LIMITATIONS OF THE CONSTANT PHOTOCURRENT METHOD - A COMPREHENSIVE EXPERIMENTAL AND MODELING STUDY

We present experimental and modeling results for the subgap absorption coefficient of intrinsic and doped hydrogenated amorphous silicon (a- Si:H) in order to explore the limitations of the constant photocurrent method (CPM). To properly model the subgap absorption coefficient, we have developed a simulation computer program including all the possib le optical and thermal transitions between gap and extended states. Ta il states are assumed to be either donor-or acceptorlike and midgap st ates are assumed to be amphoteric. The defect-pool model is also incor porated in our analysis. We have fitted several experimental dc-CPM sp ectra by using a standard set of parameters for the density of states. Our analysis demonstrates that in undoped samples the true absorption coefficient and the density of midgap states are underestimated by th e CPM measurement. This is due to a nonconstant electron lifetime in t he energy range between 0.8 and 1.2 eV. On the other hand, we show tha t in n-doped or p-doped samples, the CPM provides the correct answer. The energy range for the Fermi level where the CPM fails in a-Si:H is clearly established. (C) 1998 American Institute of Physics.