Resonant shake-up satellites in photoemission at the Ga 3p photothreshold in GaN

Photoemission spectra recorded near the Ga 3p photothreshold from GaN have been found to contain satellites of the main Ga 3d emission line. The intensity of these satellites resonate at this threshold, and are associated with a 3d⁸ state. The correlation energies and binding energies for the satellite multiplet have been measured for the satellite and related Auger transitions. The satellite multiplet contains additional constant binding energy features not observed in previous studies of other Ga compounds. The present results are compared with those for GaP and GaAs.     

The synthesis of highly oriented GaN nanowire arrays

Highly oriented GaN nanowire arrays have been achieved by the catalytic reaction of gallium with ammonium. The resulting materials were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). SEM images show that the resulting materials are nanowire arrays with a uniform length of about 10 μm. XRD, EDS, TEM and SAED indicate that the nanowire arrays are single-crystal hexagonal GaN with a wurtzite structure. They have diameters of 10 to 20 nm. Received: 2 October 2002 / Accepted: 7 October 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. E-mail: wwwangjc@sina.com     

Surface plasmon assisted photoemission from Au nanoparticles on graphite

The resonant multiple excitation of collective modes in metallic nanoparticles using ultrashort laser pulses leads to an enhanced multiphoton photoemission from the particles. This effect is here demonstrated for the surface-plasmon resonance of Au nanoparticles on graphite. The shape of the photoemission spectra is explained by multiphoton photo-assisted thermionic emission from the nanoparticles and resonant emission via the image-potential state on graphite. Tuning the photon energy between 1.7 eV and 3.2 eV allows the identification of an enhancement of the photoemission yield at 2.1±0.1-eV photon energy that is attributed to the resonant excitation of the surface plasmon in the Au nanoparticles. This identification of the surface-plasmon excitation in this energy range is also supported by electron energy loss spectroscopy. Received: 8 August 2001 / Revised version: 13 September 2001 / Published online: 10 October 2001     

Time-resolved two-photon photoemission spectroscopy of HOPG and Ag nanoparticles on HOPG

Received: 6 November 1998     

X-Ray Photoelectron Spectroscopy and Reflection High Energy Electron Diffraction of Epitaxial Growth SiC on Si（100） Using C60 and Si

The formation of silicon carbide upon deposition of C60 and Si on Si（100） surface at 850^o C is studied via x-ray photoelectron spectroscopy and reflection high energy electron diffraction （RHEED）. The C ls, O ls and Si 2p core-level spectra and the RHEED patterns indicate the formation of 3C-SiC.     

Isotopic shift in angle-resolved photoemission spectra of Bi2Sr2CaCu2O8 due to quadratic electron-phonon coupling

In connection with the experiment on oxygen isotope effect of Bi₂Sr₂CaCu₂O₈ with the angle-resolved photoemission spectroscopy (ARPES), we theoretically study the isotope-induced band shift in ARPES by the Hartree-Fork and quantum Monte Carlo methods. We find that this band shift can be clarified based on a quadratically coupled electron-phonon (e-ph) model. The large ratio of band shift versus phonon energy change is connected with the softening effect of phonon, and the positive-negative sign change is due to the momentum dependence of the e-ph coupling.     

The electronic structure of KMnO4 investigated by photoemission and electron-energy-loss spectroscopy

From photoemission and electron-energy-loss data the following picture of KMnO₄, with Mn^VII (with a formal charge state Mn⁷⁺ (3d ⁰)) tetrahedrally surrounded by four O^2–-ions, is deduced: strong covalent bonding between Mn^VII and O^2– leads to a considerable occupation of the Mn-3 d shell. The ground state of the (MnO₄)^–1 molecule is an orbital and spin singlet as seen by the absence of any multiplet splitting in the Mn core levels. The valence band shows a four peak structure extending form 4 eV to 8 eV below the Fermi energy. The first peak at 4.2 eV has mainly O-2p character. The remaining peaks are of strongly mixed Mn-3d/O-2p character due to the covalent bonding. This mixing decreases with increasing binding energy. The electron energy loss data show a variety of structures between 2 eV and 10 eV independent of the primary electron energy which defines them as dipole allowed charge-transfer transitions. An additional excitation at 1.8 eV decreases quickly in intensity with increasing electron energy which classifies it as a dipole or spin forbidden transition in the compound. This energy is close to the value of 1.6 eV reported for the activation energy observed in electrical transport data. The results are compared to quantum chemical molecular orbital calculations of the (MnO₄)^–1 molecule.Physics Department, Allahabad University Allahabad 211002, India     

