Dense pair plasma generation by two laser pulses colliding in a cylinder channel

An all-optical scheme for high-density pair plasmas generation is proposed by two laser pulses colliding in a cylinder channel. Two dimensional particle-in-cell simulations show that, when the first laser pulse propagates in the cylinder,electrons are extracted out of the cylinder inner wall and accelerated to high energies. These energetic electrons later run into the second counter-propagating laser pulse, radiating a large amount of high-energy gamma photons via the Compton back-scattering process. The emitted gamma photons then collide with the second laser pulse to initiate the Breit–Wheeler process for pairs production. Due to the strong self-generated fields in the cylinder, positrons are confined in the channel to form dense pair plasmas. Totally, the maximum density of pair plasmas can be 4.60 × 10~(27)m~(-3), for lasers with an intensity of 4×10~(22)W·cm~(-2). Both the positron yield and density are tunable by changing the cylinder radius and the laser parameters. The generated dense pair plasmas can further facilitate investigations related to astrophysics and particle physics.     

Structure and luminescence of a-plane GaN on r-plane sapphire substrate modified by Si implantation

We show the structural and optical properties of non-polar a-plane GaN epitaxial films modified by Si ion implantation. Upon gradually raising Si fluences from 5×10¹³ cm^-2 to 5×10¹⁵ cm^-2, the n-type dopant concentration gradually increases from 4.6×10¹⁸ cm^-2 to 4.5×10²⁰ cm^-2, while the generated vacancy density accordingly raises from 3.7×10¹³ cm^-2 to 3.8×10¹⁵ cm^-2. Moreover, despite that the implantation enhances structural disorder, the epitaxial structure of the implanted region is still well preserved which is confirmed by Rutherford backscattering channeling spectrometry measurements. The monotonical uniaxial lattice expansion along the a direction (out-of-plane direction) is observed as a function of fluences till 1×10¹⁵ cm^-2, which ceases at the overdose of 5×10¹⁵ cm^-2 due to the partial amorphization in the surface region. Upon raising irradiation dose, a yellow emission in the as-grown sample is gradually quenched, probably due to the irradiation-induced generation of non-radiative recombination centers.     

电磁波在非磁化等离子体中衰减效应的实验研究

针对等离子体隐身技术在航空航天领域的良好应用前景, 开展垂直入射到具有金属衬底的非磁化等离子体中电磁波衰减特性的理论与实验研究. 利用WKB方法对电磁波衰减随等离子体参数的变化规律进行了理论分析. 利用射频电感耦合放电方式产生稳定的大面积等离子体层, 搭建了等离子体反射率弓形测试系统, 进行了电磁波在非磁化等离子体中衰减效应的实验研究. 利用微波相位法和光谱诊断法, 得到不同放电功率下的等离子体电子密度, 其范围为8.17×10⁹–7.61× 10¹⁰ cm^-3. 本实验获得的等离子体可以使2.7 GHz 和10.1 GHz电磁波分别得到一定的衰减, 且电磁波衰减的理论与实验结果符合较好. 结果表明, 提高等离子体电子密度和覆盖均匀性有利于增强等离子体对电磁波的衰减效果.     

N极性GaN/AlGaN异质结二维电子气模拟

通过自洽求解薛定谔方程和泊松方程,较系统地研究了GaN沟道层、AlGaN背势垒层、Si掺杂和AlN插入层对N极性GaN/AlGaN异质结中二维电子气(2DEG)的影响,分析表明,GaN沟道层厚度、AlGaN背势垒层厚度及Al组分变大都能一定程度上提高二维电子气面密度,AlGaN背势垒层的厚度和Al组分变大也可提高二维电子气限阈性,且不同的Si掺杂形式对二维电子气的影响也有差异,而AlN插入层在提高器件二维电子气面密度、限阈性等方面表现都较为突出,在模拟中GaN沟道层厚度小于5nm时无法形成二维电子气,超过20nm后二维电子气面密度趋于饱和,而AlGaN背势垒厚度超过40nm后二维电子气也有饱和趋势,对均匀掺杂和delta掺杂而言AlGaN背势垒层Si掺杂浓度超过5×10~(19)cm~(-3)后2DEG面密度开始饱和,而厚度为2nmAlN插入层的引入会使2DEG面密度从无AlN插入层时的0.93×10~(13)cm~(-2)提高到1.17×10~(13)cm~(-2)。     

