蒙特卡罗方法模拟薄膜电致发光器件中碰撞离化的作用

基于经验赝势法得到的能带结构数据，采用分段多项式拟合获得ZnS能带结构的解析表达式 ，建立解析能带模型.使用建立的模型计算得到各能谷的态密度和总的散射速率,并与文献 的计算结果进行了对比，验证该解析能带模型既具有非抛物型多能谷能带模型运算速度快、 使用方便的优势,又具有与采用全导带模型相近的计算精度.进一步利用该模型进行蒙特卡罗 模拟，得到第一导带和第二导带中电子数随电场强度的变化、不同电场中能量分布函数以及 包含与不包含碰撞离化情况下电子能量随时间变化的曲线.讨论在外加电场下，电子在导带 内各个能谷间和 关键词： 蒙特卡罗模拟 解析能带模型 多项式拟合 碰撞离化     

Ga掺杂ZnO的电子结构与电性能的研究

采用密度泛函理论广义梯度近似第一性原理计算的方法研究了n型Ga掺杂的纤锌矿结构氧化物ZnO的晶格结构、能带结构和态密度,在此基础上分析了其电性能.计算结果表明,掺杂ZnO氧化物晶格a,b轴增大,c轴略有减小;Ga掺杂ZnO氧化物两能带之间具有0.6eV的直接带隙,需要载流子(电子)跃迁的能隙宽度较未掺杂的ZnO氧化物减小;掺杂体系费米能级附近的态密度大大提高,其能带主要由Gas态、Zns态和Os态电子构成,且他们之间存在着强相互作用,其中Gas态电子对导带贡献最大.电输运性能分析结果表明,Ga掺杂ZnO氧化物导电机构由Znp-Op电子在价带与导带的跃迁转变为Gas-Znd-Os电子在价带与导带的跃迁,这也表明Gas态电子在导电过程中的重要作用;掺杂体系费米能级附近的载流子有效质量较未掺杂体系增大,且价带中的载流子有效质量较大,导带中的载流子有效质量较小.     

用Monte Carlo方法模拟闪锌矿相ZnS电子的输运特性

用Monte Carlo方法模拟闪锌矿相(zinc blende)ZnS电子的输运特性.实验采用的是非抛物线模型计算电子的能带结构,模拟包含了声学声子散射,极性光学声子散射,压电散射,电离杂质散射,能谷间散射以及自散射等散射机理.通过模拟得到了ZnS材料的平均漂移速度、平均电子能量随电场强度变化的曲线图,以及总散射率随电子能量变化图,并将结果与文献报道的模拟结果[1]进行比较得出:本实验方法具有模型简单,计算速度快,获得结果比较准确的优点.     

电致发光加速层二氧化硅的电子高场迁移率

根据非晶态半导体的能带理论，讨论了分层优化薄膜电致发光方案中非晶二氧化硅加速层中的电子在高电场中的输运行为.研究结果表明：在高电场下，由于电场的存在降低了陷阱之间的平均势垒高度.在费密能级附近处的杂质及缺陷定域态和导带尾定域态中，电子的输运主要表现为电场增强的热辅助式跳跃传导；而在导带扩展态中，电子的输运仍像晶态半导体那样表现为共有化运动.此外，以实验数据为基础，计算出了非晶二氧化硅中电子的迁移率、最小金属电导率、导带迁移率边界状态密度及费密能级处的状态密度. 关键词：     

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。     

Y、Zr、Nb掺杂ZnS半导体第一性原理计算

本文采用第一性原理研究了Y,Zr,Nb在Zn位掺杂ZnS半导体的能带结构和态密度.研究结果表明:Y,Zr,Nb掺杂体系的价带顶与导带底都在布里渊区的G点,为直接带隙半导体材料,掺杂对带隙宽度影响不大,掺杂结构的导带向低能区移动;Y,Zr,Nb在Zn位掺杂的Zn S的费米能级从价带顶移至导带底,说明掺杂后ZnS半导体材料从p型转变为n型,同时费米能级处出现数条杂质能级;Y,Zr,Nb掺杂体系的总态密度的贡献主要来源于Zn 4s、Y,Zr,Nb的4d 5s以及S 3p相互作用.     

