Single-layer Steps Research Articles

The role of microfaceting in heteroepitaxial interfaces

We investigate the effects of microfaceting on the orientation and interface structure of fcc films on fcc substrates by molecular dynamics simulations and atomic resolution electron microscopy. For (110) substrates, the simulations reveal a misorientation between film and substrate lattices that undergoes a sudden change when interfacial facets approach the “magic size”, where a lattice dislocation minimizes the strain energy by compensating the misfit between film and substrate. For a large misfit, the angle of misorientation varies between zero and several degrees, depending on the size and sequence of microfacets. Experimental observations of interfaces in Ag/Ni near the Ni(110) orientation uncover the presence of magic-size {111} facets and show how microfaceting controls the partitioning of misfit dislocations. For facets smaller than the magic size, misfit may be compensated by partial rather than perfect dislocations. In vicinal {hhl} interfaces, made of {111} terraces separated by single-layer steps, a partial dislocation per terrace leads to film growth in the heterotwin orientation. This concept explains a range of previous results on interface structures in a variety of heteroepitaxial systems.

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.

Study of Si(100) surface step convergence kinetics

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 650oC, 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. Keywords:: Molecular-beam epitaxy, reflection high-energy electron diffraction, surface, terraces, steps, kinks.

Sn-induced [formula omitted] reconstruction on the C-incorporated Si(001)-4[formula omitted] off surface

The reconstruction induced by Sn atoms on the C-incorporated Si(001)-4∘ off surface has been studied using scanning tunneling microscopy and synchrotron photoemission spectroscopy. When 0.6-ML Sn atoms were deposited on the Si(001)-4∘ off surface with subsurface C atoms at 630 °C, the whole (001) terraces were covered with a single c(4×4) reconstruction, where each Sn addimer bridges two Si dimers, reducing the density of dangling bonds to 0.5 ML. Such a reconstruction is different from the Sn-induced c(8×4) reconstruction formed on the clean Si(001) terraces without subsurface C atoms, where Sn dimers replace Si dimers and every fourth dimer is vacant. In both cases, the adjacent dimers buckle out of phase and the DB steps are debunched into single-layer steps. The reconstruction change from c(8×4) to c(4×4) is explained by the surface stress exerted by the subsurface C atoms much smaller than Si atoms.

Theory of silicon spin qubit relaxation in a synthetic spin-orbit field

We develop the theory of single-electron silicon spin qubit relaxation in the presence of a magnetic field gradient. Such field gradients are routinely generated by on-chip micromagnets to allow for electrically controlled quantum gates on spin qubits. We build on a valley-dependent envelope function theory that enables the analysis of the electron wave function in a silicon quantum dot with an arbitrary roughness at the interface. We assume the presence of single-layer atomic steps at a Si/SiGe interface and study how the presence of a gradient field modifies the spin-mixing mechanisms. We show that our theoretical modeling can quantitatively reproduce results of experimental measurements of qubit relaxation in silicon in the presence of a micromagnet. We further study in detail how a field gradient can modify the EDSR Rabi frequency of a silicon spin qubit. While this strongly depends on the details of the interface roughness, interestingly, we find that adding a micromagnet on top of a spin qubit with an ideal interface can even reduce the EDSR frequency within some interval of the external magnetic field strength.

Asymmetric [formula omitted] twin boundary and migration mechanism in hexagonal close-packed titanium

Classical twinning theory predicted that a simple shear was able to create a perfect, twinned structure without the need of atomic shuffles for (112¯1)[112¯6¯] twinning mode in hexagonal close-packed (HCP) metals, and the elementary twinning dislocation should be a two-layer zonal. However, it was revealed in the literature that this particular twinning mode could not have mirror symmetry between the parent and twin, and shuffles should be involved during twin boundary (TB) migration. These conflicting reports indicate that the (112¯1)[112¯6¯] twinning mechanism has not been completely resolved, and what configuration of twinning dislocation mediates twin growth is not well understood either. In this work, atomistic simulation is conducted to further understand the twinning mechanism in titanium. Novel structural analyses are performed to reveal how the parent lattice is transformed to the twin lattice. The results show that the mirror symmetry of a perfect, unrelaxed TB breaks down after relaxation, because of the very high repulsive force between the atom pairs on mirrored positions with a spacing less than half of the lattice constant. As a result, the stacking sequence of the twin differs from that of the parent after relaxation. During twin growth, each {11¯00} prismatic plane of the parent splits into two layers and reorganize into the prismatic planes of the twin, such that the twin prismatic are no longer aligned with the parent prismatic and the mirror symmetry breaks down. This process is accomplished by atoms shuffling in the opposite direction that is perpendicular to the twinning shear. The atomic shuffles spread over multiple consecutive twinning planes to minimize the shuffling magnitude on each twinning plane. Thus, the actual TB is no longer a single twinning plane, but a multi-layer interface zone. Careful examination shows that no well-defined twinning dislocation core can be identified, although single-layer steps can still be observed.