A simulation study on p-doping level of polymer host material in P3HT:PCBM bulk heterojunction solar cells

In this study,we investigate the influence of doping on the charge transfer and device characteristics parameters in the bulk heterojunction solar cells based on poly(3-hexylthiophene)(P3HT) and a methanofuUerene derivative(PCBM).Organic semiconductors are also known to be not pure and they have defects and impurities,some of them are being charged and act as p-type or n-type dopants.Calculations of the solar cell characteristics parameters versus the p-doping level have been done at three different n-dopings(N_d) that consist of 5 × 10~(17) cm~(-3),10~(18) cm~(-3),and 5 × 10~(18) cm~(-3).We perform the analysis of the doping concentration through the drift-diffusion model,and calculate the current and voltage doping dependency.We find that at three different n-dopant levels,optimum p-type doping is about N_p = 6 × 10~(18) cm~(-3).Simulation results have shown that by increasing doping level,V_(oc) monotonically increases by doping.Cell efficiency reaches its maximum at somewhat higher doping as FF has its peak at N_p = 3 × 10~(18) cm~(-3).Moreover,this paper demonstrates that the optimum value for the p-doping is about N_p = 6 × 10~(18) cm~(-3) and optimum value for n-dopant is N_d = 10~(18) cm~(-3),respectively.The simulated results confirm that doping considerably affects the performance of organic solar cells.     

A two-component model for the electron distribution function in ahigh-current pseudospark or back-lighted thyratron

Temperature, energy, and densities of two electron distribution function components, including an isotropic bulk part and an anisotropic beam, are analyzed for a hydrogen pseudospark and/or back-lighted thyratron switch plasma with a peak electron density of 1-3×10¹⁵ cm^-3 and peak current density of ≈10⁴ A/cm². Estimates of a very small cathode-fall width during the conduction phase and high electric field strengths lead to the injection of an electron beam with energies ⩾100 eV and density of 10¹³-10¹⁴ cm^-3 into a Maxwellian bulk plasma. Collisional and radiative processes of monoenergetic beam electrons, bulk plasma electrons and ions, and atomic hydrogen are modeled by a set of rate equations, and line intensity ratios are compared with measurements. Under these high-current conditions, for an initial density n_H2=10¹⁶ cm^-3 and electron temperature of 0.8-1 eV, the estimated beam density is ≈10¹³ -10¹⁴ cm^-3. These results suggest the possibility of producing in a simple way a very high-density electron beam     

温稠密钛电导率计算

温稠密物质是惯性约束核聚变、重离子聚变、Z箍缩动作过程中物质发展和存在的重要阶段. 其热力学性质和辐射输运参数在聚变实验和内爆驱动力学模拟过程中有至关重要的作用. 本文通过建立非理想Saha方程, 结合线性混合规则的理论方法模拟了温稠密钛从10^-5-10 g·cm^-3, 10⁴ K到3×10⁴ K区间的粒子组分分布和电导率随温度密度的变化, 其中粒子组分分布由非理想Saha方程求解得到. 线性混合规则模型计算温稠密钛的电导率时考虑了包括电子、原子和离子之间的多种相互作用. 钛的电导率的计算结果与已有的爆炸丝实验数据相符. 通过电导率随温度密度变化趋势判断, 钛在整个温度区间, 密度0.56 g·cm^-3时发生非金属相到金属相相变. 对于简并系数和耦合系数的计算分析, 钛等离子体在整个温度和密度区间逐渐从弱耦合、非简并状态过渡到强耦合部分简并态.     

退火对AlGaInP/GaInP多量子阱 LED外延片性能的影响

采用LP-MOCVD技术在n-GaAs衬底上生长了AlGaInP/GaInP多量子阱红光LED外延片.研究表明退火对外延片性能有重要影响.与未退火样品相比,460℃退火15min,外延片p型GaP层的空穴浓度由5.6×10¹⁸cm^-3增大到6.5×10¹⁸cm^-3,p型AlGaInP层的空穴浓度由6.0×10¹⁷cm^-3增大到1.1×10¹⁸cm^-3.但退火温度为780℃时,p型GaP层和p型AlGaInP层的空穴浓度分别下降至8×10¹⁷cm^-3和1.7×10¹⁷cm^-3,且Mg原子在AlGaInP系材料中的扩散加剧,导致未掺杂AlGaInP/GaInP多量子阱呈现p型电导.在460～700℃退火范围内,并没有使AlGaInP/GaInP多量子阱的发光性能发生明显变化.但退火温度为780℃时,AlGaInP/GaInP多量子阱的发光强度是退火前的2倍.     

