Fundamental Investigation of Charge Transport and Memory Switching in Amorphized Phase-Change Nanowires

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Abstract:Technical: The PI will study the electrical properties of phase-change nanowires (NW) self-assembled from Ge-Sb-Te alloys, which are important materials for their use in non-volatile random access memory devices. Chalcogenide materials (e.g., Ge-Sb-Te alloys) have been dominant in the field of nonvolatile optical and electrical storage applications because of their reversible crystalline-amorphous phase transition that is signified by large changes in the optical reflectivity and electrical resistivity. The realization of these advantages in memory device applications is, however, still limited with requirements for high scalability, low-power consumption and fundamental understanding of electrical transport, threshold switching and recrystallization mechanism from the amorphous phase. These challenges motivate the design and understanding of nanostructured materials with sub-lithographic features based on bottom-up approach. During the project they will develop NW-based experiments to systematically understand fundamental properties of size-dependent nanoscale electrical switching and phase transitions that are important in order to instruct the design of future memory devices. The evolution of the phase-change properties especially for amorphous phase nanostructured glass as a function of size has not been fully explored, mostly due to the lack of material systems that can be prepared in a controllable fashion with sufficient size control and without damaging the surfaces that occurs in top-down lithographic techniques. The study of NW phase-change materials will provide valuable information on the size-scaling of the phase-change mechanism down to sub-20 nm lengthscales that cannot be easily obtained from top-down patterned systems. The proposed research will be built upon the recent breakthroughs in the PI's laboratory in the area of phase change NWs, with demonstration of memory switching and remarkable size-dependent properties. The important questions that they will seek to obtain answers are: what is the conduction mechanism in the amorphous phase and its size and composition dependence; what is the mechanism of threshold switching and nucleation from amorphous to crystalline state; what role does stress or electronic relaxations play in temporal drift behavior of the amorphous phase. The PI will combine novel synthesis, structural characterization with detailed electrical measurements to answer these intriguing questions.To accomplish the objectives, the following approach will be undertaken:1) Synthesis of complex chalcogenide nanowires with precise control over their chemical composition and size. Capping of NWs with dielectric materials to prevent surface oxidation.2) Study the conduction mechanisms in amorphous state of phase change nanowire devices.3) Temporal drift behavior of phase change nanowires in the amorphous state4) Nucleation and threshold switching and their statistics in amorphous phase nanowires.Non-technical: Bottom up approach to self assembled nanostructures presents a unique way of creating highly efficient nanosystems that will have functionalities that are not possible with any conventional technology. The development of such a memory system will have tremendous impact on a variety of applications such as cheaper and highly efficient computer random access memory systems, and ubiquitous portable devices such as ipods and digital cameras. Research and educational activities will be integrated by the involvement of undergraduates in the research program, incorporating new research results in the teaching module, and training high school and college teachers from the Philadelphia district with student population from minority and underrepresented sections.