Rapid thickness mapping of free-standing smectic films using colour information of reflected light

ABSTRACT A practical method to rapidly obtain the 2D thickness distribution in free-standing smectic films is developed based on the colour information of reflected image of the film. The red(R), green(G) and blue(B) colour coordinates from each pixel of a colour camera is normalised by the local intensity and is then quantitatively analysed in reference to the theoretical reflection spectra evaluated as a function of the film thickness. Results from the optical system employing a standard stereomicroscope can clearly discriminate a single-layer step over a wide range of thicknesses and provide thickness values with a precision better than 1 nm.

Ab initio study for adsorption and desorption behavior at step edges of AlN(0001) and GaN(0001) surfaces

The adsorption and desorption behavior of adatoms at step edges of AlN(0001) and GaN(0001) surfaces is investigated on the basis of ab initio calculations. Our calculations for single layer step edges along the [11̅00] direction reveal that the structure of the step edge depends on the growth condition. Furthermore, the adsorption behavior of adatoms close to the step edges is found to be dependent on the structure of step edges. The calculated results of adatom behavior suggests the possibility to control the morphology of GaN(0001) and AlN(0001) surfaces by growth parameters such as V/III ratio and temperature.

Ab initio study for adsorption and desorption behavior at step edges of GaN(0001) surface

The adsorption and desorption behavior of adatoms at step edges of GaN(0001) surface is investigated on the basis of ab initio calculations. Our calculations of single layer step edges along the [11¯00] direction reveal that the structure of step edge depends on the growth condition. Furthermore, the adsorption behavior of Ga and N adatoms close to the step edges is found to be dependent on these structures. Under moderately Ga-rich and N-rich conditions, Ga adatoms are preferentially incorporated at the step edge with low adsorption energy (−3.7 eV) and the Ehrlich-Schwoebel barrier (ESB) is recognized. On the other hand, the ESB for Ga adatoms is negligibly small under Ga-rich condition. These results suggest that island formation preferentially occurs away from single layer steps under Ga-rich condition, reasonably consistent with experimental results.

Friction at single-layer graphene step edges due to chemical and topographic interactions

Although graphene is well known for super-lubricity of its basal plane, friction at its step edge is not well understood. In this study, friction of a single-layer graphene step edge was studied using atomic force microscopy (AFM) in vacuum and humid air conditions. At a 0.34 nm thick graphene step edge, friction varies drastically depending on whether it is exposed at the topmost surface or covered under other graphene layers. The friction response of the step edge buried under one layer of graphene can be fully explained with the topographic effect only; in contrast, the exposed step edge exhibits both topographic and chemical contributions to friction. Chemical characterizations suggest that the exposed graphene step edge is terminated with C–OH groups, which can interact with the AFM tip surface through hydrogen bonding interactions and thus increase friction. The chemical interactions at the exposed step edge significantly amplify the topographic effect. When the step edge is covered by more than one layer of graphene, friction is not sensitive to the 0.34 nm height change. This must be due to the stiffness of multilayer graphene and the height changes gradually at the step edge. These findings will advance fundamental knowledge of the frictional behaviors of graphene.

Formation and morphological evolution of self-similar 3D nanostructures on weakly interacting substrates

Vapor condensation on weakly interacting substrates leads to the formation of three-dimensional (3D) nanoscale islands (i.e., nanostructures). While it is widely accepted that this process is driven by minimization of the total film/substrate surface and interface energy, current film-growth theory cannot fully explain the atomic-scale mechanisms and pathways by which 3D island formation and morphological evolution occurs. Here, we use kinetic Monte Carlo simulations to describe the dynamic evolution of single-island shapes during deposition of Ag on weakly interacting substrates. The results show that 3D island shapes evolve in a self-similar manner, exhibiting a constant height-to-radius aspect ratio, which is a function of the growth temperature. Furthermore, our results reveal the following chain of atomic-scale events that lead to compact 3D island shapes: 3D nuclei are first formed due to facile adatom ascent at single-layer island steps, followed by the development of sidewall facets bounding the islands, which in turn facilitates upward diffusion from the base to the top of the islands. The limiting atomic process which determines the island height, for a given number of deposited atoms, is the temperature-dependent rate at which adatoms cross from sidewall facets to the island top. The overall findings of this study provide insights into the directed growth of metal nanostructures with controlled shapes on weakly interacting substrates, including two-dimensional crystals, for use in catalytic and nanoelectronic applications.

