Showing papers on "Photoemission spectroscopy published in 1999"

Evidence for Quantum Critical Behavior in the Optimally Doped Cuprate Bi2Sr2CaCu2O8+δ

Abstract: The photoemission line shapes of the optimally doped cuprate Bi(2)Sr(2)CaCu(2)O(8+delta) were studied in the direction of a node in the superconducting order parameter by means of very high resolution photoemission spectroscopy. The peak width or inverse lifetime of the excitation displays a linear temperature dependence, independent of binding energy, for small energies, and a linear energy dependence, independent of temperature, for large binding energies. This behavior is unaffected by the superconducting transition, which is an indication that the nodal states play no role in the superconductivity. Temperature-dependent scaling suggests that the system displays quantum critical behavior.

Many-Body Effects in Angle-Resolved Photoemission: Quasiparticle Energy and Lifetime of a Mo(110) Surface State

Abstract: Recent investigations of strongly correlated electron systems have questioned the validity of one of the most fundamental paradigms in solid state physics— Fermi liquid theory. The latter picture is based on the existence of “quasiparticles,” or single-particle-like low energy excitations which obey the exclusion principle and have lifetimes long enough to be considered as particles. Strictly speaking, the quasiparticle concept is restricted to zero temperature and a narrow region around the Fermi level [1], but its usefulness often continues to finite temperatures, and energies away from the Fermi level [2]. Indications for possible non-Fermi-liquid behavior have been found in some organic one-dimensional conductors [3] and in the normal state of high temperature superconductors [4]. A whole variety of experimental techniques have been employed in the search for such behavior, including resistivity measurements [5], infrared spectroscopy [6], scanning tunneling spectroscopy [7], and time-resolved two-photon photoemission [8]. Angle-resolved photoemission spectroscopy (ARPES) has an advantage, in that the energy and lifetime of the photohole are directly observable in the experiment. ARPES in principle measures the quasiparticle spectral function [9]:

Differences in nature of defects between SrBi2Ta2O9 and Bi4Ti3O12

Abstract: X-ray photoemission spectroscopy measurements were executed to compare the nature of defects in SrBi2Ta2O9 (SBT) and Bi4Ti3O12 (BTO) films. In the SBT film, it was found that the oxygen ions at the metal–oxygen octahedra were much more stable than those at the Bi2O2 layers. On the other hand, for the BTO film, oxygen vacancies could be induced both at the titanium–oxygen octahedra and at the Bi2O2 layers. We suggested that the difference in stability of the metal–oxygen octahedra should be related to different fatigue behaviors of the SBT and the BTO films.

Determination of the sp3/sp2 ratio of a-C:H by XPS and XAES

Abstract: X-ray photoelectron spectroscopy and X-ray excited Auger electron spectroscopy were used to determine some of the properties of diamond like amorphous carbon films deposited using saddle field glow discharge of methane. By applying the chemical shift technique and curve fitting technique to the C1s peak in the X-ray photoelectron spectrum of the film, we were able to determine the sp 3 /sp 2 bonding ratio of the films. The ratio from this method was in agreement (±1%) with that calculated from the X-ray excited Auger electron spectrum of the same film. The method was also applied to a-C:H films doped with different impurities (boron and phosphorus) in amounts varying from 1% to 20%. We found that the sp 3 /sp 2 ratio of the film was dependent on the deposition parameters and on types of impurities and their concentrations.

Performance of the grating-crystal monochromator of the ALOISA beamline at the Elettra Synchrotron

Abstract: The new beamline ALOISA, now operational at the Elettra Synchrotron, is designed for surface studies by means of several experimental techniques: surface x-ray diffraction and reflectivity, photoemission spectroscopy, photoelectron diffraction, e−-Auger coincidence spectroscopy. A new monochromator has been specifically designed and realized for this multipurpose beamline: it makes use of a channel-cut Si crystal dispersive element for the 3–8 keV range and of a plane mirror-plane grating element for the 200–2000 eV range. Both dispersive elements share the same optical system. In the low energy range (200–900 eV) the spectral resolving power exceeds 5000 while maintaining a throughput higher than 1010 photons/s/200 mA/0.02% BW. In the case of the N2 1s→π* and Ne 1s→3p transitions, the extremely high signal-to-noise ratio of the absorption spectra allowed a very accurate determination of the corresponding natural linewidth (116±2 and 250±10 eV, respectively). Moreover, the vibrational structure of the CO–...

