Lowest energy sites

The energy surface for H° was discussed in Section III. 1. The bond center was the lowest energy site, 0.3 eV lower in energy than the T site (Van de Walle etal., 1989). The corresponding defect levels were schematically illustrated in Fig. 6. 

From the discussion of location of hydrogen and muonium in the lattice, it is clear that the bond-center position is the lowest-energy site in many semiconductors. In GaAs and Si, this site has only recently been experimentally associated with anomalous muonium, which is relatively immobile (Kief et al., 1987, 1988). The agreement between theoretical results for hyperfine parameters (Van de Walle, 1990) and experiment allows an unambiguous identification of Mu with the bond center. Other models, such as the vacancy model proposed by Sahoo etal. (1985, 1989), are no longer considered acceptable candidates for Mu. ... 

Small organic molecules doped into polymer binders have also been employed in DEED devices. Coating is done using inexpensive solution techniques. A commonly used polymer is polyvinylcarbazole [50-52], which is a hole transport material with moderate mobility. Polysilanes have also been used as hole transport agents [53]. Common laser dyes may be employed as the emissive molecular dopant, for example coumarins [50, 51], rhodamines [50] or pyrans [51, 53]. Similar considerations of energetics and concentration apply to doped polymers as to small molecule dopant-host systems the dopant should provide the lowest energy site for the exciton and concentration quenching should be minimized. 

In close packed metal systems there are two sites available, see Fig 6.18 the constricted tetrahedral sites or the more spacious octahedral sites. However, even after only partly filling the lowest energy sites, the... 

Table reproduced with permission from Taylor [136]. National Association of Corrosion Engineers. Reference states are the bare Fe( 110) surface and (a) H2 for H, (b) Qj for d, (c) NH3 for NH with H2 energies used for hydrogen balance, and (d) H2O for OH with H2 energies used for hydrogen balance. Preferred (lowest energy) sites are highlighted in bold face. 

Numbering based on crystallographic ordering specified by Newsam et al. " Relative energy with respect to the lowest energy site found in this study (in this case, A11-03). 

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