First-principles study of metal-semiconductor contact between MX2 (M = Nb,Pt; X = S,Se) monolayers

First principles calculations are performed to investigate the structural and electronic properties of MX₂ (M = Nb, Pt; X = S, Se) monolayers and their van der Waals (vdW) heterostructures. The dynamical stability of monolayers and vdW heterostructures is confirmed by binding energy and phonon spectra. An indirect band gap nature is found for PtS₂ and PtSe₂ monolayers while NbS₂, NbSe₂ and all vdW heterostructures are metals. The intrinsic electronic properties of both NbX₂ and PtX₂ are well preserved due to weak vdW contact. It is demonstrated that a p-type Schottky contact with a small barrier height is formed at NbX₂-PtX₂ interface. The zero tunnel barrier and higher potential drop across the interface in these contacts imply large transfer of charge carriers across the interface, making them potential candidates in nanoelectronic device applications.     

Band structure of non-ideal semiconductor superlattices

An ideal superlattice is an array of two (or more) alternating layers of materials with a single period, fixed barrier height and infinitely abrupt interfaces. A real superlattice differs from an ideal one in many aspects and this affects the band structure. They include unsharp interfaces, interface disorder, small fluctuations in thicknesses of quantum wells (barriers) and in the potential barrier heights from layer to layer. The band structure of superlattices in these realistic cases have been investigated. The relevance of these non-ideal cases to the shifting of the ground-state energy of the electrons, holes, and the effective energy gap are also discussed.     

铁磁-高Tc氧化物超导结中的粗糙界面散射效应对隧道谱的影响

考虑到铁磁层中的磁交换作用,以及界面的粗糙散射效率,利用推广的Blonder-Tin-kham-Klapwijk理论模型,计算铁磁－高Tc氧化物超导结中的隧道谱,研究表明：（1）铁磁层中的磁交换能能压零偏压电导峰的高度,并能感应出零偏电导凹隐;（2）粗糙界面散射除了高有压低零偏压电导峰,还有使零偏压凹隐处感应出一中心峰,该结果能较好地解释最近的两篇关于La2/3Ba1/3MnO3/DyBa2Cu3O7与La2/3Ba1/3MnO3/YBa2Cu3O7-δ结中隧道谱的实验报道。     

界面空间电荷对双层有机电致发光器件效率的影响

本文建立了双层有机电致发光器件中载流子在有机/有机界面复合的无序跳跃理论模型.计算表明:①内界面处电子和空穴的有效势垒高义决定OLEDs中的电子和空穴密度的分布,而电子与空穴密度又决定了电场强度的大小;且复合效率随着有效势垒高度的增加而增加;②当电压较低时,复合效率随载流子有效跳跃距离的增加而增加;当电压较高时,复合效率随载流子的有效跳跃距离的增加而减少;③当界面场强差较小时,有机层界面场强突变增大,复合效率增大,当界面场强差达临界值时,复合效率反而随着界面场强差的增大而减小.该理论模型可较好地解释相关的实验现象.     

掺杂对金属-MoS_2界面性质调制的第一性原理研究

采用基于密度泛函理论的第一性原理赝势平面波方法,计算了卤族元素掺杂对金属-MoS_2界面性质的影响,包括缺陷形成能、电子能带结构、差分电荷密度以及电荷布居分布.计算结果表明:卤族元素原子倾向于占据单层MoS_2表面的S原子位置;对于单层MoS_2而言,卤族元素的掺杂将在禁带中引入杂质能级以及导致费米能级位置的移动.对于金属-MoS_2界面体系,结合Schottky-Mott模型,证明了卤族元素的掺杂可以有效地调制金属-MoS_2界面间的肖特基势垒高度.发现F和Cl原子的掺杂将会降低体系的肖特基势垒高度.相比之下,Br和I原子的掺杂却增大了体系的肖特基势垒高度.通过差分电荷密度和布居分布的分析,阐明了肖特基势垒高度的被调制是因为电荷转移形成的界面偶极矩的作用导致.研究结果解释了相关实验现象,并给二维材料的器件化应用提供了调节手段.     

应变纤锌矿GaN/AlxGa1-xN柱形量子点中类氢施主杂质态结合能的压力效应

考虑应变,在有效质量、有限高势垒近似下,变分研究了纤锌矿GaN/AlxGa1-xN柱形量子点中类氢施主杂质态结合能随流体静压力、杂质位置及量子点结构参数(量子点高度、半径、Al含量)的变化关系．结果表明,类氢施主杂质态结合能随流体静压力增大而增大,且在量子点尺寸较小时,流体静压力对杂质态结合能的影响更为显著．受流体静压力的影响,杂质态结合能随量子点高度、半径的增加而单调减少,且变化趋势加剧;随Al含量增加而增大的趋势变缓．无论是否施加流体静压力,随着类氢施主杂质从量子点左界面沿材料生长方向移至右界面,杂质态结合能在量子点的右半部分存在一极大值．流体静压力使得极大值点向量子点中心偏移．     

Evaluation of parameters of the potential barrier at the metal/polymer interface during recovery of the structure of nanocrystalline nickel

The influence of nanostructured nickel on the parameters of the potential barrier for conduction electrons at the metal/polymer interface has been investigated. It has been shown that conductivity in the multilayer nickel-polymer-vanadium system changes during annealing of nanocrystalline nickel at temperatures of the onset of the recovery of the structure. It has been established that the formation of the structure with a grain size of 100 nm in nickel leads to a noticeable decrease in the potential barrier height at the metal/polymer interface.     

