Z. Fang

Physical mechanisms of endurance degradation in TMO-RRAM

We report, for the first time, three types of endurance failure behaviors in TMO based RRAM. New physical mechanisms are proposed to clarify the physical origins of these endurance failures. A physically-based optimized switching mode is developed to improve the endurance of TMO-RRAM. A significantly enhanced endurance of >10⁹ switching cycles was demonstrated in the HfO<inf>x</inf>/TiO<inf>x</inf>/HfO<inf>x</inf>/TiO<inf>x</inf> devices.

Temperature Instability of Resistive Switching on

In this letter, the temperature instability of HfOx -based resistive switching memory is investigated. It is observed that with the increase of high temperature (up to 100 °C in this work), the leakage current of high-resistance state would increase, and the set/reset voltages would decrease. In addition, multibit switching exhibited at room temperature might not be retained with the increase of temperature. All these phenomena can be correlated with oxygen-vacancy-related trap formation and annihilation.

\hbox{HfO}_{x}

In this work, the impact of Ti electrodes on the electrical behaviour of HfO2-based RRAM devices is conclusively clarified. To this aim, devices with Pt, TiN and Ti electrodes have been fabricated (see Fig. 1). We first provide several experiments to clearly demonstrate that switching is driven by creation-disruption of a conductive filament. Thus, the role of TiN/Ti electrodes is explained and modeled based on the presence of HfOx interfacial layer underneath the electrode. In addition, Ti is found responsible to activate bipolar switching. Moreover, it strongly reduces forming and switching voltages with respect to Pt-Pt devices. Finally, it positively impacts on retention. To support and interpret our results we provide physico-chemical measurements, electrical characterization, ab-initio calculations and modeling.

-Based RRAM Devices

Atomic layer deposited (ALD) HfO2 resistive-switching random access memory devices with high uniformity, self-compliance, and forming-free behavior are demonstrated. Through comparative experiments, we find that appropriate deposition techniques and annealing conditions lead to self-compliance. The forming-free behavior originates from the oxygen deficiency due to the metal doping layer. High uniformity, by first-principle calculation, is caused by Ge doping in the HfO2, which lowers the oxygen-vacancy formation energy.

Experimental and theoretical study of electrode effects in HfO 2 based RRAM

The retention failure of bipolar oxide-based resistive switching memory is investigated. A new physical model is proposed to elucidate the typical retention failure behavior of the oxide-based resistive switching memory with a sudden resistance transition, which is quite different from that of the traditional memories. In the new proposed model, the temperature- and bias-dependent failure probability and failure time of the devices can be quantified. A temperature- and voltage-acceleration method is developed to evaluate the retention of resistive switching memories.

Highly Uniform, Self-Compliance, and Forming-Free ALD

In this letter, a resistive random access memory based on Ni electrode/HfOx, dielectric/n+ Si substrate structure is demonstrated, which can be integrated with Si diode as selector for application in crossbar architecture. The unipolar device shows well-behaved memory performance, such as high ON/OFF resistance ratio (>; 103), good retention characteristics (>; 105 s at 150 °C), satisfactory pulse switching endurance (>; 105 cycles), and a fast programming speed of about 50 ns. More importantly, it also exhibits almost 100% device yield on a 6-in wafer.

\hbox{HfO}_{2}

Based on the retention failure mechanism of high resistance state due to reconstruction of oxygen vacancy filament in the rupture region, a physical model is proposed to quantify the retention failure behavior of oxide-based RRAM devices, supported by experiments. A new data retention evaluation methodology is proposed to predict the failure probability and lifetime of the memory devices.

-Based RRAM With Ge Doping

In this paper, we report a high performance, forming-free and self-rectifying unipolar HfOx based RRAM fabricated by fab-available materials. Highlight of the demonstrated RRAM include 1) CMOS technology friendly materials and process, 2) excellent self-rectifying behavior in LRS (>103 @ 1 V), 3) forming-free unipolar resistive switching, 4) wide read-out margin for high density cross-point memory devices (number of word-line >106 for worst case condition).

Modeling of Retention Failure Behavior in Bipolar Oxide-Based Resistive Switching Memory

In this letter, a unipolar resistive switching random access memory (RAM) based on NiSi/HfOx/TiN structure is demonstrated, which is compatible with NiSi S/D in advance CMOS technology process. Highlights of the demonstrated resistive RAM include the following: 1) CMOS-technology-friendly materials and process; 2) excellent self-rectifying behavior in low-resistance state (>; 103 at 1 V); 3) well-behaved memory performance, such as high on/off resistance ratio (>; 102) and good retention characteristics (>;105 s at 125 °C ); and 4) wide readout margin for high-density cross-point memory devices (number of word lines 106 for the worst case condition).

A High-Yield

In this letter, we report positive bias-induced V_th instability in single and multilayer graphene field effect transistors (GFETs) with back-gate SiO₂ dielectric. The ΔV_th of GFETs increases as stressing time and voltage increases, and tends to saturate after long stressing time. In the meanwhile, it does not show much dependence on gate length, width, and the number of graphene layers. The 1/f noise measurement indicates no newly generated traps in SiO₂/graphene interface caused by positive bias stressing. Mobility is seen to degrade with temperature in- creasing. The degradation is believed to be caused by the trapped electrons in bulk SiO₂ or SiO₂/graphene interface and trap generation in bulk SiO₂.