石墨烯沟道全自旋逻辑器件开关特性

由于石墨烯的电导率相比典型的金属材料更大,自旋弛豫时间更长,自旋轨道相互作用更弱,从而在相同的注入电流情况下,自旋电流在石墨烯材料中的耗散作用更弱.基于自旋传输和磁化动力学耦合模型,研究了石墨烯沟道全自旋逻辑器件的开关特性.结果显示,在相同的电源电压下和器件尺寸下,石墨烯沟道材料的全自旋逻辑器件磁矩翻转时间比Cu沟道更短,流入输出纳磁体的自旋电流更大.同时,长度越短、宽度越窄的沟道其开关时间更短,功耗更小.在满足磁体磁矩翻转的临界开关电流的情况下,石墨烯沟道的可靠工作长度也显著大于Cu沟道.所以石墨烯材料是相比于金属材料更理想的沟道材料.另外,通过合理选择沟道尺寸,能进一步降低器件开关时间和功耗.上述结论为全自旋逻辑器件的优化设计与应用提供了理论参考.     

高功率微波作用下热载流子引起n型金属-氧化物-半导体场效应晶体管特性退化研究

高功率微波(HPM)通过使半导体器件特性退化和功能失效,从而干扰电子系统无法正常工作. 针对金属氧化物半导体(MOS)器件的HPM效应, 建立了高功率微波引起n型金属氧化物半导体场效应晶体管(nMOSFET)特性退化的物理过程与模型. 器件仿真结果中nMOSFET的输出特性曲线显示栅极注入HPM引起器件特性退化,包括阈值电压正向漂移、 饱和电流减小、跨导减小等;结合物理模型分析可知, HPM引起的高频脉冲电压使器件进入深耗尽状态, 热载流子数目增多,热载流子效应导致器件特性退化. MOS器件的HPM注入实验结果显示,器件特性曲线、器件模型参数变化趋势与仿真结果一致, 验证了HPM引起nMOSFET特性退化的物理过程与模型.     

45 nm MOSFET毫米波小信号等效电路模型参数提取技术

为了有效地表征45 nm MOSFET毫米波频段下的电学特性,研究了其高频等效电路的建模方法。基于45 nm MOSFET的器件物理结构及其导纳参数分析,通过综合考虑器件的本征物理特性、管脚及测试寄生特性,提出了一种准静态近似的高频等效电路模型及其参数直接提取的高精度简化算法,以此来统一表征模型参数从强反型区到弱反型区的偏置依赖性,并使之在不同偏置条件下的特性表征具有良好的连续性,以便于移植到商业仿真设计自动化工具中。通过ADS2013仿真工具的散射参数模拟结果与测量数据的一致性比较,验证了所建模型的实用性及其参数提取算法的准确性, 并表征了45 nm器件的偏置依赖性。     

Enhancement of THz absorption in monolayer graphene for light at Brewster angle incidence

The enhancement of absorption in graphene for light at Brewster angle incidence is investigated. It is achieved by placing a graphene on a resonant Brewster filter that incorporating a spacer layer. The absorber presents above 50% absorption at resonance, which is attributed to the excitation of guided mode resonances. The electromagnetic field intensity distributions are illustrated to intuitively confirm the physical mechanism of such phenomenon. Moreover, the influence of geometric parameters on absorption is investigated to provide a useful guidance for practical fabrication. Besides, it is found that the absorption properties not only can be controlled by adjusting the incident angle but also can be dynamically tuned by changing the Fermi level. Last, the graphene absorption can be easily extended to multichannel by only an increase in the thickness of spacer. The results open new avenues for combining graphene with general guided mode resonance structure to enable novel optoelectronics device applications.     

Effect of graphene grain boundaries on MoS_2/graphene heterostructures

The grain boundaries of graphene are disordered topological defects, which would strongly affect the physical and chemical properties of graphene. In this paper, the spectral characteristics and photoresponse of MoS_2/graphene heterostructures are studied. It is found that the blueshift of the G and 2 D peaks of graphene in Raman spectrum is due to doping. The lattice mismatch at the graphene boundaries results in a blueshift of MoS_2 features in the photoluminescence spectra, comparing to the MoS_2 grown on SiO_2. In addition, the photocurrent signal in MoS_2/hexagonal single-crystal graphene heterostructures is successfully captured without bias, but not in MoS_2/polycrystalline graphene heterostructures.The electron scattering at graphene grain boundaries affects the optical response of MoS_2/graphene heterostructures. The photoresponse of the device is attributed to the optical absorption and response of MoS_2 and the high carrier mobility of graphene. These findings offer a new approach to develop optoelectronic devices based on two-dimensional material heterostructures.     