Combined photoemission/in vacuo transport study of the indium tin oxide/copper phthalocyanine/N,N′-diphenyl-N,N′-bis(l-naphthyl)-1,1′biphenyl-4,4″diamine molecular organic semiconductor system

Abstract: Ultraviolet photoemission spectroscopy (UPS) was used to study the indium tin oxide/copper phthalocyanine (CuPc) and CuPc/N,N′-diphenyl-N,N′-bis(l-naphthyl)-1,1′biphenyl-1-4,4″diamine interfaces, which are commonly used as an anode/hole injection layer/hole transport layer combination in organic light emitting devices. In order to assess the validity of the transport barriers measured using UPS, in vacuo I–V measurements have been performed on simple devices grown and measured in the same system as the samples studied using UPS. I–V characteristics were modeled using numerical simulations. The parameters used in the simulated curves which best fit the measured I–V characteristics agree quantitatively with the UPS measured barriers.

Numerical simulation of high-order above-threshold-ionization enhancement in argon

Abstract: Resonant enhancement of high-order peaks in the photoelectron spectrum of argon at 800 nm is studied by numerical precision integration of the time-dependent Schr\"odinger equation in the single-electron approximation. It is shown that electrons in the backscattering region of the spectrum are almost exclusively due to resonances. Wave-function analysis shows that there are two types of resonant states: high-angular-momentum states that stay away from the nucleus and mainly decay by emission of low-energy electrons, and states that are located near the polarization axis. These latter states predominantly decay through violent collision with the ionic core, and are responsible for the enhancement of high-energy photoelectrons.

Band offsets for ultrathin SiO2 and Si3N4 films on Si(111) and Si(100) from photoemission spectroscopy

Abstract: High resolution soft x-ray photoelectron spectroscopy with synchrotron radiation is used to study the interfaces of SiO2/Si(111), SiO2/Si(100), Si(111)/Si3N4, and SiO2/Si3N4 for device-quality ultrathin gate oxides and nitrides. The thin oxides and nitrides were grown by remote plasma deposition at a temperature of 300 °C. Aftergrowth samples were further processed by rapid thermal annealing for 30 s at various temperatures from 700 to 950 °C. The Si(111)/Si3N4 samples were air exposed and formed a thin ∼6 A SiO2 layer with a Si(2p) core-level shift of 3.9 eV, thus allowing us to study both the Si(111)/Si3N4 and SiO2/Si3N4 interfaces with a single type of sample. We obtain band offsets of 4.54±0.06 eV for SiO2/Si(111) and 4.35±0.06 eV for SiO2/Si(100) with film thicknesses in the range 8–12 A. The Si(111)/Si3N4 nitrides show 1.78±0.09 eV valence-band offset for 15–21 A films. This value agrees using the additivity relationship with our independent photoemission measurements of the nitride–oxide valence-ba...

Direct and indirect excitation mechanisms in two-photon photoemission spectroscopy of Cu(111) and CO/Cu(111)

Abstract: It is demonstrated that the dependence of the two-photon photoemission ~2PPE! yield on the polarization of the exciting laser light provides detailed information about the excitation mechanism and the orientation of transition dipole moments in the 2PPE process. In particular, it is possible to distinguish between a dir ct two-photon excitation process, where both electronic transitions are induced by the electric fields at the surface, and anindirect mechanism, where the first excitation step occurs in the substrate. In the latter process the intermediate state in 2PPE is populated by scattering of photoexcited hot electrons from the substrate, which are subsequently photoemitted by the second laser pulse. The analysis is applied to 2PPE from clean and CO covered Cu~111!. Furthermore, we have derived analytical expressions for the 2PPE signal based on the optical Bloch equations for a three-level system excited with continuous light beams. They allow us to calculate 2PPE spectra of surface states for a variety of cases. @S0163-1829 ~99!02008-1#

