Spot Profile Analysis Low Energy Research Articles

Ordered growth of nanodots on a pre-structured metallic template: Au/N/Cu(0 0 1)

We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) the growth of gold particles on the N/Cu(0 0 1) self-organized surface. This template consists of nitrogen islands separated by bare Cu lines and forming a regular 2D array of period 5 nm. When Au is evaporated onto this surface, it mainly grows at the intersections between the Cu lines. The islands organization reproduces then the substrate 2D ordering. However, if the substrate temperature is too low, islands form everywhere. On the contrary, if the substrate temperature is too high, some nucleation sites are empty. By following the intensity of the diffraction satellites during the growth, we have observed that the ordering of the Au particles is optimum when the substrate temperature is between 210 and 290 K. Using both an analytical treatment based on the rate equations and kinetic Monte-Carlo simulations, we have determined the activation energy for the diffusion process and the energy of the traps.

Temperature dependence of rippled corrugations induced on the Rh(1 1 0) surface via ion sputtering

Metal surfaces can be easily nanopatterned via ion sputtering: mounds or ripples can be created depending on the surface symmetry and temperature. However, in many cases these structures are unstable at room temperature and above, due to the adatom fast diffusion. This fact prevents the use of such systems as substrate or nanostamps for a technological implementation. In this paper we present a spot profile analysis low energy electron diffraction (SPA-LEED) study on the nanopatterning of a Rh(110) single crystal. Like the other (110) metal surfaces, previously investigated, also Rh(110) shows for increasing temperatures a transition between different rippled morphologies. The main advantage of this system is its stability at room temperature. From SPA-LEED data we can measure the structural features (average periodicity and local faceting) of the observed rippled structures.

Determination of the coincidence lattice of an ultra thin Al 2O 3 film on Ni 3Al(1 1 1)

Spot profile analysis low energy electron diffraction (SPA-LEED) and low temperature scanning tunneling microscopy (LT-STM) measurements were performed on an ultra thin alumina film grown at 1000 K in an oxygen atmosphere on Ni 3Al(1 1 1). By the aid of these two experimental techniques it has been shown that the alumina film exhibits a large superstructure with a lattice constant of 4.16 nm. The unit cell of this superstructure has a commensurate (√67 × √67)R47.784° relation to the Ni 3Al(1 1 1) substrate lattice.

Chopped sample heating for quantitative profile analysis of low energy electron diffraction spots at high temperatures

Spot profile analysis low energy electron diffraction (SPA-LEED) is one of the most versatile and powerful methods for the determination of the structure and morphology of surfaces even at elevated temperatures. In setups where the sample is heated directly by an electric current, the resolution of the diffraction images at higher temperatures can be heavily degraded due to the inhomogeneous electric and magnetic fields around the sample. Here we present an easily applicable modification of the common data acquisition hardware of the SPA-LEED, which enables the system to work in a pulsed heating mode: Instead of heating the sample with a constant current, a square wave is used and electron counting is only performed when the current through the sample vanishes. Thus, undistorted diffration images can be acquired at high temperatures.

Self-organisation of chemisorbed systems: elastic origin and kinetic factors

Using grazing incidence X-ray diffraction (GIXD), we have determined the atomic relaxations near the surface in the self-organised N/Cu(100) and O/Cu(110) systems. These periodic relaxations give rise to diffraction satellites whose intensity can be quantitatively measured. The comparison between the experiments and molecular dynamics simulations allows us to determine the difference between the surface stress of the chemisorbed phase and the surface stress of the bare surface, which is at the origin of self-organisation. In the case of O/Cu(110), we have analysed the kinetics of self-organisation by spot profile analysis low energy electron diffraction (SPA-LEED) and compared the experiments to Monte Carlo simulations taking into account the elastic interactions in the system.

