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

Abstract

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 density of 50kAcm−2 for an incident intensity of 75MWcm−2. For injection current produced in CdSe with circularly polarized 690nm excitation, electrons are injected with an average speed of 9kms−1; this is ∼3% of the group velocity for electrons excited with the same energy. The corresponding values for CdS excited at 410nm are 20kms−1 and 2%. From the temporal characteristics of the terahertz emission for injection currents in CdS we deduce that the electron momentum scattering time is <100fs, consistent with mobility studies.