Surface reconstruction switching induced by tensile stress of [formula omitted] steps: From Ba/Si(0 0 1)-[formula omitted] to Ba/Si(0 0 1)-4° off-[formula omitted

The alkaline-earth metal adsorption on Si(001) has attracted much interest for finding a proper template in the growth of high-κ and crystalline films. Up to now on the flat Si(001) surface with double domains and single-layer steps, the adsorbed Ba atoms are known to induce the 2×3 structure through removing two Si dimers and adding a Ba atom per unit cell in each domain. In the present investigation, the Si(001)-4° off surface with DB steps and single domains has been employed as a substrate and the reconstruction at the initial stage of Ba adsorption has been investigated by scanning tunneling microscopy and synchrotron photoemission spectroscopy. On this vicinal and single domain terrace, a novel 3×2 structure rotated by 90° from the 2×3 structure has been found. Such a 3×2 structure turns out to be formed by adding a Ba atom and a Si dimer per unit cell. This results from the fact that the adsorbed Ba2+ ions with a larger ionic radius relieve tensile stress on the original Si dimers exerted by the rebonded atoms at the DB step.

Spatially resolved scanning tunneling spectroscopy of single-layer steps on Si(100) surfaces

Single-layer steps at Si(100) surfaces/interfaces present significant challenges to the quantitative characterization of buried dopant devices as well as the accurate imaging and relocation of fabricated quantum structures. We demonstrate the detailed spatially resolved scanning tunneling spectroscopy study across monolayer step edges on Si(100) surfaces and quantitative determination of the local density of state distributions and behavior of the band gap at step edges. The influence on the local electrostatic environment due to step edge states has been quantified while accounting for the effects of scanning tunneling measurement conditions. The dangling bond states on Si(100) surfaces are utilized as a fingerprint to quantify the local band bending landscape and to make corrections to the experimentally observed surface state energy levels and band gap values at the step edge regions. We observe a significant band gap narrowing behavior along a rebonded single-layer type B step edge on a degenerately boron-doped $p$-type Si substrate.

Effect of Preoperative Occlusal Matrices on the Vickers Microhardness of Composite Disks Polymerized with QTH and LED Lamps.

This study aimed to assess the reliability of the preoperative occlusal matrix technique in terms of the surface Vickers microhardness (VMH) of the underlying composite restorative material. Two hundred microhybrid composite cylinders were built up and light-cured in a single-layer step, forming two experimental groups (N = 100) according to their heights (1.5 mm/2 mm). Each group was divided into five subgroups (N = 20) depending on the matrix thickness (no matrix/0.5 mm/1 mm/2 mm/3 mm). Half the specimens per subgroup (N = 10) were randomly polymerized with a quartz-tungsten-halogen (QTH) light-curing unit (LCU). The remaining half were cured using a light-emitting diode lamp. The top and bottom samples' sides were tested for VMH at 1 hour and 24 hours post-curing using a universal VMH machine. A multiple analysis of variance with repeated measurements for the "surface" factor and the Student-Newman-Keuls test were run (α = 0.05). Bottom/top microhardness ratios were compared with the empirically accepted limit (0.8). Surface topography was analyzed under a scanning electron microscope. The thinnest matrices provided the significantly best VMH values. LCU, disc height, and time also contributed to VMH. At 24 hours, 2-mm high discs polymerized with QTH resulted in inadequate microhardness ratios when 1-mm thick to 3-mm thick matrices were used. The thinnest matrices are the most recommendable ones. The esthetics and occlusal reproducibility achieved with customized occlusal matrices fabricated before cavity preparation have been widely demonstrated. However, their effect on the physical properties of the restorations deserves further investigation. Although more studies are necessary, the thinnest matrices seem to be the most suitable to preserve the composite surface VMH and the curing depth.

Atomic surface structure of Si(1 0 0) substrates prepared in a chemical vapor environment

Subsequent III–V integration by metal-organic vapor phase epitaxy (MOVPE) or chemical vapor deposition (CVD) necessitates elaborate preparation of Si(100) substrates in chemical vapor environments characterized by the presence of hydrogen used as process gas and of various precursor molecules. The atomic structure of Si(100) surfaces prepared in a MOVPE reactor was investigated by low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) available through a dedicated, contamination-free sample transfer to ultra high vacuum (UHV). Since the substrate misorientation has a fundamental impact on the atomic surface structure, we selected a representative set consisting of Si(100) with 0.1°, 2° and 6° off-cut in [011] direction for our study. Similar to standard UHV preparation, the LEED and STM results of the CVD-prepared Si(100) surfaces indicated two-domain (2×1)/(1×2) reconstructions for lower misorientations implying a predominance of single-layer steps undesirable for subsequent III–V layers. However, double-layer steps developed on 6° misoriented Si(100) substrates, but STM also showed odd-numbered step heights and LEED confirmed the presence of minority surface reconstruction domains. Strongly depending on misorientation, the STM images revealed complex step structures correlated to the relative dimer orientation on the terraces.

