Impurity Absorption Spectroscopy in ${}^{28}\mathrm{S}\mathrm{i}$: 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.

Figure 1

Four representative donor absorption lines observed in ${}^{28}\mathrm{S}\mathrm{i}$ at a sample temperature of 1.8 K are shown. (a) The transition from the ground state of the P donor to the $2p_0$ excited state, with an observed FWHM of $0.034{\mathrm{c}\mathrm{m}}^{-1}$. (b)–(d) Absorption transitions from the Li donor ground state to the $2p_0$, $2p_{\pm }$, and $4p_{\pm }$ excited states, respectively. (a)–(d) All have an identical energy width of $0.8{\mathrm{c}\mathrm{m}}^{-1}$. The spectra were collected at an instrumental resolution of $0.014{\mathrm{c}\mathrm{m}}^{-1}$ FWHM.

Figure 2

Absorption lines 4, 4B, and 6 of the B acceptor are compared between Si (top) and ${}^{28}\mathrm{S}\mathrm{i}$ (bottom). Lines 4 and 6 are significantly narrower in ${}^{28}\mathrm{S}\mathrm{i}$, revealing a new, identical doublet structure, with a splitting of $0.15{\mathrm{c}\mathrm{m}}^{-1}$. The 4B transition is broad in both spectra, indicating the dominance of excited state lifetime broadening for this state, even in natural Si. The middle spectra show the excellent agreement between the ${}^{28}\mathrm{S}\mathrm{i}$ spectra, convoluted with the previously calculated [14] isotopic broadening of the B ground state, and the natural Si results.