Edge luminescence spectra of acceptors in Si: Implications for multiexciton complexes

We present high-resolution photoluminescence data for Si doped with Al, Ga, and In. We observe emission lines due to recombination of electron-hole pairs in bound excitons and satellite lines which have been interpreted in terms of complexes of several excitons bound to an impurity. The bound-exciton luminescence in Si:Ga and Si:Al consists of three emission lines due to transitions from the ground state and two low-lying excited states. In Si:Ga, we observe a second triplet of emission lines which precisely mirror the triplet due to the bound exciton. This second triplet is interpreted as due to decay of a two-exciton complex into the bound exciton. Similar results are found for Si:Al and Si:In. The energy of the lines shows that the second exciton is less tightly bound than the first in Si:Ga. Other lines are observed at lower energies. The bearing of these results on the question of the existence and properties of bound multiexciton complexes is discussed.