Extrinsic surface states

The surface states considered so far are related to the clean and well-ordered surface of a crystal and they are called therefore intrinsic surface states. Defects or impurities located at a surface can form electronic states whose wave functions are localized in their vicinity and thus at the surface too. They are called extrinsic surface states. Such states do not possess a 2D translational symmetry and an electron occupying such a state cannot propagate along the surface. 

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Surface states can be divided into those that are intrinsic to a well ordered crystal surface with two-dimensional periodicity, and those that are extrinsic [25]. Intrinsic states include those that are associated with relaxation and reconstruction. Note, however, that even in a bulk-tenuinated surface, the outemiost atoms are in a different electronic enviromuent than the substrate atoms, which can also lead to intrinsic surface states. Extrinsic surface states are associated with imperfections in the perfect order of the surface region. Extrinsic states can also be fomied by an adsorbate, as discussed below. 

The dangling and the surface ion-induced states are intrinsic surface states that are characteristic of individual semiconductors. In addition, there are extrinsic surface states produced by adsorbed particles and siuface films that depend on the enviromnent in which the siuface is exposed. In general, adsorbed particles in the covalently bonded state on the semiconductor surface introduce the danglinglike surface states and those in the ionically bonded state introduce the adsorption ion-induced surface states. In electrochemistiy, the adsorption-induced surface states are important. 

The selection on an empirical basis of collective-electron factors or active-site concepts or some combination thereof in order to account for the activity of surfaces in catalysing various processes has obvious disadvantages. Possibilities for a more systematic approach to the integration of collective-electron and localised-state aspects of surface structure have developed from theoretical treatments of intrinsic and extrinsic surface states, respectively. Models based on such developments, by reason of their relative novelty, have not yet been as widely applied as collective electron or active-site models to interpret catalytic activity of various surfaces and still less to considerations of sensitivity to irradiation. However, an abbreviated consideration of such surface state models is deemed essential here both as a basis for assessing their possible relevance in the explanation of radiation-induced effects and as an illustration of the integration of electronic and localised state aspects into a common framework. 

Various theoretical approaches with differing degrees of rigour are emerging for the characterisation of extrinsic surface states and their effects upon surface properties. Thus the successful application of lattice... 

Figure 2.29 Influence of semiconductor band bending (due to equilibration with intrinsic or extrinsic surface states) on the energetic position of the EDC and of the measured core-level binding energy, Ee. with respect to Ep (defining Eb = 0) the...

Among surface states, there are some that originate simply from the sudden discontinuity in the ciystal lattice these are intrinsic surface states. They are sorted, depending on their source, into two categories Tamm states, which are caused by lattice deformation, and Schockley states, caused by the unsaturated bonds on the surface. There also appears on the real surfaces extrinsic surface states due to the presence of foreign species on the surface of the solid, namely adsorbed atoms or molecules originating from a gaseous phase. 

The strategy to modify surface states, and thus the Vg and SRV, is based on interaction of chemically grafted molecules with these states. The key is to find molecules that will modify the semiconductor surface chemistry in a way that involves the surface states. In this respect, the origin of the surface states should be considered. Intrinsic surface states originate from the termination of the crystal bulk and the breaking of chemical bonds at the surface, whereas extrinsic surface states originate from crystal imperfections, such as missing surface atoms, line defects, or... 

The situation becomes even more complex if surface states, that is, additional energy levels within the bandgap, are present as illustrated in Figure 2.2. In general, two types of surface state are distinguished, that is, intrinsic and extrinsic. Intrinsic states are associated with the original semiconductor surface, whereas extrinsic surface states result from the interaction with an external ambient, such... 

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