Impurity absorption spectroscopy of the deep double donor sulfur in isotopically enriched silicon

The chalcogen deep double donor sulfur (S) has been studied extensively with optical methods in natural silicon ( Si nat ) . Recently, it was shown that the spectroscopic linewidths of absorption transitions from shallow impurities like boron and phosphorus is limited by inhomogeneous broadening due to a Si isotope effect which is removed with the use of highly enriched 28Si. In this work we extend these results to deep centers and show that the true homogeneous linewidths of many transitions can only be revealed in isotopically enriched Si. The S + 1 s ( T 2 ) Γ 7 transition exhibits a full width at half maximum (FWHM) of only 0.008 cm - 1 in 28Si—more than one order of magnitude sharper than in Si nat , and substantially narrower than the sharpest transitions of B and P in 28Si. Hence, it is the narrowest absorption line ever seen for impurity states in semiconductors. We also report dramatically narrower absorption transitions for the neutral S 0 center, together with higher excited states than previously observed. The isotope shifts of S absorption transitions between Si nat , 28Si and 30Si are also reported.