InP(110)理解面的能带弯曲

用XPS测得了真空解理后InP样品（110）表面能带弯曲的动态过程,并对引起InP表面能带弯曲的可能原因进行了讨论．排除了本征表面态、真空中残留气体和X射线辐射等原因,认为解理过程在表面产生的缺陷和解理后表面晶格弛像产生的缺陷可能是导致能带弯曲的原因．     

含空位和杂质缺陷的LiFePO4电子结构的第一性原理计算

采用基于密度泛函理论的第一性原理研究了含空位和杂质缺陷的LiFePO_4电子结构,通过能带、态密度、布居分布分析,阐明缺陷及阴离子掺杂对材料电化学性能的影响,为LiFePO_4的结构设计和实验研究提供理论基础。结果表明,Li、Fe和O空位型缺陷对LiFePO_4的带型变化影响较小,禁带中无新的导带,禁带宽度有一定程度缩小,有利于电子的传导,但总能量上升,造成结构的不稳定性,在实际高温制备过程中,可能产生少量杂相,影响LiFePO_4正极材料的电化学性能;P空位缺陷对LiFePO_4的带型影响同样较小,但在禁带中产生了两条新的导带,禁带宽度明显变窄,有利于电子的传导,虽然总能量上升,造成结构的不稳定性,但在实际高温制备过程中,可能产生微量有利于电化学性能的杂相;F掺杂LiFePO_4的带型出现了明显的变化,半导体类型由p型转变为n型,极大地促进了电子的导电性,总能量下降,结构稳定,对LiFePO_4正极材料的电化学性能有正面的影响。     

表面悬挂键对GaAs纳米线掺杂的影响及其钝化

利用第一性原理计算方法研究了表面悬挂键对GaAs纳米线掺杂的影响及其钝化．计算结果显示,不论是闪锌矿结构还是纤锌矿结构,GaAs纳米线表面Ga原子上带正电荷的表面悬挂键都是一类稳定的缺陷,并且这种稳定性不会随着纳米线直径的变化而变化．这种表面悬挂键会形成载流子陷阱中心从而从p型掺杂的GaAs纳米线俘获空穴,使得纳米线的掺杂效率下降．和NH3相比,NO2 具有足够的电负性来俘获GaAs纳米线表面悬挂键上的未配对电子,从而有效地钝化GaAs纳米线的表面悬挂键,提高纳米线的p型掺杂效率,并且这种钝化特性不会随着纳米线直径的变化而改变．     

Li掺杂ZnO的电子结构与电性能的研究

采用密度泛函理论平面波超软赝势方法研究了p型Li掺杂的纤锌矿结构ZnO的能带结构、态密度和电荷分布,并分析了Li掺杂ZnO的电输运性能.结果表明,Li掺杂ZnO具有1.6eV的直接带隙,且为p型半导体,体系费米能级附近的态密度大大提高,在导带和价带中都出现了由Li电子能级形成的能带,其费米能级附近的能带主要由Li的s态、Zn的p态、Zn的d态和O的p态电子构成,且他们之间存在着强相互作用.电输运参数和电输运性能分析结果表明,Li掺杂的ZnO氧化物价带和导带中的载流子有效质量均较大;其载流子输运主要由Li的s态、Zn的p态和O的p态电子完成;Li掺杂有望改善ZnO的电输运性能.     

采用第一性原理研究非金属掺杂BiOCl的电子结构和光吸收性质

基于密度泛函理论的第一性原理分别研究了不同浓度Br和I掺杂BiOCl体系的能带结构、态密度、形成能和光学性质.研究结果表明,由于Br的4p和I的5p轨道作用,Br和I掺杂可在一定程度上降低BiOCl的禁带宽度,拓宽BiOCl的光吸收范围.Br和I掺杂BiOCl的形成能计算结果表明,Br掺杂BiOCl的稳定性高于I掺杂体系.对于B,C,N,Si,P和S掺杂BiOCl体系,掺杂能级的形成主要由掺杂元素的np轨道贡献,使BiOCl吸收带边红移至可见光区.而S掺杂则位于价带顶位置,有效地降低了BiOCl禁带宽度,使BiOCl响应波长出现红移,且未形成中间能级,不易成为俘获陷阱,因此S掺杂将是一种提高BiOCl可见光光催化活性的改性方法.     

