High field behaviour of chalcogenide glasses

An important feature, which stimulated great interest in amorphous chalcogenides is related to the high field electrical response. It was 

When an electron is excited from its bound state, the hole left behind and the excited electron form a coloumbicaliy interacting metastable pair, known as exciton . 

The structure of the memory switching materials in the ON state have been examined and found to be crystalline. The crystallized portions exhibit metallic conductivity or behave like small band gap semiconductors. Correspondingly, their optical properties like 

I-V curve of the OFF state generally exhibits three different regions of resistance behaviour as the voltage increases. In the first portion, the behaviour is Ohmic, while in the second it exhibits Poole-Frenkel 

If the structural state arrived at Vth is a metastable structural state which is also metallic, it corresponds to threshold switching. But if at Vth the field induced rearrangement is such that the structure tumbles over into a deeper minimum corresponding to a crystalline state, it results in a permanent ON state. Superlinear state corresponds to an unstable structural state in this approach. The mechanism considers heating as incidental rather than as a cause for switching. 

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