具有双周期势的石墨烯超晶格电子带隙和传输特性的研究

运用传输矩阵理论,研究了双周期势石墨烯超晶格(GSL)的电子带隙和传输特性。结果表明,这种准周期结构存在一个新的狄拉克点,并且它的位置恰好位于零平均波数带隙处;与传统的布拉格带隙相比,这种带隙的位置对晶格常数和入射角度的变化不敏感。讨论了入射角度对电子在双周期势中传输特性的影响,预言了双周期势中可控的电子传输过程,定量分析了允带宽度与周期总数目的幂指数关系。     

含有一个方势垒单层石墨烯的隧穿特性

利用传递矩阵的方法研究了含有一个方势垒的单层石墨烯的隧穿特性,得到了透射概率与入射粒子费米能以及势垒宽度、势垒高度的变化关系,并且计算了势垒的结构参数及入射粒子费米能对低温电导的影响.这些结果可以为设计基于石墨烯材料的纳米器件提供理论参考.     

反平行磁电垒结构中二维电子气体的输运性质

 本文对反平行磁电垒结构中二维电子气体的输运性质进行了详细的研究.利用传递矩阵的方法计算了电子气体隧穿磁电垒结构的传输概率和电导率,讨论了电势垒高度的变化对传输概率和电导率的影响.结果表明,该系统中不存在自旋过滤和自旋极化现象,电子气体可实现理想的共振隧穿,波矢过滤等.     

带有AB环的T型结构中的电子输运性质

采用一维量子波导理论研究了带有AB环的类T型结构中的电子输运。在满足波函数的连续性以及电流的闭合性的情况下,得出了透射系数的解析表达式,并根据此表达式分析了电子的输运特性。结果显示：左右端的透射系数随磁通的变化产生典型的AB振荡。当改变中间的入射点的位置X时,左出射端的透射系数的大小随之改变,右输出端除 附近外,均产生与磁通无关的全反射现象。入射点位置X的变化对透射系数的影响出现一种周期性的调制现象。体系结构轻微的偏离对系统的输运特性影响不大。     

Resonant tunneling in graphene microstructures

We present results for the resonant electronic transmission through graphene-based single and double barriers as a function of the incident wave vector, the widths and heights of the barriers, and the separation between them. Resonant features in the transmission result from resonant electron states in the wells or hole states in the barriers and strongly influence the ballistic conductance of the structures.     

基于石墨烯的太赫兹光电功能器件研究进展

作为一种新型光电材料,石墨烯独特的能带结构和电子输运特性,使其与太赫兹科学有着密切的内在关系：石墨烯内部的等离子体振荡频率在太赫兹频段;人为调谐石墨烯的禁带宽度在0~0.3 eV时,正好覆盖太赫兹频段;光电导率的外部可控性等,这些特点使得石墨烯有望成为太赫兹频段新一代高性能设备研制的基础。最近的研究显示,石墨烯在太赫兹波产生、调控、检测等光电功能器件的研制中取得了很好的成果。重点介绍了基于石墨烯的太赫兹光电功能器件,包括太赫兹源器件、可控调控器件及检测器研究的最新进展,并对这一快速发展的研究领域进行了展望。     

The Mott diode as a heterodyne receiver element

This paper considers a novel doping profile for Schottky barrier mixer diodes called the Mott barrier. The structure consists of a metal-semiconductor junction in which the semiconductor's epitaxial layer is very lightly doped and thin enough so that it remains depleted even under substantial forward bias. It has been proposed that Mott barrier diodes will generate less noise and have lower series resistance-junction capacitance products than standard Schottky diodes, thus increasing the sensitivity and cut-off frequency of heterodyne receivers. In this paper, the band structure and electron transport properties of the Mott diode are evaluated. This analysis shows that the Mott diode actually will have a large series resistance-junction capacitance product and excessive hot electron noise, making it a poor candidate for high-frequency applications. Experimental results are presented which substantiate these conclusions.     

电子在一维方势垒顶部的透射

文中对入射电子能量等于一维方势垒高度时，电子对势垒的透射情况进行了理论分析，导出了这一条件下的电子透射系数与势垒高度及势垒宽度之间的关系，并证明了电子对一维方势垒的透射系数Ｔ是入射电子能量Ｅ的连续函数。     

负－零－正折射率超常介质平板结构中的透射横向位移特性分析

针对光波在负－零－正折射率超常介质平板结构中的传播,利用稳态相位法研究了光学Dirac点附近的透射横向位移特性。讨论了横向位移随入射角度、频率和平板厚度的变化关系,发现该结构中的横向位移可以达到波长的几十甚至几百倍之多并且在Dirac点附近能够实现正负变化。进一步研究了全反射情形下的横向位移特性和光子隧穿现象,证实了横向位移的Hartman效应。另外,由于材料的特殊线性色散,发现横向位移在靠近临界角时,随着角度增大而减小;而在远离临界角时随着角度增大而增大。研究结果将在集成光学和光学器件方面产生应用可能,也将进一步促进石墨烯量子结构中电子传播的类光学现象研究。     