Graphene-based Mach–Zehnder nanophotonics interferometer working as a splitter/switch and as a multiplexer/demultiplexer

We developed and are presenting a graphene-based nanophotonic Mach–Zehnder Interferometer (MZI), which can operate as a signal follower, switch and splitter and as a multiplexer/demultiplexer. Due to the excellent electrical/optical parameters inherent to the graphene, we showed that the device we are presenting can works in several different ways, which can not be supported by MZI based on conventional materials. It is worth mentioning that the operations of the device we have developed take into account the electrical/optical parameters of the graphene, which provide greater versatility and efficiency compared to the MZIs manufactured with conventional materials. In addition, these parameters can be controlled via, for example, gate voltage, so that many operations can be performed in parallel, which is also not possible through the use of conventional materials. Due to its manometric dimensions, this MZI can be integrated within photonic integrated circuits, so that we can use this device in dense wavelength division multiplexing optical communications.     

Issues with characterizing transport properties of graphene field effect transistors

The transport properties of graphene field effect transistors are typically characterized using a conventional test structure consisting of graphene on silicon dioxide with deposited metal contacts. Two of the primary parameters affecting the total resistance of this structure are the channel mobility and contact resistance. A simple model is used to describe the impact of these parameters on total device resistance and experimentally extract them. Important issues related to characterizing the transport properties of graphene field effect transistors are presented and discussed.     

Electrical behaviors of ZnO/graphene/Si and ZnO/graphene/PS composite structures obtained by spraying different volumes of graphene solution

In this paper, a novel ZnO/graphene/porous silicon hybrid device is fabricated and its electrical behaviors are studied along with a ZnO/graphene/silicon device. Graphene (G) is prepared by exfoliation of graphite foil in aqueous solution of inorganic salt. Porous silicon (PS) is fabricated by electrochemical etching of p-type silicon (Si). Graphene is deposited on the surface of Si and PS substrates by thermal spray pyrolysis method. ZnO rods are grown on the samples by using catalyst-free chemical vapor transport and condensation method. The current–voltage relationships of ZnO/G/Si and ZnO/G/PS devices are studied under different volumes of graphene solution. The results reveal the distinctive features of the I–V characteristics of the two devices for different volumes of graphene solution under room light as well as UV illumination.     

The influence of the band structure of epitaxial graphene on SiC on the transistor characteristics

We fabricated high-mobility field-effect transistors based on epitaxial graphene synthesized by vacuum graphitization of both the Si- and C-faces of SiC. Room-temperature field-effect mobilities >4000 cm²/V s for both electrons and holes were achieved, although with wide distributions. By using a high-k gate dielectric, we were able to measure the transistor characteristics in a wide carrier density range, where the mobility is seen to decrease as the carrier density increases. We formulate a simple semiclassical model of electrical transport in graphene, and explain the sublinear dependence of conductivity on carrier density from the view point of the few-layer graphene energy band structure. Our analysis reveals important differences between the few-layer graphene energy dispersions on the SiC Si- and C-faces, providing the first evidence based on electrical device characteristics for the theoretically proposed energy dispersion difference between graphene synthesized on these two faces of SiC.     

Characteristics analysis of quantum wire infrared photodetectors under both dark and illumination conditions

This paper presents a theoretical analysis for the characteristics of quantum wire infrared photodetectors (QRIPs). Mathematical model describing this device is introduced. Maple 4 software is used to device this model. The developed model is used to investigate the behavior of the device with different values of performance parameters such as number of quantum wire layers, lateral characteristic size, and temperature. The modeling results are validated against experimental published work and full agreements are obtained. Several performance parameters are tuned to enhance the performance of these quantum photodetectors through the presented modeling. The resultant performance characteristics and comparison among both quantum well infrared photodetectors (QWIPs) and QRIPs are presented in this work. From the obtained results we notice that the total dark current in the QRIPs can be significantly lower than that in the QWIPs. Moreover, main features of the QRIPs such as the large gap between the induced photocurrent and dark current open the way for overcoming the problems of quantum dot infrared photodetectors (QDIPs).     

