Strongly-driven coarsening of height-selected Pb islands on Si(1 1 1)

A theoretical model is proposed to describe the rapid coarsening observed for Pb islands on a Si(1 1 1) surface where classical kinetics breaks down. In this system, quantum size effects produce mesa-like Pb islands with chemical potentials depending strongly on their heights, in addition to the usual dependence on the step curvature. Furthermore, a dense wetting layer enables fast mass transport between islands. Incorporating these features, our theoretical model predicts evolution of the island height distribution in good agreement with experiments.     

Structural study of Co/Pd(1 1 1) and CO/Co/Pd(1 1 1) by surface X-ray absorption fine structure spectroscopy

The structure of the Co thin films on Pd(1 1 1) and the effect of the CO adsorption on Co thin films were studied by Co K-edge surface X-ray absorption fine structure (XAFS). The polarization dependences of the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra indicate that Co thin films grow in the fcc stacking mode on Pd(1 1 1) up to 12 ML. The analysis of the nearest neighbor shell shows little mechanical strain at the interface, indicating that Co atom does not grow pseudomorphically on Pd(1 1 1). There is no alloy-like structure at the interface. CO adsorption causes no structural change of the Co thin films but modifies the Co surface electronic state. These structural studies provide deep insight in the magnetic property of the Co thin films on Pd(1 1 1).     

Spectroscopic interpretation for Na-induced phase transitions on Na/Si(1 1 1)3 × 1

Na adsorption at room temperature causes the Na/Si(1 1 1)3 × 1 surface with Na coverage of 1/3 monolayer (ML) to transit into the Na/Si(1 1 1)6 × 1 surface at 1/2 ML and sequentially into the Na/Si(1 1 1)3 × 1 surface at 2/3 ML. The phase transition was studied by Si 2p core-level photoemission spectroscopy. The detailed line shape analysis of the Si 2p core-level spectrum of the Na/Si(1 1 1)3 × 1 surface (2/3 ML) is presented and compared to the Na/Si(1 1 1)3 × 1 surface (1/3 ML) which is composed of Si honeycomb chain-channel structures. This suggests that as additional Na atoms form atomic chains resulting in the Na/Si(1 1 1)3 × 1 surface (2/3 ML), the inner atoms of the Si honeycomb chain-channel structure is buckled due to the additional Na atoms.     

Structural and statistical analysis of Yb/Si(1 1 1) and Eu/Si(1 1 1) reconstructions

We have performed the structural and statistical analysis of Yb/Si(1 1 1) and Eu/Si(1 1 1) surfaces in the submonolayer regime utilizing low-energy electron diffraction and scanning tunneling microscopy (STM). The almost identical series of one-dimensional chain structures (e.g., 3 × 2/3 × 1, 5 × 1, 7 × 1, 9 × 1, and 2 × 1 phases) are found in order of increasing metal coverage for both adsorbed systems, however, only the Eu/Si system reveals the ‘√3’-like reconstruction before the 2 × 1 endpoint phase. The atomic models of chain structures are proposed and discussed. In particular, our results suggest the odd-order n×1 (n=5,7,9,…) intermediate reconstructions to incorporate the Seiwatz chains and honeycomb chains with the proportion of m:1, where . The statistical analysis of STM images is carried out to examine the correlation of atomic rows on Eu/Si and Yb/Si surfaces. It is found that Eu stabilizes more ordered row configuration compared to Yb, which can be explained in terms of indirect electronic interaction of atomic chains or/and different magnetic properties of adsorbed species.     

Four-fold magnetic anisotropy in a Co film on MgO(0 0 1)

The development of devices based on magnetic tunnel junctions has raised new interests on the structural and magnetic properties of the interface Co/MgO. In this context, we have grown ultrathin Co films (≤30 Å) by molecular-beam epitaxy on MgO(0 0 1) substrates kept at different temperatures (T_S). Their structural and magnetic properties were correlated and discussed in the context of distinct magnetic anisotropies for Co phases reported in the literature. The sample characterization has been done by reflection high energy electron diffraction, magneto-optical Kerr effect and ferromagnetic resonance. The main focus of the work is on a sample deposited at T_S=25 °C, as its particular way of growth has enabled a bct Co structure to settle on the substrate, where it is not normally obtained without specific seed layers. This sample presented the best crystallinity, softer magnetic properties and a four-fold in-plane magnetic anisotropy with Co〈1 1 0〉 easy directions. Concerning the samples prepared at T_S=200 and 500° C, they show fcc and polycrystalline structures, respectively and more intricate magnetic anisotropy patterns.     

