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A survey of biophysical and biomedical applications of free-electron lasers (FELs) is presented. FELs are pulsed light sources, collectively operating from the microwave through the x-ray range. This accelerator-based technology spans gaps in wavelength, pulse structure, and optical power left by conventional sources. FELs are continuously tunable and can produce high-average and high-peak power. Collectively, FEL pulses range from quasicontinuous to subpicosecond, in some cases with complex superpulse structures. Any given FEL, however, has a more restricted set of operational parameters. FELs with high-peak and high-average power are enabling biophysical and biomedical investigations of infrared tissue ablation. A midinfrared FEL has been upgraded to meet the standards of a medical laser and is serving as a surgical tool in ophthalmology and human neurosurgery. The ultrashort pulses produced by infrared or ultraviolet FELs are useful for biophysical investigations, both one-color time-resolved spectroscopy and when coupled with other light sources, for two-color time-resolved spectroscopy. FELs are being used to drive soft ionization processes in mass spectrometry. Certain FELs have high repetition rates that are beneficial for some biophysical and biomedical applications, but confound research for other applications. Infrared FELs have been used as sources for inverse Compton scattering to produce a pulsed, tunable, monochromatic x-ray source for medical imaging and structural biology. FEL research and FEL applications research have allowed the specification of spin-off technologies. On the horizon is the next generation of FELs, which is aimed at producing ultrashort, tunable x rays by self-amplified spontaneous emission with potential applications in biology.

We report angle-resolved photoemission data from single crystals of C60 cleaved in UHV. Unlike the other forms of pure carbon, the valence band spectrum of C60 consists of many sharp features that can be essentially accounted for by the quantum chemical calculations describing individual molecules. This suggests that the electronic structure of solid C60 is mainly determined by the bonding interactions within the individual molecules. We also observe remarkable intensity modulations of the photoemission features as a function of photon energy, suggesting strong final state effects. Finally, we address the issue of the band width of the HOMO state of C60. We assert that the width of the photoemission peak of C60 does not reflect the intrinsic band width because it is broadened by the non 0-0 transitions via the Franck-Condon principle. Our view point provides a possible reconciliation between these photoemission data and those measured by other techniques.

Surface extended x‐ray adsorption fine structure (SEXAFS) has been used to investigate the structure of Sb on the Si(001) 2×1 surface. The coverage of Sb which remains after annealing thick layers at 375 °C, previously reported to be one monolayer (ML), is found in this work to form a disordered overlayer with three dimensional Sb clusters. This finding is concluded from the Sb L3 absorption spectra which are similar for this coverage to that of bulk Sb. After a 550 °C anneal, Auger electron spectroscopy, and scanning tunneling microscopy (STM) show that about 1 ML of Sb remains. Phase and amplitude analysis of the Sb L3 edge SEXAFS shows that the remaining Sb atoms occupy a modified bridge site with a Si–Sb bond length of 2.63±0.04 A. The Sb atoms form dimers with a Sb–Sb bond length of 2.91±0.04 A, which is almost identical to the bulk Sb–Sb bond length of 2.90 A. The Sb atoms lie 1.74±0.06 A above the Si(001) surface. STM confirms the dimer structure of the Sb overlayer.

High-quality ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ superconducting single crystals, annealed at different oxygen partial pressures, have been studied using angular-resolved x-ray photoelectron spectroscopy with a resolution higher than that used in any previous study. Two states of the oxygen, separated by \ensuremath{\approxeq}0.7 eV, are unambiguously observed. Examining these components at different angles makes it possible to distinguish bulk from surface components. Using this capability we discover that annealing under lower oxygen partial pressure (1 atm) results in oxygen intercalation beneath the Bi-O surface layer of the crystal, whereas for higher-pressure anneals (12 atm) additional oxygen is found on the Bi-O surfaces. This steplike intercalation mechanism is also confirmed by the changes observed in the Cu and Bi core lines as a function of the annealing oxygen partial pressure.

