Metastability light-induced defects

While most of the research in metastable defect formation has focussed on light-induced defects, there has recently been growing interest in thermally generated defects. Smith and Wagner (1985 Smith et al., 1986) extended the proposed Staebler-Wronski mechanism of electron-hole recombination via band tail states, resulting in the formation of dangling... 

The role of impurities in the defect creation is also controversial. There is no doubt that the density of metastable defects increases when the concentration of oxygen or nitrogen is above about 1 at% (Stutzmann, Jackson and Tsai 1985). However, the likely explanation is that alloying changes the network disorder to allow easier defect creation, rather than the impurity being associated directly with the light-induced defect. Samples of a-Si H still show the effect even when the impurity density is greatly reduced. 

The reversible photoinduced changes in dark conductivities are consistent with metastable changes in the gap states of a-Si H, but it is not yet clear whether a unique type of light-induced defect can account for the wide range of conductivity changes that have been reported for a-Si H fUms. This question can be answered only after a more detailed characterization has been carried out on both the different photoinduced conductivity changes and the gap states. 

In the original studies of the S-W effect on undoped and doped a-Si H films, the principal characteristics of the S-W effect and the presence of metastable defects in a-Si H were established (Staebler and Wronski, 1977 Wronski, 1978 Staebler and Wronski, 1980). It was found that the large light-induced conductivity changes are a bulk phenomenon that occurs between what may be considered a thermally stable state A and a new metastable conductivity state B. State A is perfectly reproducible and is independent of previous exposures to light. It is obtained after the a-Si H film is annealed (in the dark) at temperatures above 150 C and then cooled to room temperature. The annealing time required for state A depends on the... 

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