The influence of the substrate temperature on the photovoltaic properties of spray-deposited CdS:In thin films

Polycrystalline and highly transparent CdS:In thin films were produced by the spray pyrolysis (SP) technique at different substrate temperatures ranging from 350 to 490 °C on glass substrates. The effect of the substrate temperature on the photovoltaic properties of the films was investigated by studying the transmittance measurements, X-ray diffraction (XRD) patterns, scanning electron microscope (SEM) observations and the I-V plots. The transmittance measurements were used to estimate the band gap energy by the linear fit of (αhν)² versus hν. The band gap energy was found to be slightly increasing with the substrate temperature. XRD diffractograms show that a phase transition from the cubic to the hexagonal phase occurs by increasing the substrate temperature, beside more orientation of crystal growth. Also they show that complex cadmium compounds are still present till T_s ≈ 460 °C after which they practically disappear. From the linear I-V plots the resistivity was estimated and found to be strongly decreasing with the substrate temperature.     

Surface photovoltage and photoelectron spectra of GaP

Surface photovoltage (SPV) and photoelectron spectra (PES) of crystalline n-GaP wafers have been studied at 300 K. The magnitude of the surface potential (V_s) decreases in the presence of photons with energy more than the band gap, however the magnitude of V_s increases in the presence of photons with sub band gap energy. The SPV spectrum is helpful in understanding the rigid shift of PES spectra of n-GaP wafers towards higher kinetic energy in the presence of secondary white light from a tungsten lamp.     

The electronic structure of KMnO4 investigated by resonant photoemission

The valence band of potassium permanganate (KMnO₄) was investigated by means of resonant photoemission spectroscopy (ResPES) at the Mn2p, Mn3p and O1s edge. These data confirm the previous conclusions of a strong deviation from a purely ionic charge distribution in this compound. The ResPES data are in agreement with previous results about the character of the individual valence band states. A simple cluster model is used to explain qualitatively the different structures seen in the valence band spectra and their dependence on the photon energy.     

Influence of Grain Size on Electrical and Optical Properties of InP Films

InP film samples were prepared by spray pyrolysis technique using aqueous solutions of InCl₃ and Na₂HPO₄, which were atomized with compressed air as carrier gas onto glass substrates at 500°C with different thicknesses of the films. It is found that the resistivity of the polycrystalline films strongly depends on the grain size. It is observed that the grain size of the films increase with the decrease of the energy band gap and strain of the film. The changes observed in the energy band gap and strain related to the film grain size of the films are discussed in detail.     

Jahn--Teller Distortion of the Zinc Vacancy in ZnGeP2

A symmetry analysis and a simple dangling bond model are presented for the VZn^- in ZnGeP2, identifying a possible Jahn-Teller distortion mechanism which could naturally explain the localization of the defect wavefunction on two of the nearest-neighbouring P atoms, as deduced for the electron nuclear double resonance experiments.     

Temperature and Thickness Effects on Electrical Properties of InP Films Deposited by Spray Pyrolysis

InP film samples are prepared by spray pyrolysis technique using aqueous solutions of InCl₃ and Na₂HPO₄, which are atomized with compressed air as carrier gas onto glass substrates at 500°C with different thicknesses of the films. The structural properties of the samples are determined by x-ray diffraction (XRD). It is found that the crystal structure of the InP films is polycrystalline hexagonal. The orientations of all the obtained films are along the c-axis perpendicular to the substrate. The electrical measurements of the samples are obtained by dc four-probe technique on rectangular-shape samples. The effects of temperature on the electrical properties of the InP films are studied in detail.     

Effects of Dislocation on High Temperature Transport Characteristics of Unintentionally Doped GaN

High temperature transport characteristics of unintentionally doped GaN have been investigated by means of high temperature Hall measurements from room temperature to 500^o C. The increment of electron concentration from room temperature to 500^o C is found to vary largely for different samples. The dispersion of temperature dependence of electron concentration is found to be directly proportional to the density of dislocations in GaN layers calculated by fitting the FWHM of the rocking curves in x-ray diffraction measurements （XRD）. The buildup levels in persistent photoconductivity （PPC） are also shown to be directly proportionM to the density of dislocations. The correlation of XRD, Hall and PPC results indicate that the high temperature dependence of electron density in unintentional doped GaN is directly dislocation related.     

