Deep electron traps in As2Se3 single crystals studied by photoconductivity and TSC measurements

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Abstract
The dependence of the steady-state photoconductivity on temperature, illumination and photon energy has been investigated over wide ranges. At low temperatures, excitonic effects reduce the photosensitivity near the absorption edge. Thermal dissociation of the excitons points to a binding energy of 50–70 meV. Measurements of thermally stimulated currents reveal three efficient electron traps, at 0·41 eV, 0·47 eV and 0–6 eV, of densities in the range of 1016−1017 cm−3. The high density of traps leads to a temperature-independent photoconductivity for T < 200 K, whereas at higher temperatures the shift of the Fermi level through these traps results in a sublinear dependence of the current on illumination, and in a photosensitivity that increases with temperature. Non-stationary photocurrents are studied in the sub-band gap region. Filling of the traps at low temperature extends the photosensitivity down to 0·6 eV. Comparison of optical and thermal excitation of trapped electrons points to considerable electron-phonon coupling with polaron energies of 0·2 eV and 0·4 eV for the traps at 0·41 eV and 0·60 eV, respectively. The traps and recombination centres are discussed in view of recent considerations on bonding defects in chalcogenides.