Effect of interface roughness on the density of states of finite barrier height quantum wells

We calculate the density of states of a 2D electron gas in finite barrier height quantum wells with the explicit inclusion of the interface roughness effect. By using Feynman path-integral method, the analytic expression is derived. The results show that the 2D density of states is dependent on the RMS of the fluctuation potential. The interface roughness causes localized states below the subband edge. We also apply the theory to model the finite barrier height quantum wells in Al_xGa_1?xAs/GaAs.     

铝-金刚石界面电子特性与界面肖特基势垒的杂化密度泛函理论HSE06的研究

宽带隙半导体金刚石具有突出的电学与热学特性,近年来,基于金刚石的高频大功率器件受到广泛关注,对于金属-金刚石肖特基结而言,具有较高的击穿电压和较小的串联电阻,所以金属-金刚石这种金半结具有非常好的发展前景.本文通过第一性原理方法去研究金属铝-金刚石界面电子特性与肖特基势垒的高度.界面附近原子轨道的投影态密度的计算表明:金属诱导带隙态会在金刚石一侧产生,并且具有典型的局域化特征,同时可以发现电子电荷转移使得Fermi能级在金刚石一侧有所提升.电子电荷在界面的重新分布促使界面形成新的化学键,使得金属铝-氢化金刚石形成稳定的金半结.特别地,我们通过计算平均静电势的方法得到金属铝-氢化金刚石界面的势垒高度为1.03 eV,该值与金属诱导带隙态唯像模型计算的结果非常接近,也与实验值符合得很好.本文的研究可为金属-金刚石肖特基结二极管的研究奠定理论基础,也可为金刚石基金半结大功率器件的研究提供理论参考.     

异质结界面电荷对突变InP/InGaAs异质结双极晶体管热场发射影响研究

异质结界面电荷的存在改变了异质结的内建势, 这引起了界面势垒尖峰高度和形状的扰动, 从而使异质结界面载流子的输运产生相应的变化, 最终导致异质结双极晶体管 (HBT) 性能的改变. 基于热场发射-扩散模型, 对异质结界面电荷对InP/InGaAs HBT性能的改变做了研究, 得到结论是正极性的界面电荷有利于InP/InGaAs HBT的直流和高频特性的改善, 而负极性的界面电荷则使器件的直流和高频特性变差. 关键词： InP/InGaAs异质结双极晶体管 界面电荷 内建势 热场发射     

尺寸效应对微通道内固液界面温度边界的影响

采用非平衡分子动力学方法模拟不同浸润性微通道内液体的传热过程,分析了尺寸效应对固液界面热阻及温度阶跃的影响.研究结果表明,界面热阻随微通道尺寸的变化可分为两个阶段,即小尺寸微通道的单调递增阶段和大尺寸微通道的恒定值阶段.随着微通道尺寸的增加,近壁区液体原子受对侧固体原子的约束程度降低,微通道中央的液体原子自由移动,固液原子振动态密度近似不变,使得尺寸效应的影响忽略不计.上述两种阶段的微通道尺寸过渡阈值受固液作用强度与壁面温度的共同作用:减弱壁面浸润性,过渡阈值向大尺寸区域迁移;相较于低温壁面,高温壁面处的过渡阈值更大.增加微通道尺寸,固液界面温度阶跃呈单调递减趋势,致使壁面温度边界和宏观尺度下逐渐符合.探讨尺寸效应有助于深刻理解固液界面能量输运及传递机制.     

ELECTRON-INTERFACE PHONON SCATTERING IN ASYMMETRIC SEMICONDUCTOR QUANTUM WELL STRUCTURES

Electron-interface phonon scattering rates in asymmetric single quantum well and step quantum well structures are calculated by means of the interaction Fr?hlich-like Hamiltonian between an electron and interface optical phonons in a four-layer heterostructure given re-cently. The intrasubband and intersubband electron scattering rates are given as functions of quantum well width, step width and step height. We have found that the electron scattering depends strongly on the potential parameters and the usual selection rules for these tran-sitions are broken down in asymmetric heterostructures; the interface LO modes are more important than the interface TO modes for the electron-interface phonons scattering in het-erostructures; the intrasubband scattering rates are insensitive functions of step width and step height, and the intersubband scattering rates are complicated functions of step height and step width in step quantum wells. Moreover, we have also observed that the scattering rates for intrasubband and intersubband transitions have no obvious changes in the case that the first or second subband energy level crosses the step height in a step quantum well.     