电离辐射对部分耗尽绝缘体上硅器件低频噪声特性的影响

本文针对辐射前后部分耗尽结构绝缘体上硅(SOI)器件的电学特性与低频噪声特性开展试验研究. 受辐射诱生埋氧化层固定电荷与界面态的影响, 当辐射总剂量达到1 M rad(Si) (1 rad = 10^-2 Gy)条件下, SOI器件背栅阈值电压从44.72 V 减小至12.88 V、表面电子有效迁移率从473.7 cm²/V·s降低至419.8 cm²/V· s、亚阈斜率从2.47 V/dec增加至3.93 V/dec; 基于辐射前后亚阈斜率及阈值电压的变化, 可提取得到辐射诱生界面态与氧化层固定电荷密度分别为5.33×10¹¹ cm^{- 2}与2.36×10¹² cm^-2. 受辐射在埋氧化层-硅界面处诱生边界陷阱、氧化层固定电荷与界面态的影响, 辐射后埋氧化层-硅界面处电子被陷阱俘获/释放的行为加剧, 造成SOI 器件背栅平带电压噪声功率谱密度由7×10^{- 10} V²·Hz^-1增加至1.8×10^-9 V² ·Hz^-1; 基于载流子数随机涨落模型可提取得到辐射前后SOI器件埋氧化层界面附近缺陷态密度之和约为1.42×10¹⁷ cm^-3·eV^-1和3.66×10¹⁷ cm^-3·eV^-1. 考虑隧穿削弱因子、隧穿距离与时间常数之间关系, 本文计算得到辐射前后埋氧化层内陷阱电荷密度随空间分布的变化.     

The atmospheric-pressure plasma jet: a review and comparison toother plasma sources

Atmospheric-pressure plasmas are used in a variety of materials processes. Traditional sources include transferred arcs, plasma torches, corona discharges, and dielectric barrier discharges. In arcs and torches, the electron and neutral temperatures exceed 3000°C and the densities of charge species range from 10¹⁶-10¹⁹ cm^-3. Due to the high gas temperature, these plasmas are used primarily in metallurgy. Corona and dielectric barrier discharges produce nonequilibrium plasmas with gas temperatures between 50-400°C and densities of charged species typical of weakly ionized gases. However, since these discharges are nonuniform, their use in materials processing is limited. Recently, an atmospheric-pressure plasma jet has been developed, which exhibits many characteristics of a conventional, low-pressure glow discharge. In the jet, the gas temperature ranges from 25-200°C, charged-particle densities are 10 ¹¹-10¹² cm^-3, and reactive species are present in high concentrations, i.e., 10-100 ppm. Since this source may be scaled to treat large areas, it could be used in applications which have been restricted to vacuum. In this paper, the physics and chemistry of the plasma jet and other atmospheric-pressure sources are reviewed     

Electric and plasma characteristics of RF discharge plasma actuation under varying pressures

The electric and plasma characteristics of RF discharge plasma actuation under varying pressure have been investigated experimentally. As the pressure increases, the shapes of charge–voltage Lissajous curves vary, and the discharge energy increases. The emission spectra show significant difference as the pressure varies. When the pressure is 1000 Pa,the electron temperature is estimated to be 4.139 e V, the electron density and the vibrational temperature of plasma are peak4.71×10~(11)cm~(-3) and 1.27 e V, respectively. The ratio of spectral lines I391.4/peak I380.5which describes the electron temperature hardly changes when the pressure varies between 5000–30000 Pa, while it increases remarkably with the pressure below 5000 Pa, indicating a transition from filamentary discharge to glow discharge. The characteristics of emission spectrum are obviously influenced by the loading power. With more loading power, both of the illumination and emission spectrum intensity increase at 10000 Pa. The pin–pin electrode RF discharge is arc-like at power higher than 33 W, which results in a macroscopic air temperature increase.     

