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we have systematically investigated the energetics of metastable defect creation via hydrogen rebonding in order to shed some light on the microscopic origin of the defect creation process in a-Si:H [3]. Surprisingly, we found that the defect formation energy is determined by the bond-length deviation of the weak Si-Si bond. This indicates that the bond length disorder may be the dominant factor in controlling the total defect density. This qualitative information enables us to evaluate the equilibrium and the light-induced defect density, under reasonable assumptions of the distribution of the Si-Si bond lengths. In this paper, we develop the rate equations of defect generation utilizing the distribution of H-induced defects, and taking into account the two most important processes: generation and annealing of defects by both thermal and light excitations. The equilibrium defect density and its energy distribution agree well with experimental results, including the small activation energy. The magnitude of the predicted saturated light-induced defect density at room temperature is around 1017, when the experimental defect generation rates are used, also in reasonable agreement with experimental results for device quality a-Si:H thin films. DEFECT FORMATION ENERGY We study the role of hydrogen in creating metastable defects, by transferring the H from a Si-H site to a weak Si-Si bond leaving behind a dangling bond (Si*) and producing a 407

Mat. Res. Soc. Symp. Proc. Vol. 377 0 1995 Materials Research Society

H-defect complex at the weak Si-Si bond. This is schematically the reaction, Si - H + Si - Si=

. Si * +Si - H- Si.

(1)

This fundamental process has been proposed as a mechanism for defect equilibration in a-Si:H networks by Smith and Wagner [(4and Street and Winer [5]. We have performed tight-binding molecular dynamics calculations of the formation energy of reaction (1). The structural configurations resulting from H-insertion are fully relaxed with a steepest descent minimization procedure. The calculated defect formation energy includes the large relaxation energy involved in the H-insertion process. This relaxation energy would not be properly included in previous approaches where the formation energy is approximated from differences in one-electron levels [4]. The calculations are done on computer-generated a-Si:H models, with a range of Si-Si bond-lengths.

Tight-binding models of Si-H interactions that describe well properties of

H in c-Si and a-Si:H have been utilized [6]. The primary conclusion is that the formation energy of the defect reaction scales almost linearly with the bond-length of the weak Si-Si bond, and the formation energy is E -_Eo - aAR

(2)

where AR is the Si-Si bond length deviation from the average value. Statistical analysis of the calculations indicate an energy E0 = 2.5eV and a 6.3eV/A. In normal length Si-Si bonds the defects have a high formation energy (larger than 2.0 eV). However for severely elongated Si-Si bonds (AR > 0.3A), the formation energy is less than 0.8 eV-