肖特基势垒对铁磁/有机半导体结构自旋注入性质的影响

理论研究了铁磁/有机半导体肖特基接触时的电流自旋极化注入,并讨论了电流自旋极化率随界面处肖特基势垒高度、有机半导体层中特殊载流子及其迁移率、界面附近掺杂浓度的变化关系.通过计算发现,寻找在势垒区中载流子迁移率比较大的有机半导体材料对实现有效的自旋注入是必要的;同时还发现,由于铁磁/有机半导体接触而形成的肖特基势垒不利于自旋注入.因此要想实现有效的自旋注入,界面附近必须采用重掺杂来有效减少势垒区的宽度,且势垒的高度要限制在一定的范围内.     

周期场时间导数Ornstein-Uhlenbeck噪声Fokker-Planck方程的小参数展开求解

通过引入变量,周期场中内部时间导数Ornstein-Uhlenbeck噪声驱动的布朗运动可用高维Fokker-Planck方程来描述. 上述系统不能直接应用通常的小参数展开和势谷中心展开近似求解. 用一种变通的小参数展开方法近似求解了系统的Fokker-Planck方程,结果适用于小势垒高度、中等关联时间和较大的相空间区域,近似解析解可获得系统的改进. 关键词： Fokker-Planck方程 周期势 时间导数Ornstein-Uhlenbeck噪声 小参数展开     

The effect of capping layers on low-angle Bragg peaks from copper-cobalt multilayers

Using simulations based on the Parratt theory for grazing incidence X-ray specular reflectivity, we show that small variations in the thickness of the gold cap used to prevent oxidation in Co/Cu multilayers can have a dramatic effect on the height and position of the low-order Bragg peaks from these multilayers. The height of the specular Bragg peak cannot be used as a measured of interface roughness nor its position used to determine multilayer period unless the cap thickness is known or determined precisely by simulation.     

Effects of the electric field shape on nano-scale oxidation

An axisymmetric continuum model for oxide growth by the scanned probe oxidation technique is presented. The model includes equations describing the electric fields, hydroxyl and hydrogen ion concentrations, and oxide free boundaries defining the system. The governing system of partial differential equations tracks ion transport in the liquid and oxide layers and includes reactions at the substrate/oxide interface. Further, space charge effects are considered near the substrate/oxide interface. Two liquid configurations, semi-infinite layer and hemispherical drop of liquid, are examined to determine the potential in the liquid region. The AFM tip is modeled as either a point or a ring source of charges. The asymptotic limit of a small aspect ratio (height to width) oxide feature is used to reduce the governing equations to a quasi-one-dimensional system to determine the ion transport. The solution of the reduced system leads to an evolution equation for the oxide thickness and radius. Numerical simulations of the evolution equation predict oxide features that qualitatively agree with experimental observations. A parametric study is conducted to determine the influence of key operating and material parameters on the thickness and radius of the oxide, and the electric and ion concentration fields in the system.     

Dynamic response of electrons and dispersion force fields near bimetallic interfaces

The induced potential due to an oscillating charge source near the interface of two metals whose electron densities differ slightly, is calculated in the hydrodynamic approximation. Results are given for both planar and spherical interfaces. The potential is used to evaluate the dispersion force field on an atomic system near the interface. It is shown that the force field is divergence-free at very small separation from the planar interface if the electron density profile is diffuse.     

On reducing interface curvature computation errors in the height function technique

A detailed analysis of the errors involved in computing the interface curvature from volume fraction distributions using a height function technique is presented. An improved version of the height function technique is proposed, based on introducing a correction of the height function discretization error estimated from the local osculating spheres at interface points. By using this error correction and an appropriate discretization of the partial derivatives of the height function, a substantial improvement in the accuracy of the interface curvature computation can be efficiently achieved.     

Influence of parameters of the potential barrier at the metal/polymer interface on the electronic switching in the metal/polymer/metal structure

The influence of the first-order phase transition on the parameters of the potential barrier at the indium/polymer interface has been investigated. It has been established that the phase transition occurring in the metal initiates switching of the polymer insulator into a high-conductivity state. Performed investigations have shown that the main charge transfer mechanism in the metal-polymer-metal structure at high temperatures is the current caused by the electron thermionic emission. The analysis of current-voltage characteristics has demonstrated that the first-order phase transition in indium leads to the variation in the potential barrier height at the metal/polymer interface by Δφ ≈ 0.18 eV. It is this phenomenon that is responsible for the electronic switching.     

Interface polarons in a realistic heterojunction potential

The ground states of interface polarons in a realistic heterojunction potential are investigated by considering the bulk and the interface optical phonon influence. A self-consistent heterojunction potential is used and an LLP-like method is adopted to obtain the polaron effect. The numerical computation has been done for the Zn_1-xCd_xSe/ZnSe system to obtain the polaron ground state energy, self energy and effective mass parallel to the interface. A simplified coherent potential approximation is developed to obtain the parameters of the ternary mixed crystal and the energy band offset of the heterojunction. It is found that at small Cd concentration the bulk longitudinal optical phonons give the main contribution for lower areal electron densities, whereas the interface phonon contribution is dominant for higher areal electron densities. The interface polaron effect is weaker than the effect obtained by the three dimensional bulk phonon and by the two dimensional interface phonon models. Received 17 September 1998