Au掺杂硅纳米线的稳定性和电子结构

采用基于密度泛函理论的第一性原理的方法, 对Au掺杂[100]方向氢钝化硅纳米线(SiNWs)不同位置的形成能、能带结构、态密度及磁性进行了计算, 考虑了Au占据硅纳米线的替代、四面体间隙和六角形间隙的不同位置. 结果表明: Au偏爱硅纳米线中心的替代位置. Au掺杂后的硅纳米线引入了杂质能级, 禁带宽度变窄. 对于Au替代掺杂, 杂质能级主要来源于Au的d、p态和Si的p态, 由于Au的d态和Si的p态的耦合, Au掺杂硅纳米线具有铁磁性. 对于间隙掺杂, 杂质能级主要来源于Au的s态, 是非磁性的. 另外, 根据原子轨道和电子填充模型分析了其电子结构和磁性.     

ZnSe掺Cu与Zn空位缺陷的稳定性、电子结构与光学性质

采用基于密度泛函理论框架下的第一性原理平面波超软雁势方法, 对ZnSe闪锌矿结构本体、掺入p型杂质Cu(Zn_0.875Cu_0.125Se)及Zn空位(Zn_0.875Se)超晶胞进行结构优化处理. 计算并详细分析了缺陷体系的形成能和三种体系下ZnSe材料的态密度、能带结构、集居数、介电和吸收光谱. 结果表明: 在Zn空位与Cu掺杂ZnSe体系中, 由于空位及杂质能级的引入, 禁带宽度有所减小, 吸收光谱产生红移; 单空位缺陷结构不易形成, Zn_0.875Se结构不稳定, Cu掺杂ZnSe结构相对更稳定.     

过渡金属元素Sc、Cr和Mn对Mg2Ge掺杂的第一性原理研究

基于密度泛函理论(DFT)的第一性原理计算,研究了过渡金属元素Sc、Cr和Mn掺杂对Mg₂Ge晶体光、电、磁性质的影响。结果表明,Sc掺杂能使Mg₂Ge的费米能级进入导带,呈n型简并半导体;Cr和Mn掺杂能使Mg₂Ge能带结构和态密度在费米能级附近产生自旋劈裂而形成净磁矩,表现为半金属磁体和稀磁半导体,体系净磁矩均来自杂质原子3d轨道电子及其诱导极化的Ge4p态和Mg2p态自旋电子。与本征Mg₂Ge相比,掺杂体系静态介电常数增大,扩展了吸收光谱,提升了近红外光波段吸收能力。     

镨掺杂Bi2Sr3Co2O9+δ电子结构的XPS研究

研究了Pr掺杂Bi2Sr3Co2O9+δ体系的X射线光电子能谱,发现随着体系中Pr掺杂量的增大,内层电子结合能有缓慢增大的趋势,但掺杂量达到0.6时,测得的电子结合能减少0.4 eV左右,与分析材料中费米能级变化引起的结合能变化趋势相反.考虑到掺杂量达到0.6时,材料发生金属-绝缘体转变,所得到的表观结合能减少可能是绝缘体样品相对于样品底座存在正的电势差所引起的.     

掺铁TiO2纳米微粒的制备及光催化活性

采用溶胶-凝胶法制备了不同掺铁量的TiO2光催化剂,运用TG-DTA,XRD,SEM,DRS和测定光电导等方法对催化剂进行了表征.以高压汞灯为光源,罗丹明B为目标降解物,对其光催化活性进行了研究.实验结果表明,掺铁的TiO2比纯TiO2具有更好的催化活性.其原因:掺杂的铁作为受主捕获电子,使TiO2的n型半导体降低了光电导,控制了空穴和电子复合;同时掺杂的Fe3 可能形成杂质能级,由于掺杂能级处于禁带之中,使较长波长的光子也能被吸收,从而扩展吸收光谱的范围,增强了对可见光的吸收.     

Energy level and band alignment for GaAs-alkylthiol monolayer-Hg junctions from electrical transport and photoemission experiments

A series of p- and n-GaAs-S-C(n)H(2n+1) || Hg junctions are prepared, and the electronic transport through them is measured. From current-voltage measurements, we find that, for n-GaAs, transport occurs by both thermionic emission and tunneling, with the former dominating at low forward bias and the latter dominating at higher forward bias. For p-GaAs, tunneling dominates at all bias voltages. By combining the analysis of the transport data with results from direct and inverse photoemission spectroscopy, we deduce an energy band diagram of the system, including the tunnel barrier and, with this barrier and within the Simmons tunneling model, extract an effective mass value of 1.5-1.6m(e) for the electronic carriers that cross the junctions. We find that transport is well-described by lowest unoccupied and highest occupied states at 1.3-1.4 eV above and 2.0-2.2 eV below the Fermi level. At the same time, the photoemission data indicate that there are continua of states from the conduction band minimum and the valence band maximum, the density of which varies with energy. On the basis of our results, it appears likely that, for both types of junctions, electrons are the main carrier type, although holes may contribute significantly to the transport in the p-GaAs system.     

