Poloidal spin up and electric-field generation related to displacement current and neoclassical transport

Summary In accordance with the conventional orderings of neoclassical theory, poloidal and toroidal accelerations with constant parallel flow can be driven by heat transport in the absence of external momentum input and with vanishing parallel viscous stress. In a transient phase in which the heat transport is the primary source of the time dependence, the torque generating the rotation is provided at third order in the adiabatic expansion by the surface-averaged (non-ambipolar) displacement current, which is also responsible for charge build-up and for the radial electric field. The heat transport equation has been solved in a narrow layer interfaced with the intensely heated plasma core through heat flux continuity, assuming neoclassical multicollisional coefficients with self-consistent suppression mechanism of anomalous transport. Starting from low temperature in the edge layer, a strong temperature gradient, a mass poloidal rotation in the ion direction and a strongly negative sheared radial electric field can be generated, in agreement with the observations, and reach a stationary state after a displacement current-dominated triggering phase (intrinsically non-ambipolar) lasting few milliseconds. Momentum input becomes important on longer time scale and is responsible for the toroidal rotation, decoupled from temperature gradient and for a further development of the radial electric field. The results show the ability of edge transport processes to adapt flexibly to a high temperature imposed on the inner side of the edge layer and support the view that the edge processes are an integral part of a more fundamental global process involving possibly an internal bifurcation of state.     

Kinetics of quasiballistic transport in nanoscale semiconductor structures: is the ballistic limit attainable at room temperature?

Electron transport in nanoscale semiconductor structures is theoretically investigated to answer the question of whether or not the ballistic limit is really attainable under room temperature operation. The semiclassical Boltzmann transport equation is solved analytically under the relaxation time approximation for n(+)-n-n(+) test structures. We demonstrate that the solution of the Boltzmann transport equation exhibits a boundary layer structure near the potential barrier and thus the scatterings in the active region cannot be neglected even in nanoscale structures, as far as they are operated at room temperature under high applied voltages.     

Endogenous magnetic reconnection and associated high energy plasma processes

An endogenous reconnection process involves a driving factor that lays inside the layer where a drastic change of magnetic field topology occurs. A process of this kind is shown to take place when an electron temperature gradient is present in a magnetically confined plasma and the evolving electron temperature fluctuations are anisotropic. The width of the reconnecting layer remains significant even when large macroscopic distances are considered. In view of the fact that there are plasmas in the Universe with considerable electron thermal energy contents this feature can be relied upon in order to produce generation or conversion of magnetic energy, high energy particle populations and momentum and angular momentum transport.     

锡基钙钛矿太阳能电池载流子传输层的探讨

锡基钙钛矿太阳能电池可避免铅元素对环境带来的污染,近年来已成为光伏领域的研究热点.本文以SCAPS-1D太阳能电池数值模拟软件为平台,对不同电子传输层和不同空穴传输层的锡基钙钛矿太阳能电池器件的性能进行数值仿真对比,从理论上分析不同载流子传输层的锡基钙钛矿太阳能电池的性能差异.结果显示,载流子传输层与钙钛矿层的能带对齐对电池性能至关重要.电子传输层具有更高的导带或电子准费米能级以及空穴传输层具有更低的价带或空穴准费米能级时,对电池输出更大的开路电压有促进作用.另外,当电子传输层的导带高于钙钛矿层导带或钙钛矿层的价带高于空穴传输层的价带时,钙钛矿层与载流子传输层界面形成spike势垒,界面复合机制相对较弱,促使电池获得更佳的性能.当Cd0.5Zn0.5S和MASnBr3分别作为电子传输层和空穴传输层时,与其他材料相比,获得了更优的输出特性:开路电压Voc=0.94 V,短路电流密度Jsc=30.35 mA/cm^2,填充因子FF=76.65%,功率转换效率PCE=21.55%,可认为Cd0.5Zn0.5S和MASnBr3是设计锡基钙钛矿太阳能电池结构合适的载流子传输层材料.这些模拟结果有助于实验上设计并制备高性能的锡基钙钦矿太阳能电池.     

Measurements of electron transport in foils irradiated with a picosecond time scale laser pulse

The heating of solid foils by a picosecond time scale laser pulse has been studied by using x-ray emission spectroscopy. The target material was plastic foil with a buried layer of a spectroscopic tracer material. The laser pulse length was either 0.5 or 2 ps, which resulted in a laser irradiance that varied over the range 10(16)-10(19) W/cm(2). Time-resolved measurements of the buried layer emission spectra using an ultrafast x-ray streak camera were used to infer the density and temperature conditions as a function of laser parameters and depth of the buried layer. Comparison of the data to different models of electron transport showed that they are consistent with a model of electron transport that predicts the bulk of the target heating is due to return currents.     