Graphene growth process modeling: a physical–statistical approach

As a zero-band semiconductor, graphene is an attractive material for a wide variety of applications such as optoelectronics. Among various techniques developed for graphene synthesis, chemical vapor deposition on copper foils shows high potential for producing few-layer and large-area graphene. Since fabrication of high-quality graphene sheets requires the understanding of growth mechanisms, and methods of characterization and control of grain size of graphene flakes, analytical modeling of graphene growth process is therefore essential for controlled fabrication. The graphene growth process starts with randomly nucleated islands that gradually develop into complex shapes, grow in size, and eventually connect together to cover the copper foil. To model this complex process, we develop a physical–statistical approach under the assumption of self-similarity during graphene growth. The growth kinetics is uncovered by separating island shapes from area growth rate. We propose to characterize the area growth velocity using a confined exponential model, which not only has clear physical explanation, but also fits the real data well. For the shape modeling, we develop a parametric shape model which can be well explained by the angular-dependent growth rate. This work can provide useful information for the control and optimization of graphene growth process on Cu foil.     

Study of graphene Maxwell nanofluid flow past a linearly stretched sheet: A numerical and statistical approach

This study aims to unfold the significance of numerous physical parameters such as magnetic field, heat absorption, thermal radiation, viscous and Joule dissipations, etc. on the flow of graphene Maxwell nanofluid over a linearly stretched sheet with considerations of momentum and thermal slip conditions. The prevailing mathematical equations are reformed into extremely nonlinear coupled ordinary differential equations (ODE) utilizing similarity variables and then the equations are solved numerically by the scheme of Runge-Kutta Fehlberg method along with the shooting technique. The variations in graphene Maxwell nanofluid velocity and temperature owing to different physical parameters are shown via numerous graphs whereas numerical values of skin friction coefficients and Nusselt numbers are illustrated and reported in different tables. In addition, statistical approach is followed for the multiple regression estimation analysis on the numerical findings of wall velocity gradient and local Nusselt number and are reported in tabular form to demonstrate the relationship among the heat transfer rate and physical parameters. Our results reveal that the graphene Maxwell nanofluid velocity gets reduced owing to enhancement in magnetic field, angle of inclination of magnetic field, porosity and unsteadiness parameters whereas behavior of nanofluid velocity is reversed due to Maxwell parameter. Further, it is noticed that the heat transfer rate of nanofluid is augmented owing to heat absorption, radiation and thermal slip parameters while it is reduced due to increase in viscous dissipation and unsteadiness parameters. The numerical results of the paper are validated by making comparisons with the earlier published paper under the restricted conditions and we found an excellent agreement with those results. A careful review of research papers reported in literature reveals that none of the authors has attempted this problem earlier although the thoughts and methodology explained in this paper can be anticipated to lead to enormously prolific connections across disciplines.     

Graphene/Ag2ZnSnSe4诱导p-n结薄膜太阳电池数值模拟

银锌锡硒(Ag2ZnSnSe4)是一种禁带宽度为1.4 eV的n型半导体材料.本文提出一种由n型Ag2ZnSnSe4与石墨烯(Graphene)组成的Graphene/Ag2ZnSnSe4诱导p-n结薄膜太阳电池,并借助wxAMPS软件对电池的物理机理和性能影响因素进行模拟研究.模拟结果表明,高功函数的石墨烯与n型Ag2ZnSnSe4半导体接触时,Ag2ZnSnSe4吸收层的前端能带向上弯曲,在n型Ag2ZnSnSe4吸收层表面诱导形成p型Ag2ZnSnSe4反型层,p型Ag2ZnSnSe4和n型Ag2ZnSnSe4组成p-n同质结.模拟发现石墨烯和背接触的功函数会影响载流子的分离、输运和收集,严重影响器件性能,石墨烯功函数达到5.5 eV,背接触功函数不高于4.4 eV,都有利于提高器件性能.Ag2ZnSnSe4吸收层的掺杂浓度主要影响器件的短路电流,而Ag2ZnSnSe4吸收层的体内缺陷对器件整体性能产生影响.在石墨烯和背接触功函数分别为5.5和3.8 eV,Ag2ZnSnSe4吸收层的掺杂浓度和缺陷密度分别为1016和1014 cm–3时,Graphene/Ag2ZnSnSe4诱导p-n结薄膜太阳电池能够取得高达23.42%的效率.这些模拟结果为设计新型高效低成本太阳电池提供了思路和物理阐释.     

