Carrier dynamics in sulfur-hyperdoped silicon studied by time-resolved terahertz spectroscopy

Intermediate-band photovoltaics have been proposed to enhance efficiencies of solar cells by harvesting additional energy from sub-bandgap photons. One proposed method for fabricating an IB material is by introducing deep-level dopants at concentrations above the insulator-to-metal transition (IMT). Theory suggests that as the dopant states become delocalized, the non-radiative recombination is suppressed and the lifetime recovery enables photo-generated carriers to be harvested. We use optical-pump/terahertz-probe spectroscopy to study carrier dynamics of sulfur-hyperdoped silicon and test whether lifetime recovery is possible in this material system. S-hyperdoped silicon exhibits strong sub-bandgap light absorption and IMT at S concentration above $2 \times 10^{20}$ cm$^{-3}$. Previous photoconductivity study suggests the lifetime is less than 130 ps for samples at concentrations below IMT. Time-resolved THz spectroscopy is suitable for studying carrier dynamics on short time scales. We use a 400-nm fs-laser pulse to generate carriers and by monitoring the transmission of the THz probe as a function of time, we extract the carrier dynamics and mobility all-optically.