Stepped Si Research Articles

Stability of monodomain III-V crystals and antiphase boundaries over a Si monoatomic step

Here, we compare the stabilities of different III-V crystals configurations on stepped Si substrates, with or without anti-phase boundaries, for abrupt and compensated interfaces, using density functional theory. Thermodynamic stability of the different heterostructures is analyzed with an atomic scale description of charge densities distribution and mechanical strain. We show that the configuration where a III-V crystal adapts to a Si monoatomic step through change of charge compensation at the hetero-interface is much more stable than the configuration in which an antiphase boundary is formed. This study thus demonstrates that antiphase boundaries commonly observed in III-V/Si samples are not originating from Si monoatomic step edges but from inevitable kinetically driven coalescence of monophase 3D III-V islands.

Implementation of the Su-Schrieffer-Heeger Model in the Self-Assembly Si-In Atomic Chains on the Si(553)-Au Surface.

Indium-decorated Si atomic chains on a stepped Si(553)-Au substrate are proposed as an extended Su-Schrieffer-Heeger (SSH) model, revealing topological end states. An appropriate amount of In atoms on the Si(553)-Au surface induce the self-assembly formation of trimer SSH chains, where the chain unit cell comprises one In atom and two Si atoms, confirmed by scanning tunneling microscopy images and density functional calculations. The electronic structure of the system, examined through scanning tunneling spectroscopy, manifests three electron bands within the Si-In chain, accompanied by additional midgap topological states exclusively appearing at the chain's end atoms. To elucidate the emergence of these topological states, a tight-binding model for a finite-length-extended SSH chain is proposed. Analysis of the energy spectra, density of states functions, and eigenfunctions demonstrates the topological nature of these self-assembled atomic chains.

Cu thin film growth on stepped Si substrate: Effects of incident energy and thermal annealing

The existence of defects on the substrate surface is unavoidable and has a significant impact on the determination of surface properties. Among the prevalent types of defects, steps play an important role. In this study, molecular dynamics simulations are employed to explore the growth process of a thin film of Cu on a Si (001) substrate with a step. This research investigates how incident energy and thermal annealing affect the growth mode, surface roughness, film-substrate interface, and dislocation evolution. The results indicate that during the initial stage of the growth, the nucleation phenomenon was observed on terraces. The simulation indicates that the Cu atoms occur in an island growth mode on the stepped substrate. It is found that when the incident energy is increased and thermal annealing is performed, the surface curvature and roughness are reduced. Furthermore, the intermixing thickness at the interface increases due to the variation of the incidence energy. A dislocation analysis revealed that some dislocations disappeared before and after annealing, and a perfect dislocation appeared by increasing the incident energy.

Effect of stepped Si (001) substrate on Cu thin film growth

The growth processes of Cu thin film on stepped Si(001) substrate were investigated using molecular dynamics simulation. The modified embedded atom method was used to describe the atomic interaction between Cu-Cu, Si-Si, and Si-Cu. In this study, four different Si(001) substrate configurations were examined: (i) flat Si(001) substrate; (ii) stepped Si surface with 3-monoatomic layers step; (iii) Stepped Si surface with 5-monoatomic layers step; (iiii) stepped surface with 7-monoatomic layers. Our aim here is to investigate the effect of stepped substrate on the structure, the surface roughness, and the morphology of deposited Cu thin film. The results show that the Cu film obtained has a crystalline structure based on the radial distribution function. In addition, the morphology of Cu film is not smooth for the different stepped substrates. More precisely, the surface roughness increases when the substrate presents a step and rises with the augmentation of the step height. On the other hand, our results reveal that the penetration of Cu atoms in the simplest case of the flat configuration is limited to the top layer of the substrate. While for the stepped substrate, our findings show that the penetration in the stepped substrate is more important and deeper within the upper terrace compared to the lower terrace. Furthermore, the numerical calculations demonstrate that the step height has no significant effect on the penetration of Cu atoms on the Si(001) stepped substrate. These results are appropriate for the deposition of copper atoms into the stepped substrate of silicon.

Smallest Stable Si/SiO_{2} Interface that Suppresses Quantum Tunneling from Machine-Learning-Based Global Search.

While the downscaling of size for field effect transistors is highly desirable for computation efficiency, quantum tunneling at the Si/SiO_{2} interface becomes the leading concern when approaching the nanometer scale. By developing a machine-learning-based global search method, we now reveal all the likely Si/SiO_{2} interface structures from thousands of candidates. Two high Miller index Si(210) and (211) interfaces, being only ∼1 nm in periodicity, are found to possess good carrier mobility, low carrier trapping, and low interfacial energy. The results provide the basis for fabricating stepped Si surfaces for next-generation transistors.