Electronic properties of the organic semiconductor hetero-interface CuPc/C60

We present a study of the electronic properties of the interface between the well-established molecular organic semiconductor copper phthalocyanine (CuPc) and the fullerite C₆₀ using photoelectron spectroscopy and the Kelvin-probe (KP) method. Upon deposition of CuPc on C₆₀, we found interfacial shifts of the vacuum level indicating the formation of a dipole layer, while band bending is found to be negligible. The interface dipole of 0.5 eV measured with KP is close to the difference between the work functions of bulk CuPc and C₆₀. No evidence for a chemical interaction at the interface is concluded from the absence of additional features in the core-level spectra at the earliest stages of deposition. The energy-level alignment diagram at the CuPc/C₆₀ interface is derived.     

Electronic structure of icosahedral alloys: The case of Al65Cu20Os15

Measurements with photoemission spectroscopy in the photon-energy range 35–130 eV have been used to determine the valence band of the stable icosahedral Al₆₅Cu₂₀Os₁₅. Resonant photoemission near the Os 5p 5d and 4f 5d transitions has been employed to show that the feature in the valence band with the maximum intensity at 1.5 (1) eV below the Fermi level is predominantly of the Os 5d character. This has been additionally verified by conducting the photoemission measurements in the constant-initial-state mode and by using the effect of the Cooper minimum in the photoionization cross section of the Os 5d orbitals. The valence band feature with the maximum intensity at 3.7 (1) eV below the Fermi level has been shown as being due mainly to the states of the Cu 3d character. The Os 5d and Cu 3d empirical partial density of states have been determined from the photoemission spectra. The decrease of intensity towards the Fermi level has been interpreted as being indicative of the presence of a theoretically predicted pseudogap around the Fermi level. It has been indicated, however, that the Fermi cut-off also contributes to the observed intensity decrease. It has been demonstrated that the energy resolution of the spectroscopic measurements performed so far on quasicrystals was not high enough to unambiguously determine the presence of such a pseudogap. No unusual features in the valence band of icosahedral Al₆₅Cu₂₀Os₁₅, which could be ascribed to its quasiperiodic nature, have been observed within the resolution of the experiment. High energy-resolution spectroscopic measurements were also shown to be essential to observe the theoretically predicted spikiness of the density of states in quasicrystals. A critical review of published spectroscopic data on the electronic structure of quasicrystals has also been presented.     

The kinetic energy dependent effective Debye temperature for CoS2 (100)

The effective Debye temperatures of the highly spin-polarized material CoS₂ were measured using temperature dependent low energy electron diffraction and shown to be dependent upon electron kinetic energy. The normal dynamic motion of the (100) surface results in the effective surface Debye temperature of compared to a bulk Debye temperature of . Similar values for the bulk Debye temperature have been obtained through LEED I(V) analysis and core level photoemission with a lower value for the bulk Debye temperature found from heat capacity measurements.     

Electron structure and electron dynamics at InSb(111)2×2 semiconductor surface

The conduction band electronic structure and the electron dynamics of the clean InSb(111)2×2 surface have been studied by laser based pump-and-probe photoemission. The results are compared to earlier studies of the InSb(110) surface. It is found that both the energy location and the time dependence of the photoexcited structures are very similar for the two surfaces. This indicates that the dominant part of the photoemission signal in the conduction band region is due to excitations of electrons in the bulk region and that the surface electronic states play a minor role. The fast decay of the excited state, τ∼12 ps, indicates that diffusion of hot electrons into the bulk is an important mechanism. Received: 9 May 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +46-0824/913-1, E-mail: gm@matphys.kth.se     

Blue shift in the luminescence spectra of MEH-PPV films containing ZnO nanoparticles

Luminescence properties of nanocomposites consisting of ZnO nanoparticles in a conjugated polymer, poly [2-methoxy-5-(2′-ethyl hexyloxy)-phenylene vinylene] (MEH-PPV), were investigated. Photoluminescence measurements reveal a blue shift in the emission spectrum of MEH-PPV upon incorporation of ZnO nanoparticles into the polymer film while the emission is increasingly quenched with increasing ZnO concentration. In contrast, the structure of the polymer and its conjugation length are not affected by the presence of ZnO nanoparticles (up to 16 wt% ZnO) as revealed by Raman spectroscopy. The blue shift and photoluminescence quenching are explained by the separation of photogenerated electron-hole pairs at the MEH-PPV/ZnO interface and the charging of the nanoparticles.     

Oxygen chemisorption on Cu(110): A combined study by second-harmonic spectroscopy and photoemission

We have studied the formation of the addedrow (2×1)O overlayer on Cu(110) using Second-Harmonic Generation (SHG). To characterize the electronic properties of the surface, simultaneous observations with LEED and angle-resolved photoemission were performed. We are able to interpret our results in terms of transitions between surface bands of Cu(110) and Cu(110)-(2×1)O, respectively.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May. 30 to June 1, 1994