Near-interface oxide traps in 4H–SiC MOS structures fabricated with and without annealing in NO

Near-interface oxide traps(NIOTs)in 4H–Si C metal–oxide–semiconductor(MOS)structures fabricated with and without annealing in NO are systematically investigated in this paper.The properties of NIOTs in Si C MOS structures prepared with and without annealing in NO are studied and compared in detail.Two main categories of the NIOTs,the"slow"and"fast"NIOTs,are revealed and extracted.The densities of the"fast"NIOTs are determined to be 0.76×10~(11)cm~(-2)and0.47×10~(11)cm~(-2)for the N_2 post oxidation annealing(POA)sample and NO POA sample,respectively.The densities of"slow"NIOTs are 0.79×10~(11)cm~(-2)and 9.44×10~(11)cm~(-2)for the NO POA sample and N_2POA sample,respectively.It is found that the NO POA process only can significantly reduce"slow"NIOTs.However,it has a little effect on"fast"NIOTs.The negative and positive constant voltage stresses(CVS)reveal that electrons captured by those"slow"NIOTs and bulk oxide traps(BOTs)are hardly emitted by the constant voltage stress.     

Fe+注入植物种子射程实验的Monte-Carlo仿真

研究表明,TRIM程序运算结果与实验测量离子注入种子的射程分布数据相差甚远.本文根据种子微结构的特点,综合考虑多种因素,设计种子微结构模型和运算程序,用Monte-Carlo仿真不同能量（110 keV,20 keV,200 keV）、不同注量（2×10¹⁶ ions·cm^-2,5×10¹⁶ ions·cm^-2,10¹⁷ ions·cm^-2,2×10¹⁷ ions·cm^-2）的Fe⁺注入花生、彩棉、小麦种子的射程分布,结果显示本设计程序仿真的结果与实验测量数据较为吻合.所获得的注入离子与种子微结构相互作用的随机抽样模拟运算方法,为离子注入与生命体相互作用的理论研究提供新的思路.     

A cold thermionically emitting dusty air plasma formed by radiativeheating of graphite particulates

Formation of an atmospheric pressure dusty air plasma is explored experimentally in this paper. The plasma is created by seeding an air flow with graphite particles and irradiating the particulates with a focused CO₂ laser beam. The graphite particles are, thus, heated to thermionically emitting temperatures, and average particle temperatures and average particle number densities are measured. The presence of charges is inferred both from these measured quantities using a simple theoretical transient model, and experimentally by applying a dc bias across the irradiated region. It is found that an electron density of ~6.7 × 10⁵ cm^-3 (6.7 × 10¹¹ m^-3) can be produced at steady state in the presence of O₂. This value can be increased to 3.6 × 10⁷ cm^-3 (3.6 × 10¹³ m ^-1) in the ideal case where an electron attachment to O₂ is suppressed and where a lower work function particulate is used     

In-plane oriented CH_3NH_3PbI_3 nanowire suppression of the interface electron transfer to PCBM

One-dimensional nanowire is an important candidate for lead-halide perovskite-based photonic detectors and solar cells. Its surface population, diameter, and growth direction, etc., are critical for device performance. In this research,we carried out a detailed study on electron transfer process at the interface of nanowire CH_3 NH_3 PbI_3(N-MAPbI_3)/Phenyl C61 butyric acid methyl-ester synonym(PCBM), as well as the interface of compact CH_3 NH_3 PbI_3(C-MAPbI_3)/PCBM by transient absorption spectroscopy. By comparing the carrier recombination dynamics of N-MAPbI_3, N-MAPbI_3/PCBM,C-MAPbI_3, and C-MAPbI_3/PCBM from picosecond(ps) to hundred nanosecond(ns) time scale, it is demonstrated that electron transfer at N-MAPbI_3/PCBM interface is less efficient than that at C-MAPbI_3/PCBM interface. In addition, electron transfer efficiency at C-MAPbI_3/PCBM interface was found to be excitation density-dependent, and it reduces with photo-generation carrier concentration increasing in a range from 1.0 × 1018 cm~(-3)–4.0 × 1018 cm~(-3). Hot electron transfer,which leads to acceleration of electron transfer between the interfaces, was also visualized as carrier concentration increases from 1.0 × 10~(18) cm~(-3)–2.2 × 10~(18) cm~(-3).     

