Excited Impurity States and Transient Photoconductivity in Cobalt-Doped Silicon

Abstract

We have studied extrinsic optical absorption and transient photoconductivity due to the two deep impurity levels that cobalt introduces in silicon. The absorption and photoconductivity spectra show that the acceptor level 0.52 eV below the conduction-band edge and the donor level 0.38 eV above the valence-band edge are two different charge states of the same cobalt center, rather than independent states. The long-wavelength optical absorption spectra and the transient response times of positive and negative photoconductivity can be explained in terms of various excited states of neutral, and of positively charged cobalt centers. These excited states lead to a modification in the kinetics of the previously proposed two-level model of negative photoconductivity.