H. Ibach

The role of surface stress in reconstruction, epitaxial growth and stabilization of mesoscopic structures

Abstract The role of stress at solid surfaces in a vacuum environment and in contact with a liquid electrolyte in reconstruction, epitaxial growth and self-organization or stabilization of mesoscopic structures is critically reviewed. Starting from the definitions of the surface stress, the status of our current understanding of the physical origin of the surface stress on clean surfaces and of the changes of the stress upon deposition of adsorbates or foreign material is discussed. Changes in the surface stress can be measured using the cantilever bending method. The relevant equations for the elastic bending are derived. It is shown that the data analysis in previous work needs revision. Several examples are presented where the surface stress is either the driving force for the reconstruction of the surface or is at least facilitating the reconstruction. The traditional concept of the critical thickness of a heteroepitaxially grown pseudomorphic film is based on the consideration of the elastic strain energy caused by the misfit of the film and substrate lattices. It is shown that the interface stress contributes significantly to the elastic energy and hence to the stability or instability of pseudomorphic thin films. Inhomogeneous surface stresses can give rise to a self-organization of mesoscopic structures on surfaces. The theoretical background for this phenomenon is reviewed and examples are discussed.

Formation of a well-ordered aluminium oxide overlayer by oxidation of NiAl(110)

We have investigated the electronic and geometric structure of a thin oxide film grown by oxidation on NiAl(110) using electron spectroscopic techniques, i.e., LEED, EELS, XPS and ARUPS. This film is inert to adsorption of, respectively reaction with many molecules up to temperatures of about 800 K. It is well ordered as deduced from the LEED pattern and covers the whole surface. We find that the oxide film is about 5 A thick, consisting of aluminium oxide as shown by EELS, XPS and ARUPS. It is most likely formed of two aluminium layers and two quasihexagonal oxygen layers with oxygen surface termination. Since the oxide film is rather thin it only shows a two-dimensional band structure which has been investigated using ARUPS. For the electronic levels of the oxide strong periodic dispersions are observed with bandwidths compatible to dispersion bandwidths calculated for the ΓX direction of α-Al2O3.

Vibrational study of the initial stages of the oxidation of Si(111) and Si(100) surfaces

Using high-resolution electron energy loss spectroscopy (EELS) the vibrations of Si(111) and Si(100) surfaces in the early stages of oxidation have been investigated. Three different stages of oxidation, the last being the formation of a thin layer of vitreous SiO2 are identified when the surfaces are held at a temperature of 700K during the exposure with molecular oxygen. We show that also the first two stages involve atomic oxygen in bridging positions between silicon atoms. Small exposures at low temperatures (100 K) produce vibrational features of a different, possibly molecular, species. For higher exposures at the same temperature the spectrum again develops the characteristics of atomic oxygen and the molecular species eventually disappears. Exposure at room temperature leads to a mixture of atomic and molecular oxygen for smaller exposures and to purely atomic oxygen for exposures greater than ∼ 102 L. At room temperature even exposures as high as ∼ 1011 L do not produce the spectrum of vitreous SiO2. The same is found for the “natural”, room temperature grown, oxide layer on silicon wafers which we have studied by introducing the sample into the spectrometer through an air-lock. Annealing of the wafer to 700 K produced the characteristic spectrum of vitreous SiO2. The results are discussed in comparison with previous work.

The preexponential factor in desorption — CO on Ni(111)

Abstract Adsorption-desorption data are conventionally analysed in terms of an adsorption energy and a preexponential factor. The separation is however not unique but requires at least an a priori assumption, or knowledge, of the temperature dependence of the preexponential factor. For CO on Ni(111) the microscopic properties are sufficiently well understood in order to calculate the preexponential in the low coverage limit. Good agreement with experimental data is found. Assuming the same temperature law as calculated for the low coverage limit to persist for higher coverages an exponential increase of the preexponential factor with increasing coverage is deduced from the data.

Physics of Surfaces and Interfaces

Structure of Surfaces.- Basic Techniques.- Basic Concepts.- Equilibrium Thermodynamics.- Statistical Thermodynamics of Surfaces.- Adsorption.- Vibrational Excitations at Surfaces.- Electronic Properties.- Magnetism.- Diffusion at Surfaces.- Nucleation and Growth.- Appendix: Surface Brillouin Zones.- References.- Subject Index.- List of Common Acronyms.

