Spectroscopy of excitons and shallow impurities in isotopically enriched silicon—electronic properties beyond the virtual crystal approximation

Recent high resolution spectroscopic studies of excitonic and impurity transitions in high-quality samples of isotopically enriched Si have dramatically expanded our understanding of the effects of isotopic composition on the electronic properties of semiconductors. Prior to these studies on Si, the results for other semiconductors, focusing mainly on the isotopic dependence of the electronic band gap energy EG in the T→0 limit, could all be explained within the virtual crystal approximation (VCA), in which only the average isotopic mass was relevant. Remarkably, not only were the effects of isotopic randomness observable in natural Si (when compared to enriched 28Si), but the random isotopic distribution present in natural Si was found to be the true source of what had been thought of as ‘fundamental’ spectroscopic limits in Si, including the linewidths of bound exciton emission lines and impurity absorption transitions, and the ‘intrinsic’ acceptor ground state splitting. Many of these transitions are far narrower in highly enriched 28Si than in the most perfect natural Si, challenging existing spectroscopic methods, and opening up new possibilities for precision measurements, and for the observation of new physics.