Showing papers on "Photoemission spectroscopy published in 1986"

Low Energy Electrons and Surface Chemistry

Abstract: Basic Concepts Auger Electron Spectroscopy X-Ray Photoelectron Spectroscopy (XPS) Ultraviolet Photoelectron Spectroscopy (UPS) Electron Spectroscopy with Noble Gas Ions and Metastable Atoms Appearance Potential Spectroscopy Inverse Photoemission (IPE, BIS) Electron Energy Loss Spectroscopy (ELS, EELS) Low Energy Electron Diffraction (LEED) X-Ray Absorption Fine Structure (EXAFS) Vibrational Spectroscopy (HREELS, EELS) Electron and Photon Stimulated Desorption Appendix: Fundamental Constants Properties of Selected Elements Line Positions in XPS Using Al-Kd Radiation XPS Atomic Sensitivity Factors Kinetic Energies of Auger Electrons Relative Auger Sensitivity Factors Character Tables Characteristic Group Frequencies Abbreviations and Acronyms.

Investigations on hydrophilic and hydrophobic silicon (100) wafer surfaces by X-ray photoelectron and high-resolution electron energy loss-spectroscopy

Abstract: The paper reports on surface spectroscopy measurements of silicon single-crystal wafers which have been treated in order to obtain hydrophilic and hydrophobic surfaces, respectively. The wafers are characterized in terms of the oxidation behaviour in air (“native oxides”), their surface chemical composition and the chemical bonds involved. It is shown that the oxide on hydrophilic wafers mainly grows in the cleaning agent and consists of hydrated SiO2 through all stages of the growth. On a hydrophobic surface, however, the oxidation begins with the formation of a lower oxidation state which turns into SiO2 on storage in air. The thickness of the oxides on both surface types reaches 1.4–1.5 nm. Both the chemical shift in photoelectron spectroscopy and the frequency of the asymmetric Si-O-Si vibration in electron energy loss spectroscopy support the assumption of a reduced bonding angle of the oxygen bridge. Hydrophilicity is caused by singular and associated OH groups on the surface. Singular groups could be detected up to 700 K. There are hints that OH groups stabilize the oxide during heating. The hydrophobic state is mainly characterized by Si-H and Si-CH x groups on the surface, whereas Si-F exists only in minor quantities. Si-H groups were stable up to approximately 900 K in UHV. Si-CH x dehydrogenizes at temperatures between 500 and 700 K leaving SiC on the surface.

Atomic autoionization following very fast dissociation of core-excited HBr.

Abstract: Photoelectron spectroscopy excited by monochromatic synchrotron radiation (68-80 eV range) is used to study the Br $3d$ excitation in HBr. The transition to an antibonding orbital is shown to produce a resonant state whose repulsive nature has been observed directly. A two-step relaxation process involving a fast neutral dissociation followed by the autoionization of the excited fragment has been shown for the first time.

Metal polyimide interface: A titanium reaction mechanism

Abstract: Chemical bonding and reaction of in situ deposited Ti on a series of polyimide surfaces have been investigated. X‐ray photoelectron spectroscopy has been used to probe site specific information for bonding of various functional groups of polyimide molecules, while ultraviolet photoelectron spectroscopy has been applied to monitor the change of the Ti d bands and the polyimide molecular orbitals near the Fermi energy as a function of overlayer coverage. At low coverages, the polyimide surfaces react strongly with Ti and strong charge transfer via the carbonyl group occurs. As the coverage increases, a Ti–C bond formation has been identified. A detailed mechanism for the reaction of Ti with polyimides is described, and this mechanism is confirmed by model pyromellitimide reaction with Ti0 suspension in solution.

Spectroscopy of image-potential states with inverse photoemission.

Abstract: Cu(100), Cu(110), Ag(111},Au(111), and Au(100}), a semimetal [Sb(100}],and a layered compound {1T-TiS2). Data for Au{111) show that image-potential states exist even in the presence of bulk states. The coupling with bulk states is weak as indicated by the narrow {&0.1 eV) width of the image states. The binding energies of the lowest state cluster around 0.7 eV for a variety of metal and semimetal surfaces indicating a universal phenomenon. This value is close to the hydrogenic binding energy of —, 6 Ry -0.85 eV for the n =1 state, which indicates that the simple Coulombic image potential dominates the energy balance.

Photoemission spectroscopy for the spin-degenerate Anderson model.

Abstract: A procedure that calculates excitation properties for the Anderson model is reported and applied to photoemission. In the mixed-valence regime, hybridized in the ground state, the one- and two-electron impurity configurations generate two spectral resonances. In the Kondo regime, a phenomenological expression describes the narrow peak at the Fermi level.

