Dark and photogenerated ac conductivity inAs2S3single crystals

There has been continuing speculation that the origin of frequency-dependent ac conductivity commonly observed in amorphous insulators may lie in some broad spectrum of relaxation processes specifically characteristic of long-range disorder. However, ac conductivity exhibiting this type of ${\omega }^s$ ($s\sim 1$) behavior is observed in both amorphous and single crystal ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ as well as in anthracene crystals, suggesting the familiar picture of losses controlled by electronic dissipation in localized states, which need not arise from disorderinduced potential fluctuations but can simply reflect the existence of defects, impurities, and microinterfaces. Combining accompanying $b$-axis dielectric measurements on ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ with optical data enables us to estimate for the low-frequency dielectric constants (12.1, 5.9, 10.6) along the $a$, $b$, and $c$ axes, respectively. Analysis of photogenerated ac conductivity in ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ under conditions of bulk illumination leads to an estimated $\mu \tau$ product in the ${10}^{-8\mathrm{}}$-${10}^{-9\mathrm{}}$-${\mathrm{cm}}^2$/V range while comparison with drift-mobility data leads to a nominal estimate of 100 nsec for the recombination lifetime. It is demonstrated that the shape of spectral dependence of photogenerated ac conductivity can vary with ac bias frequency. Finally ac conductance and capacitance in ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ under illumination with strongly absorbed light are found to exhibit dc bias-field dependence which appears to depend on crystal thickness.