SA Step Research Articles

Изучение кинетики сближения ступеней поверхности Si(100)

In this work, the convergence kinetics investigations of the SA and SB steps on Si(100) substrates with inclination 0.5o and 0.1o were carried out. Analysis of the time dependence of reflection high-energy electron diffraction (RHEED) intensity was used to establish the growth kinetics character on vicinal Si(100) surfaces. It is shown that, in a Si flow at the growth rate of 0.37 ML/s, the step convergence velocity has a decreasing exponential dependence with the temperature increase. It is determined that the single-domain surface formation velocity increases with an increase in the terrace width on the surface, which may be due to the partial participation of growth due to the formation of two-dimensional islands. Above a temperature of 650°C, the dominant growth mode is due to the step movement and the single-domain surface formation velocity decreases with an increase in the terrace width. Thus, the single-layer step convergence is determined by both the MBE growth conditions and the Si(100) substrate orientation. The convergence of SA and SB steps of the Si(100) surface is explained by the slowdown of the step SA motion, which is associated with complex permeability mechanisms and a kink formation of steps. It is assumed that the reason for the slowdown of the step convergence with increasing temperature is an increase in the kink density at the SA step, which reduces the step SA permeability coefficient.

Subcritical damping of S step energy on Si(001) vicinals by lowering terrace stress

Abstract Si(001) vicinals with monoatomic steps present an alternation of (1 × 2) and (2 × 1) terraces and SA and SB steps. Our calculations, by using MEAM potential, show a surface stress difference between neighbouring terraces. By both energy minimization and lowering terrace stress, we observe a deformation transfer between neighbouring terraces. The deformation lowers the stress of the higher stress terrace. But the deformation logarithmically varies with step–step distance L. In theory of energy minimization, the great difficulty is to accept, or not, the logarithmic divergence for large L. We show that the logarithmic variation with L is accompanied by the vanishing of the SA step energy. The terrace–terrace deformation transfer disappears when the SA step energy is reduced to zero. This explains the large roughness of the step SB in experiment, because it is perpendicular to SA. This paper clearly confirms that surface stress and surface energy are different quantities to be considered in equilibrium of surfaces. The minimization of the surface energy is accompanied by the lowering of the surface stress for the higher stress terrace. Consequently, this terrace is contracted, while the other terrace is expanded in order to render the sum of equilibrium deformations over the period including the two terraces nul.

Electronic Control of the Tip-Induced Hopping of an Hexaphenyl-Benzene Molecule Physisorbed on a Bare Si(100) Surface at 9 K

In this work, we show that the hopping directivity of individual hexaphenyl-benzene (HPB) molecules physisorbed along the SA step edge of a bare Si(100)-2×1 surface can be reversibly controlled with a periodic hopping length. This is achieved by using the tunnel electrons of a low temperature (9 K) scanning tunneling microscope (STM). A statistical analysis of the electronic excitations applied at various positions on the HPB molecule reveals that the hopping process is related to a strong decrease of the tunnel junction conductance. This process is associated with a charge transfer from the silicon surface to the HPB molecule leading to a hopping mechanism that occurs in two sequential steps. The first step of the hopping process involves the formation of an HPB– anion that triggers the molecular motion into a metastable state. The second step is related to the neutralization of the HPB– anion which provokes the manipulation of the molecule to its final steady position. Our experimental data are supported by the calculations of the relaxed molecule using the density functional theory on the Si(100) surface that takes the van der Waals forces interactions into account. Additional calculations of the HPB– anion orbitals depict the spatial localization of the extra charge inside the HPB molecule and the relative energies of the HPB– molecular orbitals. Finally, our study shows that the hopping direction can be optimized by positioning the STM tip at specific locations along the hopping pathway.

Development of Noncontact Atomic Force Microscopy Operating at Low Temperatures

Noncontact atomic force microscopy (NC-AFM) using frequency modulation detection method has been widely used to investigate the various surfaces with atomic resolution. In this paper, we introduce the measurement technique of NC-AFM operating at low temperatures (LTs). First, we theoretically discuss the enhancement of the force sensitivity in NC-AFM operating at LTs. Then, we present the design and performance of LT-NC-AFM using fiber optic interferometer with quick sample and cantilever exchange mechanism. We also show the present status of the LT-NC-AFM imaging. In detail, we show the experimental results to investigate the influence of the surface stress around an SA step of Si(001) surface onto the buckled dimer at 5 K. We demonstrate that the LT-NC-AFM has a capability to detect the surface stress with atomic resolution.

Influence of Surface Stress on the Phase Change in a Si(001) Step Measured by LT-NC-AFM

Using the ability of the noncontact atomic force microscopy (NC-AFM) to image and perturb the Si(001) dimer configuration, we investigate the influence of surface stress around an SA step of Si(001) on the buckled dimer at 5 K. The NC-AFM first detected the different buckled dimer phases between the terraces: asymmetric c(4×2) phase in the upper terrace and symmetric p(2×1) one in the lower terrace appeared in both topographic and dissipation images. In addition, we found, for the first time, the dimers by the step in the upper terrace compress by 0.036±0.013 along dimer bond direction from the averaged profile analysis of the images. Variation in the surface dimer structure reflected by the surface stress was demonstrated from the dissipation image analysis, which points to the possible exploration of the surface stress by means of NC-AFM.