The photoelectron spectrum of water in the 30 to 140 eV photon energy range

Abstract: Synchrotron radiation from the German storage ring BESSY has been used to study the photoelectron spectrum of the water molecule. Asymmetry parameters and relative photoionization cross sections have been measured for all four molecular orbitals (2a1, 1b2, 3a1, and 1b1) in the 30–140 eV photon energy range. The results have been compared with the Xα calculations of Roche, Salahub, and Messmer and good agreement was found. Comparison was also made with a number of previous measurements, including those obtained by (e,2e) spectroscopy. Finally, the inner valence (2a1) region has been studied at 60 and 100 eV photon energies and the relative intensities of some of the satellites were found to change as the exciting energy was varied.

Synchrotron‐radiation excited carbon 1s photoemission study of Cr/organic polymer interfaces

Abstract: The chemical nature of the adhesive interface between Cr and three polymers has been investigated using synchrotron‐radiation excited photoemission from the polymer carbon 1s core levels. PMDA–ODA polyimide (PI), an important polymer in the electronics industry, was studied along with two simple polymers, PMPO and PVMK, which contain structural fragments of the complicated PI protomer. The carbon 1s spectra were decomposed into distinct peaks which were assigned to specific chemical species and monitored as a function of Cr coverage. Our results confirm that the carbonyl groups on these polymer surfaces are targets for reactive metals during the initial stages of metallization. This initial reaction stage was followed by gradual formation of an electron‐rich, carbidelike, carbon species.

Photoelectron spectroscopy of the negative ion SeO

Abstract: We have recorded the photoelectron spectrum of SeO− using a newly constructed negative ion photoelectron spectrometer. The adiabatic electron affinity of SeO is determined to be 1.456±0.020 eV. Values of ν00(a 1Δ–X 3Σ−0+) and ΔG1/2(a 1Δ) are found to be 5530±200 and 916±35 cm−1, respectively, in substantial accord with previous measurements. The negative ion parameters determined in this work are: B‘e(SeO−) =0.4246±0.0050 cm−1 which leads to r’e(SeO−)=1.726±0.010 A, ω‘e(SeO−)=730±25 cm−1, ω’e x‘e(SeO−)=2±4 cm−1, and D0(SeO−)=3.84±0.09 eV. In addition, the spectroscopic parameters of SeO− are compared with those of the electronically analogous negative ions: O−2, SO−, and S−2.

Enhancement of Si oxidation by cerium overlayers and formation of cerium silicate

Abstract: The oxidation of Si(100) surfaces covered with 3 A Ce was studied by high-resolution photoemission spectroscopy. The oxygen uptake was found to be enhanced by orders of magnitude by the presence of Ce. Because of chemical reaction between Ce and Si, a mixed oxide with a Ce:Si:O composition ratio of 1:1:3 is formed. The sharp photoemission spectra of this oxide indicate the formation of a well-defined compound in which both Ce and Si are in a 3+ oxidation state.

Characterization of β-SiC surfaces and the Au/SiC interface

Abstract: The chemistry of the β‐SiC surface has been studied with Auger electron spectroscopy (AES) and x‐ray photoemission spectroscopy (XPS). The chemically etched surface was found to be nearly stoichiometric, with only a small amount of adsorbed oxygen, which appears to be localized on the Si states. Ion sputtering caused the surface to become Si rich. Compared to Si, the β‐SiC surface was very resistant to oxidation by air exposure at room temperature and when annealed in O2 at 300 °C. In addition, essentially no reaction occurred between Au and etched β‐SiC even after extensive heat treatment.

Autoionization and isotope effect in the threshold photoelectron spectrum of 12CO2 and 13CO2

Abstract: Threshold photoelectron spectra of 12CO2 and 13CO2 have been obtained from the onset at 900 A (13.75 eV) to 620 A (20 eV) at a 9 meV resolution using dispersed synchrotron radiation with a 0.3 A bandpass. A number of forbidden and/or previously unobserved transitions have been assigned for the X, A, B, and C states of CO+2. Photoelectron spectra by electron time of flight at selected excitation wavelengths in the Franck–Condon gap region below the A state, reveal the autoionization of neutral valence states as well as Rydberg series converging to the A 2Πu and B 2Σ+u states. The photoelectron energy distribution obtained upon excitation of the Rydberg states is consistent with that given by the Bardsley–Smith model for autoionization, while the superexcited valence states decay via a resonance autoionization process proposed previously by Guyon, Baer, and Nenner. These valence states are also responsible for the strong enhancement of vibrationally forbidden transitions, such as (000)→(010). Finally...

