基于间接跃迁模型的p+-GaAsSb费米能级研究

重p型掺杂GaAsSb广泛应用于InP HBT基区材料,重掺杂影响GaAsSb材料的带隙和费米能级等重要参数,这些参数对设计高性能HBT起着关键作用。本文通过间接跃迁模型研究了p⁺-GaAsSb材料的荧光性质,以及费米能级与Sb组分的关系。由于费米能级与空穴有效质量m_h和空穴态密度n_h存在函数关系,我们通过荧光测量并计算了空穴有效质量m_h和空穴态密度n_h,研究结果表明m_h和n_h共同主导了费米能级的变化。     

p型Na掺杂各向异性ZnO的电子性质与热电输运性质

采用密度泛函理论的第一性原理计算研究了p型Na掺杂各向异性ZnO的能带结构、光学性质、介电性质、总态密度和不同原子的分态密度,并系统分析了其热电输运性质。计算分析结果表明,p型Na掺杂ZnO为p型直接带隙半导体,其带隙增大到1.3eV,其对于光子的吸收限向低能量光子移动,体系费米能级附近的态密度大幅度提高,这主要是p态电子贡献的;在费米能级附近的导带和价带中都出现了新的能级,这些新的能级主要由Nas、Nap、Znp、Znd和Op电子形成,且他们之间存在着强耦合相互作用。Na掺杂ZnO的电输运性质具有各向异性;其价带和导带中的载流子有效质量均较大;载流子输运主要由Nas、Nap、Znp和Op电子完成。     

Na掺杂CaMnO_3晶体材料的结构调控、电子结构与光吸收性质的研究

在平面波和赝势法基础上研究了Ca位Na掺杂的钙钛矿型CaMnO_3晶体材料的晶体结构、电子结构和光吸收性质,分析了Na掺杂CaMnO_3晶体材料的电输运过程。结果表明,Na掺杂Ca位使CaMnO_3的晶胞三轴长度均增大,Na掺杂在CaMnO_3中引入了晶格畸变,且晶格畸变是各向异性的。Na掺杂之后,CaMnO_3晶体材料内的O-Mn-O形成的八面体逐步向两个O顶点方向拉长和扭曲。未掺杂和Na掺杂CaMnO_3带隙宽度分别为0.713eV和0.686eV,均呈现明显的半导体特性。在费米能级附近,s态电子对CaMnO_3的态密度贡献最小,p态电子对费米能级以下的态密度贡献最大,d态电子对费米能级以上的态密度贡献最大。Mnp态电子对费米能下方能级形成贡献较大,而Mnd态电子对费米能上方能级形成贡献最大,Op态电子对费米能下方能级形成贡献较大,而Op态电子对费米能上方能级形成贡献最小。与未掺杂CaMnO_3相比,Na掺杂CaMnO_3晶体材料吸收能量向低能量移动,且存在两个强吸收峰,其在6.15eV附近的吸收最强。     

超晶格SnO2掺Cr的电子结构和光学性质的研究

基于密度泛函理论的第一性原理,采用全势线性缀加平面波方法(FPLAPW)和广义梯度近似(GGA)来处理相关能,计算了Cr掺杂SnO₂超晶格的电子态密度、能带结构、介电函数、吸收系数、反射率和折射率.研究表明由于Cr的掺入,超晶格SnO₂在费米能级附近形成了新的电子占据态,出现了不连续的杂质能带,这是由Cr-3d态和O-2p,Sn-5s态电子所形成.介电谱在0—5.5 eV之间时出现了三个新的介电峰,在高能区介电谱主峰位置发生蓝移,峰值强度减小.吸收谱、反射谱和折 关键词： 超晶格 第一性原理 态密度 电子结构     

微晶硅薄膜带隙态及微结构的研究

研究了氢化微晶硅薄膜费米能级以上的带隙态密度分布与薄膜微结构关系.采用拉曼谱和红外谱表征不同H稀释比制备的微晶硅薄膜的微结构.薄膜带隙态密度分布由调制光电流的相移分析技术测得.采用三相模型（非晶相、晶相和界面相）分析了薄膜带隙态密度与薄膜微结构的关系.结果表明,材料的带隙态密度随着界面相的增加而增加,当界面体积分数达到最大时,薄膜的带隙态密度也最大,即材料的带隙态密度与界面体积分数正相关. 关键词： 带隙态 界面相 微晶硅 调制光电流     

