V. Zielasek

Defects in epitaxial insulating thin films

Defects on thin epitaxial insulator films of NaCl(100), KCl(100), and MgO(100) generated during growth and by electron bombardment are investigated by high-resolution spot profile analysis in low-energy electron diffraction (SPALEED), photoelectron spectroscopy (XPS, UPS), electron energy-loss spectroscopy (EELS), and thermal desorption spectroscopy (TDS). All three insulators contain morphological defects: NaCl overgrows the monatomic Ge(100) steps in a carpet-like mode, whereas KCl grown on NaCl(100) forms a regular array of stacking faults up to three monolayers of KCl, and the MgO film grown on Ag(100) reveals a broadened (1 ? 1) pattern in LEED due to the formation of mosaics. In EELS, surface colour centres, produced by electron bombardment, on NaCl induce losses at 2.1 eV for FS centres and at 1.5 eV for MS centres and on KCl induce losses at 1.6 eV and 1.0 eV, respectively. Close to room temperature, high electron exposures result in additional losses in the band gap due to surface and bulk plasmons of Na and K clusters. A similar anion vacancy defect structure on MgO with losses at 2.1 eV and 3.3 eV can be produced by incomplete desorption of metallic Mg from the MgO(100) surface. TDS experiments show that colour centres increase the binding energy of Kr physisorbed on NaCl(100) by about 25%. The concentration of point defects was determined by titration with noble gases in thermal desorption.

ELS-LEED study of electronic excitations on Ag(110) and Ag(111)

An energy loss spectroscopy-low-energy electron diffraction (ELS-LEED) study of the electronic excitation spectrum of Ag(110) and Ag(111) is presented. We find that for both surfaces, the linear term dominates surface plasmon dispersion at small transferred momentum, q‖ and that it is responsible for the anisotropy, the quadratic term being isotropic. The slopes are thereby considerably smaller than reported for Ag(001), confirming that the latter surface behaves abnormally. Surface plasmon damping is nearly q‖-independent for q‖<0.1 A−1 and grows thereafter strongly in a face- and direction-dependent way. For Ag(110) surface plasmon damping depends linearly on crystal temperature T over the range 90 < 500 K. For Ag(111), on the contrary, the dependence with T is linear only for T < 350 K and increases more rapidly thereafter. The effect is possibly related to the population depletion of the filled Shockley state near \gG.

ELS-LEED study of the surface plasmon dispersion on Ag surfaces

Abstract An energy loss spectroscopy-low energy electron diffraction (ELS-LEED) study of surface plasmon dispersion on Ag(111) and Ag(110) is presented. This new technique allows us to improve the resolution in k -space, which constitutes the major limiting factor for studying surface plasmon dispersion. The uncertainty in the determination of the dispersion coefficients is thus improved by more than a factor of two with respect to conventional HREELS. In accord with previous literature we find for both Ag faces a positive dispersion, which is anisotropic with respect to the crystal face and, for Ag(110), to the crystal azimuth. The quadratic term, on the contrary, is isotropic.

K adsorption on Ag(110) : effect on surface structure and surface electronic excitations

The growth of K adlayers on Ag(110) was studied by energy loss spectroscopy‐low energy electron diVraction (ELS‐LEED) which allows the analysis of both elastically and inelastically scattered electrons. Ordered (1◊n) missing row reconstructions and disordered adsorption were investigated. We find that extended surface reconstruction takes place already at 3% K coverage generating a (1◊3) missing row structure. Elastic spot profile analysis shows that a long range correlation between K adatoms is present along the 11:0 direction on the reconstructed surface, implying the stabilization of the reconstruction also at long distances from the K adatoms. With increasing coverage [(1◊2) and (1◊3) structures] the distance between the K adatoms along 11:0 decreases and the adsorption sites become correlated also along 001 in contrast with the behaviour of K on Ni, Cu and Pd. For disordered K adsorption we observe that a weak correlation between the adatoms along both directions is already present in the background. The inelastic intensity shows only one marked loss at the surface plasmon frequency, indicating that in the investigated coverage range both K-induced s and p levels are empty. Ag surface plasmon dispersion and damping are aVected by the presence of K in accord with the electronic mechanism generating the reconstruction. For the ordered (1◊n) reconstructions the structural anisotropy is enhanced, while the anisotropy of Ag surface plasmon dispersion is reduced. K adsorption strongly aVects in particular the quadratic term of the dispersion along 001, which drops by 60% already at 3% K coverage while the linear term remains initially unaVected independently of surface reconstruction. Above 30% K coverage also the linear term decreases and the anisotropy present for the surface plasmon dispersion of the bare Ag(110) is removed.

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.

Surface morphology changes due to adsorbates and due to electron bombardment

Low index surfaces are in most cases stable even after adsorption, whereas vicinal surfaces may be only stable without or even with adsorbates. Since these phenomena are rather frequently reported, some carefully studied examples are discussed here, to show the importance of the effect and possible reasons for the sometimes dramatic morphology changes. Metals on semiconductors and metals are well studied and therefore the best examples for the present purpose. Since vacancies at the surface may act as point defects in a similar way, f-centers on NaCl are included into the discussion.

Multiple losses in off-specular electron energy loss spectra of thin NaCl films individually resolved in energy and momentum

Abstract A LEED system which provides high resolution in energy and momentum simultaneously has been used to record electron energy loss spectra of thin NaCl films epitaxially grown on Ge(100). While measurements in specular geometry show energetically equidistant dipole losses due to single and multiple excitations of the surface phonon polariton, off-specular spectra show multiple loss features which evidently are the result of a combination of one impact and several dipole scattering processes with individually resolved energy and momentum.