Oxygen effects on the interfacial electronic structure of titanyl phthalocyanine film: p-Type doping,band bending and Fermi level alignment

The effect of oxygen doping on titanyl phthalocyanine (TiOPc) film was investigated by ultraviolet photoelectron spectroscopy (UPS). The electronic structure of the interface formed between TiOPc films deposited on highly oriented pyrolytic graphite (HOPG) was clearly different between the films prepared in ultrahigh vacuum (UHV) and under O₂ atmosphere (1.3 × 10⁻² Pa). The film deposited in UHV showed downward band bending characteristic of n-type semiconductor, possibly due to residual impurities working as unintentional n-type dopants. On the other hand, the film deposited under O₂ atmosphere showed upward band bending characteristic of p-type semiconductor. Such trends, including the conversion from n- to p-type, are in excellent correspondence with reported field effect transistor characteristics of TiOPc, and clearly demonstrates that bulk TiOPc film was p-doped with oxygen. In order to examine the Fermi level alignment between TiOPc film and the substrate, the energy of the highest occupied molecular orbital (HOMO) of TiOPc relative to the Fermi level of the conductive substrate was determined for various substrates. The alignment between the Fermi level of conductive substrate and Fermi level of TiOPc film at fixed energy in the bandgap was not observed for the TiOPc film prepared in UHV, possibly because of insufficient charge density in the TiOPc film. This situation was drastically changed when the TiOPc film exposed to O₂, and clear alignment of the Fermi level fixed at 0.6 eV above the HOMO with the Fermi level of the conducting substrate was observed, probably by p-type doping effect of oxygen. These are the first direct and quantitative information about bulk oxygen doping from the viewpoint of the electronic structure. These results suggest that similar band bending with Fermi level alignment may be also achieved for other organic semiconductors under practical device conditions, and also call for caution at the comparison of experimental results obtained under UHV and ambient atmosphere.     

Electrochemical impedance study of GaAs surface charge modulation through the deprotonation of carboxylic acid monolayers

Modifications to the space charge region of p+ and p-GaAs due to surface charge modulation by the pH-induced deprotonation of bound carboxylic acid terminal monolayers were studied by electrochemical impedance spectroscopy and correlated to flat-band potential measurements from Mott-Schottky plots. We infer that the negative surface dipole formed on GaAs due to monolayer deprotonation causes an enhancement of the downward interfacial band bending. The space charge layer modifications were correlated to intermolecular electrostatic interactions and semiconductor depletion characteristics.     

Density Functional Theory Calculations Revealing Metal‐like Band Structures and Work Function Variation for Ultrathin Gallium Arsenide (111) Surface Layers

Density functional theory (DFT) calculations have been performed on tunable numbers of gallium arsenide (100), (110), and (111) planes for their electron density of states (DOS) plots and the corresponding band diagrams. The GaAs (100) and (110) planes show the same semiconducting band structure with tunable plane layers and a band gap of 1.35 eV around the Fermi level. In contrast, metal‐like band structures are obtained with a continuous band structure around the Fermi level for 1, 2, 4, 5, 7, and 8 layers of GaAs (111) planes. For 3, 6, and 9 GaAs (111) planes, the same semiconducting band structure as seen in the (100) and (110) planes returns. The results suggest the GaAs {111} face should be more electrically conductive than its {100} and {110} faces, due to the merged conduction band and valence band. GaAs (100) and (110) planes give a fixed work function, but the (111) planes have variable work function values that are smaller than that obtained for the (100) and (110) planes. Furthermore, bond length, bond geometry, and frontier orbital electron number and energy distribution show notable differences between the metal‐like and semiconducting plane cases, so the emergence of plane‐dependent electronic properties have quantum mechanical origin at the orbital level. GaAs should possess similar facet‐dependent electronic properties to those of Si and Ge.     

