Yuhui He

Impact of inhomogeneous strain on the valence band structures of Ge-Si core-shell nanowires

We report on a theoretical study of the valence band structures of germanium-silicon core-shell nanowires based on a 6times6 kldrp model. We take into account the inhomogeneous strain effects induced by the lattice mismatches between germanium and silicon. We find that the top subband ends drift back to Gamma point, and the effective masses of more subbands begin to decrease when the shell thickness increases.

Performance Evaluation of GaAs–GaP Core–Shell-Nanowire Field-Effect Transistors

We evaluate the performance of GaAs-GaP core-shell (C-S)-nanowire (NW) field-effect transistors by employing a semiclassical ballistic transport model. The valence-band structures of GaAs-GaP C-S NWs are calculated by using a kldrp method including the strain effect. The calculations show that the strain causes substantial band warping and pushes valence subbands to move up. We demonstrate that the on current can be enhanced with the strength of strain induced in the core, but an extremely thin equivalent oxide thickness may suppress the effect of the strain-induced current improvement. The achieved results can provide a design guide for optimizing device performance.

Spin-current shot noise in mesoscopic conductors

In this paper, we present a method to investigate the spin-current shot noise in mesoscopic conductors by using scattering matrix theory and Green’s function technique. We first derive a general expression for the spin-current noise in the zero-frequency limit and extract the shot-noise component by considering the zero-temperature limit. The expression indicates that the spin-current shot noise in one lead is caused by the transmissions to the spin-resolved states in this lead and the interferences of these transmissions. As an application, we simulate the spin-current shot noise in a spin transistor and discuss its dependence on the device parameters and the bias voltages applied to the transistor. The knowledge we gain from this study will help researchers to evaluate the spin-current shot noise in future spintronic devices.

Time-Dependent Transport in Low-Dimensional Systems—A Numerical Solution Using the Nonequilibrium Green's Functions

In this paper, we present a novel numerical solution to analyze time-dependent transport in low-dimensional systems, such as one-dimensional (1-D) quantum dot and quasi-one-dimensional (Q1D) carbon nanotube systems, by using the nonequilibrium Greens functions (NEGF). The novelty of proposed approach is to jointly handle the NEGF in both the time-domain and the real-space-domain in a recursive fashion. The time-domain recursive approach is a straightforward approach to solve time-dependent transport problems, while the real-space recursive approach makes the calculations feasible for arbitrary-length 1-D and Q1D systems. To verify our proposed algorithm, we apply this method to explore the transient and ac transport properties of a sample 1-D quantum-dot array system. We will present in this paper the simulated electrical current curves, J (t), in response to various pulses and sinusoid waveforms. From these simulation results, we can obtain the delay and distortion information. We will then discuss how the length of a quantum-dot array and the hopping energy affect the transport behavior. The knowledge we gain from this project will help researchers to evaluate the electrical properties of 1-D and Q1D materials. The knowledge can also benefit the making of time-dependent 1-D and Q1D nanoelectronic devices

AC conductance of finite-length carbon nanotubes

We propose a nonequilibrium Greens function approach to calculate the ac conductance of various finite-length carbon nanotubes. The simulated ac conductance differs significantly from that for infinite-length carbon nanotubes. At the low-frequency limit, the profiles of the quantized conductance are still observable in the finite-length carbon nanotubes, but many more peaks appear on the conductance curves. We also show that the conductance of finite-length carbon nanotubes oscillates as a function of the ac frequency. The dependence of the oscillation on the lengths, helicities and defects of the carbon nanotubes are also investigated. The knowledge we gain from this research will help us make carbon-nanotube-based interconnects or other ac devices in the future.

Radial Boundary Forces-Modulated Valence Band Structure of Ge (110) Nanowire

For the radial boundary force induced in the process, the strain energy distribution and strain tensor components in Ge (110) nanowire (NW) are calculated by finite element method. Based on the strain distribution, we compute valence band structures with different radial forces. As increasing force values, top valence subbands shift downwards. The influence on the corresponding effective masses and density of states are also investigated.

GaAs-GaP core-shell nanowire transistors: A computational study

We evaluate the performance of GaAs-GaP core-shell nanowire field effect transistors by employing a semiclassical ballistic transport model and a k·p calculation of the valence band structures including the strain effect. We find that the strain will induce substantial modulation on the nanowire valence band structures and this modulation will push more conduction channels into the bias window as the shell thickness increases. We analyze its impact on the transistor performance, and our simulation results indicate that in order to achieve a good ON/OFF current ratio the epitaxial shell should be grown thin enough.

An ultra-wideband bandpass filter using EBG structure

The design and modeling of a compact microwave ultra-wideband bandpass filter (BPF) are presented, by cascading three coplanar-waveguide (CPW) resonators. Electromagnetic band gap (EBG) structures are utilized in the proposal design to pursue good RF performances. The BPF is operated from 13.5 to 25 GHz with good stop rejection and sharp roll-off frequency. Its stop rejection levels are -20 dB in both the lower and upper stop-band.

Impact of Thickness and Deposition Temperature of Gate Dielectric on Valence Bands in Silicon Nanowires

a The strain distribution and strained valence band structure in silicon nanowire with varied thicknesses and deposition temperatures of gate dielectric are discussed in detail in this work. Our calculation indicates that valence subbands are dependent on the structure and process parameters. Strain has little effects in (001) orientation. But in Si (110) nanowire, the valence subbands shift upper and warp remarkably as the gate dielectric becomes thicker. Taking thermal residual strain into consideration, the strained valence subbands go to higher energy positions compared to NW without the residual strain. The different deposition temperature by a certain process slightly influences the valence bands. Strain effects on densities of states and effective masses are also investigated.

Shell Strain Effects on Valence Band Structure and Transport Property in Ge/Si 1-x Ge x Core-Shell Nanowire

education, Beijing 100871, China *Email: gang_du@ime.pku.edu.cn;xyliu@ime.pku.edu.cn Computer Center of Peking University, Beijing 100871, China Introduction To date, Ge NW based devices are of renewed interest owing to higher bulk mobility with good process compatibility. An active area has been improving those NW devices. The realization of core-shell structures with lattice misfit strain offers intriguing chances for improving NW performance. The introduction of strain effects near interfaces can tailor valence band structures. [4] And the optimization of shell materials or compositions during NW heteroepitaxy process is possible to modulate the device property. [2] Si1-xGex as a shell based NW device is demonstrated a high performance recently. [5] However, there is a paucity of theoretical study on the strained device performance with a Si1-xGex shell in core-shell NWs. In this work, we calculate the varied strain induced by thickness or Ge contents in shell via finite element method. Then, the strained valence band structure is computed by full 6×6 Luttinger-Kohn Hamiltonian. [6-7] The hole mobility is calculated using modified Kubo-Greenwood formula. [8] The results can help to be a guideline to design NW FETs with core-shell structures. Simulation Methodology Based on continuum elastic model, [9] the Si1-xGex shell strain energy expression is as follows: