光電所所訊

Dual-functional Memory and Threshold Resistive Switching Based on the Push-Pull Mechanism of Oxygen Ions

Professor Si-Chen Lee’s laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所李嗣涔教授

The resistive random access memory (RRAM) becomes more and more important as the candidate to overcome the technological limitations for next-generation non-volatile memory. We have developed a special device structure (a-TiO_x)/(Ag NPs)/(poly-TiO_x) sandwiched between indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) electrode as shown in Fig. 1 (a) which performs dual functions, i.e., memory switching (RRAM device) and threshold switching (Selector device). When used for memory switching, the device is forming-free with low operation voltage (< ±1 V) and self-compliance to current up to 50 μA as shown in Fig. 1(b). This reduced the damage of forming process on the device endurance. When the device is used for threshold switching as shown in Fig 1(c), the reduction of the threshold currents to -2 μA and 0.1 μA in negative and positive voltage regions, respectively, is beneficial for improving the device selectivity to solve the sneak current problem in the crossbar memory arrays. It is found that the push and pull actions of the oxygen ions in the top amorphous and bottom polycrystalline TiO_x films due to voltage sweeps can account for the dual-functional properties in a single device structure, as illustrated in Fig. 2.

Fig. 1. Device structure and electrical properties of the dual functional switching device.

Fig. 2. Push-pull mechanism of oxygen ions migration in the memory switching and threshold switching modes.

Y. J. Huang, S. C. Chao, D. H. Lien, C. Y. Wen, J. H. He and S. C. Lee, “Dual-functional Memory and Threshold Resistive Switching Based on the Push-Pull Mechanism of Oxygen Ions”, Nature Sci. Rep. 6, 23945, 2016.

Multi-Bent-Section Nano-Antennas

Professor Hung-chun Chang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所張宏鈞教授

Nano-antennas are a class of nano-structures that can produce “hot spots” in a subwavelength gap of the structure due to plasmonic effects under incident electromagnetic fields and have been extensively investigated in the visible and infrared wavelength range both theoretically and experimentally. Bowtie nano-antennas or bowtie antennas (BAs), as depicted in Fig. 1, have been a popular structure. The resonant wavelength λ_res of the BA is proportional to the antenna length. For a given footprint size, achieving longer λ_res is useful in applications within the near- or mid-infrared range. We have recently proposed multi-bent-section nano-antennas (MBSNAs) which are dipole nano-antennas with multi-bent-sections and demonstrate that such simple structures can achieve long λ_res’s with high gap fields. The study was based on our in-house developed 3D finite-difference time-domain (FDTD) method [IEEE Photon. J. 7, 4501108 (2015)]. Referring to the BA of Fig. 1 as the basic structure, the bowtie-shaped gold film with thickness d is supported by a silica substrate with relative permittivity of 2.25, and it is air above the structure. The function of the gold is described by the Drude model [Phys. Rev. B. 71, 085416 (2005)]. The antenna footprint area is kept the same to be 450 nm × 450 nm, with the thickness d fixed to be 36 nm and the antenna gap area fixed to be 30 nm × 30 nm. Four MBSNAs with different shapes of the gold region as shown in the top of Fig. 2, each with the same d and the gap area as aforementioned, have been simulated. Every section of each antenna is with the same width of 30 nm. It can be seen the antenna structure is very simple even for the fourth one. The results of Fig. 2 is to be compared with the reference BA for which the resonance occurs at l = 1260 nm and the corresponding gap-field enhancement is 31. The proposed structures are also with wide bandwidth in the enhancement spectrum. By numerically solving the modal effective indices of the plasmonic waveguide having its cross-section as that of the gold line with the substrate of the MBSNA, the λ_res’s for the bent gold-line waveguide length were determined and found to be in good correlation with those of the peaks in Fig. 2. (Yu-Tsung Chen and Hung-chun Chang, in Proc. OPTIC 2015.)

Figure 1. Schematic of the 3-D representation for a bowtie nano-antenna.

Figure 2. Upper part: four different shapes of the gold region of the MBSNAs. Lower part: Gap enhancement factor versus the wavelength for the four MBSNAs. Each MBSNA has the same footprint size of 450 nm × 450 nm.