Carrier Dynamics in an Intermediate-Band Semiconductor by THz Transient Photoconductivity*

Multiband semiconductors may form the basis of efficient intermediate band solar cells, if sufficiently long carrier lifetimes can be engineered. THz Time-Resolved Photoconductivity was used to probe carrier dynamics in GaP_0.5 As_0.5-x N_x bulk intermediate-band semiconductors with x<0.05. The decay of photoconductivity after excitation is consistent with bimolecular electron-hole recombination with recombination constant r = 3 10^-8 cm³/s. This rapid recombination poses a challenge for solar energy applications. The carrier mobility is observed to decrease from 280 to 65 cm²/Vs as the nitrogen concentration is increased from x=0 to 0.036. We also observe a suppression of the low-frequency conductivity (f<1THz) that may be due to sample inhomogeneity. In these measurements a femtosecond optical pump pulse excites electron-hole pairs, and a delayed THz pulse measures the change in conductivity.