Photo-generated shift and injection currents in wurtzite semiconductors.
Contributors: Van Driel, Henry.
Published: 2005
Language: eng
Number of pages: 102
Subjects: Physics Theses.
Description: Subsequent experiments measured both the shift and injection currents in CdSe and CdS via the emitted THz radiation following excitation by sub-ps optical pulses. Shift and injection current were created in CdSe by near bandgap optical excitation and in CdS by 3.0 eV photons. The THz radiation from both shift and injection currents in both CdSe and CdS follows the expected dependence on the optical polarization and sample orientation. All of the THz signals show the expected linear power dependence. The measured shift and injection current tensor amplitudes for CdSe at 1.78 eV are sigmazzz  = 1.5 x 10-6A V-2  and etaxzx = 1.5 x 10 8A V-2s-1, respectively. The shift and injection currents vanish for excitation photon energies below the electronic bandgap. For CdS, excited at 3.0 eV, the measured shift and injection current tensor amplitudes are sigmazzz = 8 x 10-6A V-2 and eta xzx = 4 x 108A V-2s -1, respectively.Two different types of photocurrents, shift and injection currents, are generated in the wurtzite semiconductors CdSe (Eg = 1.75eV) and CdS (Eg = 2.5eV). Shift and injection currents are electrical currents induced via a second-order optical nonlinearity in semi-conductors when excited with above bandgap optical radiation. Shift currents arise from a difference between the centre of charge for the valence and conduction states, while injection currents are generated due to a quantum interference between polarization-dependent absorption pathways.Injection currents were first measured in CdSe via silver electrodes which were deposited on the semiconductor sample surface. The voltage was measured across the electrodes when the sample surface between the electrodes was illuminated. Excitation with a 5 mW cw HeNe laser at 1.96 eV results in a voltage of 110 muV, providing an estimate of the injection current tensor element etaxzx of &sim;5 x 107A V-2s-1.The temporal behavior of the currents was measured by the THz radiation, and in principle a difference between the shift and injection currents is expected. However, the temporal resolution of the CdSe experiments is limited by the optical pulse width of 200 fs. The CdS experiments used shorter optical pulses of 60 fs, but the temporal resolution is limited by the electro-optic sampling crystals. No temporal difference between the EO traces from shift and injection current in CdS was measured. This implies that the momentum scattering time is less than 100 fs.
Notes: Thesis (Ph.D.)--University of Toronto, 2005.

Electronic version licensed for access by U. of T. users.

Source: Dissertation Abstracts International, Volume: 66-06, Section: B, page: 3189.