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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.
KeywordsSemiconductor Surface Substrate Atom Nominal Coverage Surface Dipole LEED Pattern
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- 1.See, for example, Greenwood and Earnshaw .Google Scholar
- 2.The general concepts, direct observations, and indirect manifestations of precursor-mediated surface reactions have been reviewed by Weinberg .Google Scholar
- 3.See, for example, Hand and Holloway , Nielsen et al. , and references cited therein.Google Scholar
- 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 .Google Scholar
- 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.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. .Google Scholar
- 7.Adsorbed cesium was found to reduce the work function of metals by approximately the same amount [Langmuir and Taylor 1932].Google Scholar