Impurity-induced enhancement of parity-forbidden optical intracenter transitions of shallow donors in silicon

Different defects in silicon crystals cause lattice distortion that may violate selection rules for certain intracenter optical transitions of hydrogen-like donor centers. Perturbations due to large concentration of substitutional donors enhance all parity-forbidden atomic transitions, but cause large concentration broadening of the transition lines, prohibiting observation of transitions at close energies. Substitutional residual carbon atoms in silicon induce, in contrast, a weak-to-moderate broadening of donor absorption lines enabling the resolution of some parity-forbidden intracenter transitions in impurity absorption spectra at moderate donor densities. Binding energies of several series of s- and d-type states were obtained by resolving of intracenter transitions terminating in these states, by low-temperature infrared absorption spectroscopy in carbon-rich and/or heavily doped silicon crystals.