Derivative of Electron Density in Non-Equilibrium Green's Function Technique and Its Application to Boost Performance of Convergence

The non-equilibrium Green＇s function （NEGF） technique provides a solid foundation for the development of quantum mechanical simulators. However, the convergence is always of great concern. We present a general analytical formalism to acquire the accurate derivative of electron density with respect to electrical potential in the framework of NEGF. This formalism not only provides physical insight on non-local quantum phenomena in device simulation, but also can be used to set up a new scheme in solving the Poisson equation to boost the performance of convergence when the NEGF and Poisson equations are solved self-consistently. This method is illustrated by a simple one-dimensional example of an N＋＋ N＋ N＋＋ resistor. The total simulation time and iteration number are largely reduced.     

Schottky Barrier Formation at a Carbon Nanotube-Scandium Junction

Recent experiment shows that scandium （Sc） can make a good performance contact with carbon nanotubes （CNTs） to fabricate n-type field effect transistor （n-FET）. We study the Schottky barrier （SB） of scandium （Sc） and palladium （Pd） with a （8,0） single-wall CNT （SWCNT） using first-principles calculation. It is found that the p-type SB height （SBH） of the Pd-CNT contact is about 0.34 eV, which is in good agreement with the experimental data. For the Sc-CNT contact, an n-type contact is formed and the SBH is about O.08eV in agreement with the experimental observations. Our calculation demonstrates that by contacting CNT with Pd and Sc, p-FET and n-FET can be fabricated, respectively. The dipole effect at the interface is used to explain our result.     

Numerical Analysis of Alternating-Current Small-Signal Response in Graphene Nanoribbons

Alternating-current small-signal admittances of armchair graphene nanoribbons are investigated using the method of non-equilibrium Green＇s function. The calculated ac admittances show an oscillatory response between inductive and capacitive behaviors, which is a result of the finite length of the graphene nanoribbon. The effects of hydrogen-passivated edges on ac response are demonstrated. At low frequency, the edge effects transform the inductive behavior in a metallic graphene nanoribbon into a capacitive one. Finally, the effects of variations in the width and bandgap of a graphene nanoribbon on its dynamic response are investigated.     

考虑温度效应的沥青混合料粘弹性力学模型

提出一种同时考虑时间和温度效应的沥青混合料本构模型,基于传统的Burgers模型,得到新模型的粘弹性力学参量.采用4种温度下的实验数据对模型参数进行标定,运用新的本构模型对实验结果进行拟合,拟合曲线比传统的Burgers模型拟合曲线更加吻合实验数据,能更好地反应沥青混合料的物理力学行为,同时新模型的物理意义更明确.本模型为进一步研究沥青路面的路用性能和破坏机理提供了新的理论工具,丰富了粘弹性力学理论.     

Time-Dependent Transport in Nanoscale Devices

A method for simulating ballistic time-dependent device transport, which solves the time-dependent Sehrǒdinger equation using the finite difference time domain （FDTD） method together with Poisson＇s equation, is described in detail. The effective mass Schrǒdinger equation is solved. The continuous energy spectrum of the system is discretized using adaptive mesh, resulting in energy levels that sample the density-of-states. By calculating time evolution of wavefunctions at sampled energies, time-dependent transport characteristics such as current and charge density distributions are obtained. Simulation results in a nanowire and a coaxially gated carbon nanotube field-effect transistor （CNTFET） are presented. Transient effects, e.g., finite rising time, are investigated in these devices.     

Supercell Approach in Tight-Binding Calculation of Si and Ge Nanowire Bandstructures

Energy bandstructures of [100] oriented Si and Ge quantum nanowires with various cross-sections are calculated by using the sp^3d^5s^＊ tight-binding model with a supercell approach. Results are compared with those obtained by the first principles method （i.e., density functional theory, or DFT）. The differences in the bandstructure between silicon and germanium nanowires are analysed and it is shown that germanium keeps indirect-bandgap and the silicon nanowire along the [100] direction becomes direct-bandgap when the wire diameter shrinks. It is shown in comparison with the available experimental data that the tight-binding method is adequate in predicting the bandstructure parameters relevant to the carrier transport in mesoscopic nanowire devices and is far superior to the DFT method in terms of computational cost.     

A Closed-Form Model for Position-Dependent Potential across the Channel in DG-MOSFETs

A closed-form model for electrostatic potential distribution in the direction normal to the channel for double-gate （DG） MOSFETs is presented. The effects of doping （NA for nMOS） and minority carriers both are taken into account for the first time, in solving Poisson＇s equation analytically. Excellent agreement between model-predicted results and numerical device simulation is achieved for a wide range of body thickness, light or high channeldoping, under various bias conditions. This complete closed form for position-dependent potential distribution has wide applications for MOS compact modelling and device design.     

新型酸性离子液体催化的Knoevenagel缩合反应

研究了一类新型酸性离子液体催化的Knoevenagel缩合反应,结果表明,该催化剂适用于一系列芳香/杂环醛与α-取代活性亚甲基化合物间的反应,反应可在室温下进行,反应时间短且收率良好(92%~98%).该反应体系操作简单,产物易分离.提出了可能的反应机理并对机理进行了验证.该离子液体重复使用5次后仍具有较高的反应活性.     

Modelling of Chirality-Dependent Current-Voltage Characteristics of Carbon-Nanotube Field-Effect Transistors

Current-voltage characteristics of ballistic carbon-nanotube field-effect transistors are characterized with an iterative simulation program. The influence of carbon-nanotube chirality and diameter on the output current is considered. An analytical current-voltage expression under the quantum capacitance limit and low-voltage application is derived. Our simulation results are compared with actual measurement data.