Oxidation study of GaN using x-ray photoemission spectroscopy

Abstract: The oxidation of GaN at room temperature was investigated using x-ray photoemission spectroscopy. The onset of oxidation is observed at 105 Langmuir (L=10−6 Torr s) of oxygen exposure. This is seen both in changes in the percent composition of oxygen and in shifts in the core spectrum of Gallium. The oxidation saturates at an exposure of 108 L. Detailed core level analysis shows that at this exposure most of the surface Ga atoms have been oxidized. The results indicate that the oxidation of GaN is a kinetically limited process restricted to the surface and the underlying bulk is not strongly perturbed. This is in sharp contrast with GaAs where oxidation begins at the same level of exposure and then continues for all further exposures as subsurface layers are oxidized.

Charge-Density-Wave Mechanism in 2 H − NbSe 2 : Photoemission Results

Abstract: The layered charge-density-wave (CDW) material $2H\ensuremath{-}{\mathrm{NbSe}}_{2}$ was studied by angle-resolved photoelectron spectroscopy. We present the first experimental mapping of the Fermi surface (FS) portions involved in the CDW transition. From this and additional data on the conduction band dispersion near the Fermi level we conclude that the CDW instability in this material is driven by FS nesting and not by saddle point singularities.

Negative Ion Photoelectron Spectroscopy Studies of Organic Reactive Intermediates

Abstract: Nearly 100 years ago, Moses Gomberg introduced the scientific community to the study of organic radicals,1 concluding this classic manuscript with the comment that, “This work will be continued and I wish to reserve the field for myself.” Gomberg’s studies of triphenylmethyl radical were only the beginning of intense research into the properties of the newly discovered trivalent carbon, and despite Gomberg’s proclamation, work was carried out by scientists all over the world. By the middle of the century, new experimental techniques such as matrix isolation2 and flash photolysis3 allowed for the direct spectroscopic study of these transient species. Armed with this array of new tools, in 1964 Porter and Ward politely requested,4 in reference to Gomberg, that “after the respectable period of more than half of a century, we may perhaps be permitted to look further into the problem”. With this comment, they ushered in the modern era of organic radical spectroscopy. Since that time, spectroscopic studies using techniques, including infrared, visible and ultraviolet absorption spectroscopy, Raman spectroscopy, and electron spin resonance spectroscopy, have lead to a much better understanding of the physical properties of many reactive intermediates, including carbenes and radicals. Over the past decade, we have applied the technique of negative ion photoelectron spectroscopy (NIPES) to such systems. In this gas-phase experiment, a beam of mass-selected negative ions is intersected with an intense ultraviolet laser beam, resulting in photodetachment of an electron (Figure 1). A portion of the photodetached electrons are collected and energy-analyzed, giving the photoelectron spectrum, which is a plot of the number of photoelectrons detected as a function of electron kinetic energy (eKE). It is frequently useful to plot the number of electrons versus electron binding energy (eBE), the difference between the laser energy and eKE. The advantage of such a plot is that the positions of features observed in the spectrum are not dependent on the energy of the laser used in the experiment. The information that can be obtained from negative ion photoelectron spectroscopy studies is multifold. Accurate values of vibrational frequencies for the isolated, gas-phase molecule can be determined from vibrational progressions observed in the spectra. Moreover, vibrational frequencies for negative ions can often be obtained from the positions of “hot bands”, i.e., transitions from excited vibrational states of the ion. However, the most important information obtained from photoelectron spectra often turns out to be the electron affinity and electronic state term energies for the neutral intermediate. Electron affinities are of particular value in calculating bond dissociation energies, as illustrated in the thermochemical cycle shown in eq 1. If the gas-phase acidity of a molecule

