Impurity Absorption Spectroscopy inSi28: The Importance of Inhomogeneous Isotope Broadening

We report high-resolution infrared absorption spectra of the neutral donors phosphorus and lithium, and the neutral acceptor boron, in isotopically pure ${}^{28}\mathrm{S}\mathrm{i}$ crystals. Surprisingly, many of the transitions are much sharper than previously reported in natural Si. In particular, the $2p_0$ line of phosphorus in ${}^{28}\mathrm{S}\mathrm{i}$ has a full width at half maximum of only $4.2\mu \mathrm{e}\mathrm{V}$, about 5 times less than the narrowest $2p_0$ line previously reported for natural Si, making it the narrowest shallow impurity transition yet observed. The widely held assumptions that the impurity transitions previously reported in high quality samples of natural Si revealed the true, homogeneous linewidths, are thus shown to be incorrect. The sharper transitions in ${}^{28}\mathrm{S}\mathrm{i}$ also reveal new substructures in the boron and lithium spectra.