A NEW MECHANISM FOR SUPERLINEAR PHOTOCONDUCTIVITY WITH RELEVANCE TO AMORPHOUS-SILICON

The phenomenon of superlinear photoconductivity in which the photocurrent varies as a power of the photoexcitation intensity greater than one, corresponding to an increase in carrier lifetime with increasing intensity, has been well known in a variety of crystalline semiconductors for many years. Description of the phenomenon in these materials requires two kinds of competing recombination centers: (1) one with a very small capture cross section for the majority carrier and a large cross-section ratio for minority carrier (Coulomb attractive) to majority carrier (neutral or Coulomb repulsive) capture, and (2) another with a larger capture cross section for majority carriers. This article describes a new mechanism for superlinear photoconductivity that involves only a single multivalent defect, such as the metastable dangling bond defect in amorphous silicon, which can give rise to superlinear photoconductivity provided that the capture cross-section ratio of Coulomb attractive to neutral capture is sufficiently small. Careful examination of the variation of photoconductivity with photoexcitation intensity in samples suitably selected for dark Fermi level position is capable of providing additional information about the capture cross-section ratios for metastable defects in amorphous silicon.