Electronic properties of cleaved CdTe(110) surfaces

Photoemission yield spectroscopy measurements were performed on a set of n- and p-doped CdTe single crystals. The surfaces were obtained by cleavage in ultrahigh vacuum and characterized by low energy electron diffraction and Auger electron spectroscopy. On clean and properly cleaved surfaces, no band bending was found, neither on n- nor on p-type samples, showing the absence of intrinsic surface states in the gap. The ionization energy is found at 5.80±0.05 eV. Oxygen adsorption removes defect-induced surface states on the valence band side of the gap and develops a band bending on n-type samples which indicates the presence of acceptor surface states in the gap down to 0.70 eV below the conduction band edge. The ionization energy remains constant.     

Electronic structure of SiO2(111) thin film

The electronic structure of (111) surface of β-crystobalite is investigat ed using the empirical tight binding method. Our calculations identify surface states in the conduction band, band gap and valence band. The surface state formed from silicon-s and p_z orbitals, which is believed to account for the structure in the O K excitation spectra, lies in the band gap. It is seen that oxygen adsorption on the surface removes surface states and gives rise to a sharp peak at about — 3.8 eV below the valence band edge.     

Surface photovoltage spectroscopy of electronic surface states on cleaved germanium(111) surfaces

Surface photovoltage (SPV) measurements on UHV cleaved Ge(111) surfaces at 100 K are reported for photon energies 0.4 < ?ω < 1 eV. The SPV spectra are sensitive to surface treatment. Upon annealing to temperatures above 200°C, which is accompanied by a reconstruction change from the (2 × 1) to an (8) superstructure, the SPV spectrum shows 2 shoulders below band gap energy with threshold energies near 0.4 and 0.45 eV. These structures are interpreted in terms of electronic transitions from the valence band into empty surface state levels which are related to the (8) superstructure. Adsorbed oxygen and water vapor both cause new similar transitions from the valence band into empty surface states at 0.08 eV below the bottom of the conduction band.     

Subthreshold photoemission from copper nanoclusters on the SiO2 surface

Subthreshold photoemission from copper nanoclusters formed on the SiO₂ surface has been observed under irradiation of the surface by photons in the 3.1–6.5-eV energy range. The average size of copper nanoclusters on the silicon oxide surface is 250–500 nm. Besides the conventional photoemission from the filled Shockley surface state (SS), strong photoemission has been recorded at incident photon energies of 0.5 eV below the work function of the copper surface. This emission is assumed to be generated in direct electron transitions from the SS state to the unfilled electron surface states formed by the Coulomb image potential, followed by escape from these states into vacuum.     

Oxide states spectroscopy of real Ge surfaces

The electronic structure of surface oxide has been investigated by measurements of internal photoemission and photoconductivity measurements for p-type single crystalline Ge specimens with real surfaces. The photo-response curves show bulk and surface effects which are separated by differential methods. Emission from the bulk into oxide states or from oxide states into the bulk result in positive or negative changes of surface conductivity. The oxide states are distributed in energy with maxima near the conduction-band edge and the valence-band edge of the oxide. Changes of the electronic structure are observed by X-irradiation and storage in laboratory air.     

p-sd Shell gap reduction in neutron-rich systems and cross-shell excitations in 20 O

Excited states in 20O were populated in the reaction 10Be(14C,alpha) at Florida State University (FSU). Charged particles were detected with a particle telescope consisting of 4 annularly segmented Si surface barrier detectors and gamma radiation was detected with the FSU gamma detector array. Five new states were observed below 6 MeV from the alpha-gamma and alpha-gamma-gamma coincidence data. Shell model calculations suggest that most of the newly observed states are core-excited 1p-1h excitations across the N=Z=8 shell gap. Comparisons between experimental data and calculations for the neutron-rich O and F isotopes imply a steady reduction of the p-sd shell gap as neutrons are added.     

Energy distributions of field-emitted electrons from silicon: Evidence for surface states

Energy distributions of field-emitted electrons were obtained from field evaporated and annealed silicon tips. Electrons are emitted from levels below the Fermi level but above the valence band. The shape of the distributions depends on the crystallographic orientation of the emitting surface. The results are intepreted in terms of emission from a band of surface states which overlaps with the band gap.     

Origin of Schottky barriers in gold contacts on GaAs110

Gold contacts on n-type GaAs(110) have been investigated using scanning tunneling microscopy and spectroscopy in cross-sectional configuration. In spatially resolved current voltage spectroscopy the Schottky barrier potential is visible. We find signatures of delocalized gap states at the interface decaying into the semiconductor and observe a defect density at the interface below 3 x 10(13) cm(-2). Both findings support that the Fermi level pinning at the Au/GaAs(110) interface is dominated by metal-induced gap states.     

