Investigations of the photoconductivity in hexagonal selenium single crystals

The photoconductance of three hexagonal Se single crystals has been investigated by measuring the d.c. current in the direction of the c-axis with the voltage across the crystal maintained at 10 V. The spectral dependence of the photoconductivity has been determined at 80 ºK and 300 ºK. The observed dependence of the steady state photoconductance delta-G* on the quantum flux F (light intensity divided by quantum energy) at large íntensities can be expressed as delta-G* - F# where # is a constant between 0.15 and 0.29. The decay of the photoconductance was always very slow and nonexponential. The rise depended very much on the light intensity used. Using a small intensity at 80 K an S-type photoconductance rise curve delta-G (t) = delta-G* (1 - e-(t-t0)/tau) was obtained with values for the delay time t0 and the time constant tau as large as several hours. Thermally stimulated trap emptying has been studied during the warming after illumination at 80 ºK. The temperature corresponding to the maximum of the thermally stimulated conductance curve decreased continuously from 175 ºK to 95 ºK with increasing intensity of the previous excitation. Optical quenching has been observed during the decay at 80 K with quantum energies from 0.8 to 1.65 eV when the quantum energy of the primary excitation was 1.85 eV. The results are in good agreement with the barrier model proposed for selenium by Stuke. However, owing to the many unknown parameters in this model the results obtained cannot prove its validity