Electrical switching studies on Ge–Te–Tl chalcogenide glasses: Effect of thallium on the composition dependence of switching voltages

Abstract

Bulk Ge 17 Te 83−x Tl x glasses (0 ≤ x ≤ 13), have been found to exhibit memory type electrical switching. The switching voltages (also known as threshold voltage -V th ) of Ge 17 Te 83−x Tl x glasses are found to decrease with increasing thallium content. The rate of decrease of V th is greater at lower concentrations and V th falls at a slower rate for higher thallium concentrations (x ≥ 6). The addition of thallium to the Ge-Te network fragments the covalent network and introduces ionic nature to it; the reduction in network connectivity leads to the decrease in switching voltages with thallium content. The decrease in the glass transition temperatures of Ge 17 Te 83−x Tl x glasses with increasing thallium concentration supports the idea of decrease in network connectivity with Tl addition. The more metallic nature of Tl also contributes to the observed reduction in the switching voltages of Ge 17 Te 83−x Tl x glasses with Tl content. Further, there is an interesting correlation seen between the threshold voltage V th and the average bond energy, as a function of Tl content. In addition, the switching voltages of Ge 17 Te 83−x Tl x glasses have been found to decrease with sample thickness almost linearly. The set-reset studies indicate that the Ge 17 Te 81 Tl 2 sample can be switched for more than 10 cycles, whereas other glasses could not be reset beyond two switching cycles.

High pressure studies on the electrical resistivity of Ge–Te–Tl glasses

“…These pressures reflect a nearly linear decrease with the thallium content. Electrical switching [19] as well as ADSC and other studies [20,21] on this series of samples indicate that the thallium is present with two valencies, a trivalent thallium and a monovalent thallium. As a function of the thallium content, it is first incorporated as a Tl 3+ species but on further addition it is present as Tl 1+ .As the thallium content increases, the increasing monovalent species tends to fragment the network; this may lead to enhanced compression with the quasi-metallic transition occurring at lower pressures.…”

“…These samples were reported to undergo memory type of electrical switching. [19] Memory switching is accompanied by a phase change from amorphous to crystalline. The increase in Tl 1+ with composition was thought to aid this type of switching due its effect of fragmenting the network.…”

Thermal diffusivity measurements on ternary Ge–Te–Tl glasses using Photo-thermal Deflection method: Effect of thallium addition

“…Also, the earlier studies [22,23] reveal that thallium plays an interesting role in the Ge-Te-Tl glassy system, entering as two species with different valencies. It resides as a covalent Tl 3 + species at lower concentrations and as an ionic Tl 1 + at higher concentrations.…”

“…The effect of thallium on the glassy network has been central in understanding the composition dependence of the electrical switching and thermal parameters of Ge 17 Te 83 − x Tl x glasses [22,23]. It has been suggested that thallium enters the network with two types of valencies, as a trivalent Tl 3 + species which is covalent in nature and a monovalent Tl 1 + which is more ionic.…”

“…In the present study, thermal diffusivity measurements using the PTD method have been undertaken on Ge 17 Te 83 − x Tl x glasses (0 ≤x ≤13) to understand the factors contributing to the composition dependence of α and to see if any correlation exists between the present thermal diffusivity data and the previous electrical switching and thermal studies [22,23]. Also, the earlier studies [22,23] reveal that thallium plays an interesting role in the Ge-Te-Tl glassy system, entering as two species with different valencies.…”