Correlation of x-ray absorption and x-ray photoemission spectroscopies in amorphous carbon nitride

Abstract: The nature of bonding in amorphous carbon nitride was studied by synchrotron-radiation photoemission spectroscopy and near-edge x-ray-absorption fine structure. The experimental data are compared with theoretical calculations of chemical shifts, and the relative importance of initial and final state effects is discussed. Bond lengths are determined from the position of the ${\ensuremath{\sigma}}^{*}$ resonance and the obtained results suggest ${\mathrm{sp}}^{2}$ hybridization. The experimental and theoretical evidence presented in this paper supports another interpretation of the photoemission spectra of amorphous carbon nitride.

Scanning Tunneling Spectroscopy of Mott-Hubbard States on the 6H-SiC(0001) √3 × √3 Surface

Abstract: Scanning tunneling spectra have been measured on the 6H-SiC(0001)√3 × √3 surface for both pand n-type material. With the use of exceptionally low tunnel currents, the tunneling spectra reveal distinct bands of empty and filled states, separated by 2.0 eV. The states are located at the same spatial position, thereby supporting a silicon adatom model which predicts a Mott-Hubbard type density of states. Silicon carbide is a wide bandgap semiconductor with properties that make it useful in high temperature electronics and other areas. SiC crystals exist with various stacking arrangements of the atomic planes; of particular interest are the cubic 3C polytype and hexagonal 4H and 6H polytypes. Much work has been done on characterizing the 6H-SiC (0001) surface as a surface suitable for epitaxy and device development. Several workers have studied the atomic structure of the surface using the scanning tunneling microscope (STM) [1,2]. A set of reconstructions of the Si terminated (0001) surface has been discovered, one of which, the √3 × √3, has evoked much experimental [3-5] and theoretical [6-9] interest. Theoretically, the lowest energy model for this reconstruction consists of Si adatoms at T4 positions on a Si terminated bulk crystal. Here, three of the Si adatom bonding orbitals are backbonded to Si atoms. The fourth bonding orbital extends into vacuum with only one electron in it. Local density functional calculations for this structural model [6,8] predict a half-filled, and hence metallic band arising from the dangling bond. More refined computations [7] performed after the photoemission [4] and inverse photoemission [5] spectra became available, employed a two-dimensional Hubbard model. These calculations indicate that the energy levels of this surface consist of a filled and an empty band, separated by a Hubbard gap of U=1.6 eV, thus producing a semiconducting density of states (DOS). Such a surface would have at each site a single localized electron, forming a 2-D system of spins which can take two values on a triangular lattice. This problem is of great theoretical interest because such systems can be frustrated and form 2-D spin glasses. This structure has been questioned in the literature, and several alternatives have been proposed [1,3,10,11] but experimental results to date are unable to distinguish between these models. Photoemission spectroscopy (PES) experiments on n-type 6H-SiC (0001) wafers which show a √3 × √3 low energy electron diffraction (LEED) pattern reveal a fully-filled band 1.2 eV below the Fermi level [4]. Inverse photoemission spectroscopy (IPES) [5] on the same surface shows an empty surface state 1.1 eV above the Fermi level. Both these results are in moderate agreement with recent theoretical predictions of a Mott-Hubbard ground state for this surface [7]

Observation of strong band bending in perylene tetracarboxylic dianhydride thin films grown on SnS2

Abstract: Perylene tetracarboxylic dianhydride (PTCDA) thin films were grown in several steps on tin disulfide (SnS2) single crystals and characterized by combined x-ray and ultraviolet photoemission spectroscopy (XPS), (UPS) in order to characterize the frontier orbital line-up and the interface dipole at their interface. Due to the large difference between the work functions of PTCDA (4.26 eV) and SnS2 (5.09 eV) this experiment represents a model system for the investigation of band bending related phenomena in organic semiconductor heterojunctions. Our results show that the equilibration between the Fermi levels of both materials in contact is achieved almost solely by band bending (bulk charge redistribution) in the PTCDA layer. No significant interface dipole was detected which means that the PTCDA molecular orbitals and the SnS2 bands align at the vacuum level corresponding to the electron affinity rule. Our experiments clearly demonstrate the importance of an additional XPS measurement which (in most cases) ...

