Distributions of formation energies - the weak bond model

3 Distributions of formation energies - the weak bond model 

The thermodynamic model in the previous section neglects the distribution of formation and gap state energies and the specific processes by which weak bonds are converted into defects. The analysis can be reformulated to take into account the distributions by generalizing expression (6.24), 

A physical model for the formation energies is more easily addressed for undoped a-Si H because the dopants do not have to be included. The origin of the distribution is the disorder of the silicon network. Defect generation is expected to occur when a silicon-silicon bond 

The weak bond model assumes that the terms in the square bracket in Eq. (6.39) are negligible, so that 

The distribution of formation energies therefore follows the density of valence band tail states. The dangling bond energy level is estimated to be about 0.8 eV above the valence band mobility edge and the band tail extends at least 0.5 eV into the gap, so that the energies are reasonably consistent with the known defect densities. The only justification for the model is that the creation of the defect occurs without a large 

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