Plasma parameters within the cathode spot of the vacuum arc

The composition of the vacuum arc plasma for five elements (Cd, Mg, Al, Ni, and Mo) is calculated by the Saha equation, which assumes local thermodynamic equilibrium conditions within the ionization region of the cathode spot(s). The lowering of the ionization potential due to the high density of charged particles is considered. By matching the computed and the measured plasma ionic composition, the electron density and the temperature are estimated. The experimental plasma compositions can be approximated only at a high electron density (10¹⁹-10 ²¹ cm^-3) and at electron temperatures in the range of a few electronvolts     

基于自吸收量化的激光诱导等离子体表征方法

为了表征激光诱导等离子体的定量特征参数,提出了一种谱线自吸收量化的方法,通过获得分析元素谱线的半高全宽来量化谱线自吸收程度,进而得到等离子体的特征参数,包括电子温度、元素含量比以及辐射物质的绝对数密度.与传统激光诱导击穿光谱定量分析方法相比,新方法由于计算过程与谱线强度弱相关,所以分析结果基本不受自吸收效应的影响,同时也无需额外的光谱效率校准.基于铝锂合金的实验结果表明,该方法能够实现精确的相对定量分析和等离子体的特性诊断.     

金属与半导体Ge欧姆接触制备、性质及其机理分析

金属与Ge材料接触时界面处存在着强烈的费米钉扎效应, 尤其与n型Ge形成的欧姆接触的比接触电阻率高, 是制约Si基Ge器件性能的关键因素之一. 本文对比了分别采用金属Al和Ni 与Si衬底上外延生长的p型Ge和n型Ge材料的接触特性. 发现在相同的较高掺杂条件下, NiGe与n型Ge可形成良好的欧姆接触, 其比接触电阻率 较 Al接触降低了一个数量级, 掺P浓度为2×10¹⁹ cm^-3时达到1.43×10^-5 Ω·cm². NiGe与p型Ge接触和Al接触的比接触电阻率相当, 掺B浓度为4.2×10¹⁸ cm^-3时达到1.68×10^-5 Ω·cm². NiGe与n型Ge接触和Al电极相比较, 在形成NiGe过程中, P杂质在界面处的偏析是其接触电阻率降低的主要原因. 采用NiGe作为Ge的接触电极在目前是合适的选择. 关键词： 金属与Ge接触性质 NiGe 比接触电阻率     

等离子体密度对激光拉曼放大机理的影响

等离子体中的背向拉曼散射机理可以用来产生超短超强的激光脉冲. 本文采用粒子模拟方法模拟研究了等离子体密度对激光拉曼放大过程的影响. 研究发现, 过低的等离子体密度会导致等离子体波提前波破而降低能量转换效率; 而过高的等离子体密度又会导致其他不稳定性的快速增长, 限制作用距离和输出能量. 因此, 拉曼放大机理的最佳等离子体密度应处于等离子体波破的密度阈值附近, 可以获得最高的能量转换效率和能量输出. 另外, 空间频谱分析显示放大激光的强度饱和主要来自于自相位调制不稳定性的发展. 利用10¹³ W·cm^-2的抽运激光脉冲, 模拟证实拉曼放大机理可有效地将种子激光的强度从10¹³ W·cm^-2 放大到10¹⁷ W·cm^-2, 脉宽压缩到40 fs, 且能量转换效率达到58%.     

S—Ka频段电磁波在等离子体中传输特性的实验研究

在感应耦合等离子体风洞上开展了等离子体中电磁波传输特性实验研究,获得了不同频率电磁波在等离子体中的传输衰减.通过微波诊断技术,获得了等离子体射流的电子数密度和碰撞频率.通过矢量网络分析仪和标准增益天线组成的电磁波传输特性测试系统,获得了电磁波经过等离子体之后的衰减,研究了电子数密度范围7.0×10~(10)-1.0×10~(13)cm~(-3)、等离子体碰撞频率在109 Hz量级的等离子体对2.6—40 GHz不同频率电磁波传输特性的影响,分析了经典传输理论和薄层理论预测结果与实验结果的差异.该实验工作为等离子体中电磁波传输特性的理论研究和数值仿真提供了基础数据.