A photoabsorption, photodissociation and photoelectron spectroscopy study of NH3 and ND3

Abstract: The absolute photoabsorption, photoionization and photodissociation cross sections and the photoionization quantum efficiency of ammonia and deuterated ammonia have been measured from the ionization threshold to 25 eV using a double ion chamber and monochromated synchrotron radiation. The photoabsorption spectrum displays extensive vibrational progressions associated with Rydberg series converging onto excited vibrational levels of the 2A2?? state. New structure has been observed for ND3 in the 10.0-11.3 eV range, and vibrational progressions due to transitions into the , and Rydberg states have been recorded with improved resolution. Features have been observed, for the first time, in the photoabsorption spectra of NH3 and ND3 due to Rydberg series converging onto the ? 2E ionization threshold, and interpretations for some of these features have been proposed based upon the corresponding photoelectron spectra. The He I excited NH3+(1a2??)-1 2A2?? photoelectron band has been studied experimentally at a resolution of 3 meV and two vibrational progressions, each involving excitation of the 2+ mode, have been observed. The vibrational lines in the main progression show a complex structure associated with rotational excitations. This structure changes gradually in a way that can be explained by the variation of the H-N-H bond angle with the 2+ mode. The effective bond angle has been found to be 120? for v2+ = 0, and similar to that of the neutral ground state near v2+ = 6. The second progression, of weak lines, has been interpreted tentatively as being due to n2++4+. The 4+ mode is doubly degenerate and the excitation of a single quantum has been explained by vibronic coupling with the ? 2E state. In addition, He II excitation has been used to record the entire valence shell photoelectron spectrum.

Organic semiconductor interfaces: Discrimination between charging and band bending related shifts in frontier orbital line-up measurements with photoemission spectroscopy

Abstract: Gaq3 is a promising luminescent organic semiconductor for applications in organic light emitting diodes. The frontier orbital alignment at the tris (8-hydroxyquinolinato) gallium (Gaq3)/Pt organic Schottky contact was determined by combined x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) measurements. A Gaq3 thin film was deposited in several steps on a previously Ar+ sputtered pure Pt foil. After each growth step, the sample was characterized by XPS and UPS. The combination of XPS and UPS measurements allows the precise evaluation of the interface dipole independent from the simultaneously occurring band bending at the interface and charging artifacts. The measurements show that the Pt/Gaq3 interface has a strong dipole of 0.71 eV indicating the transfer of negative charge from Gaq3 to Pt. Due to the large work function difference between Pt and Gaq3, strong band bending occurred. At Gaq3 coverages higher than 128 A strong charging shifts occurred in the overlayer related emission lines which...

Photoelectron spectroscopy of the CH3N− ion

Abstract: We have observed the negative ion photoelectron spectrum of the methylnitrene ion, CH3N−, and measured the electron affinity of methylnitrene, EA(CH3N)=0.022±0.009 eV. In addition to detaching the methylnitrene anion to the ground state of CH3N(X 3A2), we also detect the first electronically excited state of methylnitrene, a 1E. We measure the singlet/triplet splitting to be ΔE(a 1E−X 3A2)=1.352±0.011 eV. The photoelectron spectrum of CH3N a 1E contains relatively sharp vibronic structure. Unlike the spectra from H2CC−, the photoelectron spectra for CH3N− show no evidence for a barrier separating the rearrangement of singlet methylnitrene to methyleneimine, 1[CH3N] → CH2=NH.

