High on/off current ratio in ballistic CNTFETs based on tuning the gate insulator parameters for different ambient temperatures

A theoretical study is presented on the on/off current ratio limits for a ballistic coaxially-gated carbon nanotube field effect transistor (CNTFET) with highly doped source/drain regions. Based on changes in gate insulator dielectric constant and thickness, the current ratio has been estimated at different ambient temperatures. Decreasing the gate insulator thickness after a certain value around 3 nm causes the current ratio to degrade drastically. Although the higher dielectric constant values have a fair effect on current ratio, this effect could be suppressed when the device with a low gate insulator thickness works at a low ambient temperature. The simulation results also show that the temperature drastically degrades the current ratio value; whereas in a certain range of ambient temperature, tuning the values of gate insulator thickness and dielectric constant could be very helpful. In this way, the optimum values of gate insulator thickness and dielectric constant are identified to offer the highest on/off current ratio of the device.     

Propagation of surface plasmon polaritons in anisotropic MIM and IMI structures

Electromagnetic wave propagation through waveguide structures consisting of anisotropic dielectric layers, assisted by surface plasmon-polaritons (SPPs) is theoretically studied. Dispersion relations corresponding to both short range and long range coupled SPP modes in metal/insulator/metal (MIM) and insulator/metal/insulator (IMI) structures, taking into consideration the anisotropy of the insulator, are derived and numerically solved. The dispersion has a prominent dependence on the anisotropy of the dielectric environment. The dependence of propagation on the misalignments of the optic axes of the insulator has also been investigated.     

绝缘微堆质子直线加速器关键技术研究进展

基于绝缘微堆技术的直线加速器由于其能够实现较高的粒子加速梯度,尤其在质子加速及肿瘤治疗领域的优势得到高度关注。目前该种加速器处于研发阶段,有一系列技术和工程问题有待解决。介绍了课题组在过去的两年里围绕建立一台1 MeV质子注入器原型样机在固态脉冲功率系统、绝缘微堆及质子束源等方面取得的研究进展。实现了耐压梯度接近20 MV/m的环形绝缘微堆样品,样品内径30 mm,外径50mm,厚度15mm,基本达到设计要求;固态脉冲功率系统实现了光导开关多路稳定工作模式,开关直流偏置耐压达到20kV,采用激光二极管触发同步系统在15路同步时实现了低于1ns的抖动,输出300kV的电压脉冲,输出电压脉冲宽度10ns;进行了低能质子加速束流动力学的初步分析和模拟工作,模拟结果表明采用微堆结构可以实现质子束的有效加速和传输。     

Influence of the structure parameters on the gain and noise figure in silicon parametric amplifiers based on SOI Rib waveguides

Gain and Noise figure (NF) characteristics in dual-pump parametric amplifier based on silicon on insulator (SOI) Rib waveguides are numerically investigated in the presence of nonlinear losses. The impact of structure parameters of the silicon optical parametric amplifiers (SOPAs) on the gain and the NF are also analyzed. The results show that both the height and the width of the silicon on insulator (SOI) can affect the gain and the NF of SOPAs. 354 nm bandwidth (3 dB) and 8.135 maximum gain can be achieved by tailoring the structure parameters of the SOI rib waveguides. Moreover, the dispersion and the effective mode area of SOI are also analyzed.     

聚酰亚胺电导率随温度和电场强度的变化规律

介质深层充电对航天器安全运行构成了重大威胁.以聚酰亚胺为代表的此类聚合物绝缘介质的电导率受温度影响显著,又因为充电过程中局部产生强电场(10~7V/m量级),因此,其电导率模型需要综合考虑温度和强电场的影响,这对介质深层充电的仿真评估意义重大.已有的两类模型,不是低温区间不适用,就是没有充分考虑强电场的影响.基于跳跃电导理论,本文分析对比了现有电导率模型,提出了适用于较宽温度范围且合理考虑强电场增强效应的电导率新模型,并采用某型聚酰亚胺电导率测试数据做出验证.此外,为了提高新模型在强电场下的低温适用范围,尝试对强电场因子中的温度做变换,取得了满意的效果.参数敏感度分析表明新模型在电导率拟合与外推方面具有参数少、适用性强的优势.     

