Chung-Wei Hsu

Bipolar Nonlinear

A bipolar nonlinear selector to suppress the sneak current in the crossbar array has been fabricated using a simple Ni/TiO2/Ni metal-insulator-metal structure. The highly nonlinear current-voltage characteristics are realized by the Schottky emission over the Ni/TiO2 barriers. The series connection with an HfO2-resistive memory device shows reproducible bipolar resistive switching. The maximum array size with at least 10% read margin is projected to exceed megabits. This letter demonstrates the promise of the compact one selector-one resistor (1S1R) cell structure for high-density crossbar array applications.

\hbox{Ni/TiO}_{2}\hbox{/}\hbox{Ni}

Lack of a suitable selection device to suppress sneak current has impeded the development of 4F2 crossbar memory array utilizing stable and scalable bipolar resistive-switching. We report a high-performance nonlinear bipolar selector realized by a simple Ni/TiO2/Ni MIM structure with a high current density of 105 A/cm2, and a Ni/TiO2/Ni/HfO2/Pt vertically stacked 1S1R cell capable of gigabit memory implementation. Furthermore, the demonstration of 1S1R array fabricated completely at room temperature on a plastic substrate highlights the promise of future extremely low-cost flexible nonvolatile memory.

Selector for 1S1R Crossbar Array Applications

A high-density 3D synaptic architecture based on self-rectifying Ta/TaOx/TiO2/Ti RRAM is proposed as an energy- and cost-efficient neuromorphic computation hardware. The device shows excellent analog synaptic features that can be accurately described by the physical and compact models. Ultra-low energy consumption comparable to that of a biological synapse (<;10 fJ/spike) has been demonstrated for the first time.

One selector-one resistor (1S1R) crossbar array for high-density flexible memory applications

Three-dimensional vertical resistive-switching random access memory (VRRAM) is the most anticipated candidate for fulfilling the strict requirements of the disruptive storage-class memory technology, including low bit cost, fast access time, low-power nonvolatile storage,and excellent endurance. However, an essential self-selecting resistive-switching cell that satisfies these requirements has yet to be developed. In this study, we developed a TaOx/TiO2 double-layer V-RRAM containing numerous highly desired features, including: (1) a self-rectifying ratio of up to 10³ with a sub-μA operating current, (2) little cycle-to-cycle and layer-to-layer variation, (3) a steep vertical sidewall profile for high-density integration, (4) forming-free and self-compliance characteristics for a simple peripheral circuit design, and (5) an extrapolated endurance of over 10¹⁵ cycles at 100 °C. Furthermore, the switching and self-rectifying mechanisms were successfully modeled using oxygen ion migration and homogeneous barrier modulation. We also suggest the new possibility of monolithically integrating working and storage memory by exploiting a unique tradeoff between retention time and endurance.

3D synaptic architecture with ultralow sub-10 fJ energy per spike for neuromorphic computation

We report the first demonstration of a flexible one diode–one resistor (1D1R) resistive-switching (RS) memory cell capable of high-density crossbar array implementation at an extremely low cost. A Ti/TiO2/Pt diode with a large rectifying ratio and a stable Ni/HfO2/Pt unipolar RS memory element have been fabricated on a polyimide substrate using only room-temperature processes. No significant degradation of the rectifying ratio of the TiO2 diode and the cycling variations, retention, and read disturb immunity of the HfO2 memory was observed in the bending state. The series 1D1R cell shows highly reproducible unipolar RS because of the low reset current of the HfO2 memory, which greatly mitigates the adverse effect of diode series resistance. Furthermore, the 1D1R cell can effectively suppress read interference and realize a crossbar array as large as 512 kbit.

Homogeneous barrier modulation of TaOx/TiO2 bilayers for ultra-high endurance three-dimensional storage-class memory.

To be compatible with 3-D vertical crossbar arrays, a TiO2/HfO2 bilayer resistive-switching memory (RRAM) cell sandwiched between Ni electrodes is developed. The proposed device has numerous highly desired features for the implementation of 3-D vertical RRAM, including: stable bipolar resistive switching; forming free; self-compliance; self-rectification; multiple resistance states; and room-temperature process. The resistive switching and current rectification are attributed to oxygen vacancy migration in HfO2 and potential barrier modulation of the asymmetric TiO2/HfO2 tunnel barrier. The rectification ratio up to 103 is capable of realizing a single-crossbar array up to 16 Mb for future high-density storage class memory applications.

Flexible One Diode--One Resistor Crossbar Resistive-Switching Memory

In this article, we comprehensively review recent progress in the ReRAM cell technology for 3D integration focusing on a material/device level. First we briefly mention pioneering work on high-density crossbar ReRAM arrays which paved the way to 3D integration. We discuss the two main proposed 3D integration schemes—3D horizontally stacked ReRAM vs 3D Vertical ReRAM and their respective advantages and disadvantages. We follow with the detailed memory cell design on important work in both areas, utilizing either filamentary or interface-limited switching mechanisms. We also discuss our own contributions on HfO2-based filamentary 3D Vertical ReRAM as well as TaOx/TiO2 bilayer-based self-rectifying 3D Vertical ReRAM. Finally, we summarize the present status and provide an outlook for the nearterm future.

Bipolar RRAM With Multilevel States and Self-Rectifying Characteristics

The 3D double-layer vertical RRAM with ultralow sub-μA operating current and high self-rectifying ratio over 103 has been demonstrated for the first time. This Ta/TaOx/TiO2/Ti interfacial switching device overcomes the intrinsic trade-off between operating current and variability in filamentary RRAMs and shows promising potential for high-density data storage.

3D resistive RAM cell design for high-density storage class memory—a review

We demonstrate the good-performance In0.5Ga0.5As-based metal-oxide-semiconductor capacitor (MOSCAP) on GaAs substrate using metal-organic chemical vapor deposition technique. In0.5Ga0.5As film grown on GaAs substrate is proved to be high quality with threading dislocation density as low as 106 cm-2. The performance of the MOSCAPs is comparable to that of In0.53Ga0.47As/InP-based devices grown by molecular beam epitaxy technique. The devices show a nice capacitance-voltage response, with small frequency dispersion. The parallel conductance contours show the free movement of Fermi level with the gate bias. Acceptable interface trap density Dit values of 5 × 1011-2 × 1012 eV-1 · cm-2 in the energy range of 0.64-0.52 eV above the InGaAs valence band maximum in In0.5Ga0.5As/GaAs MOSCAPs obtained by conductance methods were shown.

3D vertical TaO x /TiO 2 RRAM with over 10 3 self-rectifying ratio and sub-μA operating current

To be compatible with 3-D vertical crossbar arrays, a TiO2/HfO2 bilayer resistive-switching memory (RRAM) cell sandwiched between Ni electrodes is developed. The proposed device has numerous highly desired features for the implementation of 3-D vertical RRAM, including: stable bipolar resistive switching; forming free; self-compliance; self-rectification; multiple resistance states; and room-temperature process. The resistive switching and current rectification are attributed to oxygen vacancy migration in HfO2 and potential barrier modulation of the asymmetric TiO2/HfO2 tunnel barrier. The rectification ratio up to 103 is capable of realizing a single-crossbar array up to 16 Mb for future high-density storage class memory applications.