Silicon before the bonds break

The diamond structure of silicon can be destroyed by a femtosecond-laser pulse in a process that is called nonthermal melting. Using a supercell of 800 atoms, we have performed ab initio molecular dynamics simulations on laser-excited potential energy surfaces for electronic excitation densities around the threshold for nonthermal melting. By introducing a quantitative measure for the resemblance of the atomic paths below and above this threshold, we show that the directions of the atomic motions are the same within 98% during 150–200 fs. The atomic pathways below the melting threshold, before the bonds break, are therefore quantitatively closely related to the atomic motions during the first stages of nonthermal melting.     

SrTiO3中形成级联能级的理论分析

以Bi和Cu掺杂为例,通过基于密度泛函理论的电子结构的计算,分析了SrTiO₃体系中形成级联能级的可能性.结果表明,Bi掺杂和Cu掺杂都可以在SrTiO₃的禁带中引入杂质能带,Bi和Cu的共同掺杂可引入两条杂质能带.通过在两条杂质能带之间级联激发,价带顶的电子可以受激跃迁到导带底.采用无辐射跃迁的简单模型,分析指出电子通过级联激发从价带顶受激跃迁到导带底的概率远远高于直接从价带顶跃迁到导带底的概率.这种级联激发可以有效提高导带中的载流子浓度. 关键词： 级联能级 密度泛函 掺杂     

First-principles study on electronic structure of Sr2Bi2O5 crystal

The electronic structure of Sr₂Bi₂O₅ is calculated by the GGA approach. Both of the valence band maximum and the conduction band minimum are located at Γ-point. This means that Sr₂Bi₂O₅ is a direct band-gap material. The wide energy-band dispersions near the valence band maximum and the conduction band minimum predict that holes and electrons generated by band gap excitation have a high mobility. The conduction band is composed of Bi 6p, Sr 4d and O 2p energy states. On the other hand, the valence band can be divided into two energy regions ranging from −9.5 to −7.9 eV (lower valence band) and from −4.13 to 0 eV (upper valence band). The former mainly consists of Bi 6s states hybridizing with O 2s and O 2p states, and the latter is mainly constructed from O 2p states strongly interacting with Bi 6s and Bi 6p states.     

Symmetry of valence states of Heusler compounds explored by linear dichroism in hard-x-ray photoelectron spectroscopy

This study reports on the linear dichroism in angular-resolved photoemission from the valence band of the Heusler compounds NiTi0.9Sc0.1Sn and NiMnSb. High-resolution photoelectron spectroscopy was performed with an excitation energy of hν = 7.938 keV. The linear polarization of the photons was changed using an in-vacuum diamond phase retarder. The valence band spectra exhibit the typical structure expected from first-principles calculations of the electronic structure of these compounds. Noticeable linear dichroism is found in the valence band of both materials, and this allows for a symmetry analysis of the contributing states. The differences in the spectra are found to be caused by symmetry-dependent angular asymmetry parameters, and these occur even in polycrystalline samples without preferential crystallographic orientation.     

Ab initio calculations of electronic structure of anatase TiO2

This paper presents the results of the self-consistent calculations on the electronic structure of anatase phase of TiO2. The calculations were performed using the full potential-linearized augmented plane wave method (FP-LAPW)in the framework of the density functional theory (DFT) with the generalized gradient approximation (GGA). The fullyoptimized structure, obtained by minimizing the total energy and atomic forces, is in good agreement with experiment.We also calculated the band structure and the density of states. In particular, the calculated band structure prefers an indirect transition between valence and conduction bands of anatase TiO2, which may be helpful for clarifying theambiguity in other theoretical works.     

Control of the Interface Between Electron‐Hole and Electron‐Ion Plasmas: Hybrid Semiconductor‐Gas Phase Devices as a Gateway for Plasma Science