Simulation studies of current transport in metal–insulator–semiconductor Schottky barrier diodes

The current–voltage characteristics of Schottky diodes with an interfacial insulator layer are analysed by numerical simulation. The current–voltage data of the metal–insulator–semiconductor Schottky diode are simulated using thermionic emission diffusion (TED) equation taking into account an interfacial layer parameter. The calculated current–voltage data are fitted into ideal TED equation to see the apparent effect of interfacial layer parameters on current transport. Results obtained from the simulation studies shows that with mere presence of an interfacial layer at the metal–semiconductor interface the Schottky contact behave as an ideal diode of apparently high barrier height (BH), but with same ideality factor and series resistance as considered for a pure Schottky contact without an interfacial layer. This apparent BH decreases linearly with decreasing temperature. The effects giving rise to high ideality factor in metal–insulator–semiconductor diode are analysed. Reasons for observed temperature dependence of ideality factor in experimentally fabricated metal–insulator–semiconductor diodes are analysed and possible mechanisms are discussed.     

Phosphor thermometry at the surface of single reacting large-diameter spherical coke particles to characterise combustion for packed bed furnaces

Packed-bed furnaces fired with large-diameter coke are important in high-temperature material processing industries such as lime and iron production. The combustion conditions are complicated by the presence of an ash layer surrounding the coke particle that remains intact during passage through the furnace and alters oxygen diffusion and heat transfer to the reacting particle core. The objective of this study is to determine the surface temperature of this ash layer using lifetime-based phosphor thermometry during combustion of single spherical 38 mm diameter coke particles in a high temperature tube furnace. Time traces of the coke particle core temperature and mass conversion rate do not significantly differ between experiments performed with and without the phosphor layer, indicating that the presence of the phosphor particles does not alter the overall combustion behaviour. Surface temperatures of up to 950 °C are measured and correlated with the fuel mass conversion rate. When the coke particle starts to react the surface temperature is up to 100 °C higher than that of the core. As the reaction front progresses toward the centre, the core temperature exceeds the surface temperature by 200 °C due to the insulating effect of the ash layer. The surface temperature of the ash layer decreases with time due to the steadily decreasing fuel mass conversion rate. The method and results can be used to provide key validation data for shrinking-core combustion models, for example by constraining the unknown transport properties of the ash layer, thereby assisting the development of complete packed-bed furnace simulations for process optimisation.     

Effects of the surface layer of La2/3Ca1/3MnO3 thin films

The surface layer effects on transport in epitaxial La_2/3Ca_1/3MnO₃ thin films are studied. It was found that the two-probe resistance is nonlinear which is enhanced with decreasing temperature. Similar to the resistance of intrinsic La_2/3Ca_1/3MnO₃ thin films reported in the literature, the apparent dynamic contact resistance behaves semiconducting at high temperatures, passes through a peak, and displays a metallic behavior. At lowest temperatures, the curve of the contact resistance versus temperature shows a little upturn. The temperature dependent work function difference between the surface layer and the thin film underneath, together with the tunneling process across either the resulting charge depleted layer or the semiconducting surface layer is used to explain our observations.     

Magnetoresistance in half-metallic/metallic ferromagnetic junction through silicon

We report spin transport through the silicon in novel magnetic junction with half metallic as free layer and metallic as pinned layer. We used La_0.7Sr_0.3MnO₃ as free layer, FeCo as pinned layer and studied the magnetoresistance through silicon as spacer layer. We fabricated this magnetic tunnel junction using RF/DC sputtering technique over SrTiO₃ substrate. Tunneling magnetoresistance (TMR) measurement for this junction at room temperature was found to be 1.1 %. At 2 K, we found a large magnetoresistance of 396 %. TMR found to be increased with decreasing temperature. The results are discussed.     

Probing semiconductor gap states with resonant tunneling

Tunneling transport through the depletion layer under a GaAs {110} surface is studied with a low temperature scanning tunneling microscope (STM). The observed negative differential conductivity is due to a resonant enhancement of the tunneling probability through the depletion layer mediated by individual shallow acceptors. The STM experiment probes, for appropriate bias voltages, evanescent states in the GaAs band gap. Energetically and spatially resolved spectra show that the pronounced anisotropic contrast pattern of shallow acceptors occurs exclusively for this specific transport channel. Our findings suggest that the complex band structure causes the observed anisotropies connected with the zinc blende symmetry.     