Crystal Structure and Magnetic Properties of Epitaxial Cobalt Thin Films and Single-Crystal Nanostrips Grown on a Stepped Surface Si(111)-5.55×5.55-Cu with a Cu(111) Buffer Layer

Crystal Structure and Magnetic Properties of Epitaxial Cobalt Thin Films and Single-Crystal Nanostrips Grown on a Stepped Surface Si(111)-5.55×5.55-Cu with a Cu(111) Buffer Layer

Au adsorption on stepped Si(hhk)-Au surfaces

The impact of submonolayer gold adsorption on metallic atomic wires grown on terraced silicon substrates is modeled from first principles. Calculations within density functional theory predict Au adsorption at the Si step edge of the Si(553)-Au surface and moderate mobility of the Au adatoms. Although the Au adsorption at the Si step edge does not substantially modify its geometry, a large adsorbate-to-substrate electronic transfer is predicted, which causes a rearrangement of the electronic band structure. The electronic states related to the Si step edge are shifted below the Fermi energy, while an electronic gap opens between the Au-related electronic states. The gap opening leads to a strong enhancement of the Au-chain dimerization. The Au adsorption at the step edge thus modifies the surface morphology non-locally, as a consequence of the strong electronic correlation in quasi-1D systems. Au adsorption on the Si(775)-Au nanowire system occurs mainly at the Au chain, and then at the Si step edge, instead. Adsorption at the Au chain directly modifies its structure by the formation of a structural defect, while adsorption at the step edge has the same effect as in the case of the Si(553)-Au system.

Evidence for Electronically Isolated Atomic Chains: Sb–Pb Structures on the Si(553) Surface

Metallic atomic chains weakly coupled to the substrate may be a practical implementation of extreme one-dimensional quantum systems. We report a two-step method of assembling one-dimensional Sb atomic chains on an ordered stepped Si(553) surface with Pb nanoribbons. We show that Pb atoms act as a surfactant, induce the dissociation of Sb molecules, and enable the formation of monatomic chains. Combining atomistic modeling within density functional theory with scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we demonstrate that the formed Sb chains are highly strained and at the same time electronically isolated, exhibiting extremely well-defined one-dimensional free-electron like behavior.

Preparation of a Si(111) Atomically Flat Substrate via Wet Etching and Evaluation as an AFM Substrate for Observations of Isolated Chains, Crystals, and Crystallization of Isotactic Poly(methyl methacrylate) at the Molecular Level.

To observe a polymer chain deposited on a substrate by atomic force microscopy (AFM) at the molecular level, the substrate should be atomically flat and stable under laboratory conditions and adsorb polymer chains firmly. Therefore, substrates used under laboratory conditions are practically limited to mica, highly ordered pyrolytic graphite, and atomically stepped sapphire, and polymers observed by AFM at the molecular level are also limited. A silicon wafer is frequently used as a substrate for AFM observation for somewhat macroscopic observations, but the surface of the silicon wafer is too rough to observe polymer chains deposited on it at the molecular level. In this study, we prepared an atomically stepped Si(111) substrate via wet etching in NH4F and evaluated it as an AFM substrate. The Si(111) substrate was stable as an AFM substrate, and isolated poly(methyl methacrylate) (it-PMMA) chains and a crystalline monolayer deposited on the substrate were observed by AFM at the molecular level. An it-PMMA amorphous monolayer deposited on mica crystallized under high humidity, but that on the Si(111) substrate did not because of the difference in the surface nature and the crystal structure of the substrates. The Si(111) substrate was hydrophobic, and the it-PMMA monolayers could be deposited as a multilayer, which could not be formed on hydrophilic mica. The crystallization behavior of an it-PMMA amorphous multilayer and an amorphous/crystalline mixed multilayer on the Si(111) substrate was also evaluated.

Thermally Induced Crossover from 2D to 1D Behavior in an Array of Atomic Wires: Silicon Dangling-Bond Solitons in Si(553)-Au.

