Spot Profile Analysis Low Energy Electron Diffraction Research Articles

The Ten-Fold Surface of The Decagonal Al72Ni11Co17 Quasicrystal Studied by Leed, Spa-Leed, Aes and Stm.

AbstractThe ten-fold surface of the decagonal Al72Ni11Co17 (d-Al-Ni-Co) quasicrystal has been investigated using low energy electron diffraction (LEED), spot profile analysis LEED (SPA- LEED), Auger electron spectroscopy (AES) and scanning tunnelling microscopy (STM). This was done as a function of both annealing temperature and annealing time. The long-range order of the surface, as indicated by LEED, increases both as a function of annealing time and temperature. STM shows the surface to be rough and cluster-like at low annealing temperatures (≤725 K), whilst annealing to temperatures in excess of 725 K results in the formation of terraces. These terraces are small (≤ 100 Å width) at lower annealing temperatures and increase in size (100 Å ≤ x ≤ 500 Å) as the annealing temperature is increased (≥ 850 K). They are characterised by the presence of three-fold protrusions which align preferentially. STM images show single height steps as expected due to the periodicity of d-Al-Ni-Co in the z direction. To date it has not been possible to obtain atomic resolution, although this work is continuing.

Formation of an intermediate 3 × 3 phase from Pb on Si(111) at high temperature

Using high-resolution spot-profile-analysis low-energy electron diffraction (SPA-LEED) we have investigated low-coverage phases of Pb on Si(111) in the temperature range between 25 °C and 600 °C. After the annealing of 1/3 of a monolayer (ML) of Pb on Si(111) at 500 °C and cooling the sample down to room temperature, we observe a 3 × 3 phase beside the well known Pb/Si(111)(3)1/2 × (3)1/2R30° structure. The 3 × 3 phase is poorly ordered (domain size 20 Å-40 Å) compared to the (3)1/2 × (3)1/2 phase (domain size 1000 Å) as we conclude from profile analysis of the diffraction spots. The development of the 3 × 3 superstructure with increase of the annealing temperature was monitored and we observe a change of the domain size from 20 Å up to 40 Å upon annealing at 500 °C and 540 °C, respectively. After annealing at 580 °C the 3 × 3 phase has vanished and we detect only the (3)1/2 × (3)1/2 mosaic phase.

Ordering of subsurface interstitial atoms in the Ge(113) surface studied by SPA-LEED

The surface order of the reconstructed Ge(113)-(3×2) phase was investigated by SPA-LEED (Spot Profile Analysis Low Energy Electron Diffraction). The LEED pattern exhibits very sharp (3×1) spots and broadened (3×2) spots which can be traced back to a disordered phase of interstitial atoms. At temperatures below 370 K, the (3×2) spots are split in the [11̄0] direction. A quantitative determination of the domain size distributions with the domain matrix method reveals that the split profiles can be explained with a non-geometrical distribution of (3×2) domains separated by antiphase domain boundaries. This may indicate a relief of surface stress at antiphase domain boundaries in the [11̄0] direction. Thereby, a fraction of 70±13% of the surface is covered with (3×2) domains, while the remaining part consists of domain boundaries and (3×1) domains. For temperatures above 370 K, a splitting of the (3×2) spots is not observed, and the domain size distributions are geometric.

Structure and electronic properties of epitaxial metallic monolayers

Epitaxial metallic monolayers are the ideal candidates for fundamental studies of two-dimensional conduction, when they are grown on an insulating surface. Therefore, Ag and Pb films with a thickness of a few monolayers have been grown on Si(111) surfaces in ultrahigh vacuum and studied in situ at low temperatures, since silicon is an insulator at low temperatures. Conductivity depends heavily on defects; therefore, those films have been analysed with spot profile analysis of low-energy electron diffraction (SPA-LEED). They are epitaxial with a mosaic structure ±3° and a grain size that is dependent on thermal history. The conductivity at T<100 K is described within the Drude model using a mean free path equal to the film thickness. For films up to a thickness of about 2 ML and T<20 K a transition to an insulating state is observed. The plasma resonance of the Ag films also shows for the continuous films a localization within the grains of the mosaic structure. The results will be discussed with respect to possible conduction and localization mechanisms in ultrathin films.