Studies of atomic and molecular fluorine reactions on silicon surfaces

Abstract: X‐ray photoelectron spectroscopy (XPS) and an ultrahigh vacuum compatible microwave discharge effusive beam source have been used to study the reactions of atomic and molecular fluorine on Si(111) surfaces. Fluorine uptake and changes in binding energy and peak shape for the Si 2p and F 1s XPS peaks have been measured as functions of fluorine exposure. The results indicate that molecular fluorine is dissociatively chemisorbed to form an SiF2‐like surface species. This reaction saturates at approximately one monolayer surface coverage. In contrast, atomic fluorine uptake extends well beyond the monolayer regime to include several Si layers. Additionally, as the uptake increases, the reaction product becomes SiF4‐like. These findings are compared with previously reported results for XeF2 adsorption.

Initial formation of the interface between a polar insulator and a nonpolar semiconductor: CaF2 on Si(111)

Abstract: The initial stages of interface formation between CaF2 and Si(111) have been studied with both core‐level and angle‐resolved valence band photoemission spectroscopy. Both the Si 2p and Ca 3p core levels indicate that a Si–Ca bond is present at the CaF2–Si(111) interface. Annealing of thin CaF2 films at 700–800 °C results in preferential evaporation of F, and the surface undergoes a number of reconstructions until a stable Si(111):Ca 3×1 reconstruction is obtained on which there is calcium, but no fluorine. This surface exhibits a surface state with an upward dispersion of 0.4 eV towards the K point of the 1×1 surface Brillouin zone. Our results show that the CaF2 molecule can be dissociated on the Si(111) surface at typical epitaxial growth temperatures, with the Ca being more strongly adsorbed to the Si than is the F.

Photoemission spectroscopy study of the Al/SiC interface

Abstract: Aluminum layers of some nanometers thickness have been deposited onto α‐SiC crystals and studied by x‐ray and ultraviolet photoelectron spectroscopies as a function of annealing temperature. Annealing beyond 600 °C induces a complete disappearance of metallic Al on the crystal surface, penetration of Al into the bulk, and formation of aluminum carbide. Ultraviolet photoemission spectra recorded after annealing at 360 °C could be explained by a beginning of interfacial reaction. However, there is no clear experimental evidence that reaction occurs at such a low temperature.

Effects of Argon-Ion Bombardment on the Basal Plan Surface of MoS2.

Abstract: : The surface composition and structure of the basal plane surface of Molybdenum Disulfide was modified by 10 keV argon-ion bombardment and then studied with x-ray photoelectron spectroscopy. The rigid shift of both sulfur and molybdenum core levels is demonstrated to result from the formation of a small amount of Molybdenum islands on the surface, although there is no a priori evidence for changes in oxidation state at the surface after bombardment. This conclusion is suggested by recent data on the deposition of metals on the MoS2 basal plane that indicate a strong correlation between the Mo or sulfur binding energy and the electronegativity of the metal. Our results are compared with those of previous studies and are explained in terms of recent theories on the effects of surface ion bombardment that suggest that chemical/thermal effects dominate over substrate mass effects for higher projectile-ion mass and energy.

Structure and bonding at the CaF2/Si (111) interface

Abstract: High resolution core level spectroscopy with synchrotron radiation is used to determine the bonding at the epitaxial CaF2/Si (111) interface It is found that both Ca and F bond to Si at the interface inducing core level shifts of +04 eV and −08 eV, respectively Structural models with an atomically sharp interface are proposed where Ca bonds to the first layer Si and F to the second layer

A variable energy photoelectron study of the valence levels and I 4d core levels of CF3I

Abstract: Using monochromatized synchrotron radiation, gas phase photoelectron spectra of CF3I have been obtained between 21 and 100 eV photon energies. Experimental valence band branching ratios have been compared with theoretical branching ratios from MS‐Xα calculations. The generally good agreement between experiment and theory confirms the orbital assignment: 4e<4a1<1a2∼3e<2e<3a1<1e∼2a1 <1a1, in order of increasing binding energy. In contrast to CF4, MS‐Xα calculations predict two shape resonances for CF3I at 15.2 eV (e channel) and 17.2 eV (a1 channel). Experimentally, weak shape resonances are observed on e orbitals at ∼14 eV kinetic energy. The cross sections are very useful for correlating orbitals in CF3I with those of CF4. For example, the cross sections suggest that the 3e orbital in CF3I correlates with the 1t1 orbital rather than the 4t2 orbital in CF4. After the onset of the I 4d level at ∼57 eV, intershell coupling becomes important in enhancing valence band levels having high I 5p character such as ...