Vicinal Surfaces Research Articles

FMR study of the anisotropic properties of an epitaxial Fe3Si film on a Si(111) vicinal surface

The anisotropic characteristics of an iron silicide (Fe3Si) epitaxial thin magnetic film grown on a Si(111) silicon vicinal surface with a misorientation angle of 0.14° have been measured by the ferromagnetic resonance method. It has been shown that the polar and azimuth misorientation angles of the crystallographic plane of the substrate can be determined simultaneously from the angular dependences of the ferromagnetic resonance field of the epitaxial film. The effective saturation magnetization of the film Meff = 1105 G and the constant of the cubic magnetocrystalline anisotropy K4 = 1.15 × 105 erg/cm3 have been determined. The misorientation of the substrate plane leads to the formation of steps on the film surface and, as a result, to the appearance of uniaxial magnetic anisotropy of the magnetic dipole nature with the constant K2 = 796 erg/cm3. Small unidirectional magnetic anisotropy (K1 = 163 erg/cm3), which may be associated with symmetry breaking on the steps of the film and is due to the Dzyaloshinskii–Moriya interaction, has been detected.

Enhancing the reactivity of gold: Nanostructured Au(111) adsorbs CO

Low-coordinated sites are surface defects whose presence can transform a surface of inert or noble metal such as Au into an active catalyst. Starting with a well-ordered Au(111) surface we prepared by ion sputtering gold surfaces modified by pits, used microscopy (STM) for their structural characterization and CO spectroscopy (IRAS and NEXAFS) for probing reactivity of surface defects. In contrast to the Au(111) surface CO adsorbs readily on the pitted surfaces bonding to low-coordinated sites identified as step atoms forming {111} and {100} microfacets. Pitted nanostructured surfaces can serve as interesting and easily prepared models of catalytic surfaces with defined defects that offer an attractive alternative to vicinal surfaces or nanoparticles commonly employed in catalysis science.

Study of the structure of a thin aluminum layer on the vicinal surface of a gallium arsenide substrate by high-resolution electron microscopy

The results of electron-microscopy studies of a thin epitaxial aluminum layer deposited onto a misoriented gallium-arsenide substrate are reported. It is established that the layer consists of differently oriented grains, whose crystal lattices are coherently conjugated with the substrate with the formation of misfit dislocations, as in the case of a layer on a singular substrate. Atomic steps on the substrate surface are visualized, and their influence on the growth of aluminum crystal grains is discussed.

X-ray photoemission analysis of clean and carbon monoxide-chemisorbed platinum(111) stepped surfaces using a curved crystal.

Surface chemistry and catalysis studies could significantly gain from the systematic variation of surface active sites, tested under the very same conditions. Curved crystals are excellent platforms to perform such systematics, which may in turn allow to better resolve fundamental properties and reveal new phenomena. This is demonstrated here for the carbon monoxide/platinum system. We curve a platinum crystal around the high-symmetry (111) direction and carry out photoemission scans on top. This renders the spatial core-level imaging of carbon monoxide adsorbed on a ‘tunable' vicinal surface, allowing a straightforward visualization of the rich chemisorption phenomenology at steps and terraces. Through such photoemission images we probe a characteristic elastic strain variation at stepped surfaces, and unveil subtle stress-release effects on clean and covered vicinal surfaces. These results offer the prospect of applying the curved surface approach to rationally investigate the chemical activity of surfaces under real pressure conditions.

Thermodynamic properties of hydrogen–water adsorption at terraces and steps of Pt(111) vicinal surface electrodes

In this work, the effect of temperature on the adsorption states of Pt(111) vicinal surface electrodes in perchloric acid is studied through a thermodynamic analysis. The method allows calculating thermodynamic properties of the interface. In this framework, the concept of the generalized isotherm and the statistical thermodynamics description are applied to calculate formal entropies, enthalpies and Gibbs energies, ΔG¯i0, of the adsorption processes at two-dimensional terraces and one-dimensional steps. These values are compared with data from literature. Additionally, the effect of the step density on ΔG¯i0 and on the lateral interactions between adsorbed species, ωij, at terraces and steps is also determined. Calculated ΔG¯i0, entropies and enthalpies are almost temperature-independent, especially at steps, but they depend on the step orientation. In contrast, ΔG¯i0 and ωij at terraces depend on the step density, following a linear tendency for terrace lengths larger than 5 atoms. However, while ΔG¯i0 increases with the step density, ωij decreases. Results were explained by considering the modification in the energetic surface balance by hydrogen, Hads, and water, H2Oads, co-adsorption on the electrode, which in turn determines the whole adsorption processes on terraces and steps.