Surface extended x-ray-absorption fine structure (SEXAFS) has been combined with scanning tunneling microscopy (STM) to determine both the local and long-range bonding properties of the Si(001)2{times}1-Sb interface. Sb {ital L}{sub 3} edge SEXAFS shows that Sb dimers occupy a modified bridge site on the Si(001) surface with a Sb-Sb near-neighbor distance of 2.88{plus minus}0.03 A. Each Sb atom of the dimer is bonded to two Si atoms with a Sb-Si bond length of 2.63{plus minus}0.04 A. STM resolves the dimer structure and provides the long-range periodicity of the surface. Low-energy-electron diffraction of vicinal Si(001) shows that the Sb dimer chains run perpendicular to the original Si dimer chains.

It is pointed out that the change in the oxidation state of Cu in YBa2CU3O7-x with increasing x vitiates the trend in the Cu(2p) satellite intensity and hence the Cu-O charge-transfer energy. When Y is partly replaced by Ca, however, the satellite intensity and T(c) decrease with the increase in Ca content or hole concentration, just as in other cuprates.

We present results of resonant-photoemission and high-resolution x-ray-photoemission-spectroscopy (XPS) studies on single-crystalline CuO, which confirms most of the earlier data obtained from polycrystalline samples. However, some minor differences with earlier results were also observed, which are concentrated on a mainly oxygen-related feature. The important first ionization state is much better resolved in our high-resolution XPS data, and is found to have both copper and oxygen orbital character. This is consistent (though not a proof) with the theoretical prediction that the first ionization state is a {sup 1}{ital A}{sub 1{ital g}} singlet with holes on both oxygen and copper sites. The 16-eV satellite is found to show a Cu antiresonance, and is assigned to a {sup 1}{ital A}{sub 1{ital g}} singlet with both holes on the copper sites.

We have examined In–GaAs(110) contacts fabricated in situ both by depositing In at room temperature followed with annealing and by depositing In on heated surfaces [n‐ and p‐type GaAs(110) surfaces (250 and 300 °C, respectively) using synchrotron radiation photoemission. It is found that In deposition on GaAs(110) surfaces with temperature below room temperature does not induce measurable reaction, whereas reaction takes place with InAs as product when the room‐temperature‐formed interface is annealed or when In is deposited on the heated substrate. However, there is no evidence of InxGa1−xAs graded heterojunction formation at the interface in either case. Outdiffusion of the excess As from the substrate is suggested to provide a source for InAs formation instead of the previously proposed In–GaAs replacement reaction. Electrical measurements were also performed on in situ In/n‐GaAs(110) contacts before and after annealing. Reproducible high‐barrier heights were recorded (1.02±0.02 eV) with good ideality ...

Palladium deposits supported on amorphous alumina were prepared in ultrahigh vacuum and studied using transmission electron microscopy (TEM), x‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and energy loss spectroscopy (ELS). The growth of clusters with deposition time was followed using TEM. The XPS, AES, and TEM results support a consistent model for the distribution of palladium on the alumina substrate, with the amount of material within the clusters accounting for the majority of the palladium deposited. The combined results of XPS, ELS, and TEM indicate that the deposit growth with deposition time can be tentatively separated into the following size regimes: monolayer islands, ‘‘small’’ clusters and metallic three‐dimensional clusters. The size‐dependent XPS Pd 3d binding energy shifts are analyzed within the initial‐state‐final‐state framework. Component fitting of the Pd 3d spectra is consistent with assigning one of the components to the layer of interfacial Pd atoms.

GaAs band bending in the low adatom coverage regime (

Interfaces between UHV-cleaved GaP (110) and overlayers of Sn, Sb, and Bi have been studied using soft X-ray photoemission spectroscopy. The substrate core level attentuation indicates that these three interfaces exhibit Stranski-Krastanov growth. The overlayer core level spectra (Sn 4d, Sb 4d, and Bi 5d) exhibit two components for coverages up to one monolayer, indicating that the adatoms form a zigzag chain overlayer as has been reported at the Sb/GaAs interface. The Ga 3d and P 2p surface components disappear as the first monolayer develops, and the remaining bulk components are used to determine various lineshape parameters of these core levels. The Schottky barrier formation at these interfaces has also been examined. The band bending at these interfaces occurs chiefly for coverages exceeding one monolayer, when islands are forming on the ordered layer. For all three of these interfaces the final surface Fermi level position is in the neighborhood of 1.0eV above the valence band maximum.