Numerical analysis of shock wave and supersonic turbulent boundary interaction between adiabatic and cold walls

Direct numerical simulations of shock wave and supersonic turbulent boundary layer interaction in a 24° compression ramp with adiabatic and cold-wall temperatures are conducted. The wall temperature effects on turbulence structures and shock motions are investigated. The results are validated against previous experimental and numerical data. The effects of wall cooling on boundary layer characteristics are analysed. Statistical data show that wall cooling has a significant effect on the logarithmic region of mean velocity profile downstream the interaction region. Moreover, the influence of wall temperature on Reynolds stress anisotropy is mainly limited in the near-wall region and has little change on the outer layer. As the wall temperature decreases, the streamwise coherency of streaks increases. Based on the analysis of instantaneous Lamb vector divergence, the momentum transport between small-scale vortices on cold-wall condition is significantly enhanced. In addition, spectral analysis of wall pressure signals indicates that the location of peak of low-frequency energy shifts toward higher frequencies in cold case. Furthermore, the dynamic mode decomposition results reveal two characteristic modes, namely a low-frequency mode exhibiting the breathing motion of separation bubble and a high-frequency mode associated with the propagation of instability waves above separation bubble. The shape of dynamic modes is not sensitive to wall temperature.     

Metallic transport in a monatomic layer of in on a silicon surface

We have succeeded in detecting metallic transport in a monatomic layer of In on an Si(111) surface, Si(111)-sqrt[7]×sqrt[3]-In surface reconstruction, using the micro-four-point probe method. The In layer exhibited conductivity higher than the minimum metallic conductivity (the Ioffe-Regel criterion) and kept the metallic temperature dependence of resistivity down to 10?K. This is the first example of a monatomic layer, with the exception of graphene, showing metallic transport without carrier localization at cryogenic temperatures. By introducing defects on this surface, a metal-insulator transition occurred due to Anderson localization, showing hopping conduction.     

Coupled fire dynamics and thermal response of complex building structures

Simulation of the effects of severe fires on the structural integrity of buildings requires a close coupling between the gas phase energy release and transport phenomena, and the stress analysis in the load-bearing materials. The connection between the two is established primarily through the interaction of the radiative heat transfer between the solid and gas phases with the conduction of heat through the structural elements. This process is made difficult in large, geometrically complex buildings by the wide disparity in length and time scales that must be accounted for in the simulations. A procedure for overcoming these difficulties used in the analysis of the collapse of the World Trade Center towers is presented. The large scale temperature and other thermophysical properties in the gas phase are predicted using the NIST Fire Dynamics Simulator. Heat transfer to subgrid scale structural elements is calculated using a simple radiative transport model that assumes the compartment is locally divided into a hot, soot laden upper layer and a cool relatively clear lower layer. The properties of the two layers are extracted from temporal averages of the results obtained from the Fire Dynamics Simulator. Explicit formulae for the heat flux are obtained as a function of temperature, hot layer depth, soot concentration, and orientation of each structural element. These formulae are used to generate realistic thermal boundary conditions for a coupled transient three-dimensional finite element code. This code is used to generate solutions for the heating of complex structural assemblies.     

Transport mechanism of reverse surface leakage current in AlGaN/GaN high-electron mobility transistor with SiN passivation

The transport mechanism of reverse surface leakage current in the AlGaN/GaN high-electron mobility transistor(HEMT) becomes one of the most important reliability issues with the downscaling of feature size.In this paper,the research results show that the reverse surface leakage current in AlGaN/GaN HEMT with SiN passivation increases with the enhancement of temperature in the range from 298 K to 423 K.Three possible transport mechanisms are proposed and examined to explain the generation of reverse surface leakage current.By comparing the experimental data with the numerical transport models,it is found that neither Fowler-Nordheim tunneling nor Frenkel-Poole emission can describe the transport of reverse surface leakage current.However,good agreement is found between the experimental data and the two-dimensional variable range hopping(2D-VRH) model.Therefore,it is concluded that the reverse surface leakage current is dominated by the electron hopping through the surface states at the barrier layer.Moreover,the activation energy of surface leakage current is extracted,which is around 0.083 eV.Finally,the SiN passivated HEMT with a high Al composition and a thin AlGaN barrier layer is also studied.It is observed that 2D-VRH still dominates the reverse surface leakage current and the activation energy is around 0.10 eV,which demonstrates that the alteration of the AlGaN barrier layer does not affect the transport mechanism of reverse surface leakage current in this paper.     