Coupling electron‐hole (e^–‐ h⁺) and electron‐ion plasmas across a narrow potential barrier with a strong electric field provides an interface between the two plasma genres and a pathway to electronic and photonic device functionality. The magnitude of the electric field present in the sheath of a low temperature, nonequilibrium microplasma is sufficient to influence the band structure of a semiconductor region in immediate proximity to the solid‐gas phase interface. Optoelectronic devices demonstrated by leveraging this interaction are described here. A hybrid microplasma/semiconductor photodetector, having a Si cathode in the form of an inverted square pyramid encompassing a neon microplasma, exhibits a photosensitivity in the ～420–1100 nm region as high as 3.5 A/W. Direct tunneling of electrons into the collector and the Auger neutralization of ions arriving at the Si surface appear to be facilitated by an n ‐type inversion layer at the cathode surface resulting from bandbending by the microplasma sheath electric field. Recently, an npn plasma bipolar junction transistor (PBJT), in which a low temperature plasma serves as the collector in an otherwise Si device, has also been demonstrated. Having a measured small signal current gain h_fe as large as 10, this phototransistor is capable of modulat‐ing and extinguishing the collector plasma with emitter‐base bias voltages <1 V. Electrons injected into the base when the emitter‐base junction is forward‐biased serve primarily to replace conduction band electrons lost to the collector plasma by secondary emission and ion‐enhanced field emission in which ions arriving at the base‐collector junction deform the electrostatic potential near the base surface, narrowing the potential barrier and thereby facilitating the tunneling of electrons into the collector. Of greatest significance, therefore, are the implications of active, plasma/solid state interfaces as a new frontier for plasma science. Specifically, the PBJT provides the first opportunity to control the electronic properties of a material at the boundary of, and interacting with, a plasma. By specifying the relative number densities of free (conduction band) and bound (valence band) electrons at the base‐collector interface, the PBJT's emitter‐base junction is able to dictate the rates of secondary electron emission (including Auger neutralization) at the semiconductor‐plasma interface, thereby offering the ability to vary at will the effective secondary electron emission coefficient for the base surface (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Radiation-stimulated pulse conductivity of CsBr crystals

The radiation-stimulated pulse conductivity of CsBr crystals is investigated upon picosecond excitation with electron beams (0.2 MeV, 50 ps, 0.1–10 kA/cm²). The time resolution of the measuring technique is ～150 ps. It is shown that the lifetime of conduction band electrons is limited by their bimolecular recombination with autolocalized holes (V _k centers). A delay in the conduction current pulse build-up is revealed. This effect is explained within the proposed model, according to which the Auger recombination of valence band electrons and holes of the upper core band substantially contributes to the generation of conduction band electrons.     

钛原子位置对立方BaTiO3电子结构的影响

本文基于密度泛函理论(DFT)的第一原理方法,计算了Ti原子位置对BaTiO3电子结构的影响.Ti的位置变化导致晶格畸变,使电子结构发生变化;从能带结构、能态密度(DOS)、电子密度、Mulliken布居等计算结果分析表明,导带和价带主要由Ti的3d电子和O的2p电子,Ti原子位置的变化,使Ti的3d电子能量分布上移,而O的2p电子能量下移;Ti位置变化,Ti的3d电子与的sp电子形成的杂化轨道更趋向离子化,以致于使OI出现了正电荷,表明发生了的2p电子向Ti的转移;O原子电子的转移使得Ti原子在导带的3d电子能量降低,与O原子在价带的2p电子能量重叠,禁带消失;随着畸变程度提高,转移逐渐增强,使禁带宽度逐渐减小,直至完全消失.     

激光驱动晶体发射高次谐波的特性研究

通过数值模拟激光驱动下电子在周期性势阱中的动力学行为, 研究了晶体在激光场中发射高次谐波的特性. 研究发现在一定的激光波长和光强驱动下, 晶体发射的谐波谱会呈现出双平台结构, 经分析后得知第一个平台主要来自于最低导带与价带间的电流(电子-空穴对复合), 第二个平台主要来源于较高导带与价带间的电流(电子-空穴对复合), 且两个平台的截止位置处的能量都与激光场的振幅呈线性关系. 在少周期激光驱动下, 晶体谐波第二平台的截止位置与激光的载波相位呈单调变化, 由此我们提出可以利用晶体谐波第二平台的截止位置来确定少周期激光的载波相位. 进一步研究发现, 在啁啾激光驱动下, 晶体发射谐波谱的第二平台有较大变化, 第二平台的发射效率会随啁啾参数而改变, 能够通过改变啁啾激光场来提高晶体谐波第二平台的发射效率.     