Features of electron transport in relaxed Si/Si1 − x Ge x transistor heterostructures with a high doping level

The low-temperature electrical and magnetotransport characteristics of partially relaxed Si/Si_{1 ? x} Ge _x heterostructures with an electron conduction channel in an elastically strained nanoscale silicon layer are investigated. It is demonstrated that the electron gas in the system exhibits 2D properties. A dependence of the conductivity along layers in the system on the degree of elastic-stress relaxation in it is observed. To understand the observed regularities, the potential and the electron distribution over the structure layers are calculated in detail for samples with different layer strains and doping levels. For the structure with x = 0.25, the parameters of the potential barrier and characteristics of the quantum well formed in the Si layer are estimated. It is established that the characteristics of the potential formed near interfaces strongly depend on the initial parameters of the system, in particular, on the degree of the plastic relaxation of elastic stresses and on the doping level. The formation of a thin tunneling-transparent barrier near the upper interface can lead to the redistribution of electrons between the 2D and 3D conduction channels in the structure, which ensures the spread of the measured transport characteristics of the samples during the measurements. The interlayer tunneling transitions of carriers from the 2D state in the Si transport channel to the 3D state of the Si_{1 ? x} Ge _x crystal matrix, which are separated by a tunneling-transparent potential barrier near the heterointerface, were observed for the first time during transport in the direction transverse to the layer plane.     

Energy oscillations in electron transport across a triangularbarrier

Carrier transport across the semiconductor space-charge region of a silicon triangular barrier diode was investigated by a Monte Carlo simulation. Oscillations of the electron mean kinetic energy are observed as a function of position along the uphill slope of the barrier under bias. At a given point on the uphill slope, the energy distribution function shows an oscillatory behavior, with a periodicity corresponding to the optical phonon energy. These oscillations are shown to be due to the nonequilibrium dynamics of the electron interaction with optical phonons in the situation when other inelastic electron scattering processes are negligible. The energy oscillations are superimposed on a smooth cooling of the distribution in the transport toward the top of the barrier, as current flows through the system. A comparison with the thermionic theory quantifies the importance of nonequilibrium effects in short-range electronic transport     

Effect of transverse constant electric field on the electron interference effects related to the under-barrier penetration of quantum-mechanical current density in 2D semiconductor nanostructures

The effect of transverse constant electric field F on the under-barrier penetration of the local quantum-mechanical current density in a 2D semiconductor structure is theoretically studied. The structure represents two quantum wells with identical widths that are sequentially located along the propagation direction of electron wave: the first well has the rectangular cross section, and the second well exhibits a semi-infinite rectangular potential barrier with height V ₀ that is modified by the transverse electric field. When an electron wave whose energy is less than resulting height of the potential barrier V _eff is incident from the first well on the barrier under certain conditions, the coordinate-dependent exponentially decaying penetration of the local quantum-mechanical current density may take place under the barrier due to the interference of electron waves in the nanostructure. It is demonstrated that the penetration parameters depend on field strength F.     

Bottom-Up Synthesized Nanoporous Graphene Transistors

Nanoporous graphene (NPG) can exhibit a uniform electronic band gap and rationally-engineered emergent electronic properties, promising for electronic devices such as field-effect transistors (FETs), when synthesized with atomic precision. Bottom-up, on-surface synthetic approaches developed for graphene nanoribbons (GNRs) now provide the necessary atomic precision in NPG formation to access these desirable properties. However, the potential of bottom-up synthesized NPG for electronic devices has remained largely unexplored to date. Here, FETs based on bottom-up synthesized chevron-type NPG (C-NPG), consisting of ordered arrays of nanopores defined by laterally connected chevron GNRs, are demonstrated. C-NPG FETs show excellent switching performance with on–off ratios exceeding 10⁴, which are tightly linked to the structural quality of C-NPG. The devices operate as p-type transistors in the air, while n-type transport is observed when measured under vacuum, which is associated with reversible adsorption of gases or moisture. Theoretical analysis of charge transport in C-NPG is also performed through electronic structure and transport calculations, which reveal strong conductance anisotropy effects in C-NPG. The present study provides important insights into the design of high-performance graphene-based electronic devices where ballistic conductance and conduction anisotropy are achieved, which could be used in logic applications, and ultra-sensitive sensors for chemical or biological detection.     

Green‐Function Calculations of Coherent Electron Transport in a Gated Si Nanowire

We describe a detailed numerical scheme to calculate electron transport in quantum wires using the Green function formalism combined with tight‐binding orbital basis. As an example of the application, we study the electron transport in a Si nanowire containing a finite potential barrier. The effects of nonzero bias, temperature, and disorder on the barrier‐induced oscillatory conductance are investigated within the context of coherent transport model. The oscillatory behavior of the conductance as a function of the Fermi energy is found to be highly sensitive to sample disorder and limited to a very low temperature and a small bias range.     