We have performed angle-resolved photoemission experiments and density-functional band calculations for NiO and CoO. The measured oxygen bands are in excellent agreement with the calculation, but the measured cation 3d dispersion is only about 25% of the calculated one. We have also demonstrated the effects of the antiferromagnetic order on the electronic structure of these materials

A technique is demonstrated for measuring photoemission and photoabsorption spectra from areas as small as 20 µm in diameter. A magnetic projection photoelectron microscope relying on the electron imaging and magnifying properties of a diverging magnetic field is described. This photoelectron microscope is of the type originally conceived by the Turner group at Oxford University. XANES spectra from 20 µm diameter regions of high Tc superconductors and from 40 µm diameter regions at the bottom of vias etched through photoresist are presented. Small area UPS spectra of In deposited locally on GaAs are also shown. Potential exists to measure photoabsorption spectra from areas on the order of one micron square.

A review of photoemission measurements on high-temperature superconductors is presented. Earlier photoemission results and interpretations are critically examined, providing the reader with an overview of the present status of the field. Special emphasis is paid to recent progress, in particular the revelation of the superconducting gap, and angle-resolved photoemission studies of high-quality single crystals of the Bi2Sr2CaC2O8 material. Comparison with predictions of band calculations and impurity model calculations as well as the electronic structure of some transition-metal oxides is made. Finally, comments on the present status of photoemission studies of the high-temperature superconductors and future implications are given.

Gd{sub 3}Si{sub 5}, GdSi, and Gd{sub 5}Si{sub 3} were investigated with photoemission spectroscopy in the photon-energy range 40.8--149 eV by exploiting the energy dependence of the photoemission cross sections and the valence resonance at the crossing of the Gd 4{ital d}-4{ital f} threshold. The modification of the spectra versus photon energy, along with their stoichiometry dependence, show the relevance of covalent mixed Gd 5{ital d}--Si 3{ital sp} states in the formation of the chemical bond. In the region close to the Fermi level an increase of the {ital d} contribution is observed. These points are discussed in connection with the existing models of the silicide bond.

The novel properties of heavy-fermion materials involve rare earth 4f electrons or actinide 5f electrons. For the 4f electrons the spectrum measured in photoemission and bremsstrahlung isochromat spectroscopies is dominated by single-site energetics and the impurity Anderson Hamiltonian gives at the semiquantitative level a unified description of the 4f spectrum and certain low energy properties characterized by a single-site Kondo temperature TK. Current efforts for 4f electron materials are directed at finding the quantitative limits of this impurity approach by detailed fitting of spectra using hybridization matrix elements calculated within the local density approximation (LDA). Examples presented herein are CeCu2Si2 and the α and γ phases of cerium metal. For the most commonly studied 5f case of uranium no such unified picture exists. Various current approaches to understanding uranium spectra include analysis and modeling of differences between measured spectra and LDA calculations, and purely experimental approaches such as alloy and dilution studies of UxY1-xPd3 and UxY1-xAl2.

Through the use of synchrotron radiation photoemission spectroscopy (PES) and related techniques, we have gained detailed knowledge of Fermi level pinning, interfacial chemistry and disruption of GaAs for coverages up to several monolayers (ML). A link has been made between these data and that in the thick layer regime (hundreds of ML), which characterizes practical Schottky diodes. PES results for thin layers deposited at room temperature (RT) and low temperatures of about 80 K (LT) as well as thick films deposited at RT and annealed to higher temperatures are considered. At LT where GaAs disruption is minimized for thin films, metal-induced gap states seem to dominate the Fermi level pinning process except where GaAs metal reactions are strong. For RT thin and thick films, the effects of defects must be considered, and the advanced unified defect model (AUDM) is applied. In the AUDM the key defects are identified as the As Ga (double donor with levels at 0.75 and 0.5 eV above the valence band maximum) and the Ga As antisite (double acceptor) with the As Ga normally dominating due to the excess As which characterizes LEC GaAs crystals. The literature is reviewed and a number of phenomena are explained in terms of this model including the Fermi level position on MBE grown GaAs observed by Svensson et al. and the anomolously high Schottky barrier height (SBH) of thick Ga on n-GaAs observed by several groups. By performing electrical, TEM, and chemical studies of thick diodes and by evaluating the changes upon thermal annealing of diodes it is found that the AUDM successfully predicts the increase or decrease of barrier height on annealing.

