Limitations of the constant photocurrent method: A comprehensive experimental and modeling study

SCHMIDT, J.A.; RUBINELLI, F.A.

JOURNAL OF APPLIED PHYSICS

AMER INST PHYSICS

Año: 1998 vol. 83 p. 339 - 348

0021-8979

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 possible optical and thermal transitions between gap and extended states. Tail states are assumed to be either donor- or acceptorlike and midgap states are assumed to be amphoteric. The defect-pool model is also incorporated in our analysis. We have fitted several experimental dc-CPM spectra 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 the CPM measurement. This is due to a nonconstant electron lifetime in the energy range between 0.8 and 1.2 eV. On the other hand, we show that 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. © 1998 American Institute of Physics.