Valence band x-ray photoelectron spectroscopic studies to distinguish between oxidized aluminum species

Abstract: The determination of the detailed chemical nature of oxidized aluminum species is an essential requirement for the study of many important practical aspects associated with aluminum metal and its compounds. While thick oxidized films of aluminum metal can be easily characterized by x-ray powder diffraction when the films are crystalline, thin amorphous films are very difficult to characterize. In this article, a study of the valence band x-ray photoelectron spectrum of aluminum oxides, hydroxides, and oxyhydroxides is reported using monochromatic aluminum Kα X radiation. The valence band spectra obtained are shown to have significant differences for different oxidized aluminum species, and can be well understood by comparison with spectra generated from cluster and band structure calculations. This study compliments earlier published studies from this research group using achromatic radiation, and demonstrates how the use of monochromatic X radiation allows a more conclusive distinction to be made among oxidized aluminum species.

Photodissociation of gas phase I3− using femtosecond photoelectron spectroscopy

Abstract: The photodissociation dynamics of gas phase I3− following 390 nm excitation are studied using femtosecond photoelectron spectroscopy. Both I− and I2− photofragments are observed; the I2− exhibits coherent oscillations with a period of 550 fs corresponding to ∼0.70 eV of vibrational excitation. The oscillations dephase by 4 ps and rephase at 45 and 90.5 ps on the anharmonic I2− potential. The gas phase frequency of ground state I3− is determined from oscillations in the photoelectron spectrum induced by resonance impulsive stimulated Raman scattering. The dynamics of this reaction are modeled using one- and two-dimensional wave packet simulations from which we attribute the formation of I− to three-body dissociation along the symmetric stretching coordinate of the excited anion potential. The photodissociation dynamics of gas phase I3− differ considerably from those observed previously in solution both in terms of the I2− vibrational distribution and the production of I−.

SiO2/6H-SiC(0001)3×3 initial interface formation by Si overlayer oxidation

Abstract: We investigate the initial oxidation and SiO2/6H-SiC interface formation by core level photoemission spectroscopy using synchrotron radiation. The results indicate that the direct oxidation of the 6H–SiC(0001)3×3 surface leads to SiO2 formation at low temperatures (500 °C) with a nonabrupt interface having significant amounts of mixed (Si–O–C) and intermediate (Si3+,Si2+,Si+) oxidation products. In contrast, C-free and a much more abrupt SiO2/6H-SiC(0001) interface formation is achieved when predeposited Si overlayer is thermally oxidized at low oxygen exposures and low temperatures (500 °C).

Valence Electronic Structure of π-Conjugated Materials: Simulation of the Ultraviolet Photoelectron Spectra with Semiempirical Hartree−Fock Approaches

Abstract: A detailed understanding of the electronic structure of π-conjugated materials can be reached by means of two widely available semiempirical quantum-chemical methods: Austin model 1 (AM1) and intermediate neglect of differential overlap (INDO). This is illustrated by calculating the ultraviolet photoelectron spectra (UPS) of π-conjugated oligomers and polymers and comparing the theoretical data to experimental spectra. The approach is applied here to a series of compounds with varying molecular topology and chemical constitution: oligomers of p-phenylenevinylene and various derivatives, fluorinated derivatives of polyisothianaphthene, and 4,4‘-bis(m-tolyphenylamino)biphenyl (TPD). The AM1- and INDO-calculated UPS spectra are also compared to data obtained with the valence effective Hamiltonian method, which is known to provide reliable results for the simulation of UPS spectra of these types of molecules. An easily applicable procedure is proposed to obtain the best fit to the experimental spectra from ...

Band lineup of a SnS2/SnSe2/SnS2 semiconductor quantum well structure prepared by van der Waals epitaxy

Abstract: A quantum well composed of layered semiconductors and SnSe2 (Eg=1.03 eV) and SnS2 (Eg=2.18 eV) was grown in several steps by van der Waals epitaxy. After each growth step the electronic structure was characterized by ultraviolet and x-ray photoemission spectroscopy. From these measurements, bandbending and the valence-band offset were determined on both sides of the quantum well. The results show that both wells are of the same magnitude, hence indicating commutativity of the band offset. Small interface dipoles (0.11–0.19 eV) were detected at the interfaces, which could be identified as quantum dipoles.