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 state 4) Nucleation and threshold switching and their statistics in amorphous phase nanowires.
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.
We studied how electronic memory devices using a new class of memory materials called phase change materials work. There is an ever increasing demand to make memory devices that can be extremely fast, very small and be able to store information even after the power is removed from the gadget. However, it has been extremely challenging to make any memory device that have all these three attributes. For example, Flash memory (used in current phones, cameras, thumb drives etc) are very small but extremely slow. Therefore, there is a need to find new materials which can solve this problem. A particular class of memory, call phase change memory is one such candidate. It works by changing the arrangement of atoms in the device from an ordered manner (crystalline with low elelctrical resistance to current) to a random disordered manner (amorphous with higher resistance to current) by applying pulses of current. We have studied how this transformation of atoms inthe material from an ordered array to disordered state takes place and how current flows in these two states. These results are important to optimize the memory devices fabricated from phase change materials so that they replace flash memory, which will allow vast amounts of data to be downloaded or uploaded on a much fatser timescale. Intellectual Merit The research extended across several frontiers of science and engineering, with the main focus on systematically studying the electronic memory properties of phase change memory that will be extremely small (10,000 time smaller than human hair thickness) extremely fast, can retain information for much longer time and be integrated with any technology. The results from thes project have lead to new insights into the size-dependent phase change memory device switching properties of materials down to sub-20 nm length scales ( 1 nm is a billionth of a meter). The results obtained from the measurements on Ge-Sb-Te NW memory devices especially at sub-50 nm size, will be a benchmark for phase-change materials and help us understand as tp how these devices work in order to optimize their performance. Broader Impact Nanotechnology presents a unique way of creating highly efficient systems that will have functionalities that are not possible with any conventional technology. The results will impact the development of an ideal, universal memory device that will be non-volatile, ultra-dense, have the ability to read, write and erase data at fast speeds and operate using extremely low power consumption. 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 was 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.