A reconfigurable terahertz polarization converter based on metal–graphene hybrid metasurface

A metal-graphene hybrid metasurface polarization converter is designed in this Letter.The unit cell of the hybrid metasurface is composed of a butterfly-shaped structure whose branches are connected by multi-layer graphene sheets.The proposed device can be reconfigured from linear-to-circular polarization to cross-polarization by changing the Fermi energy of graphene.The simulation results show that for three-layer graphene,the device acts as a linear-to-circular polarization converter when EF=0 eV and switches to a cross-polarization converter when EF=0.5 eV.Compared with single-layer graphene,the device with three-layer graphene can maintain the cross-polarization conversion performance under low Fermi energy.Furthermore,two equivalent circuits in the x and y directions are developed to understand the working mechanism of the device.     

TCAD simulation of tunneling leakage current in CaF2/Si(111) MIS structures

We introduce a simulation technique suitable to model the tunneling leakage current in the metal(polySi)/CaF₂/Si(111) MIS structures using TCAD simulators Minimos-NT and ViennaSHE. The simulations are performed using the real physical parameters of the CaF₂/Si tunnel barrier. The results obtained for the case of near-equilibrium carrier transport are in a good agreement with experimental data and also with the simulation results yielded by our reference physical model. The obtained non-equilibrium hot-electron tunnel leakages in the hypothetical transistors with CaF₂ as a gate dielectric are comparable to those in the structures with silicon dioxide. Being an important step forward for the device application of calcium fluorite, this work opens the possibility of simulating the characteristics of different silicon-based systems with crystalline insulators.     

Spontaneous transverse field in impurity graphene

The periodic Anderson model and the average electron method are used to show that an electric field normal to an applied dc electric field can spontaneously appear in impurity-containing graphene. This effect can be related to a nonequilibrium electron subsystem in graphene. The characteristics of the spontaneous field are revealed as a function of the problem parameters.     

Thermal properties of two-dimensional materials

Two-dimensional(2D) materials, such as graphene, phosphorene, and transition metal dichalcogenides(e.g., Mo S2 and WS2), have attracted a great deal of attention recently due to their extraordinary structural, mechanical, and physical properties. In particular, 2D materials have shown great potential for thermal management and thermoelectric energy generation. In this article, we review the recent advances in the study of thermal properties of 2D materials. We first review some important aspects in thermal conductivity of graphene and discuss the possibility to enhance the ultra-high thermal conductivity of graphene. Next, we discuss thermal conductivity of Mo S2 and the new strategy for thermal management of Mo S2 device. Subsequently, we discuss the anisotropic thermal properties of phosphorene. Finally, we review the application of 2D materials in thermal devices, including thermal rectifier and thermal modulator.     

On the electronic spectrum in curved graphene nanoribbons

Using the formalism of the Dirac equation for curved space-time in the Friedmann model of a non-stationary universe, we calculate the electronic spectrum and density of states in curved graphene nanoribbons. Based on the obtained density of states we further study the current-voltage characteristics of the nanoribbonmetal tunnel junction. The dependence on the geometric characteristics of the nanoribbon has been revealed, showing a great influence of such parameters as the number of carbon atoms and the characteristic frequency of distortion.     

High efficiency solar cells with multilayered structures containing graphene

The design of antireflection coating (ARC) comprising artificial graphene for solar cells is challenging due to the new promising physical and chemical properties. Theoretical and numerical discussion of a multilayered structure solar cell model containing graphene have been presented. Transverse Electric (TE) of incident light with both normal and oblique illuminations on the proposed structure is considered. The expressions of optical fields in each region are determined by using the amplitudes of the forward and backward traveling waves. In the numerical results, the mentioned field and the excitons generation rate are computed and illustrated versus the angle of incidence, the layer thickness and the structure parameters when grapheme thickness changes.     

Design of Tunable Devices at Terahertz Frequencies Based on Quasi-Photonic Crystals Incorporated with Graphene

In this study, we present a new theoretical model including Thue-Morse and double-period sequences as quasi-photonic crystals are incorporation with graphene and investigate the transmission properties of the THz waves in both structures using a straightforward computational method. We also consider properties of nonlinear conductivity in addition to surface linear conductivity for graphene. We observe the sharp peaks and proper forbidden bands are created in the range of $0.3$ THz to $30$ THz. In addition, we find that by considering the nonlinear term of graphene and engineering the structural parameters such as the chemical potential of graphene, number of layers and the incidence wave angle, transmission levels of peaks enhance scientifically and quality factor improve considerably. These results show that it would be possible to design of high-Q tunable filters with multi-stop bands in the THz regime which can reduce the noise associated with THz frequency peaks and increase the number of sharp frequency peaks.