Theory of ferromagnetism in doped excitonic condensates

Nesting in a semimetal can lead to an excitonic-insulator state with spontaneous coherence between conduction and valence bands and a gap for charged excitations. We present a theory of the ferromagnetic state that occurs when the density of electrons in the conduction band and holes in the valence band differ. We find an unexpectedly rich doping-field phase diagram and an unusual collective excitation spectrum that includes two gapless collective modes. We predict regions of doping and external field in which phase-separated condensates of electrons and holes with parallel spins and opposing spins coexist.     

Study of the chemical bonding of titanium with silicon and oxygen by AES and SEELS

Auger electron Spectroscopy (AES) and slow electron energy loss Spectroscopy (SEELS) have been employed to study the electronic structure of Ti, TiSi₂ and TiO₂. The changes in the Auger and loss spectra when Ti chemically binds with silicon to form TiSi₂ and with oxygen to form TiO₂ have been understood as manifestations of changes in electronic participation. AES spectra show distinct changes in line shapes of transitions involving the Ti valence electrons. The SEELS spectra provide information regarding shallow core levels, valence band and the collective excitation energies of the volume and surface plasmons. By monitoring the changes in the Auger peak at 387 eV and the 3p→ 3d quasiatomic transition (at about 45 eV), the role of d-orbital occupancies are studied in Ti and its compounds. The SEELS studies in the 0-80 eV range have enabled the authors to observe the behaviour of the 3p → 3d quasiatomic transition in Ti, which persists after oxidation but almost disappears during TiSi₂ formation. The values of the plasmon losses are related to the collective behaviour of conduction electrons.     

Theory for the laser-induced femtosecond phase transition of silicon and GaAS

We analyze he femtosecond instability of the chamond lattice of silicon and GaAs, which is induced by a dense electron-hole plasma after excitation by a very imense laser pulse. We obtain that the electron-hole plasma causes an instability of both transverse acoustic and longitudinal optical phonons. So, within less than 200fs, the atoms are displaced more than 1 Å from their equilibrium position. The gap between the conduction and the valence band then vanishes and the symmetries of the diamond structure are destroyed, which has important effects on the optical reflectivity and second-harmonic generation. After that, the crystal melts very rapidly because of the high kinetic energy of the atoms. Note that mis is in good agreement with recent experiments done on Shand GaAs using a pump laser to excite a dense electron hole plasma and a probe laser to observe the resulting changes in the atomic and electronic structure.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Density response to short‐pulse excitation in gold

We investigate the dynamics of band occupation in gold after excitation with short laser pulses. Strong non‐equilibrium distributions can be created by intra‐ and inter‐band transitions driven by the absorption of photons with different wavelengths. First, we briefly discuss how photons can be used as a probe and how electrons with non‐Fermi distributions can be described. Then we focus on the relaxation stage where a temperature has been established but the occupation numbers in the different bands are not yet in equilibrium with this newly established elevated temperature. We describe the relaxation towards a system with a common chemical potential with rate equations that also include the energy transfer to the lattice or ions. Finally, we give an outlook on the optical response of the relaxing electron system to a probe pulse of radiation.     

强激光照射对6H-SiC晶体电子特性的影响

使用基于密度泛函微扰理论的第一性原理赝势法, 模拟研究了纤锌矿6H-SiC晶体在强激光照射下电子特性的变化. 研究结果表明, 电子温度T_e在升高到3.89 eV及以上后, 6H-SiC由间接带隙的晶体变为直接带隙的晶体; 带隙值随电子温度T_e升高先是增大后又快速减小, 当电子温度T_e大于4.25 eV以后, 带隙已经消失而呈现出金属特性.     

强激光照射对2H-SiC晶体电子特性的影响

使用基于密度泛函微扰理论的第一原理赝势法, 计算了纤锌矿结构2H-SiC晶体在强激光照射下的电子特性, 分析了其能带结构和电子态分布. 计算结果表明: 2H-SiC平衡晶格参数a 和c随电子温度T_e的升高逐渐增大; 电子温度在0–2.25 eV范围内时, 2H-SiC仍然是间接带隙的半导体晶体, 当T_e超过2.25 eV达到2.5 eV以上时, 2H-SiC变为直接带隙的半导体晶体; 随着电子温度升高, 导带底和价带顶向高能量或低能量方向发生了移动, 当电子温度T_e大于3.5 eV以后, 价带顶穿越费米能级; 电子温度T_e在0–2.0 eV变化时, 带隙随电子温度升高而增大; T_e在2.0–3.5 eV范围变化时, 带隙随电子温度升高而快速地减少, 表明2H-SiC晶体的金属性随电子温度T_e的继续升高而增强. 在T_e =0, 5.0 eV 处, 计算了2H-SiC晶体总的电子态密度和分波态密度. 电子结构表明T_e =0 eV 时, 2H-SiC 是一个带隙为2.3 eV的半导体; 在T_e =5.0 eV时, 带隙已经消失而呈现出金属特性, 表明当电子温度升高时晶体的共价键变弱、金属键增强, 晶体经历了一个熔化过程, 过渡到金属状态.     