Frequency-dependent deformation of liquid crystal droplets in an external electric field

Nematic droplets suspended in the isotropic phase of the same substance were subjected to alternating electrical fields of varying frequency. To keep the system at a constant nematic/isotropic volume ratio with constant droplet size, we carefully kept the temperature in the isotropic/nematic coexistence region, which was broadened by adding small amounts of a non-mesogenic liquid. Whereas the nematic droplets remained spherical at low (in the order of 10 Hz) and high frequencies (in the order of 1 kHz), at intermediate frequencies we observed a marked flattening of the droplets in the plane perpendicular to the applied field. Droplet deformation occurred both in liquid crystals (LCs) with positive and negative dielectric anisotropy. The experimental data can be quantitatively modelled with a combination of the leaky dielectric model and screening of the applied electric field due to finite conductivity.     

微管道内两相流数值算法及在电浸润液滴控制中的应用

叙述了一种模拟电介质电润湿(electrowetting on dielectric，EWOD)下的微液滴的运动的数值方法. 采用二阶投影法求解N-S方程和level set 函数，并利用零level set函数俘获液滴运动界面，在液体与固体接触的边界上，通过引入动态接触角表征电介质表面润湿性随电势的改变. 数值计算基于MAC网格，模拟了2维微管道内与固体壁面接触的变润湿性的两种液体的分界面形状、平板上的微液滴在不同电势作用下处于不同湿润性的形态，以及微管道内改变接触角液滴的运动变形过程等算例. 关键词： 电浸润 接触角 level set函数 投影法     

Corrugated-enhanced second harmonic generation in metal–insulator–metal plasmonic waveguides

Nonlinear effects such as second-harmonic generation (SHG) are important for applications such as switching and wavelength conversion. In this study, the generation of second harmonic in metal–insulator–metal (MIM) plasmonic waveguides was investigated for both symmetric and asymmetric structures. Symmetric means that the metals at the top and bottom of the dielectric layer are the same and asymmetric means that the metals at the top and bottom of the dielectric layer are different. Two different structures are considered here as plasmonic waveguide for generation of second harmonic and analyzed using finite-difference time domain method. Besides the structure has grating on both sides for more coupling between photons and plasmons. The wavelength duration of grating per length unit (number of grooves) will be optimized to reach the highest second harmonic generation. To perform this optimization, the wavelength of operation λ = 458 nm is considered. It was shown that field enhancement in symmetric MIM waveguides can result in enhancement of SHG magnitude compared to the literature values and asymmetric device results in more than two orders of magnitude enhancement in SHG compared to symmetric structure. It is also shown that the electric field of second harmonic depends on the thickness of crystal (insulator). So, its thickness is optimized to achieve the highest electric field.     

介质壁加速器高梯度绝缘子研制

与普通绝缘子相比,高梯度绝缘子(HGI)的金属层与绝缘介质层交替排列结构可以抑制真空沿面闪络过程,从而提高沿面闪络场强。使用不同绝缘介质材料和金属材料采用不同加工制备工艺制备的HGI样品其沿面闪络场强差别较大。对所制备的绝缘介质层分别为聚酰亚胺、交联聚苯乙烯和尼龙,金属层为不锈钢箔,形状分别为圆柱与圆环型的HGI样品进行了真空沿面耐压测试,并对相同尺寸的纯绝缘介质样品进行了对照实验,得到了不同材料不同制备工艺HGI样品的耐压性能差异。结合样品表面显微照片观察得到的样品表面状况,分析了影响样品沿面闪络场强的因素。     

环状烯烃聚合物基底材料的太赫兹复介电常数和损耗特性研究

太赫兹时域光谱(THz-TDS)已被用于研究包括液体,半导体,爆炸物和气体等多种材料。然而自由空间太赫兹光谱系统存在着一些检测局限性,如微量物质难以被检测、系统尺寸难以实现微型化、空气中水的强烈吸收引起的信号衰减较大等问题。为了解决这些问题,研究人员设计了基于金属波导传输线结构的太赫兹芯片集成器件,通过飞秒红外激光激励传输线上的光电导材料实现太赫兹波的产生和检测。然而,在这些芯片上传输的太赫兹信号的频谱宽度很难达到自由空间太赫兹时域光谱系统的频带宽度,一个重要原因是由于传输信号受到随频率增加的传输线损耗所导致的衰减。通常这些损耗主要由三个部分组成：导体损耗,介电损耗和辐射损耗。研究表明：使用低介电常数材料作为共面传输线的衬底,将减少这种介电常数的失配,从而避免冲击波辐射损失;使用具有低损耗角正切的基底材料可以减少太赫兹传输线的损耗。环烯烃聚合物(COP)是一种具有环状烯烃结构的非晶性透明共聚物的材料,在太赫兹波段具有很高的透射率,为了探究这种材料是否能用于共面传输线的衬底,需要通过太赫兹时域光谱技术和介电函数理论分析它在太赫兹频段的光谱和介电特性,以及对这种材料作为基底时用在太赫兹传输线的传输特性进行仿真计算分析。使用透射式太赫兹时域光谱系统,对三种COP、熔融石英和PMMA进行了光谱测试,提取了它们的透射时域信号,采用Dorney和Duvillaret等提出的物理模型计算复介电函数。实验表明：与其他两种材料相比,COP材料在1 THz处的透过率更高,可以达到94.5%,同时介电损耗和介电常数更低,其中介电损耗在1 THz处达到4.31×10-4,因此将COP作为传输线基底材料时能有效减少基底的介电损耗。同时COP材料的介电常数在0.2～2.8 THz范围内维持在约2.3的水平,也有效减弱了辐射损耗。对实验材料基底组成的共面波导传输线进行了HFSS模拟,获得了它们的正向传输衰减系数(S₂₁ parameter),并对由基底引起的介电损耗和辐射损耗进行了计算分析。模拟和计算结果也表明在同一传输线结构下,与其他材料相比COP作为基底时的损耗更小。通过太赫兹时域光谱法与介电响应分析,表明了在太赫兹波段具有较低介电常数的COP材料更适合作为太赫兹传输线结构的基底材料,它可以有效的降低因基底引起的介电损耗和辐射损耗。这为太赫兹传输线的设计过程中衬底材料的选择与应用提供了实验和理论依据。     

