Photo-induced creation of metastable defects in a-Si: H at low temperatures and their effect on the photoconductivity

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Abstract
Using undoped hydrogenated amorphous silicon (a-Si: H) containing ND = 6.7 × 1015cm-3 native dangling bond (DB) defects we found that metastable DB defects can be created with comparable efficiency at 4.2 K and 300 K by exposure to bandgap light. This shows that the creation mechanism is not thermally activated and that only rather local changes in bonding can be involved. The dependence of the metastable defect concentration ΔND on photocarrier generation rate G at 4.2 K is G0.44, its dependence on exposure time te is tme with m = 0.35 ± 0.01 at 4.2 K, 80 K and 300 K. For a given te there are about 0.7 times fewer defects created at 4.2 K and about 0.3 times fewer at 80 K than at 300 K. Defects created at 4.2 K have a spread of relaxation barriers. About 30% of the metastable defects anneal between 150 K and 300 K, the remainder below 460 K according to subgap absorption measurements. Strong effects of metastable defects on the photoconductivity [sgrave]p and its temperature dependence were observed. ΔND = 1016cm-3 additional metastable defects decrease [sgrave]p by two to three orders of magnitude between 150 K and 300 K. The relation between ND and [sgrave]p is not single-valued. Metastable defects having small anneal energies reduce [sgrave]p more strongly than more stable defects. We discuss how the properties of the defects depend on the way they were produced.