XPS study of the formation of ultrathin GaN film on GaAs(1 0 0)

The nitridation of GaAs(1 0 0) surfaces has been studied using XPS spectroscopy, one of the best surface sensitive techniques. A glow discharge cell was used to produce a continuous plasma with a majority of N atomic species. We used the Ga3d and As3d core levels to monitor the chemical state of the surface and the coverage of the species. A theoretical model based on stacked layers allows to determine the optimal temperature of nitridation. Moreover, this model permits the determination of the thickness of the GaN layer. Varying time of nitridation from 10 min to 1 h, it is possible to obtain GaN layers with a thickness between 0.5 nm and 3 nm.     

Ultrathin Fe layers on Ag (1 0 0) surface

Iron layers (0.15-10 ML thick) deposited on Ag (1 0 0) substrates were investigated by conversion electron Mössbauer spectrometry over a broad temperature range. The layers were characterized by scanning tunneling microscopy. Different forms of the layers, depending on their thickness, were observed. Minimum roughness of the layers were found at 0.15 and 10 ML thickness values. The Mössbauer spectra showed systematic thickness dependence. At low thickness values, broad doublets were observed, while above 6 ML, magnetic split spectra appeared at room temperature. At low temperatures, magnetically split spectra appeared with parameter values characteristic of Fe-Ag and Fe-Fe atomic interactions. The hyperfine split spectra indicated magnetic anisotropy and an enhanced saturation hyperfine magnetic field of ?40 T. The latter value is the highest ever measured for iron in thin layers.     

Structural and electronic properties of graphite layers grown on SiC(0 0 0 1)

Photoelectron spectroscopy, low-energy electron diffraction, and scanning probe microscopy were used to investigate the electronic and structural properties of graphite layers grown by solid state graphitization of SiC(0 0 0 1) surfaces. The process leads to well-ordered graphite layers which are rotated against the substrate lattice by 30°. On on-axis 6H-SiC(0 0 0 1) substrates we observe graphitic layers with up to several 100 nm wide terraces. ARUPS spectra of the graphite layers grown on on-axis 6H-SiC(0 0 0 1) surfaces are indicative of a well-developed band structure. For the graphite/n-type 6H-SiC(0 0 0 1) layer system we observe a Schottky barrier height of ?_B,n = 0.3 ± 0.1 eV. ARUPS spectra of graphite layers grown on 8° off-axis oriented 4H-SiC(0 0 0 1) show unique replicas which are explained by a carpet-like growth mode combined with a step bunching of the substrate.     

Growth process and surface structure of MnSi on Si(1 1 1)

The solid-phase epitaxial growth process and surface structure of MnSi on Si(1 1 1) were investigated by coaxial impact-collision ion scattering spectroscopy (CAICISS) and atomic force microscopy (AFM). For the Si(1 1 1) sample deposited with 30 monolayers (ML) of Mn at room temperature, the intermixing of Mn and Si gradually started at 100 °C and reached equilibrium at approximately 400 °C. At this equilibrium state, the Mn atoms were transformed into crystalline MnSi film. Further annealing caused the desorption of Mn atoms. We identified the structure of MnSi as cubic B20 and the crystallographic orientation relationships as Si(1 1 1)//MnSi(1 1 1) and Si[]//MnSi[]. The MnSi(1 1 1) surface was found to have a dense Si terminating layer on its topmost surface. On the other hand, 3 ML of Mn deposited on Si(1 1 1) reacted with Si even at room temperature and formed a pseudomorphic structure. This structure was transformed into MnSi after annealing. A filmlike morphology with protrusions was observed for the sample with 30 ML of Mn, while island growth occurred for the sample with 3 ML of Mn.     

Ab-initio investigation of Ni(Fe)/ZrO2(0 0 1) and Ni-Fe/ZrO2(0 0 1) interfaces

The atomic and electronic structures of Me/ZrO₂(0 0 1) interfaces, where Me is Ni, Fe or a Ni-Fe alloy, are investigated by the plane wave pseudopotential method within density-functional theory. The work of separation of metal films from oxide substrate for the O- and Zr-terminated Me/ZrO₂(0 0 1) interfaces is calculated. High adhesion at both Me/(ZrO₂)_O and Me/(ZrO₂)_Zr interfaces is found. The effect of oxygen vacancies on the adhesion at the metal-ceramic interfaces is also investigated. It is shown that Ni(Fe)-O interaction at the O-terminated interface weakens in the presence of interfacial oxygen vacancies. At interfaces with Ni-Fe alloys the adhesion depends strongly on the composition of the interfacial layers and their magnetic properties.     

