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Nonvolatile Multi-States Memories for High-Density Data Storage.

In the current information age, the realization of memory devices with energy efficient design, high storage density, nonvolatility, fast access and low cost is still a great challenge. As a promising technology to meet these stringent requirements, nonvolatile multi-state memory (NMSM) has attracted lots of attentions over the past years. Owing to the capability to store data in more than single bit (0 or 1), the storage density is dramatically enhanced without scaling down the memory cell, making memory devices more efficient and less expensive. Multiple states in a single cell also provide an unconventional in-memory computing platform beyond the von Neumann architecture and enable neuromorphic computing with low power consumption. Albeit the NMSM research has motivated and pervaded almost all existing memory technologies, there is no comprehensive review on the wide variety of NMSMs up to date. In this review, an in-depth perspective is presented on the recent progress and challenges on the device architectures, material innovation, working mechanisms of various types of NMSMs, including flash, magnetic random-access memory (MRAM), resistive random-access memory (RRAM), ferroelectric random-access memory (FeRAM), and phase-change memory (PCM). The properties and performance of these NMSMs, which are the key to realizing highly integrated memory hierarchy, are discussed and compared.