C掺杂TiO薄膜的制备及其第一性原理研究

TiO在微电子结构器件中有重要的应用前景. 本文以CO₂作反应气体, 采用直流反应磁控溅射方法成功制备出C掺杂TiO薄膜. 采用XRD, XPS和四探针电阻计对薄膜结构、成分和电阻率进行表征. 在实验结果的基础之上建立起TiO和C掺杂TiO的计算模型并采用第一性原理 方法计算其能带结构和电子态密度. 实验结果表明薄膜相结构为面心立方的岩盐结构, C取代O的阴离子掺杂为主要掺杂方式, 薄膜电阻率为52.2 μΩ·cm. 第一性原理计算结果表明, 费米能级穿过TiO的导带, TiO具有金属性导电的能带结构特征; C掺杂TiO 后, 其金属性导电的能带结构没有改变, 只是在费米面附近出现C 2p态提供的杂质能级, 杂质能级扩展了TiO的导带宽度并提高了费米面附近的电子能态密度, 从而导至TiO电导增加, 电阻率降低. 第一性原理计算结果与实验结果一致. 关键词： C掺杂TiO 直流反应磁控溅射 第一性原理 电子结构     

Mn高掺杂浓度对ZnO禁带宽度和吸收光谱影响的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波超软赝势方法, 建立了未掺杂与不同浓度的Mn原子取代Zn原子的三种Zn_1-xMn_xO超胞模型, 分别对模型进行了几何结构优化、态密度分布、能带分布和吸收光谱的计算. 结果表明: 电子非自旋极化处理的条件下, Mn掺杂浓度越小, ZnO形成能越小, 掺杂越容易, 晶体结构越稳定; Mn的掺入使得ZnO体系的杂质能带和导带发生简并化, 并且导带底和价带底同时向低能方向移动, 掺杂后的导带比价带下降得少导致禁带宽度变宽, ZnO吸收光谱明显出现蓝移现象, 计算结果和实验结果相一致. 同时, 电子自旋极化处理的条件下, 体系有磁性, 吸收光谱发生红移现象. 计算结果与相关实验结果相符合.     

固体物理教学中电子态密度的计算

电子态密度是指单位能量范围内的状态数,是与电子能带结构密切相关的一个物理量.为了计算电子的比热和晶体的输运性质,必须用较精确的方法计算出晶体的电子态密度.大多数教材中对该部分的处理通常采用简化模型,并不能反映一般情况下态密度的计算思路.本文从电子态密度的公式出发,详细说明了二维石墨烯和三维面心立方晶格态密度的计算步骤,并且对其中细节给出了基于数值计算的详细解释.     

镓掺杂氧化锌和硫化锌电子结构差异的第一性原理

运用第一性原理进行了相关计算研究Ga掺杂的ZnO和ZnS的电子结构的差异. 结果表明,LDA和LDA+U计算的结果在定性上是一致的. 掺杂Ga以后,ZnO和ZnS的费米能级处均出现杂质态. 掺杂中的ZnO,杂质态在导带是离域的. 掺杂后的ZnS,虽然p态比较离域,但其s态在费米能级处却是局域的. 前线轨道的电荷密度分布也给出了相同的信息. 交换ZnO和ZnS的晶格结构,结果不变. 局域化的Ga-s态是导致掺杂ZnS电学性能差的原因.     

