Vicinal Silicon Surfaces Research Articles

The Nature of Ferromagnetism in a System of Self-Ordered α-FeSi2 Nanorods on a Si(111)-4° Vicinal Surface: Experiment and Theory.

In this study, the appearance of magnetic moments and ferromagnetism in nanostructures of non-magnetic materials based on silicon and transition metals (such as iron) was considered experimentally and theoretically. An analysis of the related literature shows that for a monolayer iron coating on a vicinal silicon surface with (111) orientation after solid-phase annealing at 450–550 °C, self-ordered two-dimensional islands of α-FeSi2 displaying superparamagnetic properties are formed. We studied the transition to ferromagnetic properties in a system of α-FeSi2 nanorods (NRs) in the temperature range of 2–300 K with an increase in the iron coverage to 5.22 monolayers. The structure of the NRs was verified along with distortions in their lattice parameters due to heteroepitaxial growth. The formation of single-domain grains in α-FeSi2 NRs with a cross-section of 6.6 × 30 nm2 was confirmed by low-temperature and field studies and FORC (first-order magnetization reversal curves) diagrams. A mechanism for maintaining ferromagnetic properties is proposed. Ab initio calculations in freestanding α-FeSi2 nanowires revealed the formation of magnetic moments for some surface Fe atoms only at specific facets. The difference in the averaged magnetic moments between theory and experiments can confirm the presence of possible contributions from defects on the surface of the NRs and in the bulk of the α-FeSi2 NR crystal lattice. The formed α-FeSi2 NRs with ferromagnetic properties up to 300 K are crucial for spintronic device development within planar silicon technology.

Z3 Charge Density Wave of Silicon Atomic Chains on a Vicinal Silicon Surface.

An ideal one-dimensional electronic system is formed along atomic chains on Au-decorated vicinal silicon surfaces, but the nature of its low-temperature phases has been puzzling for last two decades. Here, we unambiguously identify the low-temperature structural distortion of this surface using high-resolution atomic force microscopy and scanning tunneling microscopy. The most important structural ingredient of this surface, the step-edge Si chains, are found to be strongly buckled, every third atom down, forming trimer unit cells. This observation is consistent with the recent model of rehybridized dangling bonds and rules out the antiferromagnetic spin ordering proposed earlier. The spectroscopy and electronic structure calculation indicate a charge density wave insulator with a Z3 topology, making it possible to exploit topological phases and excitations. The tunneling current was found to substantially lower the energy barrier between three degenerate CDW states, which induces a dynamically fluctuating CDW at very low temperature.

Creation and annihilation of mobile fractional solitons in atomic chains

Localized modes in one-dimensional (1D) topological systems, such as Majonara modes in topological superconductors, are promising candidates for robust information processing. While theory predicts mobile integer and fractional topological solitons in 1D topological insulators, experiments so far have unveiled immobile, integer solitons only. Here we observe fractionalized phase defects moving along trimer silicon atomic chains formed along step edges of a vicinal silicon surface. By means of tunnelling microscopy, we identify local defects with phase shifts of 2π/3 and 4π/3 with their electronic states within the band gap and with their motions activated above 100 K. Theoretical calculations reveal the topological soliton origin of the phase defects with fractional charges of ±2e/3 and ±4e/3. Additionally, we create and annihilate individual solitons at desired locations by current pulses from the probe tip. Mobile and manipulable topological solitons may serve as robust, topologically protected information carriers in future information technology.

Coexistence of two gold-induced one-dimensional structures on a single terrace of the Si(11 11 13)

Regular arrays of silicon terraces with coexisting two gold-induced one-dimensional structures have been synthesized on the Si(11 11 13) surface. For the first time, it was possible to confine the well-known 5 × 2-Au reconstruction to narrow stripes separated by atomic steps and preserving its basic properties. Moreover, each terrace consists of additional chain of gold atoms that shows the same crystallographic and electronic properties to the Si(557)-Au surface. The results obtained by scanning tunneling microscopy and angle-resolved photoemission spectroscopy supplemented by first-principles density functional theory calculations allow to identify fingerprints of 5 × 2-Au and Si(557)-Au structures occupying the same terrace. This contrasts with the whole family of gold chains on vicinal silicon surfaces and presents new paradigm in engineering of complex one-dimensional nanostructures on stepped surfaces.

