Dangling surface bond

Robertson has summarized the three recent classes of models of a-Si H deposition [439]. In the first one, proposed by Ganguly and Matsuda [399, 440], the adsorbed SiHa radical reacts with the hydrogen-terminated silicon surface by abstraction or addition, which creates and removes dangling bonds. They further argue that these reactions determine the bulk dangling bond density, as the surface dangling bonds are buried by deposition of subsequent layers to become bulk defects. 

Successive burial of hydrogen-rich surface layers leads to the formation of the a-Si H material. The large amount of hydrogen at the surface is to saturate the surface dangling bonds. The much lower hydrogen content in the bulk is due to the solubility limit of hydrogen in silicon. 

Once a particular surface structure has been determined to be stable (i.e., autocompensated), the primary factor determining the nature of the surface reconstruction is the energy that can be gained by rehybridizing the surface dangling bond charge density in response to the reduced coordination at the surface. As a consequence, a charge transfer between the atoms at the surface takes place and this results in the formation of new bonds between surface atoms adsorbed to the surface (also known as adatoms). The formation of new bonds on the surface leads to different chemical and physical properties at the surface.11... 

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 . 

Hydride surface termination has the capability for ideal surface passivation, with each hydrogen atom bonding to a single surface-dangling bond. On silicon, hydride termination has been well researched and shown to provide many advantages, including aqueous stability and limited air stability [13]. The hydride-terminated surface is also of interest as it can be used as a precursor for wet chemical reactions. 

When an oxygen anion is removed from a surface, a surface dangling bond appears, as described for Mn(CO)s, directed towards the vacancy position. In NiO this will contain two electrons. Because it is doubly occupied, in the case of coordination to Ni2+ ions, interaction with the dangling bond will be weak. However for Co3+, strong coordination of basic molecules containing occupied a lone pair orbitals becomes possible. 

An illustrative model of growth is one in which SiHg radicals both remove hydrogen from the surface and bond to surface dangling bonds. The rate equation for the concentration of surface dangling bonds... 

Fig. 10 Schematic representation of surface dangling bonds of diamond and passivation by hydrogen. ...

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