Stoichiometry and morphology of MgO films grown reactively on Ag(100)

The stoichiometry and the morphology of MgO adlayers epitaxially grown on Ag(100) are studied by means of X-ray photoelectron spectroscopy (XPS) and high-resolution spot profile analysis low-energy electron diffraction (SPA-LEED). For comparison, Mg deposited at 200 K is oxidized after and during growth, respectively. The post-oxidized film is dominated by stoichiometric MgO but shows also non-stoichiometric species. Annealing the film to 700 K reduces the latter components. Strong differences have been found for the epitaxy of both kinds of MgO films, depending on the preparation condition. The post-oxidized film shows a clear diffraction pattern with 12-fold symmetry due to MgO(100) grains in three rotational orientations only after annealing to 700 K. Depositing and oxidizing simultaneously the Mg the oxide film shows a better film quality (no metallic Mg, a small MgO 2 fraction) and epitaxy with 1×1 already at 200 K.

Ultrathin Au layers on Si(100): surface silicide formation at room temperature

The deposition at room temperature of Au films on Si(100) has been studied by a combination of techniques for thicknesses up to 10 monolayers. From low-energy ion scattering (LEIS) and Auger electron spectroscopy (AES) measurements, it is derived that the outermost layer consists of a mixture of Au and Si for the whole coverage range. The critical coverage to induce intermixing is determined to be at most 0.25 monolayer (ML), but the measurements are also consistent with an immediate onset of intermixing. The surface silicide has a disordered character as shown by spot-profile analysis low-energy electron diffraction (SPA-LEED). Using this technique it is also concluded that the interface between the surface silicide and the substrate is essentially identical to the morphology of the Si(100) surface before deposition.

Epitaxial Insulating Films

The study of insulating films is possible also with electrons, as soon as the charging up of the surface and the electron damage are avoided. Thin epitaxial films do not charge up, as long as the thickness is less than about 5 nm. Therefore the epitaxy of several insulating films could be studied using the spot profile analysis of low energy electron diffraction (SPA-LEED). The results show special features of growth, which are described as elastic bending of the growing films. For this phenomenon defects of the substrate or special growth conditions may be decisive. The features may be enhanced due to the strong binding within the ionic film compared to the weaker bonding to the substrate. Point defects are generated in NaCl films by electron bombardment with an energy over about 30 V. Since these defects have an electronic transition (color centers), they are easily quantitatively measured with electron energy loss spectroscopy. Therefore their effect on the background intensity of the elastic scattering has been studied quantitatively for the first time.

Coexistence of pseudomorphic and coincidence phases in nickel monolayers

We report a study of the structural properties of Ni monolayers on W(110) using low-energy electron diffraction (LEED), spot profile analyzing LEED (SPA-LEED) and scanning tunneling microscopy (STM) at room temperature. Below a coverage of 0.4 monolayers (ML) we find a pseudomorphic (1×1) LEED pattern. Starting from 0.4 ML, LEED experiments show a (8×1) superstructure, followed by a (7×1) superstructure at a coverage of about 0.7 ML. These superstructures are interpreted as coincidence structures. A stripe pattern characteristic of the (7×1) structure is found with STM. STM indicates a partial transformation into the (7×1) coincidence structure: ∼10% of the first monolayer in the vicinity of surface defects remains pseudomorphic. Annealing the films leads to a lateral separation of the pseudomorphic and (7×1) coincidence structures, but not to the growth of one phase in preference to the other.

The formation of mosaics during the reactive growth of MgO films on Ag(100)

The morphology of ultra-thin MgO films has been studied using Spot-Profile Analysis Low-Energy Electron Diffraction (SPA-LEED). The films are deposited at 350 K on a Ag(100) substrate oxidizing Mg in an O 2 atmosphere during deposition. The intensity oscillations of the specular beam prove that the smooth film grows almost flat in the layer-by-layer growth mode. After depositing 1 ML, MgO satellites appear in the diffraction pattern, which can be attributed to mosaics with (100) planes tilted in the 〈010〉 directions. The tilt angle θ has been obtained from analysing the satellite positions at various scattering conditions. It decreases with increasing MgO coverage Θ. The mosaic coverage equilibrates at 7 ML.

Domain structure, segregation and morphology of the Pt 3Sn(111) surface

Using spot-profile analysis low-energy electron diffraction, we have studied the domain structure and morphology of the Pt 3Sn(111) surface after sputtering at room temperature and subsequent anneal treatments which result in the formation of the previously reported ( 3 × 3 ) R30° and (2×2) structures. In addition, we present new results on the temperature-dependent composition of the outermost layer and the subsurface region that is obtained from quantitative analysis of low-energy ion scattering and Auger electron spectroscopy experiments. The combination of these results and previous experiments yields a detailed picture of the evolution of the surface region during the anneal treatments. Finally, we propose the existence of a phase transition at high temperatures between the (2×2) phase and a (2×2)′ phase (not previously observed) on the basis of reversible changes in the SPA–LEED, LEIS and AES data.