Unoccupied electronic structure and momentum-dependent scattering dynamics in Pb/Si(557) nanowire arrays

The unoccupied electronic structure of quasi-one-dimensional reconstructions of Pb atoms on a Si(557) surface is investigated by means of femtosecond time- and angle-resolved two-photon photoemission. Two distinct unoccupied electronic states are observed at $E\ensuremath{-}{E}_{\mathrm{F}}=3.55$ and 3.30 eV, respectively. Density functional theory calculations reveal that these states are spatially located predominantly on the lead wires and that they are energetically degenerated with an energy window of reduced electronic density of states in Si. We further find momentum-averaged lifetimes of 24 and 35 fs of these two states, respectively. The photoemission yield and the population dynamics depend on the electron momentum component perpendicular to the steps of the Si substrate, and the momentum-dependent dynamics cannot be described by means of rate equations. We conclude that momentum- and direction-dependent dephasing of the electronic excitations, likely caused by elastic scattering at the step edges on the vicinal surface, modifies the excited-state population dynamics in this system.

Hybrid InGaAs quantum well–dots nanostructures for light-emitting and photo-voltaic applications

Hybrid quantum well–dots (QWD) nanostructures have been formed by deposition of 7–10 monolayers of In0.4Ga0.6As on a vicinal GaAs surface using metal–organic chemical vapor deposition. Transmission electron microscopy, photoluminescence and photocurrent analysis have shown that such structures represent quantum wells comprising three-dimensional (quantum dot-like) regions of two kinds. At least 20 QWD layers can be deposited defect-free providing high gain/absorption in the 0.9–1.1 spectral interval. Use of QWD media in a GaAs solar cell resulted in a photocurrent increment of 3.7 mA cm−2 for the terrestrial spectrum and by 4.1 mA cm−2 for the space spectrum. Diode lasers based on QWD emitting around 1.1 μm revealed high saturated gain and low transparency current density of about 15 cm−1 and 37 A cm−2 per layer, respectively.

Energetics of Rutile TiO2 Vicinal Surfaces with ⟨001⟩ Steps from the Energy Density Method

Rutile TiO2 vicinal surfaces with ⟨001⟩ steps are investigated using the energy density method (EDM) based on density functional theory. In this approach, EDM provides the energy for each atom so that we can determine the stability of different step configurations correctly. Even though the energy variation due to the step is localized around the step edge, the step–step interaction is long ranged. The finite-size effect in the step–step interaction is identified using EDM. The oxygen vacancy at the step edge explains the atomic structure of the step observed by STM.

Study of a MHEMT heterostructure with an In0.4Ga0.6As channel MBE-grown on a GaAs substrate using reciprocal space mapping

The crystallographic characteristics of the design elements of a metamorphic high-electron-mobility (MHEMT) heterostructure with an In0.4Ga0.6As channel are determined based on reciprocal space mapping. The heterostructure is grown by molecular beam epitaxy on the vicinal surface of a GaAs substrate with a deviation angle from the (001) plane of 2° and consists of a stepped metamorphic buffer containing six layers including an inverse step, a high-temperature buffer layer with constant composition, and active HEMT layers. The InAs content in the layers of the metamorphic buffer is varied from 0.1 to 0.48. Reciprocal space maps are constructed for the (004) symmetric reflection and (224)+ asymmetric reflection. It is found that the heterostructure layers are characterized both by a tilt angle relative to the plane of the (001) substrate and a rotation angle around the [001] axis. The tilt angle of the layer increases as the InAs concentration in the layer increases. It is shown that a high-temperature buffer layer of constant composition has the largest degree of relaxation compared with all other layers of the heterostructure.

Tb silicide nanowire growth on planar and vicinal Si(001) surfaces

Scanning tunneling microscopy and low energy electron diffraction were used to investigate the growth of Tb silicide nanowires on Si(001) and its dependence on Tb coverage, annealing temperature, and the vicinality of the substrate. The nanowires are observed both isolated and in bundles, and while being narrower than 4nm, they reach lengths exceeding 500nm. Their appearance fits a hexagonal TbSi2 structure model with Si dimer rows on top of the nanowires. The growth of exclusive parallel nanowires was achieved on vicinal surfaces. On planar samples, the nanowire growth is accompanied by the formation of a 2×7 reconstruction that shows a wetting layer like behavior. In contrast, mainly building blocks of this 2×7 reconstruction are observed on vicinal samples.

