Separation of Mid-IR Light Trapping and Sulfur-Donor Absorption in Nanosecond-Laser Sulfur-Hyperdoped Silicon: a Way to Spatially- and Spectrally-Engineered Interband IR-Absorption

We perform IR nanosecond laser microtexturing and hyperdoping of commercial silicon wafers under a 5 mm thick carbon-disulfide liquid layer at broadly variable laser exposures from incomplete to multi-shot irradiation per spot. All these samples demonstrate that the mid-IR-range optical densities and sulfur-donor dopant vary concentrations nonmonotonously versus laser intensity and scanning speed due to surface ablation and its vapor bubble screening. This enabled us, for the first time, to distinguish the predominating MIR-light trapping in microstructured Si at high exposures and a characteristic sulfur-donor MIR-absorption at lower exposures at rather constant sulfur concentrations. The characteristic interband sulfur-donor IR-absorption exhibits increasing magnitude for larger dissolved sulfur clusters versus increase in total sulfur concentration, paving the way for spatially- and spectrally-engineered MIR-absorption in hyperdoped Si for thin-film photovoltaics.