Electronic properties of adsorbates on GaAs(001)-c(2x8)/(2x4)

A systematic experimental and theoretical study was performed to determine the causes of oxide-induced Fermi level pinning and unpinning on GaAs(001)-c(2 x 8)/(2 x 4). Scanning tunneling spectroscopy (STS) and density functional theory (DFT) were used to study four different adsorbates' (O(2), In(2)O, Ga(2)O, and SiO) bonding to the GaAs(001)-c(2 x 8)/(2 x 4) surface. The STS results revealed that out of the four adsorbates studied, only one left the Fermi level unpinned, Ga(2)O. DFT calculations were used to elucidate the causes of the Fermi level pinning. Two distinct pinning mechanisms were identified: direct (adsorbate induced states in the band gap region) and indirect pinnings (generation of undimerized As atoms). For O(2) dissociative chemisorption onto GaAs(001)-c(2 x 8)/(2 x 4), the Fermi level pinning was only indirect, while direct Fermi level pinning was observed when In(2)O was deposited on GaAs(001)-c(2 x 8)/(2 x 4). In the case of SiO on GaAs(001)-c(2 x 8)/(2 x 4), the Fermi level pinning was a combination of the two mechanisms.     

Electrostatic field and partial Fermi level pinning at the pentacene-SiO(2) interface

Monolayer islands of pentacene deposited on silicon substrates with thermally grown oxides were studied by electric force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM) in ultrahigh vacuum (UHV) after prior 10 min exposure to atmospheric ambient. On 25-nm-thick oxides, the pentacene islands are 0.5 V higher in electrostatic potential than the silicon dioxide background because of intrinsic contact potential differences. On 2-nm-thin oxides, tunneling across the oxides allows Fermi level equilibration with pentacene associated states. The surface potential difference depends on the doping of the underlying Si substrates. The Fermi level movement at the pentacene SiO(2) interface was restricted and estimated to lie between 0.3 and 0.6 eV above the pentacene valence band maximum. It is proposed that hole traps in the pentacene or at the pentacene-oxide interface are responsible for the observations.     

Electronic and chemical properties of tin-doped indium oxide (ITO) surfaces and ITO/ZnPc interfaces studied in-situ by photoelectron spectroscopy

The chemical and electronic properties of tin-doped indium oxide (ITO) surfaces and its interface with zinc phthalocyanine (ZnPc) were investigated using photoelectron spectroscopy partly excited by synchrotron radiation from the BESSY II storage ring. Preparation and analysis of ITO and ITO/ZnPc layer sequences were performed in-situ without breaking vacuum. The Fermi level position at the ITO surface varies strongly with oxygen content in the sputter gas, which is attributed to an increase of surface band bending as a consequence of the passivation of the metallic surface states of ITO. The shift of the Fermi level is accompanied by a parallel increase of the work function from 4.4 to approximately 5.2 eV. No changes in the surface dipole are observed with an ionization potential of I(P) = 7.65 +/- 0.1 eV. The barrier height for hole injection at the ITO/ZnPc interface does not vary with initial ITO work function, which can be related to different chemical reactivities at the interface.     

Formation of As2O3 during anodic dissolution of GaAs

The formation of As₂O₃ particles on GaAs surfaces, which have been anodically polarized at potentials <1 V versus SCE, has been studied using SEM, EDX, AES and XPS. Selective dissolution of GaAs occurs resulting in the formation of an As-rich surface layer. The As layer agglomerates and oxidizes on exposure to air forming As₂O₃ particles. The particle formation is dependent on test conditions, with p-GaAs forming As₂O₃ in both dark and daylight conditions; As₂O₃ only forms on n-GaAs when polarized in daylight. Polarization at corrosion potentials does not lead to particle formation, as GaAs dissolution rates are too low for surface enrichment of As.     

Deposition of tetracene on GaSe passivated Si(111)

The growth of tetracene on GaSe half-sheet passivated Si(111) is investigated under ultrahigh vacuum (UHV) using low-energy electron diffraction (LEED) and photoelectron spectroscopy (PS). A highly ordered thin-film growth was observed in the initial stages of the deposition process. All proposed structures form a coincidence lattice with the underlying substrate, due to the influence of the molecule-substrate interactions and are built up by either flat lying tetracene molecules at low coverage or tilted molecules at higher coverages. Photoelectron spectroscopy (XPS/UPS) shows that the deposited tetracene molecules induce band bending in the silicon substrate. No band bending was observed in the tetracene film, and an interface dipole potential of 0.45 eV was measured between the GaSe passivated Si(111) surface and the tetracene film.