Effective carrier lifetime in ultrashort pulse laser hyperdoped silicon: sulfur concentration dependence and practical limitations

Charge carrier lifetime is a crucial material parameter in optoelectronic devices and knowing the dominant recombination channels points the way for improvements. The effective carrier lifetime τ _eff of surface-passivated hyperdoped (hSi) and nonhyperdoped “black” (bSi) silicon by quasi-steady-state photoconductance decay (QSSPC) measurements and its evolution upon controlled wet-chemical etching are studied. Sample preparation involves the irradiation of Si by numerous ultrashort laser pulses either in SF ₆ for hSi or ambient atmosphere for bSi. Findings suggest that the hSi is composed of a double layer: 1) an amorphous resolidified top layer with about 92% of the total incorporated sulfur that accounts for the sub-bandgap absorptance and 2) a crystalline layer underneath in which sulfur concentration tails off toward the Si substrate. The effective lifetime is deconstructed by a 1D simulation to quantify the impact of the local lifetime of the defect-rich top layer, τ _top. It is found that by the QSSPC method, a maximum τ _top for 1) can be estimated. For 2), τ _top between 2 and 8 ns is estimated. The bSi sample shows a faster lifetime recovery upon etching which suggests that in hSi samples purely laser-induced defects are not limiting the carrier lifetime compared to sulfur-related defects.