Y. C. Yeo

Transport properties of pristine few-layer black phosphorus by van der Waals passivation in an inert atmosphere.

Ultrathin black phosphorus is a two-dimensional semiconductor with a sizeable band gap. Its excellent electronic properties make it attractive for applications in transistor, logic and optoelectronic devices. However, it is also the first widely investigated two-dimensional material to undergo degradation upon exposure to ambient air. Therefore a passivation method is required to study the intrinsic material properties, understand how oxidation affects the physical properties and enable applications of phosphorene. Here we demonstrate that atomically thin graphene and hexagonal boron nitride can be used for passivation of ultrathin black phosphorus. We report that few-layer pristine black phosphorus channels passivated in an inert gas environment, without any prior exposure to air, exhibit greatly improved n-type charge transport resulting in symmetric electron and hole transconductance characteristics.

Spin–orbit proximity effect in graphene

The development of spintronics devices relies on efficient generation of spin-polarized currents and their electric-field-controlled manipulation. While observation of exceptionally long spin relaxation lengths makes graphene an intriguing material for spintronics studies, electric field modulation of spin currents is almost impossible due to negligible intrinsic spin-orbit coupling of graphene. In this work, we create an artificial interface between monolayer graphene and few-layer semiconducting tungsten disulphide. In these devices, we observe that graphene acquires spin-orbit coupling up to 17 meV, three orders of magnitude higher than its intrinsic value, without modifying the structure of the graphene. The proximity spin-orbit coupling leads to the spin Hall effect even at room temperature, and opens the door to spin field effect transistors. We show that intrinsic defects in tungsten disulphide play an important role in this proximity effect and that graphene can act as a probe to detect defects in semiconducting surfaces.

A High-Yield

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}_{x}

Ultrathin black phosphorus, or phosphorene, is a two-dimensional material that allows both high carrier mobility and large on/off ratios. Similar to other atomic crystals, like graphene or layered transition metal dichalcogenides, the transport behavior of few-layer black phosphorus is expected to be affected by the underlying substrate. The properties of black phosphorus have so far been studied on the widely utilized SiO2 substrate. Here, we characterize few-layer black phosphorus field effect transistors on hexagonal boron nitride—an atomically smooth and charge trap-free substrate. We measure the temperature dependence of the field effect mobility for both holes and electrons and explain the observed behavior in terms of charged impurity limited transport. We find that in-situ vacuum annealing at 400 K removes the p-doping of few-layer black phosphorus on both boron nitride and SiO2 substrates and reduces the hysteresis at room temperature.

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

New findings of interface trap passivation effect in negative bias temperature instability (NBTI) measurement for p-MOSFETs with SiON gate dielectric are reported. We show evidence to clarify the recent debate: the recovery of V/sub th/ shift in the passivation phase of the dynamic NBTI is mainly due to passivation of interface traps (N/sub it/), not due to hole de-trapping in dielectric hole traps (N/sub ot/). The conventional interface trap measurement methods, dc capacitance-voltage and charge pumping, seriously underestimate the trap density N/sub it/. This underestimation is gate bias dependent during measurement, because of the accelerated interface trap passivation under positive gate bias. Due to this new finding, many of previous reliability studies of p-MOSFETs should be re-investigated.

Transport properties of ultrathin black phosphorus on hexagonal boron nitride

The realization of p-n junctions in graphene, combined with the gapless and chiral nature of its massless Dirac fermions has led to the observation of many intriguing phenomena such as the quantum Hall effect in the bipolar regime, Klein tunnelling and Fabry-Pérot interferences, all of which involve electronic transport across p-n junctions. Ballistic snake states propagating along the p-n junctions have been predicted to induce conductance oscillations, manifesting their twisting nature. However, transport studies along p-n junctions have so far only been performed in low mobility devices. Here, we report the observation of conductance oscillations due to ballistic snake states along a p-n interface in high-quality graphene encapsulated by hexagonal boron nitride. These snake states are exceptionally robust as they can propagate over 12 μm, limited only by the size of our sample, and survive up to at least 120 K. The ability to guide carriers over a long distance provide a crucial building block for graphene-based electron optics.

Interface trap passivation effect in NBTI measurement for p-MOSFET with SiON gate dielectric

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).

Conductance oscillations induced by ballistic snake states in a graphene heterojunction

In this work, by using a novel HfLaO high-kappa (HK) gate dielectric, we show for the first time that with a thermal budget of 1000 degC, Fermi-Pinning in the HK-metal gate (MG) stack can be released. The effective metal work function (EWF) can be tuned by a wide range more than the requirement of bulk CMOSFETs, and also fits the future UTB-SOI CMOSFETs when Si body thickness is approaching 3 nm or less. As prototype examples, TaN gate with EWF ~3.9-4.4 eV and TaN/Pt gate with EWF ~5.5 eV are shown. In addition, by replacing HfO2 with HfLaO, high k value and low gate tunneling are maintained, BTI Vth instability is improved by one order. These new findings are correlated to the enhanced thermal stability and significantly reduced oxygen vacancy density in HfLaO compared to HfO 2 as estimated by the first-principles calculations

A Self-Rectifying

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.

\hbox{AlO}_{y}

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).