Fig. 106: a) Schematic drawing of a memory cross-point architecture in which each memory cell consists in a phase-change memory element stacked over an OTS selector device at crossing between Word-line and Bit-line of the 3D memory array. b) Typical I(V) characteristic of an OTS selector device.

When increasing the applied electrical field on the OTS material, at Vth (threshold voltage) the chalcogenide glass switches from its initial highly insulating state (OFF-state) to a metallic state (ON-state). After switching in the ON-state, upon decreasing voltage, this conductive state can be

maintained down to the holding current (Ihold). Under Ihold, the OTS material recovers its insulating OFF- state. One major parameter of OTS material is the leakage current (Ileak) at Vth/2 that directly defines

the maximum achievable size of the memory array in a cross-point architecture.

ELECTRONIC STRUCTURE, MAGNETISM AND DYNAMICS

122 ESRF

the crystalline phase of chalcogenide-based phase-change materials [4]. Such MVBs are thus responsible for a huge change in electronic density of states (eDoS) accompanied by a drop in the electronic conductivity of the materials. In addition, this OTS model establishes, for the first time, the common link between chalcogenide materials belonging to the

structural rearrangements of the amorphous phase during the application of high electric fields. In particular, the excitation of glasses under a high electric field leads to the alignment of certain bonds (see molecules displayed as insets in Figure 107) and the appearance of local structural patterns reminiscent of the new metavalent bond (MVB) recently described in

Fig. 107: EXAFS spectra obtained at the (a) Ge, (b) Se and (c) Sb K-edges for four prototypical GeSe (Sb, N) OTS thin film samples plotted with different colour lines. AIMD instantaneous snapshots of amorphous Ge30Se70 OTS glass in the (d) pristine and (e) excited state (Ge: red; Se: yellow). The increase of conductivity upon electric-field application in OTS materials can be explained by strong delocalisation of electronic states around EF wave functions (shown as blue isocurves).