液滴撞击加热壁面传热实验研究

本文采用高速摄像仪对水滴和乙醇液滴撞击加热壁面后的蒸发过程进行了实验观测, 分析了液滴撞击加热壁面后的蒸发特性参数. 实验中, 两种液体初始温度均为20 ℃, 不锈钢壁面初始温度范围为68-126℃. 水滴初始直径为2.07 mm, 撞击壁面时Weber 数为2-44; 乙醇液滴初始直径为1.64 mm, Weber数为3-88. 结果表明, 液滴受到重力、表面张力及流动性的影响, 在蒸发过程的大部分时间内, 水滴高度持续降低而接触直径几乎不变; 蒸发后期, 液滴发生回缩, 水滴的接触直径、高度和接触角出现振荡现象. 乙醇液滴的接触角随时间的增加呈现先减小随后保持不变的趋势, 而接触直径和高度则持续减小, 直到液滴完全蒸发. 液滴蒸发总时长与液体物性和壁面温度有关, 随壁面温度的升高而减小, 与液滴撞击壁面时的Weber 数无关. 同时, 随着壁面温度的升高, 液滴显热部分占总换热量的比重增大, 显热部分能量不可忽略, 本文实验条件下得到水滴的平均热流密度为0.014-0.110 W·mm^-2.     

表面等离子体激元的若干新应用

表面等离子体激元（SPPs）是在金属和介质界面传播的一种波动模式。本文首先叙述了SPPs的相关特性和激发方式,给出了一种基于表面等离子体激元共振（SPR）场增强原理产生相干极紫外辐射的方法,利用该方法可极大地提高光源的光子流量。分析了SPPs在生物及医疗领域的新应用,并对其在治疗癌症方面的技术原理进行了讨论。介绍了SPPs在新型光源和能源领域的发展和应用情况,综述了SPPs在太阳能电池、光子芯片以及集成电路方面的新工艺和新技术,包括最近几年来所取得的一些重要成果。最后讨论了SPPs在光存储方面的快速发展和巨大贡献。     

紫外表面等离激元在基于氧化锌纳米线的半导体-绝缘介质-金属结构中的输运特性研究

研究了紫外表面等离激元在半导体纳米线-绝缘介质-金属构成的波导结构中的输运问题,借助有限元方法,对这种波导所支持导模的电磁能分布、有效折射率、传播长度和有效模场面积随电磁参数和几何结构参数的依赖关系进行了分析.计算结果表明:以氧化锌纳米线作为增益介质,绝缘材料选择折射率小的空气,金属选择铝能够实现对输出光场的亚波长约束,有效模场面积达到λ2/100,同时保持低的传输损耗和高场强限制能力;有望用作纳米光源,使得相关的生物探测器件和医疗诊断设备实现更高的灵敏度和更小的体积.     