Self-organization of step bunching instability on vicinal substrate

The authors investigate quantitatively the self-organization of step bunching instability during epitaxy of Si on vicinal Si(001). They show that growth instability evolution can be fitted by power laws L∼tα and A∼tβ (where L is the correlation length and A is the instability amplitude) with critical exponents α∼0.3 and β∼0.5 in good agreement with previous studies and well reproduced by kinetic Monte Carlo simulation. They demonstrate that the main phenomenon controlling step bunching is the anisotropy of surface diffusion. The microscopic origin of the instability is attributed to an easier adatom detachment from SA step, which can be interpreted as a pseudoinverse Ehrlich-Schwoebel barrier [J. Appl. Phys. 37, 3682 (1967); J. Chem. Phys. 44, 1039 (1966)].

Behavior of Ga atoms on Si(001) surface at high temperature

The growth processes and structures of Ga layers formed on a Si(001) surface have been studied by scanning tunneling microscopy and low-energy electron-diffraction analysis. Si(001) wafers cut at 0.5° toward the [110] direction and showing a double-domain surface structure were used as substrates. Ga atoms from a Knudsen cell were deposited on substrates maintained at 600°C. In a Ga coverage range between 0.15 and 0.30 monolayer (ML), a Si(001)2×3–Ga structure composed of Ga ad-dimers rows was formed. In this coverage range, the ratio of the surface area of the TA terrace to that of the TB terrace (terraces where Si dimer rows run parallel and perpendicular to the step, respectively) was changed by step rearrangement, and biatomic steps were formed partially. Such step rearrangement can be explained by the substitution of Si atoms by Ga atoms at kinks. On the TB terrace, Ga ad-dimer rows along the SA step filled the terrace, while those along the SB step (an SA step where Si dimer rows on the upper terrace run parallel and perpendicular to the SB step) terminated partly. At a Ga coverage of about 0.48 ML, a Si(001)2×2–Ga single-domain structure occupied almost the entire surface (accounts for 97% of the whole surface area). On the terraces on the surface, Ga ad-dimer rows run parallel to step edges. Bright irregular protrusions were observed on biatomic step edges, while small hazy protrusions attributed to adatoms were observed on the ledge and edge of the step.

Adsorption and diffusion of Si adatom near single-layer steps on Si surface

By use of the empirical tight-binding (ETB) method, the adsorption and diffusion behaviours of single silicon adatom on the reconstructed Si(100) surface with single-layer steps are simulated. The adsorption energies around the SA step, nonrebonded SB step, rebonded SB step and rough SB step with a kink structure are specially mapped out in this paper, from which the favourable binding sites and several possible diffusion paths are achieved. Because of the rebonded and kink structures, the SB step is more suitable for the attachment of Si adatom than the SA step or defective surface.

Faceting of 〈010〉 steps on Si(001) and Ge(001) surfaces

We have used scanning tunneling microscopy to study the faceting of oriented steps on Si(001) and Ge(001) surfaces. The oriented steps on Si(001) tend to facet into local SA and SB step segments, whereas oriented steps on Ge(001) meander along the mean direction. We show that the step faceting behavior of the Si and Ge(001) surfaces is fully governed by the next-nearest-neighbor (NNN) interaction between the substrate dimers. Only in the case of a repulsive NNN interaction, as for Si(001), faceting occurs.

Monte Carlo simulation of decay process of pyramidal islands formed on Si([formula omitted]) surface

Decay of pyramidal islands formed on Si(100) surface has been analysed by using a Monte Carlo simulation based on a modified solid-on-solid model. An atom on the surface is more stable when it sticks an island to form a dimer and a dimer row, which results in a rectangular island elongated along the dimer row direction. The decay process is primarily determined by atom migration from step edges of the islands to the outer terrace, especially from a biatomic height Db step. The elongation of an island changes during the decay as the migration front changes from Db step to Sa step. In the final stage of the decay of the topmost layer, an island elongated along the dimer row is formed before the complete disappearance of the layer. This is an essential process for the decay of one layer in an island on Si(100).

Scanning tunneling microscope studies of boron-doped Si(001)

We have used scanning tunneling microscopy (STM) and Auger electron spectroscopy to characterize B/Si(001) surface structures and the dependence of their density and location on high-temperature annealing. While annealing of heavily B-doped Si(001) surfaces is known to result in periodic SB step “fingers” whose density depends strongly on temperature, we have found that, on the time scale of formation of the step fingers, the atomic-scale properties of these surfaces are not altered significantly by high-temperature processing. B structures were found to decorate SA step edges. Hot STM was used to study the dynamics of finger formation and the interaction between atomic-scale B/Si(001) structures and fluctuating dimer rows.