Interaction of Short Laser Pulses in Wavelength Range from Infrared to X‐ray with Metals,Semiconductors, and Dielectrics

Laser‐matter interaction is defined by an electronic band structure of condensed matter and frequency ω_L of electromagnetic radiation. In the range of moderate fluences, the energy absorbed by electrons from radiation finally thermalizes in the ion thermal energy. The thermalization processes are different for optical as compared with X‐ray quanta and for metals relative to semiconductors and dielectrics, since the light absorption and electron‐electron, electron‐ion dynamics are sensitive to the electron population in a conduction band and the width of a forbidden gap. Although the thermalization processes are different, the final state is simply a heated matter. Laser heating creates powerful stresses in a target if duration of a laser pulse τ_L is short in acoustic time scale. Nucleation and material removal take place under such stresses. Such way of removal is called here the spallative ablation. Thus the spallative ablation is an ablation mechanism universally important for qualitatively different materials and quanta (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electronic energy structure of vacancy and divacancy in SiO2

We investigated electronic energy structure of vacancy and divacancy in SiO₂, and found that oxygen vacancy and divacancy give rise to bound-states near the edge of the conduction band, whereas localized states related to the silicon vacancy occur in the valence band. Our results demonstrate that the doubly occupied oxygen vacancy state yields electrons to silicon in Si-SiO₂ junction and serves as a fixed oxide charge.     

CrI_3高压相变及其光学性质理论研究

利用第一性原理计算方法,探讨了体相CrI_3在低温斜方六面体结构(■,BiI_3-type)及高压单斜结构(C2/m,AlCl_3-type)的相变、电子结构和光学性质.计算结果显示,半导体CrI_3当压强增加到26.1GPa时,高压导致的晶格畸变致使CrI_3从相■变化到相C2/m;原子之间的错位位移,使导带处的能带发生下移,价带处的能带发生了一定程度的上移,导致带隙减小.两种相的光学性质进一步验证了这些特性.     

Effect of preheating temperatures on microstructure and optical properties of Na-doped ZnO thin films by sol–gel process

The chemical composition, crystalline structure, surface morphology and photoluminescence spectra of Na-doped ZnO thin films with different heat treatment process were investigated by X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy and a fluorescence spectrometer. The results show that preferred orientation, residual stress, average crystal size and surface morphology of the thin films are strongly determined by the preheating temperature. The effects of preheating temperature on microstructure and surface morphology have been discussed in detail. The photoluminescence spectra show that there are strong violet & UV emission, blue emission and green emission bands. The violet & UV emission is ascribed to the electron transition from the localized level below the conduction band to the valence band. The blue emission is attributed to the electron transition from the shallow donor level of oxygen vacancies to the valence band, and the electron transition from the shallow donor level of interstitial zinc to the valence band. The green emission is assigned to the electron transition from the level of ionized oxygen vacancies to the valence band.     

Raman study of Fano interference in p‐type doped silicon

As the silicon industry continues to push the limits of device dimensions, tools such as Raman spectroscopy are ideal to analyze and characterize the doped silicon channels. The effect of inter‐valence band transitions on the zone center optical phonon in heavily p‐type doped silicon is studied by Raman spectroscopy for a wide range of excitation wavelengths extending from the red (632.8 nm) into the ultra‐violet (325 nm). The asymmetry in the one‐phonon Raman lineshape is attributed to a Fano interference involving the overlap of a continuum of electronic excitations with a discrete phonon state. We identify a transition above and below the one‐dimensional critical point (E = 3.4 eV) in the electronic excitation spectrum of silicon. The relationship between the anisotropic silicon band structure and the penetration depth is discussed in the context of possible device applications. Copyright © 2010 John Wiley & Sons, Ltd.