Surface states dangling bonds

Fig. 4.15. Contours of constant charge density for Si(lll). The occupied portion of the dangling-bond surface state on Si(lll) is shown. Dots locate nuclei of surface atoms, the vacuum is above, and the charge density is in a.u.X lOL (Reproduced from Appelbaum and Hamann, 1976, with permission.)...

Figure 32. Calculated occupied densities of states for the bulk and the (100) surface of pyrite projected onto the Fe 3d orbitals, from Rosso et al. (1999a). The z axis is arbitrarily chosen to be parallel to the surface normal direction. dz2-like states are shifted to higher energy and partially depopulated at the surface. Other states with a z-component show similar trends but to lesser degrees. Nonbonding dxy states are changed very little because overlap with S 3p orbitals is unchanged in the lateral directions at the surface. The density of dx2.y2 states increases at the surface, likely indicating a shift of electron density from dangling bond dz2 states into remaining Fe-S bonds at the surface.

It should be mentioned that as well as for metals the passivation of semiconductors (particularly on Si, GaAs, InP) is also a subject of intense investigation. However, the goal is mostly not the suppression of corrosion but either the fonnation of a dielectric layer that can be exploited for devices (MIS stmctures) or the minimization of interface states (dangling bonds) on the semiconductor surface [63, 64]. 

The degree of surface cleanliness or even ordering can be determined by REELS, especially from the intense VEELS signals. The relative intensity of the surface and bulk plasmon peaks is often more sensitive to surface contamination than AES, especially for elements like Al, which have intense plasmon peaks. Semiconductor surfaces often have surface states due to dangling bonds that are unique to each crystal orientation, which have been used in the case of Si and GaAs to follow in situ the formation of metal contacts and to resolve such issues as Fermi-level pinning and its role in Schottky barrier heights. 

First, I shall describe the hydrogenation method I used and then consider the passivation of surface states and that of bulk dangling bonds, including grain boundaries, dislocations and point defects. 

The autocompensation model states that the energetically most stable surfaces are those for which all the cation-derived dangling bonds are completely empty and all anion-derived dangling bonds are completely full. Thus, this model predicts which rearrangement of atoms and which surface terminations will be stable and exist.10 Surface autocompensation is a necessary but insufficient condition for a stable structure. This means that there may be several autocompensated surfaces that are stable but not observed, presumably because some other autocompensated surfaces are more stable energetically. However, the main drawback of this model is that it cannot predict interlayer... 

Surface related properties are carrier trapping on intrinsic (due to surface dangling bonds) and extrinsic (related to adsorbates, including donor and acceptor) surface states, carrier recombination mediated by surface states [26], and mass transfer of acceptor and donor and products from/to bulk solution. 

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. 

In bulk FeSa, each Fe atom is coordinated with six S atoms and each S atom is coordinated with three Fe atoms and one S atom. Fe atom has two free electrons and S has 6 free electrons. For S atom, one electron is used to form S— S dimmer and the other 5 electrons are used to form Fe— S bond. For Fe atom, all the free electrons are shared by Fe— S bond. That is to say the electron contribution of Fe atom to each Fe— S bond is 2/6 and that of S atom is 10/6. When the crystal is cleaved from (100) directions, only Fe—S bonds is broken (see Fig. 9.6). Then the 6-fold Fe atom becomes 5-fold Fe atom and 4-fold S atom becomes 3-fold S atom. Thus, it will produce dangling bonds and surface states. 

Fig. 1.11. The nascent Si(lll) surface and its reconstruction, (a) The nascent Si(lll) surface has a threefold symmetry, with nearest-neighbor atomic di.stance 3.84 A. (b) The Si( 111) surface reconstructs immediately at room temperature to a metastable Si(lll)-2XI surface, which has a lower symmetry. Two rows of dangling bond states are formed One is filled, another is empty.

Because the bonds between the dimer Si atoms are occupied and the dangling bonds at the top-layer Si atoms are unoccupied, the STM image at positive bias and negative bias should be different. This is indeed observed (Fig. 1.15). As expected, at +1.5 V, that is, by tunneling into the unoccupied states of the Si surface, two well-separated peaks were observed on each dimer. On the other hand, at -1.5 V, that is, by tunneling from the occupied... 

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