Zhenhua Wu

The shear mode of multilayer graphene

The quest for materials capable of realizing the next generation of electronic and photonic devices continues to fuel research on the electronic, optical and vibrational properties of graphene. Few-layer graphene (FLG) flakes with less than ten layers each show a distinctive band structure. Thus, there is an increasing interest in the physics and applications of FLGs. Raman spectroscopy is one of the most useful and versatile tools to probe graphene samples. Here, we uncover the interlayer shear mode of FLGs, ranging from bilayer graphene (BLG) to bulk graphite, and suggest that the corresponding Raman peak measures the interlayer coupling. This peak scales from ~43 cm(-1) in bulk graphite to ~31 cm(-1) in BLG. Its low energy makes it sensitive to near-Dirac point quasiparticles. Similar shear modes are expected in all layered materials, providing a direct probe of interlayer interactions.

The Hartman effect in graphene

We investigate theoretically the Hartman effect in quantum tunneling through single and double barriers in a single graphene layer. The numerical results indicate that the Hartman effect in graphene depends heavily on the incident angle and the energy of the carrier in the tunneling process through single and double barriers. We find that the Hartman effect disappears for normal incidence and appears when the incident angle and energy are larger than some critical values.

Spin and momentum filtering of electrons on the surface of a topological insulator

We investigate theoretically the transport properties of Dirac fermions on the surface of a three-dimensional topological insulator. Dirac electrons can be totally reflected in front of a magnetic/electric p-n junction. For a p-n-p structure, multiple total internal reflections at the interfaces result in the bound states in the channel, which behaves like an electronic waveguide. This p-n-p like structure exhibits spin and momentum filtering features and could be used as a spin and/or charge diode.

Quantum tunneling through graphene nanorings

We investigate theoretically quantum transport through graphene nanorings in the presence of a perpendicular magnetic field. Our theoretical results demonstrate that the graphene nanorings behave like a resonant tunneling device, contrary to the Aharonov-Bohm oscillations found in conventional semiconductor rings. The resonant tunneling can be tuned by the Fermi energy, the size of the central part of the graphene nanorings and the external magnetic field.

Resonant tunneling through S- and U-shaped graphene nanoribbons

We theoretically investigate resonant tunneling through S- and U-shaped nanostructured graphene nanoribbons. A rich structure of resonant tunneling peaks is found emanating from different quasi-bound states in the middle region. The tunneling current can be turned on and off by varying the Fermi energy. Tunability of resonant tunneling is realized by changing the width of the left and/or right leads and without the use of any external gates.

A practical Si nanowire technology with nanowire-on-insulator structure for beyond 10nm logic technologies

This paper reports the design and fabrication of a practical Si nanowire (NW) transistor for beyond 10 nm logic devices application. The dependency of the DC and AC performances of Si NW MOSFETs on NW diameter (DNW) and gate oxide thickness has been investigated. A Si NW device with the scaled DNW of 9 nm and thin equivalent oxide thickness (EOT) of 0.9 nm improved both on-current and electrostatic characteristics. Finally, a Nanowire-On-Insulator (NOI) structure has been proposed to enhance the AC performance of a multiple-stacked NWs structure, which improves DC performance but has the issue of high parasitic capacitance. As a result, the simulated AC performance of a triple-NOI structure was improved by around 20% compared to conventional triple NW structure.

Reduction of RESET current in phase change memory devices by carbon doping in GeSbTe films

Phase Change Memory (PCM) has been proposed for use as a substitute for flash memory to satisfy the huge demands for high performance and reliability that promise to come in the next generation. In spite of its high scalability, reliability, and simple structure, high writing current, e.g., RESET current, has been a significant obstacle to achieving a high density in storage applications and the low power consumption required for use in mobile applications. We report herein on an attempt to determine the level of carbon incorporated into a GeSbTe (GST) film that is needed to reduce the RESET current of PCM devices. The crystal structure of the film was transformed into an amorphous phase by carbon doping, the stability of which was enhanced with increasing carbon content. This was verified by the small grain size and large band gap that are typically associated with carbon. The increased level of C-Ge covalent bonding is responsible for these enhancements. Thus, the resistance of the carbon doped Ge2Sb2Te...

In 0.53 Ga 0.47 As-Based nMOSFET Design for Low Standby Power Applications

III-V n-channel MOSFETs based on InxGa1-xAs are evaluated for low-power (LP) technology at a sub-10-nm technology node. Aggressive design rules are followed, while industry-relevant FinFET architecture is selected. We show, for the first time, quantum confinement-related leakage and performance tradeoff done self-consistently in performance evaluation using an in-house developed semiclassical tool. In this paper, we focus on In0.53Ga0.47As as the channel material, as it has been investigated heavily in the literature. Furthermore, it has a bulk bandgap EG similar to that of Ge, another highly studied complementary p-FET channel material. Higher In-content results in lower EG and hence larger band-to-band tunneling (BTBT) current, resulting in more stringent design requirements for LP applications. A comparison is done with the state-of-the-art tensile-Si (t-Si) technology, with roughly 2-GPa stress, under similar constraints LG, design rules). Thus, we show that while for 0.75 V operation, In0.53Ga0.47 As performance is limited by the BTBT and fails to outperform t-Si, it starts to perform better than t-Si below 0.7 V. VDD scaling further results in an increased performance gap between the two material systems.

Spin-polarized charge trapping cell based on a topological insulator quantum dot

We demonstrate theoretically that a topological insulator quantum dot can be formed via double topological insulator constrictions (TICs). The TICs are created by appropriate split-gate electrode patterns on the top of a HgTe/CdTe quantum well (QW) with inverted band structures. In sharp contrast to conventional semiconductor quantum dots, the presence or absence of topological insulator edge states in the proposed quantum Hall bar system leads to distinct propagating behaviors. This topological insulator quantum dot can be used as a charge and/or spin carrier trap memory element with near perfect program/erase efficiency by properly adjusting the voltages applied to the split-gates. For completeness, we also demonstrate that a small perturbation of the Rashba spin orbit interaction (RSOI) or a magnetic field in the quantum dot does not destroy the topological edge states and has negligible impact on the on-(edge)-state transport behaviors of the quantum Hall bar.

Enhancement of a cyclic endurance of phase change memory by application of a high-density C15(Ge21Sb36Te43) film

The lower cyclic endurance of Phase Change Memory (PCM) devices limits the spread of its applications for reliable memory. The findings reported here show that micro-voids and excess vacancies that are produced during the deposition process and the subsequent growth in sputtered carbon-doped GeSbTe films is one of the major causes of device failure in PCM with cycling. We found that the size of voids in C15(Ge21Sb36Te43) films increased with increasing annealing temperature and the activation energy for the growth rate of voids was determined to be 2.22 eV. The film density, which is closely related to voids, varies with the deposition temperature and sputtering power used. The lower heat of vaporization of elemental Sb and Te compared to that for elemental Ge and C is a major cause of the low density of the film. It was possible to suppress void formation to a considerable extent by optimizing the deposition conditions, which leads to a dramatic enhancement in cyclic endurance by 2 orders of magnitude in PCM devices prepared at 300oC-300W compared to one prepared at 240oC-500W without change of compositions.