Physical Vapor Transport Growth of 4H-SiC on {000-1} Vicinal Surfaces

Bulk 4H-SiC crystals were grown on 4° off-axis seeds by the physical vapor transport technique. Two completely different surface morphologies of as-grown crystals were observed by laser scanning confocal microscopy. The formation mechanisms of the different surface morphologies are proposed and discussed. We found that the facet formation and migration on the 4° off-axis seeds largely depended on the profile evolution of the temperature field in the growth cell over a long-term growth run. At the interface between the two growth regimes, some grown-in defects, including micropipes, dislocations and polytype inclusions, were frequently observed by polarizing optical microscopy and chemical etching. The stress induced by step coalescence could be responsible for the formation of these grown-in defects.

Regularity in Time for Weak Solutions of a Continuum Model for Epitaxial Growth with Elasticity on Vicinal Surfaces

The evolution equation derived by Xiang (SIAM J. Appl. Math. 63:241–258, 2002) to describe vicinal surfaces in heteroepitaxial growth is(1) where h denotes the surface height of the film, and H is the Hilbert transform. Existence of solutions was obtained by Dal Maso et al. (Arch. Rational Mech. Anal. 212: 1037–1064). The regularity in time was left unresolved. The aim of this paper is to prove existence, uniqueness, and Lipschitz regularity in time for weak solutions, under suitable assumptions on the initial datum.

Refined BCF-type boundary conditions for mesoscale surface step dynamics

Deposition on a vicinal surface with alternating rough and smooth steps is described by a solid-on-solid model with anisotropic interactions. Kinetic Monte Carlo (KMC) simulations of the model reveal step pairing in the absence of any additional step attachment barriers. We explore the description of this behavior within an analytic Burton-Cabrera-Frank (BCF)-type step dynamics treatment. Without attachment barriers, conventional kinetic coefficients for the rough and smooth steps are identical, as are the predicted step velocities for a vicinal surface with equal terrace widths. However, we determine refined kinetic coefficients from a two-dimensional discrete deposition-diffusion equation formalism which accounts for step structure. These coefficients are generally higher for rough steps than for smooth steps, reflecting a higher propensity for capture of diffusing terrace adatoms due to a higher kink density. Such refined coefficients also depend on the local environment of the step and can even become negative (corresponding to net detachment despite an excess adatom density) for a smooth step in close proximity to a rough step. Our key observation is that incorporation of these refined kinetic coefficients into a BCF-type step dynamics treatment recovers quantitatively the mesoscale step-pairing behavior observed in the KMC simulations.

Dense Mg x Ni1−x O nanocubes with special grain boundaries and paracrystalline distribution of defect clusters by pulsed laser ablation

Pulsed laser ablation of reactively sintered MgO–NiO (9:1, 1:1, 1:9 molar ratio) disks was used to fabricate dense rocksalt-type Mg x Ni1−x O in the form of submicron-sized spherical particulate and nanocubes for X-ray diffraction and transmission electron microscopic characterizations. The rapidly solidified particulates and the condensed nanocubes have internal compressive stress increasing up to ca. 1 GPa with the increases in Ni content. The nanocubes were coalesced over well-developed polar surfaces, i.e., {100}, minor {110} and their vicinal surfaces with growth ledges, to form special grain boundaries, i.e., (100) 15° and 45° twist boundaries and [100] {001}/{011} asymmetric tilt boundaries. The Mg x Ni1−x O nanocubes also showed 3-D paracrystalline distribution of defect clusters with 2 nm and 1.5–2 nm interspacing for M1N9 and M1N1 compositions, respectively, but G.P. zone-like features for M9N1 composition. The Mg x Ni1−x O particulates and nanocondensates have UV–visible absorption edge in the range 4.0–3.3 eV, decreasing with increasing Ni content for potential engineering applications.

Noncollinearity of the canted spins across ultrathin Fe films on vicinal Ag surfaces

We investigate the depth dependence of the canted magnetization in a bcc Fe ultrathin film grown on a vicinal Ag substrate. This study is performed for different Fe thicknesses, in the vicinity of the spin reorientation transition, by soft x-ray resonant magnetic reflectivity (SXRMR). Above 5.5 monolayers, the high spatial resolution of SXRMR allows us to describe a gradient of the canted magnetization. The tilt angle is larger near the Ag interface and decreases towards the surface. This noncollinearity is ascribed to a different balance of various anisotropies across the layer thickness. Below that value, a large increase of the tilt angle is observed and ascribed to the spin reorientation transition. Unlike the previous situation, the tilt angle is found to be homogeneous throughout the film. This can be related either to the out-of-plane spin reorientation associated to a reduced number of monolayers or to the $q$-space limitation of the SXRMR technique at the Fe ${L}_{3}$ edge.