第一性原理研究Fe掺杂SnO2材料的光电性质

采用基于第一性原理的线性缀加平面波(FP-LAPW)方法,研究Fe掺杂SnO₂材料电子结构和光学性质,包括电子态密度、能带结构、介电函数和其他一些光学图谱. 研究结果表明,掺Fe后材料均属于直接跃迁半导体,且呈现半金属性;随掺杂浓度增加,费米能级进入价带,带隙逐渐减小,Fe原子之间耦合作用增强;通过掺杂能够在一定程度上改变成键性质,使其具有金属键性质. 光学谱线(吸收谱、消光系数等)与介电函数虚部谱线相对应,均发生蓝移,各峰值与电子跃迁吸收有关,从理论上指出光学性质和电子结构的内在联系. 关键词： 能带结构 态密度 光学性质 介电函数     

GaN/SiC异质结的慢正电子研究

通过对不同生长厚度GaN/SiC(n-n)的慢正电子研究,发现在GaN/SiC的界面中存在大量各种缺陷并在界面两端形成两个不同方向的电场. 这些缺陷的产生和SiC衬底表面制备以及GaN和SiC不同的热膨胀系数有关. 而缺陷中大量的带状缺陷在界面中形成一个费米能级钉扎(Fermi level pinning),它的存在使界面中存在一定高度的势垒,导致在界面两端的一定区域内形成两个不同方向的电场. 用VEPFIT模拟该电场的存在,分四层(GaN/Interface/SiC1/SiC2)进行拟合,得到了很好的拟 关键词： 正电子湮没 缺陷 半导体     

TiO_2缺陷结构电子性质的第一性原理计算

本文采用第一性原理研究了O空位缺陷、Ti空位缺陷TiO_2的能带结构、总态密度、吸收谱.通过研究发现:与TiO_2超胞结构的能带相比,O空位缺陷体系的价带与导带能量均向低能区域移动,费米能级与导带底交错,呈现出n型半导体,Ti空位缺陷的TiO_2的费米能级与价带顶部的能级交错,为p型半导体材料.对于O空位缺陷TiO_2总态密度与分波态密度在低能区的态密度则主要由O的3s、3p轨道贡献的能量,而在费米能附近的态密度则主要由Ti的4d轨道贡献能量;Ti空位缺陷的态密度总态密度仍然由O的3s、3p和Ti的4d轨道贡献的能量;同时分析吸收光谱发现峰值下移较多的是钛缺陷体系,其原因在于Ti缺陷结构中未成键电子的相互作用.     

掺杂分子束外延GaAs薄膜表面和GaAs-GaAs界面的光反射调制谱

报道了用光反射调制谱(PR)测量掺杂分子束外延GaAs薄膜表面和界面电场的结果。分别用He-Ne激光和He-Cd激光作调制光,由于它们的穿透深度不同,可以有效地区分来自表面和界面的PR信号。由PR谱推算出薄膜表面和界面的电场。研究了薄膜干涉效应对调制光谱的影响,对界面电场的成因进行了分析和讨论。 关键词：     

N/B掺杂石墨烯的光学与电学性质

石墨烯因其独特的化学成键结构而拥有出色的化学、热学、机械、电学、光学特性.由于石墨烯为零带隙材料,限制了其在纳电子学领域的发展.因此,为了拓宽石墨烯的应用范围,研究打开石墨烯带隙的方法显得尤为重要.本文构建了本征石墨烯、N掺杂石墨烯、B掺杂石墨烯三种模型,研究了本征石墨烯和不同掺杂浓度下的N/B掺杂石墨烯的能带结构、电子态密度及光学与电学性质,包括吸收谱、反射谱、折射率、电导率和介电函数等.研究结果显示:1)本征石墨烯费米能级附近的电子态主要是由C-2p轨道形成,而N/B掺杂石墨烯费米能级附近的电子态主要是由C-2p和N-2p/B-2p轨道杂化形成;2)N/B掺杂可以引起石墨烯费米能级、光学与电学性质的改变,且使狄拉克锥消失,进而打开石墨烯带隙;3)N/B掺杂可以引起石墨烯光学和电学性质的变化,且对吸收谱、反射谱、折射率、介电函数影响较大,而对电导率影响较小.本文的结论可为石墨烯在光电子器件中的应用提供理论依据.     

