Weak-bond density

Equation (6.16), with a single formation energy of 0.6 eV and a weak bond density of 10 cm , predicts a defect density of 10 cm in undoped a-Si H at a growth temperature of 250 C, which is of the correct magnitude. Therefore the same parameters fit both the doped and undoped material. However, an activation energy of 0.6 eV should result in a much larger temperature dependence of than is actually observed (see Fig. 6.8). This discrepancy is also removed when the distribution of formation energies is included. 

Although the transition to difhision control is satisfactorily described in such an approach, even for these apparently simple elementary reactions the situation in reality appears to be more complex due to the participation of weakly bonding or repulsive electronic states which may become increasingly coupled as the bath gas density increases. These processes manifest tliemselves in iodine atom and bromine atom recombination in some bath gases at high densities where marked deviations from TronnaF behaviour are observed [3, 4]. In particular, it is found that the transition from Lto is significantly broader than... 

This fast removal of Si-F species can be ascribed to the weakening of the Si backbonds induced by the strong polarizing effect of F [Ubl], The weak back-bonds are then attacked by HF or H20. This reaction scheme for the dissolution process is supported by quantum-chemical calculations [Trl]. The observed dissolution valence of two for Jelectron injection current and Si-F bond density [Be22] are experimental findings that are in support of the divalent dissolution mechanism, as shown in Fig. 4.3 [Lei, Ge7, Ho6]. 

The bonding description for the 8g cation must take account of the three transannular contacts in the range 2.86-3.00 A. This structural feature is attributed to the weakly bonding 7r -7r interaction of the partially occupied 3p orbitals of the six central sulfur atoms of the eight-membered ring (Figure 12.12a). An additional interaction that contributes to the delocalisation of the positive charge onto the exo and endo sulfur atoms of 83 involves donation of electron density from the lone pair orbitals of those sulfur atoms into the empty a ... 

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