波破碎下海洋表面微波辐射率的一种模式

利用有效介质近似理论方法，本文提出了计算强风驱动下海浪破碎的海洋表面微波辐射率的一种修正模式。该模式修正Pandey等人的经验模式，通过把海浪破碎的白冠层等效为海水和空气的复合介质，并利用复合介质有效介电常数的计算方法，在该模式中充分考虑了海洋表面水滴形态、白冠层厚度和覆盖率对海洋表面微波辐射率的影响。实验表明，该模式在不同风速、不同水滴形态以及微波频率下（6.6GHz,10.7GHz,37GHz）计算的海洋表面微波辐射率与实测结果一致。     

Detailed study of NBTI characterization in 40-nm CMOS process using comprehensive models

The impact of negative bias temperature instability(NBTI) can be ascribed to three mutually uncorrelated factors, including hole trapping by pre-existing traps(?V_(HT)) in gate insulator, generated traps(?V_(OT)) in bulk insulator, and interface trap generation(?V_(IT)). In this paper, we have experimentally investigated the NBTI characteristic for a 40-nm complementary metal–oxide semiconductor(CMOS) process. The power-law time dependence, temperature activation, and field acceleration have also been explored based on the physical reaction–diffusion model. Moreover, the end-of-life of stressed device dependent on the variation of stress field and temperature have been evaluated. With the consideration of locking effect, the recovery characteristics have been modelled and discussed.     

微堆层绝缘子的研制与初步实验研究

 为发展新型介质壁加速器技术,研制了微堆层绝缘子并开展了初步的实验研究。利用有限元及粒子模拟方法,理论计算了微堆层绝缘子的沿面电场分布及电子运动轨迹,提出微堆层绝缘子的优化设计方案;采用高温层压的方法,研制出以聚全氟乙丙烯薄膜为介质层、不锈钢膜为金属层的微堆层绝缘子试样,并初步开展了纳秒脉冲下微堆层绝缘子的真空沿面闪络实验。研究结果表明：微堆层绝缘子具有良好的真空沿面闪络性能,其闪络场强可达180 kV/cm。     

电极材料及偏压极性对氧化物介质击穿行为的影响及机制

忆阻器和能量存储电容器具有相同的三明治结构,然而两个器件需要的操作电压有明显差异,因此在同一个器件中,研究操作电压的影响因素并对操作电压进行调控,实现器件在不同领域的应用是十分必要的一个工作.本文利用反应磁控溅射技术在ITO导电玻璃、Pt/Si基底上生长了多晶ZrO_2和非晶TaO_x薄膜,选用不同金属材料Au, Ag和Al用作上电极构建了多种金属/氧化物介质/金属三明治结构的电容器,研究了器件在不同偏压极性下的击穿强度.结果发现:底电极是ITO的ZrO_2基电容器在负偏压下的击穿电场比Pt电极器件稍大.不管底电极是ITO还是Pt, Ag作为上电极时器件的击穿强度均存在明显的偏压极性依赖性,正偏压下的击穿电场减小了一个数量级;相反,在Al作为上电极的Al/TaO_x/Pt器件中,正向偏压比负向偏压下的击穿电场增加了近2倍.上述器件的不同击穿行为分别可以由氧化物电极和介质界面层间氧的迁移和重排、电化学活性金属电极的溶解迁移和还原以及化学活性金属电极与氧化物界面的氧化还原反应来解释.该实验结果对有不同操作电压要求的器件,如忆阻器和介质储能电容器等在器件设计和操作方面具有指导意义.     

The apparent state of droplets on a rough surface

The factors influencing the state and wetting transition of droplets on a rough surface are both complex and obscure. The change in wetting is directly reflected by changes under the contact condition of the droplets with the surface. The recent study about the wettability of the superhydrophobic surface under the condensing condition arouses the new understanding about the apparent state of droplets on a rough surface. In this work, to validate the existence of droplets in an intermediate state, a microscale pillar topological polydimethylsiloxane (PDMS) surface was manufactured and its wettability under various conditions was studied. According to the experimental data, it is proposed that the wetting state of a rough surface may be embodied using the contact area ratio of a solid/liquid/gas droplet with the projective plane. A general calculation model for the apparent contact angle of droplets is given and expressed diagrammatically. It is found that the measured apparent contact angles of droplets at different states on the surface falls within the range predicted by our proposed equation. Supported by the National Natural Science Foundation of China (Grant No. 50606025)     

绝缘颗粒液体主体基质复合介质的非线性光学性质

利用Maxwell-Garnett近似，并结合谱表示方法，理论研究了具有一般非线性的绝缘颗粒，无规则浸入液体主体基质复合体系的非线性光学性质-在弱非线性条件下，数值研究了体系的有效线性介电函数和光学非线性随体积分数的变化，并发现体系的线性介电函数比光学非线性更依赖于绝缘颗粒组分的体积分数-在一般非线性条件下，研究体系的有效介电函数随入射光强度的依赖关系，还研究了s方向和p方向极化的总反射率的行为- 关键词： 复合介质 谱表示 非线性光学性质     

微液滴在不同能量表面上润湿状态的分子动力学模拟

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