栅氧化层介质经时击穿的逾渗模型

在研究了MOSFET栅氧化层介质经时击穿物理机制的基础上，提出了氧化层击穿的逾渗模型.认为氧化层的击穿是E′心和氧空位等深能级缺陷产生与积累，并最终形成电导逾渗通路的结果.指出在电场作用下，氧化层中产生深能级缺陷，缺陷形成定域态，定域态的体积与外加电场有关.随着应力时间的增长，氧化层中的缺陷浓度增大，定域态之间的距离缩小.当定域态之间的距离缩小到一个阈值时，定域态之间通过相互交叠形成逾渗通路，形成扩展态能级，漏电流开始急剧增大，氧化层击穿. 关键词： 栅氧化层 TDDB 逾渗 模型     

氧化石墨烯纳米带能带结构和态密度的第一性原理研究

基于密度泛函理论,采用第一性原理方法,计算了氧化石墨烯纳米带的电荷密度、能带结构和分波态密度。结果表明,石墨烯纳米带被氧化后,转变为间接带隙半导体,带隙值为0.375 e V。电荷差分密度表明,从C原子和H原子到O原子之间有电荷的转移。分波态密度显示,在导带和价带中C-2s、2p,O-2p,H-1s电子态之间存在强烈的杂化效应。在费米能级附近,O-2p态电子局域效应的贡献明显,对于改善氧化石墨烯纳米带的半导体发光效应起到了主要作用。     

Oxygen-related band gap state in single crystal rubrene

A molecular exciton signature is established and investigated under different ambient conditions in rubrene single crystals. An oxygen-related band gap state is found to form in the ambient atmosphere. This state acts as an acceptor center and assists in the fast dissociation of excitons, resulting in a higher dark and photoconductivity of oxidized rubrene. The band gap state produces a well-defined photoluminescence band at an energy 0.25 eV below the energy of the 0-0 molecular exciton transition. Two-photon excitation spectroscopy shows that the states are concentrated near the surface of naturally oxidized rubrene.     

Topological magnetic insulators with corundum structure

Topological insulators are new states of quantum matter in which surface states residing in the bulk insulating gap are protected by time-reversal symmetry. When a proper kind of antiferromagnetic long-range order is established in a topological insulator, the system supports axionic excitations. In this Letter, we study theoretically the electronic states in a transition metal oxide of corundum structure, in which both spin-orbit interaction and electron-electron interaction play crucial roles. A tight-binding model analysis predicts that materials with this structure can be strong topological insulators. Because of the electron correlation, an antiferromagnetic order may develop, giving rise to a topological magnetic insulator phase with axionic excitations.     

Benefits of oxygen in CuInSe2 and CuGaSe2 containing Se-rich grain boundaries

Using density functional theory calculation, we show that oxygen (O) exhibits an interesting effect in CuInSe₂ and CuGaSe₂. The Se atoms with dangling bonds in a Se-rich Σ3 (114) grain boundary (GB) create deep gap states due to strong interaction between Se atoms. However, when such a Se atom is substituted by an O atom, the deep gap states can be shifted into valence band, making the site no longer a harmful non-radiative recombination center. We find that O atoms prefer energetically to substitute these Se atoms and induce significant lattice relaxation due to their smaller atomic size and stronger electronegativity, which effectively reduces the anion–anion interaction. Consequently, the deep gap states are shifted to lower energy regions close or even below the top of the valence band.     

Energy distributions of photoelectrons emitted from <Emphasis Type="Italic">p</Emphasis>-GaN(Cs,O) with effective negative electron affinity

Energy distributions of photoelectrons emitted into vacuum from the valence band and the localized states in the energy gap of p-GaN(Cs, O) with effective negative electron affinity were studied. It is shown that the photothermal electron excitation from the localized states lying below the Fermi level in the energy gap of p-GaN(Cs, O) is the dominant photoemission mechanism at the low-energy photoemission threshold.     

Observation of competing order in a high-Tc superconductor using femtosecond optical pulses

We present studies of the photoexcited quasiparticle dynamics in Tl(2)Ba(2)Ca(2)Cu(3)O(y) (Tl-2223) using femtosecond optical techniques. Deep into the superconducting state (below 40 K), a dramatic change occurs in the temporal dynamics associated with photoexcited quasiparticles rejoining the condensate. This is suggestive of entry into a coexistence phase which, as our analysis reveals, opens a gap in the density of states (in addition to the superconducting gap), and furthermore, competes with superconductivity resulting in a depression of the superconducting gap.     

