High-resolution studies of sulfur- and selenium-related donor centers in silicon

High-resolution infrared absorption and photoconductivity spectra of several S- and Se-related donor centers in silicon are presented. These include isolated, probably substitutional, impurities and impurity pairs, as well as more complex centers. The spectra of the isolated impurities are consistent with $T_d$ symmetry and those of impurity pairs with $D_{3d}$ symmetry. The binding energies of excited states are recalculated in accordance with effective-mass theory. The results agree better with experiments than previously published calculations. The spectra are discussed in detail with emphasis on valley-orbit splittings of excited states in $T_d$ and $D_{3d}$ symmetry.