Calculation of binding energy from moments via Jacobi polynomials

Moments contain essential information for the density of states, and they can be readily computed within the tight-binding framework; therefore, it is desirable to obtain the bond energy from the moments. A linear-scaling moment-based potential via orthogonal polynomials (LMPO) was proposed recently by (Qin, 2010) [29]. Arbitrary orthogonal polynomials can be implemented within the LMPO. This paper will concentrate on Jacobi polynomials, and these polynomials have finite support intervals. In order to match these support intervals with the band edges of the density of states, the band edges need to be obtained first, ideally from the moments. Algorithms obtaining the band edges from the moments are analysed, and a more convergent algorithm is proposed. Then the combination of this band edge algorithm and the LMPO is examined on an s-band system and a d-band tungsten system. As demonstrated from the results, even though the band edges are not fully converged within only a limited number of moments, this combined method is able to converge to the tight-binding results in terms of binding energy. Consequently, this can significantly reduce the computation time.     

Spin susceptibilities and the theory of itinerant-electron antiferromagnetism

This article investigates the equilibrium states of antiferromagnetic itinerant-electron systems in the Hartree-Fock approximation. As a result, the spin susceptibilities are determined in the random phase approximation. The lowlying collective excitations are then obtained by finding the poles of these susceptibilities.

We start by giving a brief review of the Hartree-Fock procedure and by indicating how the susceptibilities are obtained. The density matrix approach, where the ground state is interpreted as that minimizing the energy, is used throughout. Using an effective Coulomb interaction of the Hubbard type we consider two distinct systems: a one-band system with an incommensurate spin density wave in its ground state, and a many-band simply commensurate model for f.c.c. manganese.

The first of these is such that the band structure and resulting susceptibilities can be obtained explicitly. The spin-wave energies and wave-vectors are found by a careful, small energy and momentum transfer, expansion of these susceptibilities for the case of a parabolic band. The spin-wave damping, which is shown to arise from spin-wave decay into quasiparticle quasihole pairs, is also obtained for this band structure.

For the case of f.c.c. manganese the antiferromagnetic bands are obtained from a realistic 9-band paramagnetic model by using a many-band generalization of the Hubbard interaction. The enhanced spin susceptibilities are calculated, using the tetrahedral Brillouin zone integration method, and are presented along with their associated collective excitations. The results obtained are discussed with particular reference to the many-band effects. These effects are shown to be very much dependent on the particular form of interaction used.     

Electronic structure of fluoropolymers : Theory and esca measurements

Theoretical and experimental results for the band structure of fluoropolymers are presented. Original theoretical results are obtained from the extende Hückel method of the following polymers : (I) polyethylene, (II) polyvinyl fluoride, (III) polydifluoro (1,2) ethylene-cis, (IV) polydifluoro (1,2) ethylene-trans, (V) polyvinylidene fluoride, (VI) polytrifluoroethylene and (VII) polytetrafluoroethylene. Band structures schemes and density of state histograms are calculated. Results are compared with valence band structures as revealed by E.S.C.A. spectroscopy, using a flood gun to neutralize char effects on these compounds. In this work, accent is put on the substituent effect of fluorine. Correlation between theoretical and observed density of     

Electronic and thermal properties of B2-type AlRE intermetallic compounds: A first principles study

The ground state electronic structure and thermal properties of B₂-type intermetallic compounds AlRE (RE: Pm, Sm, Eu, Tb, Gd and Dy) have been studied using a self-consistent tight-binding linear muffin-tin orbital (TB-LMTO) method at ambient as well as at high pressure. These compounds show metallic behavior under ambient condition. The band structure, total energy, density of states and ground state properties like lattice parameter, bulk modulus are calculated in the present work. The Debye-Grüneisen model is used to calculate the Debye temperature and the Grüneisen constant. The calculated results are in good agreement with the reported experimental and other theoretical results. The variation in the Debye temperature with pressure has also been reported. We present a detailed analysis of the role of f electrons of RE in the AlRE system.     