Lattice structures and electronic properties of WZ-CuInS_2/WZ-CdS interface from first-principles calculations

Using the first-principles plane-wave calculations within density functional theory, the perfect bi-layer and monolayer terminated WZ-CIS(100)/WZ-Cd S(100) interfaces are investigated. After relaxation the atomic positions and the bond lengths change slightly on the two interfaces. The WZ-CIS/WZ-Cd S interfaces can exist stably, when the interface bonding energies are-0.481 J/m~2(bi-layer terminated interface) and-0.677 J/m~2(monolayer terminated interface). Via analysis of the density of states, difference charge density and Bader charges, no interface state is found near the Fermi level.The stronger adhesion of the monolayer terminated interface is attributed to more electron transformations and orbital hybridizations, promoting stable interfacial bonds between atoms than those on a bi-layer terminated interface.     

Interface effects in spin-polarized metal/insulator layered structures

Recent advances in thin-film deposition techniques, such as molecular beam epitaxy and pulsed laser deposition, have allowed for the manufacture of heterostructures with nearly atomically abrupt interfaces. Although the bulk properties of the individual heterostructure components may be well-known, often the heterostructures exhibit novel and sometimes unexpected properties due to interface effects. At heterostructure interfaces, lattice structure, stoichiometry, interface electronic structure (bonding, interface states, etc.), and symmetry all conspire to produce behavior different from the bulk constituents. This review discusses why knowledge of the electronic structure and composition at the interfaces is pivotal to the understanding of the properties of heterostructures, particularly the (spin polarized) electronic transport in (magnetic) tunnel junctions.     

Electronic reconstruction at SrMnO3-LaMnO3 superlattice interfaces

We use resonant soft-x-ray scattering (RSXS) to study the electronic reconstruction at the interface between the Mott insulator LaMnO3 and the band insulator SrMnO3. Superlattices of these two insulators were shown previously to have both ferromagnetism and metallic tendencies [Koida, Phys. Rev. B 66, 144418 (2002)10.1103/PhysRevB.66.144418]. By studying a judiciously chosen superlattice reflection, we show that the interface density of states exhibits a pronounced peak at the Fermi level, similar to that predicted in related titanate superlattices by Okamoto et al. [Phys. Rev. B 70, 241104(R) (2004)10.1103/PhysRevB.70.241104]. The intensity of this peak correlates with the conductivity and magnetization, suggesting it is the driver of metallic behavior. Our study demonstrates a general strategy for using RSXS to probe the electronic properties of heterostructure interfaces.     

Enhancement of laser-induced field emission in tungsten due to a metastable d band

A possible theory explaining the experimentally observed large enhancement of the field emission current from a tungsten needle exposed to a laser beam is presented. Calculations based on the density functional theory indicate a large density of 5d states and less dense 6p states above the Fermi level. The incident laser beam of 2.5 eV pumps the electrons to an unoccupied 6p band, some of which then de-excite to 5d states at lower energies, forming metastable states since the radiative decay from 5d states to the ground state 6s band is forbidden. On the other hand, the tunneling time from the metastable state is shorter than that from the ground state because of a smaller surface barrier, which may lead to the observed enhancement of the emitted current.     

Fermi Arcs and DC Transport in Nanowires of Dirac and Weyl Semimetals

The transport properties and electron states in cylinder nanowires of Dirac and Weyl semimetals are studied paying special attention to the structure and properties of the surface Fermi arcs. The latter make the electric charge and current density distributions in nanowires strongly nonuniform as the majority of the charge density is accumulated at the surface. It is found that a Weyl semimetal wire also supports a magnetization current localized mainly at the surface because of the Fermi arcs contribution. By using the Kubo linear response approach, the direct current (DC) conductivity is calculated and it is found that its spatial profile is nontrivial. By explicitly separating the contributions of the surface and bulk states, it is shown that when the electric chemical potential and/or the radius of the wire is small, the electron transport is determined primarily by the Fermi arcs and the electrical conductivity is much higher at the surface than in the bulk. Due to the rise of the surface-bulk transition rate, the relative contribution of the surface states to the total conductivity gradually diminishes as the chemical potential increases. In addition, the DC conductivity at the surface demonstrates noticeable peaks when the Fermi level crosses energies of the surface states.     

