Surface dangling state

In addition to the electron energy bands and impurity levels in the semiconductor interior, which are three-dimensional, two-dimensional localized levels in the band gap exist on the semiconductor surface as shown in Fig. 2-28. Such electron levels associated with the surface are called surface states or interfacial states, e . The siuface states are classified according to their origin into the following two categories (a) the surface dangling state, and (b) the surface ion-induced state. 

Fig. 2-30. Surface dangling states and surface ion-induced states (a) surface dangling donor (DL-B) and acceptor (DL-AB) leveb on covalent bonding semiconductors, (b) surface cation-induced acceptor (SCL) and surface anion-induced donor (SAL) levels on ionic bonding semiconductors.

On semiconductors that are partially ionic and partially covalent, such as transition metal oxides, the surface ion-induced and the surface dangling states may coexist together. 

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. 

However, if the molecules of 5 had R alkyl chains longer than Me, the steric hindrance prevented 100% substitution and IR examinations indicated a 50% less derivatization. Moreover, XPS analysis showed that the surface is partly modified by substitution of hydrogen by halogen . In the case of 5 with X = I and to some extent X = Br, the formation of X radicals (besides 12) in a secondary reaction was reported . They participate in reactions analogous to equations 21 and 22b, but with X instead of 12, and attach to the Si surface improving the electronic passivation of the surface at defect sites, sterically inaccessible to 12. A possibility that surface dangling bonds may also appear in the charged states was discussed as well . 

The optical band gap of silicon clusters increases with decreasing cluster size and all surface atoms with dangling bonds contribute to mid-gap electronic states (defects) which are assumed to quench photoluminescence. Saturation of the surface dangling bonds with hydrogen is the simplest way to eliminate defects, and therefore theoretical studies of the electronic structure of silicon clusters without defects are most easily performed on hydrogenated silicon clusters. The thermodynamic stability of these species is also of interest, because hydrogenated silicon clusters are formed in thermal and plasma CVD of silane. ... 

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.

The heterostructure problem, which has been widely studied in the bulk form, is becoming an increasingly popular topic also in the one-dimensional systems. The band alignment problem has been addressed recently in a DFT study conducted by Kagimura and coworkers (2007) where a Si-Ge interface was studied. The model for such a system is shown in O Fig. 27-8. The band states contributed by the surface dangling bonds were investigated as possible candidates for the induction of a potential well. 

Kagimura, R., Nunes, R. W., 8c Chacham, H. (2007). Surface dangling-bond states and band Lineups in Hydrogen-Terminated Si, Ge, and Ge/Si nanowires. Physical Review Letters, 98, 026801. 

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