Study of the temperature dependent trap-dominated conduction in GeSe and GeSe₂ Ovonic threshold switching selectors

Abstract

The highly non-linear current–voltage characteristics of Ovonic threshold switching selectors make them a key component in novel 3D crossbar memory architectures. The sub-threshold conduction and switching mechanism have long been a point of debate but it is generally accepted that they are trap-assisted in nature. In this paper, we study the sub-threshold conduction of both pristine and switched amorphous GeSe and GeSe2 devices across a wide temperature range spanning from up to 353 K down to 133 K. By applying a simple Poole–Frenkel model, insight can be gained into the average trap depth and concentration and how these are affected by temperature, composition, and layer thickness. It is found that the average trap depth is temperature dependent, suggesting a distribution of traps in the mobility gap with only very shallow traps contributing to the current at low temperatures. GeSe layers tend to have a significantly larger amount of traps than the GeSe2 layers, in line with the hypothesis that wrongly coordinated Ge bonds are the dominant source of trap states. For thinner films, there are larger deviations from the simple Poole–Frenkel model for conduction, indicating an increasingly important effect of the interface barriers as the layer thickness decreases. After first-fire switching, the sub-threshold conduction changes drastically. The simple Poole–Frenkel model can still be applied, but quantitative information on the number of traps can no longer be reliably obtained.