FERMI-LEVEL EFFECTS ON PHOTOCONDUCTIVITY DURING OPTICAL DEGRADATION OF AMORPHOUS-SILICON

During optical degradation and anneal of hydrogenated amorphous silicon, it is often observed that the photoconductivity changes by a larger factor and/or with a different dependence on time than the density of dangling bond defects measured by the constant photoconductivity method (CPM). It is the purpose of this paper to demonstrate that these effects can be described, for a set of degradation measurements at four different temperatures, by a simple model of photoconductivity with only one type of multivalent defect, when full recognition is made of the effects of varying Fermi level position with time because of the creation of additional dangling bond defects during the optical degradation. The lifetime decreases when the Fermi level approaches the negatively charged defect level (D(- /0)) from above, and the apparent defect density decreases as measured by CPM as the Fermi level moves down below the D(-/0) defect level. Although a total description of all photoconductivity effects might require the consideration of other types of defect, the introduction of other types of defect or defect level is not needed in the cases considered.