Study of Ag induced bimetallic (Au–Ag) nanowires on silicon (5 5 12) surfaces: Experiment and theoretical aspects

The reconstructed vicinal (high index) silicon surfaces, such as, Si (5 5 12) composes row-like structures that can be used as templates for growing aligned nanowires. By using a sub-monolayers of Ag, prior to Au deposition on reconstructed Si (5 512) surface, intermixing of Au and Ag, enhancement of aspect ratio of bimetallic Au–Ag nanowires with tunable morphology is reported. This is attributed to a combined effect of pre-grown Ag strips as nucleation centers for incoming Au ad-atoms and anisotropic Au–Ag intermixing. To achieve optimum conditions for the growth of larger aspect ratio Au–Ag nanostructures, the growth kinetics have been studied by varying growth and annealing temperatures. At ≈400 °C, the Ag diffused into silicon substrate and the inter-diffusion found to inhibit the formation of Au–Ag bimetallic nanostructures. Controlled experiments under ultra-high vacuum condition in a molecular beam epitaxy system and in-situ scanning tunneling microscopy measurements along with ex-situ scanning transmission and secondary electron microscopy measurements have been carried out to understand the bimetallic nanostructure growth. Kinetic Monte Carlo (KMC) simulations based on kinematics of ad-atoms on an anisotropic template with a solid on solid model in which the relative ratios of binding energies (that are obtained from the Density Functional Theory) have been used and the KMC simulations results agree with the experimental observations. Advantage of having bimetallic structures as effective substrates for Surface enhanced Raman spectroscopy application is demonstrated by detecting Rhodamine 6G (R6G) molecule at the concentration of 10−7M.

Direct observation of the spin polarization in Au atomic wires on Si(553)

The spin-resolved electronic band structure of Au-induced metallic atomic wires on a vicinal silicon surface, Si(553), was investigated using spin- and angle-resolved photoelectron spectroscopy. We directly measured the spin polarization of three partially filled one-dimensional metallic bands, a one-third-filled band, and the doublet of nearly half-filled bands. For the half-filled doublet, the strong apparent spin polarization was observed near the Fermi energy with a minor out-of-plane spin component. This observation is consistent with the Rashba-type spin-orbit splitting and with a recent experiment on a similar doublet of Si(557)-Au. In contrast, the one-third-filled band does not show a substantial spin polarization within the experimental accuracy, indicating a much smaller spin splitting, if any. These results are discussed for the origin of the partially filled bands and for the intriguing broken-symmetry ground state observed at low temperature.

The role of vicinal silicon surfaces in the formation of epitaxial twins during the growth of III-V thin films

We examine the role of vicinal surface steps in the formation and propagation of twins during the growth of epitaxial III-V thin films (GaAs, InP, GaSb, AlSb) on silicon substrates. This is achieved through the combined use of two-dimensional X-ray diffraction and conventional transmission electron microscopy techniques, which allow for both a macro and nano/micro characterization of the material systems. Observed is a systematic suppression of twins formed opposite to the tilt direction of vicinal substrates through a process of step-flow overgrowth of nucleated twins, and an enhancement of twins toward the tilt direction when the fastest growth planes are aligned with the step-flow. These results indicate a probable path to the enhancement of the electronic mobility of lateral devices based on III-V semiconductors on silicon.