Structural studies of Al thin layer on misoriented GaAs(100) substrate by transmission electron microscopy

AbstractA thin Al layer has been grown on a misoriented GaAs(100) substrate by molecular beam epitaxy and its structure has been studied by means of transmission electron microscopy. The metal layer is formed as grains of three orientations Al(100), Al(110) and Al(110)R. Digital analysis of dark‐field micrographs made it possible to obtain their grain sizes and relative coverage areas. By comparison with an Al layer grown on an exactly oriented GaAs(100) substrate it has been found that on the vicinal surface the relative coverage area and grain size of the Al(110)R orientation increase and the Al(100) relative coverage area decreases. This is attributed to surface atomic steps, which have been visualized in high‐resolution micrographs. The edge misfit dislocation system at the Al/GaAs interface has been revealed, which is insensitive to the substrate misorientation. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Equilibrium shape of nano-cavities in H implanted ZnO

Thermally equilibrated nano-cavities are formed in ZnO by H implantation and subsequent high temperature annealing to determine the relative surface formation energies and step energies of ZnO from reverse Wulff construction and related analysis. H adsorption, vicinal surfaces, and surface polarity are found to play an important role in determining the final thermal equilibrium shape of the nano-cavities. Under H coverage, the O-terminated surface shows a significantly lower surface formation energy than the Zn-terminated surface.

Substrate temperature optimization for heavily-phosphorus-doped diamond films grown on vicinal (001) surfaces using high-power-density microwave-plasma chemical-vapor-deposition

We have investigated the growth condition suitable for homoepitaxial diamond growth of phosphorus (P)-doped films on vicinal (001) substrates with a misorientation angle of 5° using the high-power-density microwave-plasma (MWP) chemical-vapor-deposition (CVD). The P-doped layers were grown with H2-diluted (1%) CH4 gas containing P(CH3)3 with P/C ratio of 0.99% at various substrate temperatures ranging from 960 to 1210°C by using a conventional MWPCVD system with a quartz-tube chamber. It is found from exciton-related cathodoluminescence spectra taken at ≈80K and secondary ion mass spectrometry profiles that reasonably-high-quality P-doped layers with containing a substantial amount of substitutional P donors were grown on vicinal (001) substrates only at substrate temperatures of ≈1160°C although the sample surface completely lost the original flatness with a roughness of ≈0.3nm. This indicates that the suitable process window should be rather narrow for the P-doped diamond homoepitaxial growth on the vicinal (001) substrate using the present MWPCVD with 1% CH4 source gas and that the crystalline steps should play an important role on the growth process appropriate for the P atom incorporation to substitutional sites.

Step free energy of an arbitrarily oriented step on a rectangular lattice with nearest-neighbor interactions

We have derived within the framework of a solid-on-solid model with anisotropic nearest-neighbor interactions an exact expression for the free energy of an arbitrarily oriented step edge or boundary on a rectangular two-dimensional lattice. The full angular dependence of the step free energy allows the determination of the surface free energy of any vicinal surface as well as the temperature evolution of the equilibrium island shape. Our method can also be applied to derive an expression for the boundary tension of an arbitrarily oriented domain boundary of the anisotropic nearest neighbor two-dimensional Ising model

Multiple in-plane spin reorientation transitions inFe/CoObilayers grown on vicinalMgO(001)

Step-induced antiferromagnetic (AFM) uniaxial anisotropy and its effects on the exchange coupling have been systematically investigated in the epitaxial $\mathrm{Fe}/\mathrm{CoO}$ bilayers on a $\mathrm{MgO}(001)$ vicinal surface. X-ray magnetic linear dichroism measurements proved that the atomic steps induced a strong in-plane AFM uniaxial anisotropy in the CoO film. We found that the thermal activation induced in-plane 90\ifmmode^\circ\else\textdegree\fi{} switching of CoO AFM in-plane spins. The competition among the step-induced AFM anisotropy, the interface exchange coupling, and thermal activation generate a novel multiple in-plane spin reorientation transition of the Fe magnetization, which can further provide new insights on the exchange coupling in FM/AFM systems.