Formation of Al-nanoclusters on Si(111)-surfaces

Reflection high-energy electron diffraction (RHEED), spot profile analysis low-energy electron diffraction (SPA-LEED), and scanning force microscopy (SFM) have been used to study the formation of Aluminum nanoclusters on the Si(111) surface, out of a thin film of one to three monolayers (ML) thickness, deposited by molecular beam epitaxy.

Growth and morphology of Rh deposits on an alumina film under UHV conditions and under the influence of CO

Adsorption of certain gases may alter the structure of small metal particles deposited on a surface. If the particles are prepared by vapour deposition, the morphology may also be changed by the presence of these gases during growth. We have investigated the growth and morphology of Rh under ultrahigh vacuum (UHV) conditions and under the influence of CO using a thin, well-ordered Al 2O 3 film as substrate. First, the growth modes at UHV pressures were characterized with the help of spot profile analysis low energy electron diffraction (SPA-LEED) and scanning tunnelling microscopy at two substrate temperatures: 90 and 300 K. At 300 K, we observe the decoration of domain boundaries between different antiphase domains of the film. The particles are mainly disordered, only a small fraction is crystalline. Keeping the substrate temperature at 90 K, smaller (disordered) aggregates form, which are mostly located inside the domains. If the deposits are exposed to CO, in both cases structural changes are detectable: SPA-LEED results point to a spreading of the particles, resulting in an expansion of their surface area. Exposing the sample to CO during deposition, on the other hand, leads to the formation of a carbonyl-like species in the case of large amounts of Rh. Its decomposition gives rise to a characteristic peak in the thermal desorption spectrum.

Particle size dependent CO dissociation on alumina-supported Rh: a model study

Via deposition of Rh from the gas phase onto a thin, well-ordered alumina film we have prepared various of alumina-supported Rh particle systems. The morphologies and particle sizes have been characterised with spot profile analysis LEED and STM measurements. The probabilities for thermally induced dissociation of adsorbed CO at temperatures between 350 and 500 K were determined from C 1s photoelectron spectra. For small aggregates the dissociation activity increases with increasing average particle size. After reaching a maximum for particles containing an average of 500 to 1000 Rh atoms, the fraction of dissociated CO decreases to values closer to those observed well-prepared Rh single crystal surfaces.

Interaction of rhodium with hydroxylated alumina model substrates

In order to investigate how metal growth and metal-oxide interaction depend on the chemical properties of oxide surfaces, we describe a modification procedure which allows the introduction of surface hydroxyl groups on a well-ordered Al 2O 3 film on NiAl(110). The modification — based on deposition of metallic Al and subsequent water exposure — is characterized using LEED spot-profile analysis (SPA-LEED) and high-resolution photoelectron spectroscopy (PES). Upon Al deposition, small aggregates are formed, which are oxidized completely in the final preparation step as verified via PES. The presence of OH-groups is supported by the appearance of additional Al 2p and O 1s surface features. The origin of oxide core and valence level binding energy shifts induced by the modification procedure is discussed. Growth and metal-substrate interaction of Rh deposited onto the hydroxylated Al 2O 3 film is compared to Rh growth on the non-modified oxide surface. It is shown that at 300 K nucleation preferentially occurs on modified oxide areas (SPA-LEED). Photoelectron spectroscopy of both oxide and rhodium core levels points to a direct chemical interaction between the metal and surface hydroxyl groups.

Brillouin light scattering from ultrathin iron films (abstract)

The thickness dependence of the magnetization, magnetic anisotropy, coercive forces, and g factors of an ultrathin Fe film are studied by Brillouin scattering and magneto-optic Kerr effect at room temperature. The wedge Fe film was fabricated on a Ag buffer with GaAs[100] substrate by means of molecular-beam epitaxy technique. The epitaxial growth and crystalline quality of the specimen structures were controlled by reflection high-energy electron diffraction, spot profile analysis-low energy electron diffraction (SPA-LEED), and Auger electron spectroscopy techniques. The measured magnetization and thickness dependence [1.5–16 monolayer (ML)] are compared with the theoretical predictions. Two magnetic order phases are identified, paramagnetic for 1.5 ML and magnetic for 2 ML and thicker film. It is found that g factor is 7% smaller in 1.5 ML film than that in the bulk. A nearly linear thickness dependence of coercive force in this film is observed. The establishing of the bulk in-plane anisotropy shows similar behavior as that of the magnetization, but with a delay in thickness. No apparent perpendicular magnetic anisotropy is observed at room temperature in this sample, which may be related to the nearly perfect in-plane lattice matching between the sandwiched Fe layer and Ag buffer and capping layers.

