Multilevel Nonvolatile Memories Based on Nanowire/Molecular Wire Heterostructures
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Multilevel Nonvolatile Memories Based on Nanowire / Molecular Wire Heterostructures Chao Li, Wendy Fan,1 Bo Lei, Daihua Zhang, Jie Han,1 M. Meyyapan, 1 Chongwu Zhou Department of Electrical Engineering – Electrophysics, University of Southern California, Los Angeles, CA 90089, U. S. A. 1 Eloret Corporation, MS 229-1, NASA Ames Research Center, Moffett Field, CA 94035, U.S.A.
ABSTRACT Nonvolatile nanoscale memories with ultra-long retention times have been demonstrated for both binary and multilevel applications. These devices were based on nanowires functionalized with a selfassembled monolayer of redox active molecular wires, where the bit was represented by the charge stored in the redox molecules and the nanowire conductance was used as the readout. Our devices exhibited reliable operation, on/off ratios ~ 104 and retention times ~ one month, one of the longest retention times ever achieved with nanoscale devices. These devices were further tailored for multilevel data storage with appreciable noise margins, representing a new concept for functional devices. Our work clearly demonstrates the potential of combining nanowires and molecular wires for superior performance. INTRODUCTION The past fifty years has witnessed tremendous growth in the semiconductor industry, which has been powered by the startling down-scaling of two important devices: transistors and memories. This trend, however, may soon end due to physical and technical limitations. Continued growth of the semiconductor industry will likely rely on breakthroughs in both electronic materials and also device concepts, and extensive efforts have been devoted to address these two issues. The advance in nanomaterial synthesis has brought many novel materials to people’s attention, including carbon nanotubes [1-3], nanowires [4-6] and molecular wires [7-9]. In particular, memories based on molecular wires have been demonstrated using various configurations [8,9] and a retention time up to 15 minutes has been observed by Chen et al [8]. Bistable devices have also been constructed using nanotubes [10, 11] or nanowires [12] with retention times exceeding 20 minutes. These results represent important advances for nanoelectronics; however, the retention time achieved may not qualify for nonvolatile application, where retention times up to days and months are required. Furthermore, while these devices boast nanoscale dimensions for potential high-density data storage, they mimic conventional memory devices using bistable states. It is therefore highly desirable to explore and develop new device concepts that may blaze new trails toward high-performance data storage, such as multilevel memories. We present our approach of using nanowire / molecular wire integrated systems for binary and multilevel memory applications with ultra-long retention times. Our devices are based on nanowire (mainly In2O3) field effect transistors functionalized with redox active molecular wires. The data storage was carried out by altering the redox states of the molecular wires,
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