Uniform island height selection in the low temperature growth of Pb/Si([formula omitted])-(7×7)

Self-organized, uniform-height, Pb islands with flat tops and steep edges form on Si(111)-(7×7) at low temperatures 120<T<250 K. Islands of heights differing by bilayer height increments are observed depending on growth conditions. The formation of these structures is highly unusual since at low temperatures thermal diffusion is suppressed. The origin of the regular structures is believed to be quantum size effects (i.e. effects related to the quantization of the electron energy levels in the islands). We have studied with two complementary techniques (i.e. high resolution spot profile analysis low energy electron diffraction and variable temperature scanning tunneling microscopy) how the preferred island heights depend on the growth parameters (i.e. temperature, coverage, kinetic pathway etc.). We have constructed a kinetic phase diagram in the coverage–temperature plane which indicates the type of islands formed under different growth conditions. The phase diagram can be used as a guide so the island height can be easily controlled.

Determination of domain size distributions from LEED beam profiles: an optimization scheme based on the maximum entropy method

An optimization scheme based on the maximum entropy method (MEM) is derived for the analysis of spot profiles which are measured using the spot profile analysis low energy electron diffraction (SPALEED) method. The profiles of the spots in the diffraction pattern contain information about the distribution of terrace lengths, step heights and island diameters in the surface region. For one-dimensionally disordered structures, the spot profiles for given domain length distributions can be calculated using the domain matrix method (DMM). It is shown that the reverse calculation, namely the direct determination of the domain size distributions from spot profile intensities, can be performed by combining the DMM with the MEM. An iteration scheme is developed which reveals smooth, positive domain length distributions directly from the experimental data. In principle, the scheme is not restricted to SPALEED but can be applied also to the analysis of profiles measured with surface X-ray diffraction or He beam diffraction. Model calculations for stepped surfaces demonstrate that it is possible to extract both step height distributions and terrace lengths from the spot profiles. The iteration scheme is applied to the analysis of the domain structure of the disordered (3×2) phase of Ge(1 1 3).

Breakdown of the simple kinematic approximation models in high-resolution LEED characterization of the initial growth of Si/Si(111)

We have developed a new software for spot profile analysis low energy electron diffraction experiments, which allows variation of the beam energy during a measurement. It enables following multiple diffraction peaks simultaneously. We have explored this possibility in highly accurate measurements of out-of-phase and in-phase intensity during initial growth of Si/Si(111). Under both diffraction conditions intensity oscillations have been observed, clearly demonstrating the breakdown of the usually applied simple kinematic approximation models. The effective atomic scattering factors related to atomic steps do deviate from those connected to ideal terrace sites. Our novel findings urge great caution when extracting information on the morphology of the surface either from the temporal behavior of the spot heights during growth or from the energy dependence of the central spike in the spot profile. We suggest that dynamic effects, changing upon kinetic roughening of the surface, may be important. However, additional experiments on simpler surfaces are required to fully nail down this statement.

Quasicrystalline surface order on decagonal Al 72.1Ni 11.5Co 16.4: An investigation with spot profile analysis LEED

The quasicrystalline order of a decagonal, Co-rich AlNiCo alloy was investigated using spot profile analysis low energy electron diffraction (SPA-LEED). The surface gives rise to a tenfold diffraction pattern with the peak positions being related by the golden mean, indicating quasicrystallinity in the surface region. From the half-widths of the specular spot as a function of the electron energy, the step height was determined to be (2.05±0.05) Å, a value which agrees with the bulk distance of the fivefold quasiperiodic planes of 2.04 Å. Kinematic model calculations revealed that the cluster structure of the bulk is intact also in the surface region. The existence of broad features in the diffraction pattern points to a certain degree of lateral disorder within the terraces.

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.

Growth of semiconductor layers studied by spot profile analysing low energy electron diffraction – Part II1

AbstractThe universal capabilities of high resolution spot profile analysis low energy electron diffraction for in situ studies of surface morphology and surface defects will be discussed and demonstrated. The position of the diffraction spots is used to determine lateral lattice constants, step heights and the strain state of heterosystems with a precision of 0.02 Å. With the knowledge of the spot profile we could determine island and domain size distributions – even during deposition – and correlation functions of arbitrary surface defects. The variation of the spot profile with electron energy allows the evaluation of the 3dim. reciprocal space. With this the power spectrum of surface roughness, facet orientation, or step morphology of flat and vicinal surfaces could be completely characterised.