Structure of ultrathin iron films on a highly asymmetrical substrate: [formula omitted

The morphology of epitaxial Fe films on Cu(311) is examined in situ by spot profile analyzing-low-energy electron diffraction (SPALEED). The Cu(311) substrate is characterized by a trough-like surface structure with uniaxially arranged close-packed atom rows. Deposition at 140 K leads to 3D growth of bcc Fe(211). Likewise, this overlayer orientation is highly asymmetrical: close-packed Fe atom rows are aligned parallel to those of the Cu substrate. Substrate-induced uniaxial stress causes a periodic faceting of the growth front along the row direction. Completion of coalescence takes place at 5 ML and leads to the occurrence of two structurally different fractions of the overlayer: the film is pinned to a lattice arrangement which is dictated by the substrate and is likely to correspond to a fcc-like structure. The periodic faceting of the film surface, on the other hand, enables partial relaxation toward strained bcc structure to persist in the top layers of the film.

Low-energy electron thermal diffuse scattering from Al(111) individually resolved in energy and momentum

A low-energy-electron-diffraction (LEED) system which provides high resolution in energy and momentum simultaneously has been used to measure the intensity and the angular distribution of the thermal diffuse scattering (TDS) of Al(111) within the first surface Brillouin zone. The kinematic scattering theory is shown to provide a good description of the experimental results. Especially the one-phonon scattering is seen to decrease radially with distance |${\mathbf{K}}_{\mathrm{\ensuremath{\Vert}}}$| from the (00)-Bragg rod at about 1/|${\mathbf{K}}_{\mathrm{\ensuremath{\Vert}}}$|. Here the pioneering experiments of about 30 years ago are expanded and one-phonon, multiple-phonon, and elastic scattering throughout the Brillouin zone are separated in the experimental data with the help of energy resolution. The discussion shows to what extent TDS is affecting the defect analysis using a spot profile analysis of LEED without an energy resolution.

A SPALEED structural study of lead adsorption on stepped copper surfaces Cu(211) and Cu(511)

The structure of thin films of Pb deposited on the stepped surfaces of Cu(211) and Cu(511) has been investigated by spot profile analysis low energy electron diffraction (SPALEED). For both substrates Stranski-Krastanov growth (layer by layer growth followed by island formation) has been found at 300 K which is dominated by the anisotropy of the substrate. At submonolayer coverages lead forms dense packed linear chains which are compressed by 3% compared to bulk lead on both surfaces. Annealing the leadcovered surfaces at 600 K causes a reconstruction in which the size of the surface terraces grows by a factor of 8 3 . Furthermore at lead-coverages of greater than one ML the annealed Pb Cu(511) surface shows a “checker”-pattern diffraction pattern that can be interpreted as arising from domains of a c(2 × 2) structure on the rather large (100) terraces of the reconstructed surface.

A combined SPALEED and helium-atom diffraction investigation of the structure of oxidized Cu(001)

The structure of the oxidized Cu(001) surface has been investigated at room temperature using spot profile analysis LEED (SPALEED) and helium-atom diffraction. In both measurements the observed 1 4 - order diffraction spots of the (2√2 × √2)R45° structure show a splitting which is attributed to a large number of small translationally displaced domains. Extensive Eikonal simulations of the helium atom results indicate that [ 3 2 , 1 2 ,0] a translational domain displacements are preferred. Evidence for areas with different lattice constants is also presented.

Preparation-dependent surface composition and structure of NiAl(001): SPA-LEED and NICISS study

The surface composition and structure of NiAl(001) has been investigated by 180°-neutral impact collision ion scattering spectroscopy (NICISS) and high resolution spot profile analysis of low energy electron diffraction (SPA-LEED). Long time annealing below 500 K results in a p(1 × 1) LEED pattern. The surface can be described by a defect enriched Al terminated surface. Another p(1 × 1) LEED pattern appears after flashed annealing at 1400 K and rapid cooling at room temperature. The high temperature p(1 × 1) phase is however nearly perfectly Ni terminated. It has been found that the formation of the high temperature NiAl(001)-p(1 × 1) phase is closely related to roughening of the topmost Al layer by vacancy creation at high annealing temperatures. In the intermediate annealing temperature range about 800 K surface roughening results in an Al terminated missing row surface structure exposing a c(√2 × 3√2)R45° LEED superstructure.