Phase-change electronic memory utilizes the electric field-induced reversible structural change in chalcogenide materials to switch between crystalline and amorphous phases to store information, which is fast and non-volatile. In spite of extensive investigations of the field-induce phase-change phenomena, the underlying mechanisms are quite complex and their elucidation requires the continued development of new experimental tools.
Our group has demonstrated that conventional understanding of melt-quench based amorphization process needs to be revisited. While working with single-crystalline nanowires which due to their long lengths do not typically reach high temperatures required for conventional melting of the material, we realized that alternate pathways for crystal-amorphous transformation can exist. Furthermore, nanowires due to their cylindrical geometry, conventional melting should lead to the formation of an amorphous shell around the crystalline core, which cannot explain the abrupt resistance switching as observed. We have shown that crystal-amorphous transformation in phase-change materials can be achieved through a subtle and ad lower energy costing defect-based pathway. This pathway involves creation of extended defects such as anti-phase boundaries (APBs) in GeTe and dislocations in Ge2Sb2STe5, which migrate and accumulate at a region of local inhomogeneity creating a defect template. The formation of APBs leads to polar domain inversion as revealed by optical second-harmonic generation polarimetry. Due to the accumulation of defects locally, the material first undergoes a metal-to-insulator transition followed by a structural collapse leading to amorphization without conventional melting. We utilized this understanding to pre-induce defects in the crystalline phase via controlled ion bombardment to engineer carrier localization and enhance carrier-lattice coupling in order to efficiently extract work required to introduce bond-distortions necessary for amorphization from input electrical energy itself. We demonstrated that such a strategy shows 100X improvement in amorphization current densities compared to the melt-quench strategy. The existence of multiple resistance states along with ultra-low power switching makes this approach promising for low power memory and neuromorphic computation. We will also discuss our efforts to integrate defect-engineered phase change nanowires in integrated photonics platforms for designing the next generation of reconfigurable photonic devices.
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