The self-assembly of submonolayer amounts of Au on the densely stepped Si(553) surface creates an array of closely spaced "atomic wires" separated by 1.5nm. At low temperature, charge transfer between the terraces and the row of silicon dangling bonds at the step edges leads to a charge-ordered state within the row of dangling bonds with ×3 periodicity. Interactions between the dangling bonds lead to their ordering into a fully two-dimensional (2D) array with centered registry between adjacent steps. We show that as the temperature is raised, soliton defects are created within each step edge. The concentration of solitons rises with increasing temperature and eventually destroys the 2D order by decoupling the step edges, reducing the effective dimensionality of the system to 1D. This crossover from higher to lower dimensionality is unexpected and, indeed, opposite to the behavior in other systems.

Organic Molecule Adsorption on Stepped Si–Au Surfaces: Role of Functional Group on Geometry and Electronic Structure

Stepped gold‐stabilized Si(111) surfaces offer much potential for self‐organized assembly of one‐ and two‐dimensional nanostructured arrays of organic molecules due to the presence of regular atomic chains of Au and Si on the surface, parallel to the step edges. In this theoretical study, the adsorption of various simple organic molecules with different functional groups on the Si(553)–Au surface has been investigated. It has been found that adsorption at the step edge site is favored by all species. If the step edge is first passivated by hydrogen, adsorption occurs on Si atoms on the terraces. Instead, adsorption on the gold chains is completely unfavored. The influence of organic molecule adsorption on the electronic band structure and surface metallicity is investigated.

Plasmon Standing Waves by Oxidation of Si(553)–Au

Self-assembled Au atomic wires on stepped Si surfaces are metallic, as evidenced by a one-dimensionally dispersing plasmonic excitation. Here we investigate the effects of oxidization on metallicity along such Au atomic wires on a regularly stepped Si(553) surface, by employing infrared absorption and high resolution electron energy loss spectroscopies. Our results indicate that only the Si environment undergoes oxidation, which has a remarkably small effect on the plasmon dispersion. Only close to $k_{\parallel}\rightarrow 0$ the plasmon dispersion ends at increasingly higher energies as a function of oxygen exposure, which is attributed to standing wave formation on small sections of Au wires generated by the introduction of O atoms as scattering centers, not to electronic gap opening. This interpretation is in full agreement with the findings by infrared spectroscopy and with low energy electron diffraction.

Double-layer stepped Si(1 0 0) surfaces prepared in As-rich CVD ambience

Low-defect III-V integration on Si(1 0 0) requires atomically well-ordered heterointerfaces. To this end, we study the interaction of As with Si(1 0 0) surfaces and the formation of dimerized Si(1 0 0):As surfaces in situ with reflection anisotropy spectroscopy during metalorganic chemical vapor deposition (MOCVD). Control of surface processes allows us to intentionally ‘rotate’ the dimer orientation and choose between predominantly (1 × 2) and (2 × 1) reconstructed Si(1 0 0):As surfaces both for low and higher surface misorientation. We also verify that the (2 × 1) surface reconstruction becomes energetically more favorable for vicinal Si(1 0 0):As surfaces. We explain the preparation of Si(1 0 0):As surfaces displaying an almost single-domain, (1 × 2) reconstructed surface with an RMS roughness ≤1 Å and exhibiting broad, atomically flat terraces as well as regular double-layer steps. This surface is highly suitable for subsequent low-defect III-V integration.

Controlling the Local Electronic Properties of Si(553)-Au through Hydrogen Doping.

We propose a quantitative and reversible method for tuning the charge localization of Au-stabilized stepped Si surfaces by site-specific hydrogenation. This is demonstrated for Si(553)-Au as a model system by combining density functional theory simulations and reflectance anisotropy spectroscopy experiments. We find that controlled H passivation is a two-step process: step-edge adsorption drives excess charge into the conducting metal chain "reservoir" and renders it insulating, while surplus H recovers metallic behavior. Our approach illustrates a route towards microscopic manipulation of the local surface charge distribution and establishes a reversible switch of site-specific chemical reactivity and magnetic properties on vicinal surfaces.

Formation of a Stepped Si(100) Surface and Its Effect on the Growth of Ge Islands

The transition from a two-domain to one-domain surface on a Si(100) substrate is investigated. It is demonstrated using reflection high-energy electron diffraction that at a temperature of 600°C and a deposition rate of 0.652 A/s onto a Si(100) substrate pre-heated to 1000°C and inclined at an angle of 0.35°C to the plane, a series of reflections from the 1 × 2 superstructure completely vanishes at a constant flow of Si. This is attributed to the transition of the surface from monoatomic to diatomic steps. At growth rates lower than 0.652 A/s, the transition from a two-domain to one-domain surface is also observed; with a decrease in the growth rate, the intensity ratio I2 × 1/I1 × 2 decreases and the maximum of the dependences shifts toward lower temperatures. The complete vanishing of the series of superstructural reflections after preliminary annealing at a temperature of 700°C is not observed; this series only vanishes after annealing at 900 and 1000°C. The growth of Ge islands on a Si(100) surface preliminary annealed at a temperature of 800°C is studied. It is shown that the islands tend to nucleate at the step edges. A mechanism of Ge island ordering on the Si(100) surface is proposed.