Metal/InP(110) interfaces have been systematically studied using photoelectron Spectroscopy. This work aims at understanding the physical nature of interface states and Fermi level pinning mechanisms. Clear-cut experimental evidence shows that two types of interface states are responsible for Fermi level pinning. At the disrupted interfaces which occur at room temperature, a large amount of defects are generated during various interfacial processes and pin the surface Fermi levels around 0.95 eV above the valence band maximum (VBM). On the contrary, much fewer defects are present at the nearly perfect interfaces which are obtainable at reduced temperature. In these cases, the metal induced gap states play an overwhelming role, and the Fermi level is pinned around 0.75 eV above the VBM. It is also concluded that interplay between the two mechanisms should be taken into account in order to understand the process of Schottky barrier formation.

Semiconductor surface core level shifts are studied by use of selected overlayers. It is shown that certain nonreactive overlayers fully quench the surface core level shifts of the substrate without introducing additional interfacial components. The unique possibility of separately characterizing the bulk core level lineshape allows a precise determination of the surface core level shifts. This method is illustrated for the cleavage faces of Si and InP.

The idea of LiF lenses as monochromators for inverse photoemission spectroscopy (IPES) was introduced recently. Here we report a simple, versatile, inexpensive and very efficient IPES system. The high‐yield LiF monochromator system consists of a custom‐designed LiF biconvex lens as a UV photon dispersement element (∼128 mm/eV). It focuses light below 11.8 eV into a tunable KBr‐coated spiraltron detection system. With the detector set at 11.6 eV, experiments on n‐GaAs(110) and Au/n‐GaAs(110) yield 2400 and 6000 counts/μA s, respectively, with a resolution of 0.4–0.6 eV. Investigation into the lens quality and other components suggests that better yield and resolution are achievable in the future.

Resonant photoelectron spectroscopy studies have been performed on the NiO valence band at photon energies corresponding to the Ni $2p$,$3p$,and O $1s$ absorption thresholds. Strong resonances are seen in the vicinity of the Ni $2p$ threshold, which confirm earlier conclusions from the weaker resonances seen at the Ni $3p$ threshold. No valence-band resonance is observed at the O $1s$ threshold. The analysis of this data confirms the picture of NiO as a strongly correlated charge-transfer insulator by identifying the highest-lying states as being of mainly $3{d}^{8}\mathit{L}$ final-state character. The existence of localized excited Ni states, as well as the delocalized nature of the O states, are confirmed. Comparisons with the configuration-interaction model and quasiparticle calculations are also made.

Synchrotron radiation photoemission spectroscopy suggests that the room temperature grown Au/Si(111) interface is an abrupt interface with the contact made by metallic Au and bulk Si. Si 2[ital p] core level spectra show no sign of an interface component, but only the surface reacted and bulk Si components during the growth of the interface. A surface Au-Si alloy film is first formed for Au coverages below 3 monolayers. The alloy is then stabilized into an Au[sub 3]Si-like film and detached from the Si substrate when metallic Au starts nucleation in between.

We report angle-resolved photoemission data from single crystals of C60 cleaved in UHV. Unlike the other forms of pure carbon, the valence band spectrum of C60 consists of many sharp features that can be essentially accounted for by the quantum chemical calculations describing individual molecules. This suggests that the electronic structure of solid C60 is mainly determined by the bonding interactions within the individual molecules. We also observe remarkable intensity modulations of the photoemission features as a function of photon energy, suggesting strong final state effects. Finally, we address the issue of the band width of the HOMO state of C60. We assert that the width of the photoemission peak of C60 does not reflectthe intrinsic band width.