Molecular self-alignment on pre-structured Sm/Si(1 1 1) interfaces at room temperature

Adsorption of SiPc(OH)₂ on Sm/Si(1 1 1) interface is investigated by scanning tunneling microscopy. The Sm/Si(1 1 1) interface is known to induce a high number of 1D structures in the sub-monolayer Sm coverage. We show in this study, the possibility to use these pre-structured interfaces to generate at room temperature, a 1D molecular alignment on a semi-conductor substrate.     

The epitaxial sexiphenyl (0 0 1) monolayer on TiO2(1 1 0): A grazing incidence X-ray diffraction study

A para-sexiphenyl monolayer of near up-right standing molecules (nominal thickness of 30 Å) is investigated in-situ by X-ray diffraction using synchrotron radiation and ex-situ by atomic force microscopy. A terrace like morphology is observed, the step height between the terraces is approximately one molecular length. The monolayer terraces, larger than 20 μm in size, are extended along the [0 0 1] direction of the TiO₂(1 1 0) substrate i.e. along the Ti-O rows of the reconstructed substrate surface. The structure of the monolayer and its epitaxial relationship to the substrate is determined by grazing incidence X-ray diffraction. Extremely sharp diffraction peaks reveal high crystalline order within the monolayer, which was found to have the bulk structure of sexiphenyl. The monolayer terraces are epitaxially oriented with the (0 0 1) plane parallel to the substrate surface (out-of-plane order). Four epitaxial relationships are observed. This in-plane alignment is determined by the arrangement of the terminal phenyl rings of the sexiphenyl molecules parallel to the oxygen rows of the substrate.     

Atomic structure of thin dysprosium-silicide layers on Si(1 1 1)

We report on scanning tunneling microscopy results of thin dysprosium-silicide layers formed on Si(1 1 1). In the submonolayer regime, both a and a 5 × 2 superstructure were found. Based on images taken at different tunneling conditions, a structure model could be developed for the superstructure. For one monolayer, a 1 × 1 superstructure based on hexagonal DySi₂ was observed, while several monolayers thick films are characterized by a superstructure from Dy₃Si₅.     

Theoretical investigation of the Au/Si(1 1 1)-(5 × 2) surface structure

The atomic structure of the Au/Si(1 1 1)-(5 × 2) surface has been studied by density-functional theory calculations. Two structure models, proposed experimentally by Marks et al. and Hasegawa et al., have been examined on an equal ground. In our total-energy calculations, both models are found to be locally stable and energetically comparable. In our electronic-structure analyses, however, both models fail to reproduce the key features of angle-resolved photoemission spectra and scanning-tunneling-microscopy images, indicating that the considered models need to be modified. Suggestions for the modification are given based on the present calculations.     

Core level photoemission and STM characterization of Ta/Si(1 1 1)-7 × 7 interfaces

We present the results of scanning tunneling microscopy (STM) and photoemission spectroscopy (PES) of the Ta/Si(1 1 1)-7 × 7 system after deposition of Ta at substrate temperatures from 300 to 1250 K. The coverage of Ta varied from 0.05 up to 2.5 of a monolayer (ML). STM shows that at 300 K and coverage less than 1 ML, a disordered chemisorbed phase is formed. Deposition on a hot surface (above 500 K) produces round 3D clusters randomly distributed on the surface. Cluster height and their diameter are found to change drastically with annealing temperature and the Ta coverage. Analysis of photoemission data of the Si 2p core levels shows that at room temperature and at coverage ?1 ML core level binding energy shifts and intensity variations of Si surface related components are observed, which clearly indicate that the reaction starts already at 300 K. Shifts in the binding energy, changes of the peak shapes and intensity of the Ta 4f doublet at higher temperatures can be explained by the formation of stable silicide on the surface.     

Effects of electronic confinement and substrate on the low-temperature growth of Pb islands on Si(1 0 0)-2 × 1 surfaces

The growth of Pb films on the Si(1 0 0)-2 × 1 surface has been investigated at low temperature using scanning tunneling microscopy. Although the orientation of the substrate is (1 0 0), flat-top Pb islands with (1 1 1) surface can be observed. The island thickness is confined within four to nine atomic layers at low coverage. Among these islands, those with a thickness of six layers are most abundant. Quantum-well states in Pb(1 1 1) islands of different thickness are acquired by scanning tunneling spectroscopy. They are found to be identical to those taken on the Pb(1 1 1) islands grown on the Si(1 1 1)7 × 7 surface. Besides Pb(1 1 1) islands, two additional types of Pb islands are formed: rectangular flat-top Pb(1 0 0) islands and rectangular three-dimensional (3D) Pb islands, and both their orientations rotate by 90° from a terrace to the adjacent one. This phenomenon implies that the structures of Pb(1 0 0) and 3D islands are influenced by the Si(1 0 0)-2 × 1 substrate.     