Step dynamics on Ag(111) and Cu(100) surfaces

Abstract Tunnel images of steps on Ag(111) and Cu(100) display a particular kind of micro-roughness (“frizzes”). We have investigated the frizzes as a function of time and substrate temperature. By comparison to a model for kink diffusion we show that the frizzes are caused by kink motion which is rapid on the time scale of a tunnel image. On vicinal surfaces such as Cu(1111) kinks move less rapidly.

Oxygen on Cu(100) – a case of an adsorbate induced reconstruction

We show that at and above room temperature only a single long range ordered superstructure exists on Cu(100). This is the (2 × 22)R45° structure, which corresponds to a saturation coverage of θo = 0.48 ± 0.05. A well-ordered c(2 × 2) superstructure is not observed in the investigated range of adsorption temperatures (Tads = 300–600 K). Above a critical oxygen coverage θc = 0.34 ±0.02 long range order starts to develop. The disordered adsorption region below θc = 0.34 is characterized by two different adsorption sites. One of these sites already is saturated at relatively low coverages (θo ≅ 0.15). This site appears to corresponds to a local reconstruction with the neighbouring atoms of the adsorbate atom engaging in an intralayer contraction. Diffraction studies show a first order phase transition at θc from the disordered phase to a phase with long range order. The long range ordered domains are larger than the coherence length of the EELS diffraction experiment already slightly above θc. For the “Perfect” (2 ×22)R45° structure only a single fourfold coordinated adsorption site is occupied. The large number of total symmetric vibrations as well as the observed LEED pattern show that the substrate surface is reconstructed. An assignment of the observed vibrations allows us to determine the reconstruction pattern which is characterized by a local symmetry reduction. This lowering of symmetry might be the driving force of the reconstruction pointing towards a Peierls instability or a cooperative Jahn-Teller effect.

CO vibrations on a stepped Ni surface

Abstract Electron energy loss spectra (ELS) have been measured after CO adsorption on the stepped Ni[5(111) × (110)] surface as function of substrate temperature and CO exposure. At 150 K and exposures below 0.5 L, only adsorption sites close to the step edges are occupied. Pronounced rearrangements of CO molecules over terrace sites occur by increasing the temperature to 300 K. The occupation of special step sites gives rise to an exceptionally low frequency C-O stretching vibration of 1520 cm−1. This frequency indicative of a weakened C-O bond is correlated with the previously found tendency for CO decomposition to occur on this surface during a flash desorption experiment.

Adsorbate-induced surface stress: sulfur, oxygen and carbon on Ni(100)

Abstract We have determined the surface stress induced by the adsorption of sulfur, oxygen and carbon on Ni(100). As a result of a strong adsorbate-substrate interaction of these strongly chemisorbed systems, all adsorbates induce a considerable compressive surface stress on Ni(100). The c(2 × 2)-structures of sulfur, oxygen and carbon cause a surface stress of -6600 ± 924dyn/cm, -7500 ± 1050dyn/cmand -8500 ± 1190dyn/cm, respectively. The magnitude of the oxygen- and carbon-induced surface stress is in agreement with total-energy.calculations, whereas the stress induced by sulfur is surprisingly high. Our results on C/Ni(100) support the concept of surface stress as the driving force surface reconstruction.

Structure and adsorbate-adsorbate interactions of the compressed Ni(110)-(2 × 1)CO structure

Abstract Electron energy loss spectroscopy (EELS) with a resolution down to 15 cm −1 was used to study the vibrational modes of the Ni(110)-(2 × 1)CO surface. Besides the CO-stretch and the CO-metal stretch vibration we measured a CO-rotational mode, and a “frustrated translation” mode. The observation of only these CO modes over the entire Brillouin zone favours an (bent) on-top adsorption site for the CO molecule. For the CO stretch vibration we observed a Davydov splitting. The dispersion of the CO-stretch vibration can be described by a simple model of electrostatic dipole-dipole interactions and is sensitive to a structural parameter of CO adsorption, namely the lateral displacement from the on-top position along [001]. The dispersion of the “frustrated translation” mode is caused by strong direct CO-CO interactions.