Shallow impurity absorption spectroscopy in isotopically enriched silicon

Inhomogeneous broadening due to isotopic randomness in natural Si has been shown to cause a broadening of many of the ground-state to excited-state infrared-absorption transitions of the shallow donor phosphorus and acceptor boron. Previously, it had been thought that the observed linewidths of shallow impurity transitions in silicon were at their fundamental lifetime limit. We report improved high-resolution infrared-absorption studies of these transitions in new samples of isotopically enriched ²⁸Si, ²⁹Si, and ³⁰Si. Some of the transitions in ²⁸Si show the narrowest linewidths ever reported for shallow donor and acceptor absorption transitions, and many higher excited states are now observed. The improved samples of ²⁹Si and ³⁰Si result in revised values for the dependence of shallow donor and acceptor binding energies on the average Si mass.