Conformational isomers of stilbene on Si(1 0 0)

Stilbene (1,2-diphenylethylene) has shown an intriguing isomerisation behavior and may serve as a model system for “molecular switches” incorporating a CC double bond. To evaluate the possible use of such molecules as molecular switches on semiconductor surfaces, the adsorption of cis- and trans-stilbene on Si(1 0 0) has been investigated. Identification of both isomers is achieved by differences in adsorption geometry as revealed by NEXAFS, and differences in electronic structure in the occupied and unoccupied molecular orbitals. For both isomers, bonding takes place via the CC double bond to the Si dimer atoms allowing for free movement of the aromatic rings, a necessary prerequisite for photoinduced isomerisation on the surface. Our experimental results agree well with theoretical calculations.     

Fourfold in-plane magnetic anisotropy in epitaxial Fe(1 1 0)/Cu(0 0 1) and Fe(1 1 0)/Ni(0 0 1) bilayers

Epitaxial Fe(1 1 0) films with thicknesses of 100-800 nm on Cu(0 0 1) and Ni(0 0 1) buffer layers grown on MgO(0 0 1) substrates have been fabricated. These films contain Fe(1 1 0) crystallites which are in the Pitsch orientation relationship. Magnetization and the fourfold in-plane magnetic anisotropy constants of these films have been determined by torque measurements. All the samples under study are characterized by a fourfold magnetic anisotropy with easy axes parallel to the [1 0 0] and [0 1 0] directions of Cu(0 0 1) and Ni(0 0 1) layers. The measured values of the constant for Fe(1 1 0)/Cu(0 0 1) are found to depend on deposition temperature; a maximum value of (2.5±0.1)×10⁵ erg/cm³ is reached after annealing at 600 °С. The in-plane torque measurements on Fe(1 1 0)/Ni(0 0 1) bilayers obtained at 300 °С, on the other hand, exhibit a constant value of (2.7±0.1)×10⁵ erg/cm³. Assuming an exchange interaction between the Fe(1 1 0) crystallites, which are in the Pitsch orientation relationship, the fourfold in-plane magnetic anisotropy has been calculated as 2.8×10⁵ erg/cm³. The deviations of the experimental values from the predicted one may be explained by the formation of a polycrystalline phase within the Fe(1 1 0) layer and a partial disorientation of the epitaxial crystallites.     

Band bending at the Si(1 0 0)-Si3N4 interface studied by photoreflectance spectroscopy

Photoreflectance spectroscopy has been used to measure the band bending at the p-Si(1 0 0)-Si₃N₄ interface subjected to annealing and ion implantation. Upon annealing, unimplanted interfaces exhibit a constant band bending of about 0.77 eV, even though the spectral amplitude changes due to variations in the way minority carriers are annihilated at the interface. Implantation reduces the band banding, although subsequent annealing in stages up to 900 °C progressively restores the bending to its original value through pathways exhibiting a wide range of activation barriers.     

Atom geometry of nanostructured Fe films grown on c(2 × 2)-N/Cu(1 0 0) surface: An investigation by X-ray absorption spectroscopy with multishell analysis

We investigated the growth of Fe nanostructured films on c(2 × 2)-N/Cu(1 0 0) surface with Fe K-edge X-ray absorption fine structure (XAFS) in the near edge and in the extended energy region. The high photon flux of the incident X-rays allowed us to perform multishell analysis of the XAFS oscillations for Fe coverage Θ_Fe < 1 ML. This data analysis yields a detailed investigation of the atom geometry and some insights in the film morphology. At Θ_N < 0.5 ML (N saturation coverage) there is absence of contribution to XAFS from N atoms. First shell analysis of linearly polarized XAFS gives Fe-Fe (or Fe-Cu) bond length values varying between R₁ = 2.526 ± 0.006 Å at the highest Fe coverage (3 ML ) and R₁ = 2.58 ± 0.01 Å at Θ_Fe = 0.5 ML, Θ_N = 0.3 ML, with incidence angle Θ = 35°. These values are different from the case of bcc Fe (R = 2.48 Å), and compatible with fcc Fe (R₁ = 2.52 Å) and fcc Cu (R₁ = 2.55 Å). At the Fe lowest coverage (Θ_Fe = 0.5 ML) the dependence of R₁ on the incidence angle indicates expansion of the outmost layer. Near edge spectra and multishell analysis can be well reproduced by fcc geometry with high degree of static disorder. At N saturation pre-coverage (Θ_N = 0.5 ML) the XAFS analysis has to keep into account the Fe-N bonding. The results suggest two different adsorption sites: one with Fe in a fcc hollow site, surrounded by other metal atoms as nearest neighbours, and one resulting from an exchange with a Cu atom underneath the N layer.