Specific features of photoelectron emission from palladium clusters on graphite

Subthreshold photoelectron emission was observed to emerge from palladium nanoclusters formed on pyrographite surface under irradiation by photons in the energy range 3.1–6.5 eV. The average size of the palladium nanoclusters on the pyrographite surface was 50–80 nm, and the average height, 2–4 nm. Besides conventional photoemission from states below the Fermi level, photoelectron emission was observed at the energies of photons irradiating the surface 0.9 eV below the work function of the Pd surface. It is assumed that this emission is stimulated by direct electron transitions from Pd states below the Fermi level to the unfilled electron surface states formed in the Coulomb potential of image forces (image states) and, subsequently, into vacuum. This phenomenon is assumed to originate from the contact spot field generated above the surface which is nonuniform in work function. This assumption is supported by the calculations presented in the paper.     

Abnormal phenomenon of source-drain current of AlGaN/GaN heterostructure device under UV/visible light irradiation

We report an abnormal phenomenon that the source-drain current (I_D) of AlGaN/GaN heterostructure devices decreases under visible light irradiation. When the incident light wavelength is 390 nm, the photon energy is less than the band gaps of GaN and AlGaN whereas it can causes an increase of I_D. Based on the UV light irradiation, a decrease of I_D can still be observed when turning on the visible light. We speculate that this abnormal phenomenon is related to the surface barrier height, the unionized donor-like surface states below the surface Fermi level and the ionized donor-like surface states above the surface Fermi level. For visible light, its photon energy is less than the surface barrier height of the AlGaN layer. The electrons bound in the donor-like surface states below the Fermi level are excited and trapped by the ionized donor-like surface states between the Fermi level and the conduction band of AlGaN. The electrons trapped in ionized donor-like surface states show a long relaxation time, and the newly ionized donor-like surface states below the surface Fermi level are filled with electrons from the two-dimensional electron gas (2DEG) channel at AlGaN/GaN interface, which causes the decrease of I_D. For the UV light, when its photon energy is larger than the surface barrier height of the AlGaN layer, electrons in the donor-like surface states below the Fermi level are excited to the conduction band and then drift into the 2DEG channel quickly, which cause the increase of I_D.     

Mass-selected Ag<Subscript>3</Subscript> clusters soft-landed onto MgO/Mo(100): femtosecond photoemission and first-principles simulations

The electronic structure of supported mass-selected Ag₃ clusters is analyzed by joint femtosecond photoemission spectroscopy and ab initio theoretical investigations. A wide band gap insulating magnesia ultra-thin film on Mo(100) has been chosen as substrate in order to minimize the electronic interaction between metal clusters and support. After magnesia ultra-thin film preparation no photoemission from the molybdenum substrate is observed anymore, instead very weak two photon photoemission is detected possibly originating from surface or subsurface oxide defect states. Soft-landing deposition of 2 of atomic monolayer equivalents of Ag₃ clusters results in the disappearance also of the MgO two photon photoemission signal, while a strong single photon photoemission signal is detected from states located directly below the Fermi level. The theoretical study of structural, electronic and optical properties of Ag₃ at two model sites of MgO (100), the stoichiometric MgO(100) and an F_S-center defect, based on the DFT method and the embedded cluster model provides insight into the interactions between the cluster and the support which are responsible for the characteristic spectroscopic features.     

A new model for spectral sensitization of photoconduction in zinc oxide powder

Seven organic dyes were adsorbed on photoconductive zinc oxide powder and their sensitizing efficiencies were examined. The adsorption of dye induced a change in the dark conductivity, the magnitude of which appeared to represent the sensitizing power of the dye. The initial slope of sensitized photocurrent was found to be an exponential function of σ₀, the pre-exponential factor in the conductivity equation. A model is proposed, which is consistent with all the observations. In this model, the sensitized photoinjection of electrons is assumed to occur from surface states to the bulk, depending upon the population of the surface states. A critical surface level for the spectral sensitization is introduced to account for the observed temperature effect. Surface states located below the critical energy are assumed not to contribute for the sensitization.     

Modification of the Hybridization Gap by Twisted Stacking of Quintuple Layers in a Three-Dimensional Topological Insulator Thin Film

Twisting the stacking of layered materials leads to rich new physics. A three-dimensional topological insulator film hosts two-dimensional gapless Dirac electrons on top and bottom surfaces, which, when the film is below some critical thickness, will hybridize and open a gap in the surface state structure. The hybridization gap can be tuned by various parameters such as film thickness and inversion symmetry, according to the literature. The three-dimensional strong topological insulator Bi(Sb)Se(Te) family has layered structures composed of quintuple layers(QLs) stacked together by van der Waals interaction. Here we successfully grow twistedly stacked Sb_2Te_3 QLs and investigate the effect of twist angels on the hybridization gaps below the thickness limit. It is found that the hybridization gap can be tuned for films of three QLs, which may lead to quantum spin Hall states.Signatures of gap-closing are found in 3-QL films. The successful in situ application of this approach opens a new route to search for exotic physics in topological insulators.