The chalcogen deep double donor sulfur (S) in natural silicon (nat-Si) has been studied extensively with optical methods in the past. Recently it was shown that the spectroscopic linewidth of shallow impurity absorption transitions is limited by inhomogeneous broadening due to a silicon isotope effect which is removed by the use of highly enriched 28-Si. In this work we extend these results to deep centers. Several different samples were prepared, allowing for a systematic identification of isotope effects. The observed isotope effects include the elimination of inhomogeneous broadening, energy shifts and the removal of satellite peaks. The S+ 1s(T2) transition exhibits a FWHM of only 0.008 1/cm for the Gamma-7 component, more than one order of magnitude sharper than in nat-Si and less than a quarter the width of the sharpest phosphorous transition in 28-Si. Hence it is the narrowest line ever seen for impurity states in silicon.
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