Diamond is an important semiconductor which has great potential in high temperature, high power device applications. In the fabrication process of diamond electronic device, doping ofdiamond and understanding of diamond/metal interfaces are important. As a Column V element, Sb is a possible dopant for diamond. Early work reported that Sb is incorporated into diamond by ion implantaion [1]. In addition, Sb plays an important role in Si and Ge heteroepitaxial growth. On the Si or Ge surface one ordered monolayer of Sb occupies the epitaxial sites and saturates the surface dangling bonds, which leads to uniform epitaxial growth. While diamond has the same crystal structure as both silicon and germanium, it has a drastically smaller lattice and much stronger bond. This makes it very difficult to extrapolate antimony's behavior on diamond from its behavior on either silicon or germanium. In this work, we have studied the electronic and geometric structure of Sb on diamond surfaces using photoelectron spectroscopy and low energy electron diffraction. While the exact adsorption sites could not be determined, we find that antimony strongly bonds to the diamond surface. Further, antimony behaves very differently on the diamond(100) face as compared to the diamond(111) face. We also find that neither Sb/diamond system behaves like antimony on either silicon or germanium. We attribute these results to the drastically smaller diamond lattice and the stronger C-C bond.

Photoemission spectroscopy is commonly used to study band bending at clustered metal-semiconductor interfaces. However, these band-bending data have been difficult to interpret because of the nonuniform distribution of pinning sites. In this paper the interpretation of these measurements is investigated in detail. Using three-dimensional Poisson integration programs, we examine the extent to which the band bending can be ascribed to states at the cluster-semiconductor interfaces. First, the potential variations in the semiconductor near an individual cluster are calculated. The depletion of the semiconductor along the surface is found to be significantly less than the one-dimensional depletion length. Next, the surface-potential variations are computed for systems of known cluster morphology, assuming that the pinning states are restricted to the cluster-semiconductor interfaces. From these potentials, the expected band-bending measurements are extracted and compared to experiment. At submonolayer coverages, we find that the experimental band bending is greater than can be attributed to pinning states under the clusters. Thus, this low-coverage band bending is interpreted in terms of surface states between the clusters. Defects, chemisorbed adatoms, and surface unbuckling are discussed as possible sources of these states. Above one monolayer coverage, the clusters cover enough of the surface to uniformly pin the Fermi level. Experimentally, however, a separation between the n- and p-type surface Fermi-level positions persists to the highest coverages. This separation is also interpreted in terms of intercluster surface states.

Etude par spectrometrie de photoemission des rayons X, la chimie interfaciale de Cu, Ag, Au, Ni, Pd, Al, Ga, In, Sn, et Bi deposes sur des surfaces liees de GaP(110). Influence de la reactivite de couche superieure, de la morphologie et de la metallicite sur le developpement de la barriere en relation avec deux sources de potentiels des etats de l'interface

For n- and p-doped III-V compounds, Fermi-level pinning and accompanying phenomena of the (110) cleavage surface have been studied carefully using photoemission at hv≲ 300 eV (so that core as well as valence band levels could be studied). Both the clean surfaces and the changes produced, as metals or oxygen are added to those surfaces in submonolayer quantities, have been examined. It is found that, in general, the Fermi level stabilizes after a small fraction of a monolayer of either metal or oxygen atoms have been placed on the surface. Most strikingly, Fermi-level pinning produced on a given semiconductor by metals and oxygen are similar. However, there is a strong difference in these pinning positions depending on the semiconductor: The pinning position is near (1) the conduction band maximum (CBM) for InP, (2) midgap for GaAs, and (3) the valence band maximum (VBM) for GaSb. The similarity in the pinning position on a given semiconductor produced by both metals and oxygen suggests that the states responsible for the pinning resulted from interaction between the adatoms and the semiconductor. Support for formation of defect levels in the semiconductor at or near the surface is found in the appearance of semiconductor atoms in the metal and in disorder in the valence band with a few percent of oxygen. Based on the available information on Fermi energy pinning, a model is developed for each semiconductor with two different electronic levels which are produced by removal of anions or cations from their normal positions in the surface region of the semiconductors. The pinning levels have the following locations, with respect to the VBM: GaAs, 0.75 and 0.5 eV; InP, 0.9 and 1.2 eV (all levels + 0.1 eV).

We present the results of photoemission studies of ‘‘valence disordered’’ monoclinic BaBiO3 in the photon energy range 15–120 eV. The line‐shapes of the valence band photoemission spectra and the Ba contributions to the valence band are very similar to the line shapes of the total density of states and Ba partial density of states, respectively. Oxygen resonance is observed, demonstrating the existence of empty O 2p states. These results support a more covalent rather than a simple ionic picture for the electronic states of BaBiO3.

