PHOTOINDUCED CREATION OF METASTABLE DEFECTS IN A-SI-H AT LOW-TEMPERATURES AND THEIR EFFECT ON THE PHOTOCONDUCTIVITY

Using undoped hydrogenated amorphous silicon (a-Si:H) containing N-D=6 .7 x10(15) cm(-3) native dangling bond (DB) defects we found that meta stable DB defects can be created with comparable efficiency at 4.2 K a nd 300 K by exposure to bandgap light. This shows that the creation me chanism is not thermally activated and that only rather local changes in bonding can be involved. The dependence of the metastable defect co ncentration Delta N-D on photocarrier generation rate G at 4.2 K is G( 0.44), its dependence on exposure time t(e) is t(e)(m) with m=0.35+/-0 .01 at 4.2 K, 80 K and 300 K. For a given t(e) there are about 0.7 tim es fewer defects created at 4.2 K and about 0.3 times fewer at 80 K th an at 300 K. Defects created at 4.2 K have a spread of relaxation barr iers. About 30% of the metastable defects anneal between 150 K and 300 K, the remainder below 460 K according to subgap absorption measureme nts. Strong effects of metastable defects on the photoconductivity sig ma(p) and its temperature dependence were observed. Delta N-D=10(16) c m(-3) additional metastable defects decrease sigma(p) by two to three orders of magnitude between 150 K and 300 K. The relation between N-D and sigma(p) is not single-valued. Metastable defects having small ann ealsingle-valued. Metastable defects having small anneal energies redu ce sigma(p) more strongly than more stable defects. We discuss how the properties of the defects depend on the way they were produced.