Thex charge transport mechanisms and photoconductivity of nanoporous granular CdS-CdSe
I. A. Ryzhikov1 ( ), A. L. Rakhmanov1, Y. V. Trofimov2
1Institute for Theoretical and Applied Electrodynamics RAS, 13/19, Izhorskaya, Moscow, 127412, Russia.
2Institute of Electronics National Acad. Sci. of Belorus, 220090 Minsk, Belorus.
The unusually high value of the photoconductivity effect in the nanogranular materials attracts the attention of a number of researchers. In some of such materials the ratio of "bright" conductivity, $\sigma_{ph}$, to "dark" conductivity, $\sigma_0$, achieves the values of the order of $10^3$ - $10^6$ and even higher. However, some specific features of photoconductivity in nanogranular materials is still a puzzling problem. Among such problems we would like to note a large difference between the building-up time $t_1$ of the "bright" conductivity and the relaxation time $t_2$ of the "bright" conductivity (usually $t_1 << t_2$). It should also be mentioned that the temperature dependence of the photoconductivity, $\sigma_{ph}(T)$, is rather unusual. This value increases with $T$ for some materials, which is natural for the semiconductors. However, there exists a number of systems in which $\sigma_{ph}(T)$ rapidly decreases with temperature. In the present research we study the photoconductivtiy of CdS-CdSe granular system in the temperature range 20 - 120${}^o$C. It was observed that the dependence $\sigma_{ph}(T)$ steeply decreases with $T$ for the studied samples and has a significant hysteresis. The value of $\sigma_{ph}(T)$ exhibits a large "training" effect in the course of the temperature cycling with the saturation after 3 - 4 cycles. The observed features of photoconductivity behavior are interpreted qualitatively accounting for the nanopores existing in the studied samples.
Key words: photoconductivity, temperature dependence, granular material, nanopores