Characteristics and parameter extraction for NiGe/n-type Ge Schottky diode with variable annealing temperatures

Current transport mechanism in Ni-germanide/n-type Ge Schottky diodes is investigated using current--voltage characterisation technique with annealing temperatures from 300~\duto 500~\du. Based on the current transport model, a simple method to extract parameters of the NiGe/Ge diode is presented by using the $I$--$V$ characteristics. Parameters of NiGe/n-type Ge Schottky diodes fabricated for testing in this paper are as follows: the ideality factor $n$, the series resistance $R_{\rm s}$, the zero-field barrier height $\phi _{\rm b0}$, the interface state density $D_{\rm it}$, and the interfacial layer capacitance $C_{\rm i}$. It is found that the ideality factor $n$ of the diode increases with the increase of annealing temperature. As the temperature increases, the interface defects from the sputtering damage and the penetration of metallic states into the Ge energy gap are passivated, thus improving the junction quality. However, the undesirable crystallisations of Ni-germanide are observed together with NiGe at a temperature higher than 400~\du. Depositing a very thin ($\sim $1~nm) heavily Ge-doped $n^{+}$ Ge intermediate layer can improve the NiGe film morphology significantly.     

Influence of Substrate on the Transportation Properties of Co/Alq3 Granular Films on a Si Wafer

A series of Co0.48 （Alq3）0.52 granular films were deposited on silicon substrates using the co-evaporating technique. A crossover of magnetoresistance （MR） from negative to positive was observed in the samples, due to conducting channel switching. The transport properties of samples are greatly influenced by hydrofluoric acid pretreatment, as a result, positive MR decreases drastically and the temperature dependence of resistance changes a lot near room temperature. The result indicates that the native oxide layer plays an important role in the transport mechanism. Moreover, different resistivities of Si substrates influence the current distribution of conducting channels, leading to different transport behaviors accordingly.     

Surface transport in premelted films with application to grain-boundary grooving

We present a new model of surface transport in premelted films that is applicable to a wide range of materials close to their melting points. We illustrate its use by applying it to the evolution of a grain-boundary groove in a high vapor pressure material and show that Mullins's classical equation describing transport driven by gradients in surface curvature is reproduced asymptotically. The microscopic contact angle at the groove root is found to be modified over a thin boundary layer, and the apparent contact angle is determined. An explicit transport coefficient is derived that governs the evolution rate of systems controlled by surface transport through premelted films. The transport coefficient is found to depend on temperature and diverges as the bulk melting temperature is approached.     

In-Si(111)界面上的电荷转移及铟原子的表面电致迁移现象

用反射式高能电子衍射仪首次观察到在Si(111)面上的一部分铟吸附原子在直流电场下,沿电场方向发生迁移的现象——表面电致迁移。根据所观察到的表面电致迁移过程,可以把吸附在Si(111)面上的铟原子的结合状态分成两类:紧靠着硅表面的一个单原子层铟与硅表面结合牢固,几乎不受电场影响,称为紧固层;在紧固层以上的铟层易受电场影响而发生表面电致迁移,称为迁移层。从铟原子的表面电致迁移率与温度的关系,求得表面质量迁移的激活能为0.43eV。用表面电导测量研究了In-Si(111)界面形成过程中的电荷转移现象。结果表明,吸附在硅表面的铟原子形成表面深施主能级。导致表面电致迁移的力是离化了的铟原子在电场中所受到的库仑力。 关键词：     

氨热法生长氮化镓晶体中传热传质的研究

本文利用基于非正交网格的二阶精度有限体积法,对氨热法生长过程中温度场和流场进行了模拟,其中隔板开孔率分别为10%(中心开孔5%,侧壁与隔板边缘开孔5%)和20%(中心开孔10%,侧壁与隔板边缘开孔10%).通过对流场和温度场的分析,了解了高压釜内部营养素的输运及溶液结晶的本质.结果显示在釜底的多孔介质层流动较弱,在流体层流动较强.在多孔介质层热量的输运主要通过热传导;在流体层中,流体与原料的分界处以及流体和高压釜的侧壁出现了大的温度梯度.     

Nonlinear Theory of Coupled Electron-Hole Transport

A theoretical investigation is carried out to examine the Coulomb interaction effects in linear and nonlinear transport of a coupled electron-hole system which consists of an electron layer and a hole layer separated by a potential barrier. In this we apply the balance equation method to examine the induced current in one charged layer when the other layer is subject to a constant electric field. Nonlinear balance equations are developed for both the electrons and the holes, with Coulomb interactions among them, as well as interactions of the charge carriers with phonons and impurities. Numerically obtained linear results exhibit qualitative agreement with experimental data in regard to dependencies on temperature and carrier densities. Nonlinear effects are shown to be important at low temperatures, in agreement with experimental results.