A Model Calculation of Step-Flow Growth on Single-Layer Stepped Diamond (001) Surface~!2009-05-30~!2010-02-02~!2010-04-22~!

Step-flow growths of diamond on single-layer steps of hydrogenated diamond (001) surface have been investigated using the semiempirical molecular orbital method PM5. The chemical reactions at the first stage of growth have been calculated as a function of the charges biased to the substrates. When the frontier orbits of a pair of surface hydrogen atoms of step edge by negative charge � 2 interact effectively with frontier orbits of hydrogen gases, the pair of surface hydrogen atoms is abstracted by two hydrogen gases. A dimer C2 is bonded onto the pair of vacant hydrogen sites by the chemisorption. The step-flow growth of the dimer rows seem to be proceeded by C2 under the influence of a pulsed charge (-2 and 0) biased to the single-layer stepped (001) surface.

Nanostructure formation in the initial roughening of a thin silicon sheet

Silicon-on-insulator (SOI) presents a unique model system for exploring the stability of crystalline nanomaterials in metastable configurations. We show that the initial destabilization of ultrathin SOI is related to mechanical stress, in contrast to phenomena at later times driven by the energy of the ${\text{SiO}}_{2}/\text{Si}$ interface. Stepped rectangular truncated pyramids, with lateral dimensions of tens of nanometers, are formed on the outer Si layer of ultrathin (001)-oriented SOI during heating in ultrahigh vacuum. Pyramid edges are bounded by doubled atomic steps, with corners consisting of a complex series of single-layer steps. The shape of these nanopyramids represents a balance between stress-induced roughening and the elastic interaction between steps. SOI allows the precisely known energetics of silicon surfaces to be readily adapted to materials with nanoscale dimensions.

Adsorption of Single Oxygen Atoms on Vicinal Si(001) Surfaces: Lattice Relaxation and Si Oxidation ¡¤¡¤¡¤¡¤¡¤¡¤ ¡¤¡¤¡¤¡¤¡¤¡¤

By employing ab initio electronic structure calculations, we investigated the adsorption of single oxygen atoms on vicinal Si(001) surfaces with single-layer SA and SB steps. The calculations in this study showed that the oxygen adatom preferetially adsorbs not only on the rebond sites of the rebonded step-edge Si atoms (RSAs) at the SB step but also on the backbond site of the down Si atom of the step-edge Si dimer on the upper terrace of the SB step. For more detailed information, the electronic properties of the adsorption structures of the oxygen adatoms were also examined. All these calculations showed that the lattice relaxation of the RSAs near the rebonded SB step played a signi cant role in the adsorption of the single oxygen atoms on the vicianl Si(001) surfaces with the single-layer steps.

Vertical GaP nanowires arranged at atomic steps on Si(111) substrates

We report vertical GaP nanowires self-arranged in lines formed from Au islands that were initially arranged at single-layer steps on Si(111) substrates. These Au islands are spontaneously arranged by controlling the deposition. There are three aspects to the growth of vertical GaP wires: the cosupply of TMGa and PH3, two growths at different temperatures, and a low PH3 flow rate. The grown wires are quite ordered in lines of several micrometers. Electrical measurement confirmed selective current flow at the wires and we confirmed photoluminescence from the wires. This bottom-up technique is promising for nano-hetero-device integration on Si circuits.

First-Principles Study of the Step Oxidation at Vicinal Si(001) Surfaces

By carrying out ab initio total-energy and electronic-structure calculations, possible initial oxidation structures at vicinal Si(001) surfaces with alternate single-layer SA and SB steps were studied. A new oxidation structure that is more stable than the previously proposed backbond-oxidation models on flat terraces of Si(001) was found. The new oxidation structure is located at the rebonded SB step and consists of a –Si–O– chain structure aligned along the step edge. This chain structure was found to be able to effectively reduce the number of dangling bonds (DBs) at the step edge. This indicates that the reduction of the number of the step-edge DBs plays a crucial role in the formation of the oxidation complex at steps with the local strain. For more detailed information, the electronic properties of the oxidation structure that was found therein were also determined. The calculated site-projected density of states and the scanning tunneling microscopy images of the oxidation structure were found to be clearly distinct from those of the clean vicinal Si surface.