Monovalent Adatoms

Abstract

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.