Enhanced gas barrier properties of graphene-TiO2 nanocomposites on plastic substrates assisted by UV photoreduction of graphene oxide

Reduced graphene oxide (rGO) sheets have received great attention as a key element for thin barrier films that block the permeation of water vapor and other gases. However, it remains a challenge to prepare the rGO-based barrier films on plastic substrates through a chemically benign and low temperature fabrication route. Toxic chemicals or high temperature thermal treatments that are widely used for preparing rGO need to be avoided because they can damage the underlying plastic substrates. In this study, we report the fabrication of rGO/TiO₂ composite films via an eco-friendly and low temperature ultraviolet (UV) photoreduction process and demonstrate their enhanced gas barrier properties by measuring water vapor transmission rates (WVTRs). When photocatalytic TiO₂ nanoparticles are employed, UV exposure reduces the GO/TiO₂ composite solution to form rGO/TiO₂, which is subsequently deposited on plastic substrates. The rGO/TiO₂ composites become resistant to water absorption because the UV photoreduction of GO/TiO₂ effectively removes most polar groups on the GO sheets. We confirmed that rGO/TiO₂ composites were successfully deposited onto the plastic substrate through a solution process and the barrier films led to a substantial reduction in WVTRs of the substrate. Our strategy for preparing graphene-based thin barrier films by using a UV photoreduction process enables the fabrication of solution-processed graphene-based encapsulation layers on plastic substrates with an eco-friendly and low temperature fabrication method.     

Examination of barrier layers for lead zirconate titanate thin films

The selection of a suitable electrode and barrier layer are important in the integration of Lead Zirconate Titanate (PZT) into memory circuits. Five materials were evaluated using physical analysis techniques as possible barrier layers. Glancing-angle x-ray diffraction was used as the initial assessment to determine the structure of the PZT. With either silicon or MoSi₂ as a base layer, it was not possible to obtain a perovskite PZT structure under the conditions investigated. Although the desired crystal structure could be obtained with TiN as a base material, the oxygen-rich sputtering conditions for PZT had transformed this layer to an oxide making it unsuitable as electrode. PZT grown on an indium tin oxide (ITO) coated glass substrate also had a perovskite structure and good electrical properties. Scanning electron microscopy showed good adhesion between the layers, but transmission electron microscopy and secondary ion mass spectroscopy revealed Pb-rich areas residing in the glass beneath the ITO. PZT on platinum appeared to be the only promising barrier layer studied as there was no indication of Pb diffusion into the underlying layers and the desired crystal structure was obtained.     

Stable polymer solar cells using conjugated polymer as solvent barrier for organic electron transport layer

Redissolusion of an organic film by the subsequent processing solvent is a critical issue for the fabrication of organic multilayer structures and this can limit the advanced development of organic devices for flexible/stretchable application. In this study, we present a simple, doable, and straightforward approach to overcome this problem through introducing a solvent barrier. As a demonstration, we have successfully deposited an organic photoactive material on top of an organic electron transport layer (ETL) by pre-depositing a solvent barrier layer on the ETL for fabricating inverted polymer solar cells (PSCs). Water-/alcohol-soluble and thermally cross-linkable poly[9,9-bis(6’-(N,N-diethylamino)propyl)-fluorene-alt-9,9-bis-(3-ethyl(oxetane-3-ethyloxy)-hexyl)-fluorene] (PFN-OX) is selected as the solvent barrier material. PSCs fabricated with organic-ETL/PFN-OX bilayer show improved performance in comparison with those using conventional ZnO as ETL due to the enhanced interface compatibility as well as improved charge extraction, hole blocking, and interfacial recombination properties. In addition, those devices also exhibit longer lifetime and better light-soaking stability. Therefore, the approach of introducing solvent barrier is facile to stack organic films of similar solvent preference together and the resulting structure carries enhanced interfacial electrical properties, which can have positive effects on the device performance. We believe that our proposed idea is very useful and can advance the development of flexible/stretchable devices to be applied in wearable technology.     

Ⅲ族氮化物及其二维电子气输运特性的研究进展

Ⅲ族氮化物半导体具有宽禁带和直接带隙 ,导带能谷间距大 ,强场输运特性好。Al Ga N/Ga N异质结产生高密度的二维电子气 ,屏蔽了杂质和缺陷的散射 ,改善了低场输运性能。它弥补了宽禁带半导体输运性能差的缺点 ,已研制成大功率的 HFET。利用强场下的速度过冲有望消除阴极端的速度凹坑 ,显著改进器件性能。电子气的强二维性使输运特征依赖于器件结构和工作状态 ,器件设计变为一个剪裁电子状态和输运特性的复杂工程。文中综述了 族氮化物及其二维电子气的输运特性 ,讨论了从输运特性出发 ,优化 HFET性能的问题。     

Velocity-direction dependent transmission coefficient of electron through potential barrier grown on anisotropic semiconductor

In contrast to the usual wavevector dependent transition coefficients, the velocity-direction dependent transition coefficients of an incident electron are calculated. Through a potential barrier grown on anisotropic semiconductors, the transition coefficients of an incident electron are calculated in all valleys and incident-directions. In the anisotropic semiconductor, the mathematical expressions of the electron wavevector are also derived in the framework of the incident-angle and incident-energy parameters.