SA step growth mechanism on (0 0 1) surfaces of diamond structure crystals

SA step growth mechanism of (0 0 1) surfaces of diamond structure crystals was investigated by means of quantum chemical calculations. Adatoms migrating on dimer rows on the terrace become stationary at the down side of SA steps, because of the high potential energy wall preventing the adatoms running up the SA steps. Thus, migrating adatoms have a higher probability of remaining at the down side of SA steps than at any point of the terrace. The second adatom running down the SA step or migrating on the same dimer row of the lower terrace has a high probability of meeting the first adatom remaining at the down side of the SA step, and a new dimer is formed. It was concluded that the SA step growth is a complex event; i.e., the formation of an isolated dimer at the SA step is followed by SB step growth starting from the newly produced dimer as the smallest nucleus. The SB step growth progressing along the SA step means one step propagation of the SA step growth.

Binding and diffusion of a Si adatom around the type A step on Si(001) c(4×2)

The results of a simulation are described, using a density functional method, of the diffusion of adatoms on a flat Si(001)c(4×2) surface and around one type of surface step (the SA step). These indicate that there is a moderate additional energy barrier (0.2±0.1 eV) to cross the SA step as compared to the energy for diffusion on a flat surface. The dimer-top lattice site on the lower terrace adjacent to the step edge is stabilized (by 0.15±0.1 eV) with respect to the flat surface result although the most stable binding sites near the step are unaffected. This behavior can be understood based on the disruption of dimer tilt near the step. The results suggest that adatoms are more likely to stop on lattice sites at the SA step edge than on lattice sites on the open surface. This may affect the relative dimer formation rate near the step with respect to the behavior on the flat surface even in the absence of a clear change in binding energy. The effect of the SA step terrace edge on adatom behavior is very short ranged and weak. This is consistent with the relatively small strain field and lack of change in dangling bond density associated with the step edge.

Ab initio studies of single-height Si(001) steps

We have studied the Si(001) surface with single-height steps by ab initio molecular dynamics simulations. Surface dimers were found to be unstable with respect to buckling for all geometries considered. However, the ground state reconstruction depends on the type of step. For the SA step, the c(2 × 4) geometry is induced by the step edge, while, for the SB step, the p(2 × 2) reconstruction is more stable. The binding sites and diffusion barriers for a single Si adatom were investigated via the adiabatic trajectory method. In agreement with other studies of the flat surface, fast diffusion takes place along the dimer rows. The local changes to buckling induced by the adatom are sizable and lead to changes in the activation barriers for diffusion, in particular for the path perpendicular to the dimer rows. We also investigated the diffusion of the adatom over the rebonded SB step. The calculations show that there is no additional barrier for the arrival of the adatom at the edge from the upper terrace, while a barrier of at least 1 eV exists for the arrival of the adatom from the lower edge. In step flow growth involving the rebonded SB step, most of the adatoms will thus arrive from the upper terrace.

Simulations of SI(100) Growth: Step Flow and Low Temperature Growth

ABSTRACTWe have investigated Si (100) homoepitaxy in variety of different temperature regimes. In the high temperature step flow regime, the growth properties of the different steps play an important role. The binding sites and diffusion barriers for a Si adatom moving over the buckled Si(100) surface and single-height steps were investigated with the ab initio Car-Parrinello scheme. The SA step edge was found to be a relatively poor sink for adatoms. By contrast, growth takes place much more rapidly at the SB step edges, so that one can understand the relatively fast growth observed at the ends of dimer rows. We have also investigated Si(100) homoepitaxy with classical molecular dynamics simulations at very low temperatures, where typically an amorphous deposit forms. By tuning the energy of the incoming atomic beams, one can lower the temperature of the amorphous to crystalline transition considerably and thereby enhance epitaxial growth.

Step - Step Interactions on the Vicinal GE(001) Surface

Ge(001) surfaces misoriented towards the [011] direction have been investigated with a UHV STM. The vicinal Ge(001) surface consists of alternating straight SA steps and rough SB steps. The anisotropie surface stress tensor leads to interactions between the steps, which can be characterised by assigning a force monopole and a force dipole to each step. The monopole-monopole interaction describes the repulsion, the monopole-dipole interaction the attraction between adjacent steps. From STM images the distribution of the step-step separation was extracted. Using this distribution the value of the force monopole and the force dipole has been calculated. It was found that the short range monopole-dipole interaction attracts the SB step towards the lower lying SA step, which results in the formation of DB steps at a miscut angle of 5°.

Growth Properties of the Si(100) Steps: A Molecular Dynamics Study

ABSTRACTWe have mapped out the energy surfaces seen by a single silicon adatom over the Si(100) surface and Si(100) steps, using Molecular Dynamics methods. This identifies the most likely binding sites as well as the activation energies for diffusion over the terraces and steps. We find that only the 5e step with no rebonded atoms is a good sink for adatoms - the SA, rebonded Sb and Db steps are weak sinks. Because of a higher density of binding sites and lower activation energies for surface diffusion along die step edge, we expect mat growth at the Sb and Db steps take place much more readily man at the SA step.