Shape transformation of Si1−xGex structures on ultra clean Si(5 5 7) and Si(5 5 12) surfaces

We report growth of Ge nano/micro structures on ultra clean, high vicinal silicon surfaces of Si(5 5 7) and Si(5 5 12) under two substrate heating conditions: direct current (DC) and radiative heating (RH). These were grown under ultra high vacuum conditions while keeping the substrate at a temperature of 600°C. The results for 10 monolayer (ML) and 12 ML thick Ge deposited on the above surfaces show spherical island structures for RH conditions while aligned trapezoidal structures were observed under DC conditions of heating. We find that the longer side of trapezoid structures are along <11̄0> irrespective of DC current direction. In the case of 10 ML Ge deposited on Si (5 5 7), elongated SixGe1−x nanostructures with an average length of ≈300 nm and a length/width ratio of ≈3.3 have been formed along the step edges. Under similar conditions for 10 ML Ge growth on Si(5 5 12), we found aligned SixGe1−x trapezoidal microstructures of length ≈6.25 μm and an aspect ratio of ≈3.0. Scanning transmission electron microscopy (STEM) measurements showed the mixing of Ge and Si at the interface and throughout the over-layer. Detailed electron microscopy studies (scanning electron microscopy (SEM) and STEM) reveal the structural aspects of these microstructures.

Selective functionalization of substrates through assembled nanostructures: From physics to biology

The fabrication and use of various nano-patterned substrates as templates in physics (growth experiments) and biophysics (adhesion of living cells) will be presented. Vicinal silicon surfaces which transform under thermal treatment into an array of parallel steps are used to grow high density, metallic nanostructures subsequently employed as a chemically neutral template to grow magnetic nanodots. SiC substrates are used to develop out-off-plane nanopatterning. In a different set of experiments, Ag(1 1 0) substrates are first functionalized with silicon nanostripes and serve subsequently as templates for the growth of one-dimensional cobalt nano-lines. Finally, porous alumina membranes are used as moulds to fabricate an array of ordered nano-pillars from photopolymers which subsequently serve to study the adhesion of living cells on nano-patterned surfaces.

Growth of Pb on Si(3 3 5)–Au surface

The growth, crystallographic structure and electronic properties of ultrathin Pb films grown on a vicinal silicon surface are investigated with reflection high energy electron diffraction (RHEED) and specific resistivity measurement techniques. A Si(335) surface with a perfect distribution of monoatomic steps separated with (111) terraces induced by a submonolayer amount of Au is used as a substrate. In the early stage, Pb growth is anisotropic. Apparently, the presence of steps forces the growth of short crystalline Pb chains along the steps. The layer is amorphous in the perpendicular direction. With the increasing thickness, a phase transition takes place between 3 and 4monolayers (ML) that makes crystalline order also across the terraces. A further increase in thickness causes the layer surface to repeat the substrate morphology. It consists of regularly distributed monoatomic steps and narrow (111) terraces.

Adsorption of small hydrocarbon molecules on Si surfaces: Ethylene on Si(001)

The interaction between small unsaturated hydrocarbon molecules of ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ with a vicinal silicon (001) surface is studied by means of reflectance anisotropy spectroscopy and analyzed with first principles calculations. Our results confirm that ethylene adsorbs without breaking the silicon dimers. Comparison of theoretical optical spectra with experimental data shows that the ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ molecules lay on top of the silicon dimers from low to high coverage. This occurs even though, from a purely energetic point of view, a bridge configuration would be favorable at 1 monolayer coverage.

Tuning reactive epitaxy of silicides with surface steps: Silicide quantum dot arrays on Si(111)

We have studied the nucleation and growth of cobalt silicides on vicinal silicon surfaces. The latter exhibit one-dimensional arrays of steps with narrow terrace widths ranging from 20 Å to 40 Å (miscut angle between 9 ∘ and 4.6 ∘, respectively). Submonolayer amounts of Co were deposited by thermal evaporation on such vicinal Si(111) surfaces by reactive thermal deposition, and the resulting silicide structures were analysed by scanning tunnelling microscopy. The presence of narrow terraces in the system notably alters the usual nucleation process of cobalt silicide on flat Si(111). It is observed that by varying the Co coverage and vicinal angle, the size and amount of the hexagonal silicide clusters can be controlled producing a dense array of quantum dots. This behaviour contrasts to that observed on flat substrates, where nanocluster structures are notably scarce.

