Disclosing the basis of the ratio of the firing voltage to the threshold voltage in Ovonic Threshold Switching selectors: Urbach energy

Ovonic Threshold Switching (OTS) selectors match the phase-change memory (PCM) scaling in physical and electrical properties, showing the great potential of PCM integrated with OTS in high-density crossbar arrays. Nevertheless, OTS selectors suffer from the challenge of an initial voltage (Vfire) higher than threshold voltage (Vth) in the subsequent switching process. The high Vfire/Vth ratio causes operation problems in relation to the memory operation process in the cross-point arrays and the large overshoot current taking place upon firing switching will damage the electrical properties of device. Thus, an effective mean to rapidly identify chalcogenide materials with low Vfire/Vth ratio must be seriously considered. In this study, taking the typical OTS materials Ge100-xSex as examples, we examine the characteristics of threshold switching including the firing process. Our results reveal that Urbach energy (a sign of structural disorder) is related to the Vfire/Vth ratio. The more structure disordered the chalcogenide layer in the device, the higher the Vfire/Vth ratio, which can be explained by the fact that more conductive paths are generated after the electric firing process, so that the subsequent threshold voltage reduction is greater. We provide a pre-considered basis for screening OTS materials with low Vfire/Vth ratio.