Possible atomic structures for the sub-bandgap absorption of chalcogen hyperdoped silicon

Single-crystal silicon wafers were hyperdoped respectively by sulfur, selenium, and tellurium element using ion implantation and nanosecond laser melting. The hyperdoping of such chalcogen elements endowed the treated silicon with a strong and wide sub-bandgap light absorptance. When these hyperdoped silicons were thermally annealed even at low temperatures (such as 200~400 oC), however, this extra sub-bandgap absorptance began to attenuate. In order to explain this attenuation of absorptance, alternatively, we consider it corresponding to a chemical decomposition reaction from optically absorbing structure to non-absorbing structure, and obtain a very good fitting to the attenuated absorptances by using Arrhenius equation. Further, we extract the reaction activation energies from the fittings and they are 0.343(+/- 0.031) eV for S-, 0.426(+/-0.042) eV for Se-, and 0.317(+/-0.033) eV for Te-hyperdoped silicon, respectively. We discuss these activation energies in term of the bond energies of chalcogen-Si metastable bonds, and finally suggest that several high-energy interstitial sites instead of the substitutional site, are very possibly the atomic structures that are responsible for the sub-bandgap absorptance of chalcogen hyperdoped silicon.