We have used ultraviolet photoemission spectroscopy to study the morphology of the nonreactive materials In, Ag, Sb, and Bi deposited on the GaAs(100) surface. The attenuation of the substrate core levels and the electronic structure of the overlayers yield information on the growth modes and others properties of these interfaces. We find that in all cases the behavior is significantly different from the growth on the more heavily studied (110) surface. In the case of In, we observe isolated In atoms in between large clusters, while on the (110) surface, only strong clustering is observed. Ag forms a larger number of small clusters compared to the (110) surface. This indicates that the mobility of small Ag clusters is lower, and/or the number of nucleation sites is greater on our (100) surfaces. Both Sb and Bi grow in more laminar modes, as opposed to the Stranski–Krastanov mode observed on (110). The Sb film is found to be closer to perfectly laminar. After annealing at 250 °C for 10 min, the Bi film is ...

The possibility of a defect mechanism is proposed to explain the Fermi‐level pinning in Schottky barriers on III–V semiconductor surfaces. This suggestion comes from the results of photoemission spectroscopy applied to the study of formation of metal–semiconductor barrier heights. Changes in the electronic structure and composition of the interface are studied. For Au metal overlayers on the (110) surfaces of GaAs, GaSb, and InP, the Fermi‐level pinning occurs at 0.1 or less of a monolayer coverage. Furthermore, the pinning position is roughly independent of the choice of adatom: Cs, Al, Au, or O. The partial yield structure disappears at about monolayer coverage of Au. The Au valence band had the characteristic shape for atomiclike Au, indicating that the metal was dispersed homogeneously on the surface without the formation of thick islands. Deposition of Au onto the GaSb surface causes the compound to decompose. The antimony segregates to the surface and leaves behind a nonstoichiometric interface. The...

Ultraviolet photoemission experiments have been performed on the (110) and (111) single crystal surfaces of PtCu alloy. The difference in their surface atomic arrangements gives rise to different peak shapes in the Cu 3d‐derived surface ’’density of states’’. The PtCu (110) sample, with 1.9–2.3 at. % Cu in the bulk, shows 5.6%–7.7% Cu near the surface. The PtCu(111) sample, in comparison, shows insignificant Cu surface segregation.

This paper describes a comprehensive modelling scheme for idealized rectangular transmission gratings of arbitrary dimensions placed at arbitrary angles to the irradiating beam. The beam parameters are those of an arbitrary synchrotron source radiating past a series of mirrors and slits. The geometrical effects of the mirrors and slits on the beam characteristics are calculated by the methods of phase-space analysis. The particular aspects of grating performance that are determined by the computer program are the output intensity spectrum and the power loading in the grating structure. Though gratings of any material may be analyzed by the program, the material analyzed and utilized to date has been gold. Two items of particular interest that are discussed in this paper are modelled blazing effects and modelled thermal loading of the grating structure under high flux conditions.

A room-temperature reaction between Al adsorbate atoms and a GaAs (110) surface is observed by means of soft X-ray photoemission techniques. Evidence of two states of Al which are distinct from the bulk Al metal is seen at submonolayer coverages. The sequential appearance of these states suggest that both Al chemisorbed on the surface and Al replacing Ga in the surface lattice are present. The possible influence of surface lattice reconstruction and Al proximity effects on the replacement reaction is discussed. The replacement reaction is important in the context of metal contacts to GaAs (both ohmic and Schottky barriers) as well as for GaAs—AlGaAs heterojunctions.

In this paper, a new method for measuring optical constants using transmission gratings is presented and analysed. The method, based on utilizing the properties of ultra-thin transmission diffraction gratings, is independent of the absolute intensity of the radiation source, the most critical part of the experiment being the accurate measurement and control of the diffraction grating parameters. By way of example, this paper relates the use of the new method to gold in the soft X-ray regime. By extension, it is shown that the same technique may be used on other materials, as long as they may be shaped into similar grating structures, and as long as they fulfil certain other criteria, outlined in the main body of this paper.