Nanolines on silicon surfaces

Although a wide range of periodic surface nets can be grown on low index silicon surfaces, only a few of these have quasi-one dimensional symmetry. If high index silicon surfaces, such as (553) and (557), are used instead, the surface unit cell contains steps. It is possible to fabricate a number of quasi-one dimensional nanoline systems on the terraces and some of these have nested energy bands near the Fermi level. These nano-scale systems may support exotic many-electron states produced by enhanced electron correlations and a reduction in electron screening in one spatial dimension. In this paper, our groups' experimental and theoretical studies of nanolines phases, grown on both low index and vicinal silicon surfaces are reviewed. These studies give us insight into the electronic properties of artificial nanoline structures.

Transport in defective quasi-one-dimensional arrays of chains of gold atoms on a vicinal silicon surface

The temperature dependence of electrical conductivity of quasi-one-dimensional arrays of metallic chains formed on a vicinal silicon surface, Si(553)-Au, was measured by a microscopic four-point probe method. A metal-insulator transition was observed around $160\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which was expected from the previously observed charge density wave formation at low temperatures. This behavior was completely different from that of Si(557)-Au showing an insulating character only. Although both surfaces have similar atomic and electronic structures, differences in interchain coupling and defect density may explain the difference. Inevitably introduced point defects on the surfaces block conduction along the atomic chains. However, with enough interchain coupling, current can flow by avoiding the defects and moving to the adjacent chains on the Si(553)-Au, while it is almost prohibited on the Si(557)-Au due to smaller interchain coupling.

Vicinal silicon surfaces: From step density wave to faceting

We investigate faceting mechanisms induced by electromigration in the regime where atomic steps are transparent. For this purpose we study several vicinal orientations by means of in situ (optical diffraction, electronic microscopy) as well as ex situ (atomic force microscopy, microprofilometry) visualization techniques. The data show that faceting proceeds in two stages. The first stage is short and leads to the appearance of a step density wave, with a wavelength roughly independent of the surface orientation. The second stage is much slower and leads to the formation of a hill-and-valley structure, the period of which depends on the initial surface orientation. A simple continuum model enables us to point out why the wavelength of the step density wave does not depend on the microscale details of the surface. The final wavelength is controlled by the competition between elastic step-step interaction and facet edge energy cost. Finally, the surface stress angular dependence is shown to emerge as a coarsed-grained picture from the step model.

Adsorption of 2-butyne on Si(0 0 1) at room temperature: A valence band photoemission study

We present an angle resolved ultraviolet photoemission spectroscopy study of the adsorption of 2-butyne (CH 3–C C–CH 3) on Si(0 0 1)-2 × 1 at room temperature. We recorded valence band photoemission spectra for two azimuthal positions of a vicinal silicon surface, where all the rows formed by the surface silicon dimers are parallel. The photoemission symmetry selection rules allow the determination of the orientation of the molecular orbitals. The photoemission signal of the HOMO is enhanced when the electric field is parallel to the dimer rows. This showed that the π orbital left intact after the cyclo-addition reaction of the molecule with one silicon dimer is parallel to the dimer rows. This indicates that each 2-butyne molecule adsorbs on one silicon dimer. In spite of the size of the system and the vicinity of the orbitals, the angle resolved study points out that no dispersion of the electronic bands occurs. Not all the surface dimers are reacted so some disorder still exists on the surface preventing the formation of Bloch states.

