Trapping Processes in Amorphous Selenium

The theory of small-signal current transients is applied to the study of electron trapping processes in amorphous selenium. In the experiment, a ${10}^{-8\mathrm{}}$-sec light pulse illuminated one side of the sample and produced free carriers near this surface. The free electrons were drawn across the sample in an applied electric field, and the shape of the current transients thus produced was studied. The shape of these transients indicated that electron trapping processes in vitreous selenium involve three distinct species of trap: those which control the mobility ($m$ traps), a deep trapping level ($d$ traps), and a shallow trapping level ($s$ traps). Magnitudes are given for the ratio of the $m$-trap density to the density of states in the conduction band ($\frac{N_m}{N_c}$) and for the ratio of the $s$-trap density to the density of states in the conduction band ($\frac{N_s}{N_c}$). For a selenium film which has been evaporated onto a substrate held at 38°C during the evaporation, these ratios are $\frac{N_m}{N_c}=5.2\mathrm{}\times {10}^{-4\mathrm{}}$ and $\frac{N_s}{N_c}=2.4\mathrm{}\times {10}^{-6\mathrm{}}$, respectively. It is further shown that the magnitude of these ratios decreases as the substrate temperature at which the samples are prepared is increased. The energy separation of the $s$ level and the $m$ level from the conduction-band edge depends also on the sample preparation, the separation increasing as the substrate temperature is increased. For the film prepared at 38°C, the levels are at $E_s=0.39\mathrm{}$ eV and $E_m=0.29\mathrm{}$ eV, respectively, below the conduction-band edge. The capture probability for both the $s$ and $d$ traps was measured and was found to increase exponentially with $\frac{1}{T}$ with a characteristic energy. It is suggested that the electron trapping processes in vitreous selenium are closely connected with the structural properties of the material. This is strongly indicated by a decrease in the shallow-trap density as the measuring temperature approaches the glass transition temperature.