Nonvolatile Memory Devices with Colloidal Nanoparticles Assembly

The nonvolatile memory devices such as nano-floating gate memory (NFGM) and resistive memory using self-assembled colloidal nanoparticles (NPs) will be discussed in this presentation. First, three representative methods forming NPs, i.e. Volmer-Weber growth, Stranski-Krastanow growth, and chemical synthesis of colloidal NPs are compared, and the self-assembly characteristics of colloidal NPs on planar, patterned, three-dimensional nanostructure, carbon nanotubes, and nanowires are introduced. Thanks to the nanoscale dimension, the NPs can be suitably utilized as charge storage node of flash-type NFGM. The memory function were successfully obtained using selectively assembled NPs on nanopatterns. Also, the NFGM function with core-shell NPs in oxide-semiconductor thin film transistor will be discussed. In contrast to the metallic or semiconducting NPs as charge storage node, the assembly of Pt-Fe2O3 core-shell NPs formed the charge storage node and thin tunnelling dielectric layer simultaneously. The core of NPs stored the charges and the shell as thin tunnelling dielectric layer effectively prevented the dissipation of charges. Thus, the memory operation showing threshold voltage shift by charging NPs in indium-gallium-zinc-oxide thin film transistor structure could be obtained. As another example of memory device with NPs assembly, the resistive memory characteristics of Fe2O3, NiOx, Pt-Fe2O3 core-shell NPs assemblies will be discussed; e.g. digital-type bipolar switching, threshold switching, or analogue–type switching characteristics emulating potentiation and depression motions of biological synapse. The Fe2O3 NPs with Pt electrodes showed the analogue resistance change depending on the polarity of applied voltage. Also, the Pt-Fe2O3 core-shell NPs exhibited the threshold switching, defined as switching between a high resistance state at a low voltage and a low resistance state at a high voltage. Also, the multimode switching showing both the threshold and bipolar switching was also observed in the bilayered assembly with these NPs. The NiOx NPs also showed the thickness-dependent digital or analogue switching. These characteristics are distinguished from those of typical metal-oxide thin film structures. Besides, these characteristics could be obtained on flexible substrate thanks to the use of colloidal NPs assembled by dip-coating process. All of these results demonstrate the nonvolatile memory device applications with the colloidal NPs assembly.

Tae-Sik Yoon received the B.S., M.S., and Ph.D. degrees in Materials Science and Engineering from Seoul National University, Korea, in 1996, 1998, and 2002, respectively. After working as a postdoctoral researcher at Seoul National University and University of California Los Angeles and as a senior researcher at Samsung electronics, he is currently an Associate Professor at Myongji University. His research includes the semiconductor nanostructures and devices including nonvolatile memory, neuromorphic devices, thin film transistors, substrate engineering with epitaxial structure, and the nanomaterials such as colloidal nanoparticles, epitaxially grown quantum dots, nanowires and their application.