Au-Induced Nanostructuring of Vicinal Si Surfaces

We have investigated the Au-induced nanofaceting of vicinal silicon surfaces tilted from [111] towards [112̅]. Using samples miscut 3.8°, and 8°, from [111] respectively we have used scanning tunnelling microscopy (STM) to measure the surface morphology as a function of Au coverage (0.04 to 0.44 ML). As expected, Au adsorption produces dramatic changes in surface morphology on both samples. On the 3.8° sample we find that as little as 0.04 ML of Au is sufficient to remove the faceting present on the clean surface. As more Au is deposited 1-dimensional chain structures nucleate at step edges. These chain structures eventually grow to form (775)-Au facets. At 0.17 ML we observe a surface with Si(111)7 × 7 terraces and (775)-Au nanofacets. With more Au, the (111) terraces transform from a 7 × 7 to a 5 × 2 reconstruction and at 0.4 ML the sample consists of Si(111)5 × 2-Au terraces separated by (775)-Au facets. The morphology of the 8° sample also depends critically on Au coverage. Below 0.32 ML all 8° surfaces include (775)-Au nanofacets. Above 0.32 ML, the (775)-Au facet is no longer stable and Au is accommodated on the surface via the formation of higher angle facets with smaller chain spacing. In both samples, the persistence of the (775)-Au facet reinforces the idea that it represents a low energy facet on these Au modified vicinal surfaces.

Growth of copper phthalocyanine on hydrogen passivated vicinal silicon(111) surfaces

Using ultra-high vacuum scanning tunneling microscopy (UHV-STM), we show that copper-phthalocyanine (CuPc) grows in a well ordered manner on hydrogen passivated vicinal silicon surfaces. CuPc grows one-dimensionally parallel to the monatomic steps on the vicinal silicon surface. Surprisingly, elongated clusters of the CuPc parallel to the step directions are formed even on the middle of the terraces well away from the step edges. The one-dimensional growth mode continues even after the full monolayer coverage on the substrate which results in strongly oriented growth mode of a thin film of CuPc on the vicinal silicon surfaces.

Monitoring the ordering in biomolecular films on vicinal silicon surfaces by reflectance difference/anisotropy spectroscopy

DNA base molecules, adenine, thymine, guanine, and cytosine may be employed as charge transport molecules in biomolecular electronic devices. Their electronic properties are comparable with those of inorganic wide bandgap materials, e.g. GaN with the absorption onset in the near ultra-violet (UV) range. A recent field effect transistor study based on a modified DNA base revealed that the prototype bio-transistor gives rise to a better voltage gain compared to one based on carbon nanotubes (CNTs) [G. Mauricio, P. Visconti, V. Arima, S. D’Amico, A. Biasco, E. D’Amone, R. Cingolani, R. Rinaldi, Nanoletters 3 (2003) 479]. Here, in situ reflectance difference/anisotropy spectroscopy (RDS/RAS) is employed under ultra-high vacuum (UHV) conditions for monitoring the growth of DNA base molecules on vicinal hydrogen passivated Si(1 1 1) surfaces. Such vicinal substrates consisting of steps and terraces may serves as suitable templates for molecular ordering. Indeed, RDS/RAS measurements reveal information about molecular ordering of DNA bases induced by the density of steps on silicon surfaces. All four molecules, however, behave differently on the vicinal substrates. The first transition dipole moments corresponding to adenine and thymine molecules align mainly perpendicular to the step edge direction while for guanine and cytosine they align parallel to this direction, however, only in very thin layers. The RDS/RAS signal of the guanine and cytosine layers with thicknesses above 20 nm saturates due to a loss of ordering at higher coverages. Additionally, time-resolved RDS/RAS measurements at the silicon E 2 (4.25 eV) critical point (CP) demonstrate the sensitivity to the biomolecular/inorganic interface formation.

Formation of atom wires on vicinal silicon.

The feasibility of creating atomic wires on vicinal silicon surfaces via pseudomorphic step-edge decoration has been analyzed for the case of Ga on Si(112). Scanning tunneling microscopy and density functional theory calculations indicate the formation of Ga zigzag chains intersected by quasiperiodic vacancy lines or "misfit dislocations." This structure strikes a balance between the system's drive towards chemical passivation and its need for strain relaxation in the atom chains. Spatially fluctuating disorder, intrinsic to the reconstruction, originates from the two symmetry-degenerate orientations of the zigzag chains on vicinal Si.