Monovalent Adatoms

  • Winfried Mönch
Part of the Springer Series in Surface Sciences book series (SSSUR, volume 26)


Chemical intuition expects monovalent adatoms to saturate dangling bonds at semiconductor surfaces. This generally results in the formation of both adatom-induced surface dipoles and surface states. The existence of adatom-induced surface dipoles indicates the chemical bonds between adsorbate and substrate atoms to be partly ionic and leads to a variation of the ionization energy of the semiconductor substrate. As a consequence of adatom-induced surface states, on the other hand, surface band-bending will generally be observed. Then at least part of the adatoms are ionized in order to balance the space-charge existing in the band-bending region beneath the semiconductor surface or, in other words, electrons are transferred between adatoms and a spatially extended space-charge layer. Adatom-induced surface dipoles, on the other hand, may be viewed as a polarization or a slight shift of the bond charges towards either the adsorbate or the surface atom depending on which one is more electronegative. As long as adatoms are sparsely distributed on a semiconductor surface, they will interact neither directly nor via the substrate. Then the respective adatom-induced surface states will form no two-dimensional surface bands but may rather be described as discrete energy levels.


Semiconductor Surface Substrate Atom Nominal Coverage Surface Dipole LEED Pattern 
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  1. 1.
    See, for example, Greenwood and Earnshaw [1984].Google Scholar
  2. 2.
    The general concepts, direct observations, and indirect manifestations of precursor-mediated surface reactions have been reviewed by Weinberg [1987].Google Scholar
  3. 3.
    See, for example, Hand and Holloway [1989], Nielsen et al. [1990], and references cited therein.Google Scholar
  4. 4.
    These structures are frequently also referred to as 7 × 1:X structures. The label (5(7 × 7):X was first used by Daimon and I no [1985].Google Scholar
  5. 5.
    Hydrogen molecules were recently found to adsorb dissociatively on Si(111)-7 × 7 surfaces at elevated temperatures [Bratu and Höfer 1995]. The strong temperature dependence of the initial sticking coefficient supports the model of phonon-assisted adsorption [Brenig et al. 1995].Google Scholar
  6. 6.
    A similar analysis for Ca-, Al-, As-, H-, S-, O-, and F-induced shifts of Si(2p) levels has been conducted by Himpsel et al. [1989].Google Scholar
  7. 7.
    Adsorbed cesium was found to reduce the work function of metals by approximately the same amount [Langmuir and Taylor 1932].Google Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 1995

Authors and Affiliations

  • Winfried Mönch
    • 1
  1. 1.Laboratorium für FestkörperphysikGerhard-Mercator-Universität DuisburgDuisburgGermany

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