Ferromagnetism in the Hubbard model on fcc-type lattices

The Hubbard model on fcc-type lattices is studied in the dynamical mean-field theory of infinite spatial dimensions. At intermediate interaction strength finite temperature Quantum Monte Carlo calculations yield a second order phase transition to a highly polarized, metallic ferromagnetic state. The Curie temperatures are calculated as a function of electronic density and interaction strength. A necessary condition for ferromagnetism is a density of state with large spectral weight near one of the band edges. Received: 6 August 1997 / Accepted: 20 August 1997     

Optical Properties of Neutral and Charged Low Band Gap Alternating Copolyfluorenes: TD-DFT Investigation

用含时的密度泛函(TD-DFT)方法研究了低带隙的中性和带电的交替共聚芴(Green 1),该化合物是由烷基取代芴和(1,2,5-噻吩基-3,4-硫重氮基)喹喔啉噻吩(T-TDQ-T)单元交替重复组成,对他们的激发态特性用二维(2D)和三维(3D)实空间分析方法做了进一步分析．对于中性的Green 1,分别得到其带隙、键能、激子结合能和核驰豫能．用3D跃迁密度方法对中性和带电的Green 1的跃迁偶极矩进行比较可显示出跃迁偶极矩的取向和强度;用3D电荷差异密度方法显示出激发后的中性和带电的Green 1电荷重新分布和比较,用2D实空间分析方法(跃迁密度矩阵)来研究中性和带电的Green1处于激发态时的电子空穴相干性．中性Green 1的激发态特性分别用TD-DFT和ZINDO两种方法进行了计算,比较得出电子-电子相互作用(在TD-DFT中)对激发态性质的重要影响．     

The quasibound state model for self-consistent characteristics of semiconductor intersubband devices

The quasibound state model (QBSM) for determining the self-consistent conduction band profile and space charge density of semiconductor intersubband devices is presented. This new method is based on the quasibound (QB) state resonances of quantum structures. For heterostructures, the traditional self-consistent energy continuum model (ECM) calculates space charge by integration over the entire energy continuum, weighted by Fermi–Dirac statistics. In the present approach, the continuum of energy states of the heterostructure is accurately represented by a small number of QB states, and the space charge calculations are performed only at these eigen-energies. This approach significantly reduces the computational burden associated with all self-consistent algorithms. Theoretical formulation of QBSM is compared with the traditional ECM approach. The bound (B) and QB eigenenergies of the structure are obtained by solving the single-band effective-mass Schrödinger equation using the argument principle method. The performance and the accuracy of the QBSM are evaluated for a double-barrier resonant structure and an asymmetric Fabry–Perot electron-wave interference filter. The self-consistent electron density and potential profiles calculated by the present method are shown to be in excellent agreement with the results obtained from the traditional ECM model. In addition to requiring less computational time, the present method is easily implemented and may be applied equally well to biased/unbiased, symmetric/asymmetric heterostructures.     

Electronic structure of black phosphorus studied by X-ray photoelectron spectroscopy

The X-ray photoelectron spectrum of black phosphorus has been measured for the first time. The features in the valence band spectrum are found to be in good agreement with those of the valence state density recently calculated on the basis of a pseudopotential method. The 2s and 2p core spectrum of black phosphorus is also discussed.     

High pressure study of structural and electronic properties of magnesium telluride

The full-potential linear muffin-tin orbital method (FP-LMTO) within the local density approximation (LDA) is used to calculate the electronic band structures and the total energies of MgTe in its stable (NiAs-B8) and high pressure phases. The latter provide us with the ground state properties such us lattice parameter, bulk modulus and its pressure derivatives. The transition pressure at which this compound undergoes the structural phase transition from the NiAs to CsCl phase is calculated. The energy band gaps and their volume and pressure dependence in the stable NiAs-B8 phase are investigated. The ground state properties, the transition pressure are found to agree with the experimental and other theoretical results. The elastic constants at equilibrium in both NiAs and CsCl structure are also determined.