Xuan Anh Tran

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}

A unified microscopic principle is proposed to clarify resistive switching behaviors of transition metal oxide based resistive random access memories (RRAM) for the first time. In this unified microscopic principle, both unipolar and bipolar switching characteristics of RRAM are correlated with the distribution of localized oxygen vacancies in the oxide switching layer, which is governed by the generation and recombination with dissociative oxygen ions. Based on the proposed microscopic principle, an atomistic simulation method is developed to evaluate critical memory performance, and successfully conduct the device optimization. The experimental data are well in line with the developed simulation method.

-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.

Oxide-based RRAM: Unified microscopic principle for both unipolar and bipolar switching

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.

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

In this letter, a bipolar resistive switching RAM based on Ni/AlO<i>y</i>/n⁺-Si which exhibits high potential to realize transistor-free operation for cross-bar array is successfully demonstrated. The proposed device shows well-behaved bipolar memory performance with self-rectifying behavior in low-resistance state (>; 700 at 0.2 V), a high on/off resistance ratio (>;10³), a good retention characteristic (>; 10⁴ s at 100 ^°C ), and a wide readout margin for cross-bar architecture (number of word line N >; 2⁵ for worst case condition).

\hbox{HfO}_{2}

In this paper, we study the effect of highly doped n⁺/p⁺ Si as the bottom electrode on unipolar RRAM with Ni-electrode/ HfO<i>x</i> structure. With heavily doped p⁺-Si as the bottom electrode, RRAM devices illustrate the coexistence of the bipolar and the unipolar resistive switching. Meanwhile, by substituting heavily doped n⁺ -Si, the switching behavior changes to that of the self-rectifying unipolar device. The asymmetry and rectifying reproducible behavior in a n⁺-Si/HfO<i>x</i>/Ni device resulted from the Schottky barrier of defect states in the SiO<i>x</i>/HfO<i>x</i> junction and n⁺ Si substrate, but this behavior is not seen for the p⁺-Si bottom electrode case. With rectifying characteristics and high forward current density observed in the Ni/HfO<i>x</i>/n⁺Si device, the sneak current path in the conventional crossbar architecture was significantly suppressed. We believe that the proposed structure is a promising candidate for future crossbar-type RRAM applications.

-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).

A High-Yield

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).

\hbox{HfO}_{x}

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.

-Based Unipolar Resistive RAM Employing Ni Electrode Compatible With Si-Diode Selector for Crossbar Integration

In this paper, the ambient doping effect on negative/positive bias temperature instability (NBTI/PBTI) of single layer graphene field effect transistors (FETs) is investigated. In ambient air, the ΔV_th of NBTI is comparable with that of PBTI under the same stress voltage at room temperature. The ΔV_th of NBTI appears insensitive to temperature, while the ΔV_th of PBTI increases significantly with rising temperatures, due to the thermally activated charging of ambient doped defects at the graphene/SiO₂ interface. This effect also results in an abnormal recovery of NBTI. In an ambient vacuum, the ΔV_th is much less than that in ambient air. In addition, the ΔV_th of both NBTI and PBTI decreases substantially for higher temperatures in the vacuum. The adsorbed molecules are mainly responsible for the ΔV_th under BTI stress and the back-gated graphene FETs in the air.