Effects of Resonant Phonon Interactions on Shapes of Impurity Absorption Lines

A study is made of the interaction of an electron bound to an impurity center in a semiconductor with the phonon field of the material. In particular we study the shift and shape of optical absorption lines of donor impurities in semiconductors when the excitation energy lies close to the energy of an optical phonon branch of the vibrational spectrum of the crystal. The results exhibit a variety of phenomena. The optical absorption lines may be split or broadened either symmetrically or asymmetrically. The different possibilities depend primarily on the dispersion of the phonon bands in the vicinity of the electronic excitation energy. If the phonon energy is approximately independent of the wave number, a splitting of the line arises. Particular emphasis is given to the case of bismuth donors in silicon interacting with transverse optical phonons.