Electrical switching and topological thresholds in Ge-Te and Si-Te glasses

Melt-quenched $Ge_xTe_100-x$ glasses and $Si_xTe_100-x$ glasses (15 \leq x \leq 25) have been found to exhibit memory switching, with threshold fields of the order of 4-11 kV/cm and 6-25 kV/cm, respectively. It is found that the switching voltages of $Ge_xTe_100-x$ samples increase linearly with Ge content and the composition dependence of threshold voltage $V_t$ shows a marked slope change at x=20, which has been earlier identified as the rigidity percolation threshold (RPT) of the system. Above the RPT, $V_t$ of Ge-Te glasses continues to increase with composition until the boundary of bulk glass formation (x=28). On the other hand, the switching voltages of $Si_xTe_100-x$ glasses increase with x, exhibiting abroad maximum around x=20 (RPT). The difference in the composition dependence of $Si_xTe_100-x$ and $Ge_xTe_100-x$ glasses has been understood on the basis of separation between the rigidity percolation threshold and the stoichiometric threshold $(CT_ST)$ in these samples. The present results also indicate that the turn around in the composition dependence of $V_t$ and the subsequent minimum observed in the switching voltages of chalcogenide glasses is likely to be due to $CT_ST$ and not to the chemical ordering threshold $(CT_COCRN)$.