Effect of a Stepped Si(100) Surface on the Nucleation Process of Ge Islands

Nucleation of Ge islands on a stepped Si(100) surface is studied. It is shown by diffraction of fast electrons that at a temperature of 600°C, constant flux of Si, and deposition rate of 0.652 A/s, a series of the 1×2 superstructure reflections completely disappears, if the Si (100) substrate deviated by an angle of 0.35° to the (111) face is preliminarily heated to 1000°C. The disappearance of the 1×2 superstructure reflexes is due to the transition from the surface with monoatomic steps to that with diatomic ones. Investigations of the Ge islands’ growth were carried out on the Si(100) surface preliminarily annealed at temperatures of 800 and 1000°C. It is shown that the islands tend to nucleate at the step edges.

Modulation of van der Waals and classical epitaxy induced by strain at the Si step edges in GeSbTe alloys

The present work displays a route to design strain gradients at the interface between substrate and van der Waals bonded materials. The latter are expected to grow decoupled from the substrates and fully relaxed and thus, by definition, incompatible with conventional strain engineering. By the usage of passivated vicinal surfaces we are able to insert strain at step edges of layered chalcogenides, as demonstrated by the tilt of the epilayer in the growth direction with respect of the substrate orientation. The interplay between classical and van der Waals epitaxy can be modulated with an accurate choice of the substrate miscut. High quality crystalline GexSb2Te3+x with almost Ge1Sb2Te4 composition and improved degree of ordering of the vacancy layers is thus obtained by epitaxial growth of layers on 3–4° stepped Si substrates. These results highlight that it is possible to build and control strain in van der Waals systems, therefore opening up new prospects for the functionalization of epilayers by directly employing vicinal substrates.

Sum frequency generation spectroscopy study of hydrogenated stepped Si(111) surfaces made by molecular hydrogen exposure

Hydrogen adsorption on stepped Si(111) surfaces 9.5° miscut in the [1̅1̅2] direction has been investigated in situ in a UHV chamber with a base pressure of ~10−8Pa. The H-Si(111)1×1 surface was prepared by exposing the wafer to ultra-pure hydrogen gas at a pressure of ~470Pa. Termination of hydrogen on terraces and steps was observed by sum frequency generation (SFG) with several polarization combinations such as ppp, ssp, pps, spp, psp, sps, pss and sss. Here the 1st, 2nd and 3rd symbols indicate SFG, visible and IR polarizations, respectively. ppp and ssp-SFG clearly showed only two modes: the Si-H stretching vibration terrace mode at 2082cm−1 (A) and the vertical step dihydride vibration mode at 2094cm−1 (C1). Interesting points are the appearance of the C1 mode in contrast to the previous SFG spectrum of the H-Si(111)1×1 surface with the same miscut surface angle prepared by wet chemical etching. We suggest that the formation of step dihydride and its orientation on the Si(111) stepped surfaces depend strongly on the preparation method.

One-Dimensional Plasmonic Excitations in Gold-Induced Superstructures on Si(553): Impact of Gold Coverage and Silicon Step Edge Polarization

Free charge carriers confined to atomic chains such as the gold-induced superstructures on the stepped Si(553) surface enable experimental insight into one-dimensional physics. Embedding into the higher dimensional substrate allows for additional couplings between the free charge carriers and their surroundings, which might modify the one-dimensional characteristics. The gold atom superstructures on Si(553) consist of a parallel arrangement of metallic chains from Au and Si atoms on the terraces and of parallel Si step edges with some of the Si atoms having dangling bonds with one unpaired electron. The metallic chains give rise to localized plasmonic excitations. We have studied these plasmonic resonances with infrared spectroscopy that enables the detection of resonance shifts as small as 1 meV or even less. The plasmonic behavior of the conductive chains of the high- and the low-coverage gold superstructures on Si(553) is investigated at various temperatures and additionally after filling electrons int...

The ordering of Ge islands on a stepped Si(100) surface

In the paper we investigate the Ge island nucleation on a Si(100) surface which was annealed to obtain the diatomic steps. It was observed that the islands tend to nucleate at the step edges.