Photoluminescence Spectroscopy of Polyexcitons and Thermally-Generated Defects in Silicon.

The analogy between excitons and positronium atoms has been useful in developing our understanding of solid state physics, and the existence of stable multi-electron -hole-pair species in semiconductors was postulated in 1958. Using photoluminescence near twice the band-gap energy, we made the first observation of free polyexcitons (n > 2) in a semiconductor and obtained the molecular binding energies of triexcitons and tetraexcitons in Si. Thermal annealing of oxygen-rich Si near 450 ^circC produces a variety of defects, including the well-known thermal donors seen in infrared absorption. Photoluminescence spectroscopy has been used to identify these donors, and to study in detail two centres believed to be their remnant neutral cores. The two very similar isoelectronic centres are unique in that they can bind up to four excitons.