This Materials World Network focuses on Non-volatile Phase Change Memory (PCM) is a promising alternative for future data storage systems, as it is more scale-able and requires less power than present technologies. Understanding and controlling the materials science of the phase change process while writing data is of particular importance, as it defines the ultimate operational speed and is fundamental to surmounting challenges with stored data accuracy and longevity. To investigate these issues, this joint effort between the University of Connecticut and Lancaster University in the UK leverages complementary expertise at both sites in nanometer- and nanosecond- scale property measurements. In addition to a semester abroad for the supported students, teaching modules on the topic will be developed for local high school teachers, and two international scientific symposia as well as a special journal issue will be organized on the topic of this sponsored research.
Present data storage systems are based on technologies that are difficult to scale, relatively slow, and/or energy inefficient given future storage requirements. Non-volatile Phase Change Memory (PCM), based on resistance changes for locally switchable crystalline vs. amorphous states, is a promising alternative. The materials science of this switching process is therefore of tremendous importance, especially the dynamics of nucleation and growth which defines ultimate switching speeds and is implicated in challenges with bit retention, fidelity, and fatigue. To investigate these issues, this joint US/UK effort leverages complementary expertise in quantitative electronic, mechanical, and thermal property measurements at the necessary nanometer and nanosecond scales. Accordingly, switching will be initiated electrically, thermally, and optically, and the switching pattern, speed, and mechanisms will be quantified by several variations of scanning probe microscopy as a function of composition, processing, and switching cycles.
The program features a semester abroad for the supported students, faculty exchanges, round-robin measurements, and extensive industrial collaboration. During the award, the PI?s will develop hands-on teaching modules for local high school teachers, jointly organize two international symposia, and co-edit a special journal issue on the topics of nanocharacterization and next-generation memory devices.
Intellectual Merit: Intellectually this work led to 4 major contributions for solid state memory systems such as phase change materials, including evidence of the fastest locally switched phase change bits with Scanning Probe Microscopy (SPM), a unique comparison of surface and subsurface effects caused by various switching regimes, and generally novel interfacial studies into materials properties, including the first direct evidence of solid state diffusion for lithography applications. The ultimate goal, to investigate switching (‘writing’ information) in phase change memory devices at commercially relevant length and time scales, was achieved by writing nanoscale dimension bits with voltage pulses of 20 nanoseconds or less. The energy threshold for switching was also identified, and property maps connected to these energetics revealed processing variations at the nanoscale which future device developers must correct for optimized memory system performance. The results into ‘writing’ information in these data storage systems also hinted at incomplete switching through the thickness, addressed by this 2nd project achievement of accomplishing the first depth-dependent images through phase-change bits written with SPM. These studies reveal distinct differences in the edges of the written bits depending on the information state (conducting or resistive), with implications for ultimate data storage system densities. More generally, several new experimental capabilities were developed to enable these studies. This includes measurements of nanoscale materials properties at a range of interfaces, including local stiffness, thermal conductivity, and switching dynamics. These results have important implications particularly for engineering more energy-efficient electronic devices. Lastly, solid state diffusion of silver ions into and out of chalcogenide films was directly observed for the first time, confirming spatially-averaged (i.e. non-local) observations. In this circumstance, 10-50 nm sized silver particles literally disappear as they ionically ‘dissolve’ into the film upon light exposure, and then reappear when the illumination is terminated. The dynamics of this process were directly observed, with relevance to new optical lithography methods and materials crucial to future semiconductor fabrication processes. Broader Impacts: This effort provided 4 broader impacts, ranging from the development of enhanced experimental capabilities for future research, to enhanced opportunities in science and engineering for students and professionals including international experiences for supported graduate students. Technically, new experimental capabilities have been developed that enable nanometer scale measurements of materials properties as a function of nanosecond duration pulses. As a result, processes critical to the optimization of commercial devices such as future solid state memory systems can be investigated with unprecedented combined spatial and temporal resolution. These are being employed in ongoing work for studies of future transistor, solar cells, and resistive memory systems. In terms of outreach, more than 100 high school students were exposed annually to materials science and engineering (MS&E) through hands-on demonstrations. This led to a substantial increase in awareness of, and interest in, MS&E as a possible college major for these youth according to pre/post surveys, an important goal due to a general lack of awareness amongst the general public of the MS&E discipline despite its central role in nearly all technological advances. For professionals, the PI organized 3 major symposia (MRS, MS&T, JSAP/MRS), and one entire conference (EMA), on the sponsored topics of nanoscale materials properties and data storage systems. As a result, more than 600 academic and industrial researchers had the opportunity to interact, share their progress, and plan future studies to improve or invent a range of current and future technologies. Finally, for the specific investigators in this effort which included a substantial collaboration with international partners at Lancaster University, UK, several exchange opportunities were achieved. This includes 3 graduate students spending several months at the partner institution, thereby learning a different educational system, approach to research, and skillset from the complementary labs which are now being implemented at both sites. The PI’s worked closely together throughout, including co-organizing two of the conference symposia, writing several joint publications, and developing ongoing and future projects ideas that stemmed from the NSF Materials World Network support.