Lattice structures and electronic properties of CIGS/CdS interface:First-principles calculations

Using first-principles calculations within density functional theory, we study the atomic structures and electronic properties of the perfect and defective （2VCu＋ Incu） CulnGaSe2/CdS interfaces theoretically, especially the interface states. We find that the local lattice structure of （2VCu＋ InCu） interface is somewhat disorganized. By analyzing the local density of states projected on several atomic layers of the two interfaces models, we find that for the （2VCu＋InCu） interface the interface states near the Fermi level in CulnGaSe2 and CdS band gap regions are mainly composed of interracial Se-4p, Cu-3d and S-3p orbitals, while for the perfect interface there are no clear interface states in the CulnGaSe2 region but only some interface states which are mainly composed of S-3p orbitals in the valance band of CdS region.     

Metallization and Schottky-barrier formation for Se-passivated GaAs(1 0 0) interfaces

Using a molecular dynamics DFT-LDA code, we have analyzed the Schottky-barrier formation of a Se-passivated GaAs(1 0 0)-2×1 reconstruction. In our approach we consider, first, the energetically most favorable interfaces formed by the deposition of either one or two Ga atoms per surface unit cell; then, we analyze the electron density of states and calculate the interface Fermi level and the Schottky-barrier height. We show that the height depends essentially on the very same interface geometry. In particular, the effect of exchanging Ga and Se atoms at the interface (an intermixing process) yields a normal Schottky-barrier height, while the normal passivated surface yields an ohmic contact.     

Back-to-back Schottky diodes: the generalization of the diode theory in analysis and extraction of electrical parameters of nanodevices

We report on the analysis of nonlinear current-voltage characteristics exhibited by a set of blocking metal/SnO(2)/metal. Schottky barrier heights in both interfaces were independently extracted and their dependence on the metal work function was analyzed. The disorder-induced interface states effectively pinned the Fermi level at the SnO(2) surface, leading to the observed Schottky barriers. The model is useful for any two-terminal device which cannot be described by a conventional diode configuration.     

Bias-induced reconstruction of hybrid interface states in magnetic molecular junctions

Based on first-principles calculations, the bias-induced evolutions of hybrid interface states in π-conjugated tricene and in insulating octane magnetic molecular junctions are investigated. Obvious bias-induced splitting and energy shift of the spin-resolved hybrid interface states are observed in the two junctions. The recombination of the shifted hybrid interface states from different interfaces makes the spin polarization around the Fermi energy strongly bias-dependent. The transport calculations demonstrate that in the π -conjugated tricene junction, the bias-dependent hybrid interface states work efficiently for large current, current spin polarization, and distinct tunneling magnetoresistance. But in the insulating octane junction, the spin-dependent transport via the hybrid interface states is inhibited, which is only slightly disturbed by the bias. This work reveals the phenomenon of bias-induced reconstruction of hybrid interface states in molecular spinterface devices, and the underlying role of conjugated molecular orbitals in the transport ability of hybrid interface states.     

Influence of initial surface reconstruction on the interface structure of HfO2/GaAs

We show that the bonding structures and electrical properties of the HfO₂/GaAs interface can be controlled by a choice of the reconstruction on the initial GaAs surface. Electron-beam evaporation of HfO₂ onto the c(4 × 4) surface yielded As-O bonds at the interface, while Ga-O bonds were dominant at the interfaces formed on the (2 × 4) and (4 × 6) surfaces. Influences of the initial surface reconstruction on the interface structure persisted even after annealing at 673 K. Electrical characterization of Ir/HfO₂/GaAs capacitors indicated that the interfacial As-O bonds cause weak Fermi level pinning. It was also suggested that the interfaces dominated by the Ga-O bonds have trapping states in the upper half of the GaAs bandgap.