Growth of semiconductor layers studied by spot profile analysing low energy electron diffraction – Part I 1

Abstract The universal capabilities of high resolution spot profile analysis low energy electron diffraction for in situ studies of surface morphology and surface defects will be discussed and demonstrated. The position of the diffraction spots is used to determine lateral lattice constants, step heights and the strain state of heterosystems with a precision of 0.02 Å. With the knowledge of the spot profile we could determine island and domain size distributions – even during deposition – and correlation functions of arbitrary surface defects. The variation of the spot profile with electron energy allows the evaluation of the 3dim. reciprocal space. With this the power spectrum of surface roughness, facet orientation, or step morphology of flat and vicinal surfaces could be completely characterised.

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.

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.

An investigation by spot profile analysis low energy electron diffraction of Si grown on GaAs(001)

We present the first in-situ spot profile analysis low energy electron diffraction (SPA-LEED) study of the MBE growth of sub-monolayer coverages (0 to 1 ML) of Si on GaAs(001). Changes in the surface reconstructions from c(4 × 4) via mixed c(4 × 4)/(1 × 2), (1 × 2)/(2 × 1), (1 × 2)/asymmetric (3 × 1) and asymmetric (3 × 1) to a (3 × 1) increasing Si coverage have been monitored. Results are subsequently compared with recent in-situ reflection high energy electron diffraction (RHEED) and scanning tunnelling microscopy (STM) measurements. The continuous splitting between the 1 3 - and 2 3 - order diffraction spots up to the completion of the symmetric (3 × 1) reconstruction has been examin detail. Domain sizes for each reconstruction have been quantified from the spot profiles.

STM and SPA-LEED studies of O-induced structures on Rh(100) surfaces

Oxygen adsorption on the Rh(100) surface has been studied by spot profile analysis low energy electron diffraction (SPA-LEED) and scanning tunnelling microscopy (STM). Our results show that oxygen can be reliably dosed onto the Rh(100) surface to produce distinct p(2 × 2) and c(2 × 2) phases. Additionally, the symmetry of the phase formed at saturation coverage is identified as (2 × 2)p4g rather than (2 × 2)p2gg as had been proposed previously. STM images of both the c(2 × 2) and (2 × 2)p4g phases show that the O atoms sit in alternate four-fold hollow sites. It is proposed that the difference between these two structures is a result of overall oxygen coverage. In the c(2 × 2) case we speculate that surface strain is relieved by the formation of islands, but that for the saturation coverage (2 × 2)p4g structure surface strain is reduced by the O atom burrowing into the surface, causing the characteristic clock reconstruction of the substrate Rh atoms.

The temperature dependent growth mode of nickel on the basal plane of graphite

The growth mode of metals on weakly interacting substrates is still a matter of debate. Especially for graphite this question is unsettled. In order to contribute to a clarification, we investigated the temperature dependent growth behaviour of nickel on the basal plane of graphite. Via SPA-LEED (spot profile analysis low energy electron diffraction) we found that Ni(111) islands are formed at 90 K. The formation of these islands deposited originally at 10−2 ML/s, however, is a rather slow process, which takes approximately two hours as observed in the profile of the (00) beam. The driving force for this rearrangement taking place after deposition is certainly the small mismatch between the lattice constant of Ni(111) and the basal plane of graphite. Furthermore, XPS has been used to determine the coverage and the height of the Ni(111) islands. At 300 K, much larger clusters with a different shape and structure are growing on the surface. It is likely that they represent the thermodynamically stable situation.

Scattering phases in low energy electron diffraction from spot profile analysis and from multiple scattering theory

Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) on pseudomorphic submonolayers of Fe(110) on W(110) provides experimental access to phase shift differences between wavelets scattered from Fe top-layer terraces and from the uncovered W-surface, respectively. These phase shifts are alternatively determined within a dynamical LEED calculation and agree surprisingly well with the experimental results.