Ultrafast shift and injection currents observed in wurtzite semiconductors via emitted terahertz radiation

Shift and injection currents are generated in the wurtzite semiconductors CdSe and CdS at 295K using above-band-gap (ℏω>Eg) femtosecond pulses and detected via the emitted terahertz radiation; the optical beams are normally incident on samples with the optic axis in the plane of the surface. For optical intensities up to 75MWcm−2 (or carrier density <1018cm−3) the terahertz radiation amplitude shows the expected linear dependence and also varies with optical polarization and sample orientation consistent with the third-rank tensors that govern the current generation processes in the wurtzite structure. The largest shift currents are generated along the optical axis for light polarized along that axis. In CdSe with ℏω=1.80eV (690nm), the electron shift distance is ∼40% of the 0.25nm bond length and the peak current density is 5kAcm−2 for an optical intensity of 10MWcm−2; for CdS the corresponding experiment at ℏω=3.0eV (410nm) gives a shift distance ∼80% of the 0.26nm bond length with a peak current densit...