Photogeneration of Carriers in Vitreous Selenium

The photogeneration process of mobile carriers in vitreous selenium has been studied. The number of carriers generated increases with applied field and temperature. In the high-field region, the quantum efficiency for long-wavelength light excitation (5400 Å to 6000 Å) is $\exp (\frac{\beta E^{\frac{1}{2}}}{\mathrm{kT}}-\frac{E_0}{\mathrm{kT}})$, where $E$ is the applied field, $\beta$ is a constant, $E_0$ is an activation energy, $k$ is the Boltzmann constant, and $T$ is the absolute temperature. The electric field range in which this equation applies gets smaller for short-wavelength excitation, and $\beta$ becomes wavelength-dependent. The expression is similar to that obtained for the Frenkel effect, suggesting that the photogeneration is a field-assisted thermally activated process. The activation energy $E_0$ decreases linearly with increasing photon energy of the exciting radiation and becomes a constant at higher photon energies. The departure of the quantum efficiency from the above expression in the low-field region can be attributed to a competing loss process which prevents complete collection of the generated carriers.