Optical Phonon-Assisted Transitions in Deep Acceptor Impurities in Silicon

The coupling between the lattice and the impurity bound carrier for deep monovalent acceptors in silicon, such as gallium and indium, is found to be stronger than for the shallower boron impurity. This suggests the possible existence of optical phonon-assisted transitions associated with these deep impurities. Such transitions are observed in the absorption spectrum of indium-doped silicon. The phonon-assisted transitions are superimposed on the photoionization continuum transitions of the indium acceptors. Interference effects between the phonon-assisted transition and the transition to the continuum states modify the position and line shape of the transitions. Using the phonon dispersion curves for silicon, interpretation of the results is presented. An estimate of the strength of the electron–phonon coupling in indium-doped silicon is obtained.