the value of 10−3 cm2 s−1 that is generally adopted. We also found that long jumps occur frequently in this system and their contribution cannot be neglected.

MOLECULAR DYNAMICS SIMULATIONS OF SINGLESiADATOM DIFFUSION ON THESi(001)SURFACE AND ACROSS SINGLE-LAYERSi(001)STEPS

By using the Stillinger–Weber atomic interactional potential, we have carried out molecular dynamics simulations of single Si adatom diffusing on the Si(001) surface and single-layer Si(001) steps at temperatures ranging from 1000 K to 1300 K. We have presented one new diffusion pathway of a single Si adatom diffusing on the Si(001) along the direction perpendicular to dimer rows, that can weaken the diffusion anisotropy. We have investigated the process of the single Si adatom diffusing across single-layer Si(001) steps as well and given adatom diffusion pathways of step-flow and transformation of single-layer into double-layer steps. Our results show that the exchange between an adatom and a surface atom plays an important role in the adatom diffusion process above 1000 K.

The Calculation of Adatom Diffusion Process on Si and Ge Surfaces from First-Principles

We study Si adatom diffusion process near Si/Ge stepped surfaces from first-principles. Dependence of surface growth morphology on growing conditions such as temperature is not clearly understood. We calculate Ehrlich-Schwoebel(ES) barrier of stepped surfaces from first-principles and analyze adatom diffusion process to understand growth mechanism of the surfaces. The configuration of the surfaces with ES barrier would play a key role in the diffusion process because it controls hopping rates in the presence of step edges. Our results are likely to help correctly access the adatom diffusion process on the surfaces.

Local Self-Order Observed During Chemical Vapor Deposition of Silicon Quantum Dots for Application in Nanocrystal Memories

ABSTRACTA systematic study on the Si dot formation after chemical vapor deposition on silicon oxide substrates is presented. The process has been followed from the early stages of the dot formation up to 25% of coverages. Structural characterization has been performed by means of energy filtered transmission electron microscopy, which allowed us to observe dot sizes down to 0.5 nm in radius. The nanodots are shown to be surrounded by a depleted zone, where no new Si dots are observed to nucleate. This has been attributed to the adatoms capture mechanism by pre-existing dots, during the deposition. The dot radius and the capture size are shown to collapse onto the same curve, thus indicating the scaling behavior of the process. The adatom diffusion process is shown to restrict the number of nucleation sites, the final dot size and the dot position, thus driving the process toward partial self-order.

Adatom Diffusion during Decay of 2D Island on Si(100) Studied by Kinetic Monte Carlo Simulation.

Adatom diffusion process during decay of a 2D island formed on Si(100)-2×1 surface is analyzed using a kinetic Monte Carlo simulation based on a solid-on-solid model. The diffusion is basically a mass transport on the surface; both a source and a drain of adatoms are needed. The experimental results of the decay process of a 2D island at the substrate temperature around 800 K are reproduced when a boundary condition with proper drains is adopted in the system of simulation. The decay rate depends on the boundary condition. The decay is governed by adatom diffusion process, but the process is not so simple. An adatom does not move long distance on a flat terrace. It is trapped by a vacancy created by hopping of another atom from the terrace, and the latter atom moves on the terrace instead of the former in the next step. This vacancy mediated diffusion of adatom is a key issue for understanding diffusion process on surface, especially at high temperature.

First-principles calculation on diffusion of Si adatoms on H/Si(001)-(2×1) surface

We report the result of first-principles calculations on the Si adsorption on the monohydride terminated Si(001)-(2×1) surface. It is found that the Si adatom spontaneously segregates one H atom from a surface Si dimer during adsorption, and further captures the remaining H atom of the same Si dimer during surface migration, leading to the most stable adsorption geometry. The Si adatom diffusion process on the hydrogenated Si(001) surface is complicated by the transfer of H atoms between the adatom and the Si dimer atoms. The migration of the Si adatom is extremely reduced compared with that on the bare Si(001) surface.

RHEED investigations of MBE-growth kinetics of Si on Si(111) and SiC on SiC(100)

Reflection high-energy electron diffraction has been used to study the molecular beam epitaxy-growth kinetics of Si on Si(111) and of SiC on SiC(100). By means of a simple kinematic scattering theory we have estimated diffusion barrier energy values for adatom migration and nucleation energies. Results clearly show that growth kinetics are different at low and high temperatures for both systems. At lower temperatures the nucleation can be explained by a single adatom diffusion process, whereas at higher temperatures much higher activation energies may be attributed to an island coarsening process.

Diffusion processes and growth on aluminum cluster surfaces

Diffusion processes of adatoms on icosahedral and Wulff polyhedral aluminum cluster surfaces have been studied by molecular dynamics simulations using the effective medium theory. Activation energies of diffusion mechanisms along {111} and {100} facets and from one facet to another, including different hopping and exchange processes as well as more exotic events, have been calculated. Exchange diffusion of an adatom by a chain mechanism through a {100} facet between two {111} facets and hopping diffusion across the edge between two {111} facets via a pull of another adatom on the neighbour facet are shown to play an important role. Adatoms on {111} facets are mobile already at very low temperatures, but on {100} facets diffusion starts above the room temperature as well as diffusion from {111} facets to {100} facets. Diffusion from {100} facet to other facets was not observed until at temperatures close to the melting temperatures of clusters. Dynamical simulations at different temperatures confirmed the appearance of diffusion mechanisms predicted by the activation energies.