Photoconductivity and recombination in doped amorphous silicon

Abstract

Abstract The photoconductivity αph and its dependence on incident light intensity and temperature have been investigated in a series of doped amorphous silicon specimens at a photon energy of 2 eV. Specimens were prepared by the glow discharge decomposition of silane at a substrate temperature between 500 and 550 K. Doping was achieved by the addition of controlled amounts of phosphine or diborane during deposition. The primary aim of the work has been to explore the recombination process and its dependence on the known density of state distribution using substitutional doping to control the dark Fermi levels position over a range of 0·8 eV. It is shown that [sgrave]ph attains its optimum level when at a given temperature and intensity the steady-state electron Fermi level has been moved to an energy between 0·35 and 0·30 eV below the mobility edge, ∊ c . At this stage there also occurs a transition from predominantly monomolecular to bimolecular recombination. Both effects appear to be associated with the growing negative charge in the density of state peak lying 0·4 eV below ∊c. Changes in charge distribution with doping are discussed in some detail and shown to be the basic reason for the greatly increased electron lifetime on phosphorus doping, which leads to extremely high photosensitivity in this material.