Quantitative characterization of carrier transport in nanowire photodetectors

Scanning photocurrent microscopy was used to study carrier transport processes in semiconductor nanowire photodetectors. Under high-level local carrier injection in CdS nanowire devices, spatially non-uniform photocurrent distributions were observed and explained in terms of bipolar transport with spatially separated electrons and holes. The mobility-lifetime product, (μτ)*, for both electrons and holes was determined in intrinsic CdS nanowire photodetectors under high-level injection. (μτ)* was enhanced compared to the bulk values as a result of the carrier spatial separation. Local time-resolved photocurrent measurements supported this interpretation of the enhanced (μτ)*. Global time-resolved photocurrent measurements were used to establish a 10%-90% rise time of ∼ 6ns (limited by the instrument response) and a 90%-10% fall time of ∼ 20ns for the nanowire photodetectors. A small fraction of the total photocurrent exhibited a long power-law decay attributed to carrier trapping/detrapping processes, and characteristic shallow trap energy of 60meV was extracted. Spatially uniform photocurrent profiles were observed in Si nanowire photodetectors under low-level injection conditions, consistent with unipolar minority carrier transport. Under varying biases, consistent variations in the photocurrent profiles were observed and attributed to the effect of the applied electric fields on drift and diffusion of minority carriers.