Structural and electrical characteristics of a hyperdoped silicon surface layer with deep donor sulfur states

Inhomogeneous hyperdoping of a 100-nm-thick silicon surface layer with sulfur atoms at concentrations above $2\times10^{21}\,$cm$^{-3}$ was obtained via its femtosecond laser ablation in a sulfur-containing organic solvent. Infrared transmission spectroscopy reveals distinct interband absorption peaks of donor sulfur states, which are absent in the initial crystalline silicon, and a broad absorption band of free carriers with a concentration of ${\sim}\,10^{18}\,$cm$^{-3}$. The rather low free-carrier concentration is related to equilibrium room-temperature ionization of localized donor sulfur states, preserving their nondegenerate character owing to the strong electronion binding in the donor states.