Photoluminescence of Isotopically Purified Silicon: How Sharp are Bound Exciton Transitions?

We report the first high resolution photoluminescence studies of isotopically pure Si ( $99.896\%{}^{28}\mathrm{Si}$). New information is obtained on isotopic effects on the indirect band gap energy, phonon energies, and phonon broadenings, which is in good agreement with calculations and previous results obtained in Ge and diamond. Remarkably, the linewidths of the no-phonon boron and phosphorus bound exciton transitions in the ${}^{28}\mathrm{Si}$ sample are much narrower than in natural Si and are not well resolved at our maximum instrumental resolution of $\sim 0.014{\mathrm{cm}}^{-1\mathrm{}}$. The removal of the dominant broadening resulting from isotopic randomness in natural Si reveals new fine structure in the boron bound exciton luminescence.