We present the results of a photoemission study of the annealed Ti/GaAs(110) interface, using soft x‐ray (SXPS), x‐ray (XPS), and ultraviolet (UPS) photoelectron spectroscopies to monitor the substrate and overlayer core levels and the valence band for Ti thicknesses between 6.7 and 20 monolayers and annealing temperatures between 200 and 425 °C. The SXPS data imply the formation of a stable Ti–As region near the Ti/GaAs interface that increases in width with temperature. Concomitant to this, the binding energy of the Ga changes from its room‐temperature position towards elemental Ga. The XPS data suggest the presence of elemental Ga at or near the overlayer surface. The UPS data suggest the possibility of cluster formation on the overlayer surface. In situ current–voltage (I–V) measurements were also made on thick‐film systems fabricated using similar methods to those used in the photoemission studies [i.e., cleaving, depositing metal (∼1000 A), and annealing in UHV]. The movement of Ti–As reaction produ...

Photoemission spectroscopy using synchrotron radiation has been used to investigate the electronic structure of the Bi/sub 2/Ca/sub 1/Sr/sub 2/Cu/sub 2/O/sub 8/ and the Bi/sub 2/Sr/sub 2/CuO/sub 6/ superconductors. The states near the Fermi level E/sub F/, as observed in photoemission spectroscopy in these two compounds, have less Cu character than the La/sub 1.8/Sr/sub 0.2/CuO/sub 4/ and the YBa/sub 2/Cu/sub 3/O/sub 7/ superconductors. We suggest the coexistence of bandlike p states of Bi-O origin and correlated Cu 3d O 2p bands. The p bands of Bi-O are occupied differently in the Bi/sub 2/Ca/sub 1/Sr/sub 2/Cu/sub 2/O/sub 8/ and the Bi/sub 2/Sr/sub 2/CuO/sub 6/ superconductors, resulting in a different density of states near E/sub F/ and a slightly different valency of the Bi ions in the two compounds. We point out the correlation between the difference in the density of states near E/sub F/ and the difference in the superconducting transition temperatures of the Bi/sub 2/Ca/sub 1/Sr/sub 2/Cu/sub 2/O/sub 8/ and the Bi/sub 2/Sr/sub 2/CuO/sub 6/ superconductors.

In this paper we describe the properties of the SPEAR storage ring at the Stanford Linear Accelerator Center as a synchrotron radiation source for X-ra

Photoemission studies have been made of the clean and oxidized amorphous Si surface at hv = 10·2, 21·2 and 40·8 eV. For the clean surface, no emission from surface states in the band gap is found and exposure to small amounts of oxygen results in no preferential decreases in emission near the valence band edge. We place an upper limit of 4 × 1013 surface states/cm2 in the gap and conclude that any residual states in the gap are not intrinsic to the amorphous Si vacuum interface. A small amount of oxygen induces a broad valence band peak 2 to 5 eV below the valence band maximum; this we associated with a SiOx structure. Furthermore, we find that exposure to 108 L oxygen produces valence band structure similar but not identical to SiO2 and a depletion of the SiOx emission.

A synchrotron radiation (SR) photoemission investigation of the Si(111) cleaved surface+Ag thin overlayers (0.25–2.5 monolayers (ML) has been carried out at room temperature with the Cooper minimum method and at liquid nitrogen temperature (LNT) for a critical coverage (0.5 ML) with the energy dependent escape depth method. The two experiments give independent evidences of intermixing, following chemical reaction, between Ag and Si at the very low coverages, indicating that the very early stage of the Si(111)/Ag junction is not a sharp interface but an intermixed region extended to at least two layers of material.

Using energy-loss spectroscopy of photoelectrons from a single crystal of ${\mathrm{Bi}}_{2}$${\mathrm{CaSr}}_{2}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$, we show that the electronic structure of the near-surface region is the same as that of the bulk. Utilizing the fact that photoelectrons of different elements are excited at different locations in the unit cell, we identify the energy-loss features as due to valence plasmon excitations, and one-electron excitations by comparing the photoelectron energy-loss spectra of the different elements.

New photoemission measurements show higher Schottky barrier heights (≳1.3 eV) on atomically clean GaAs(110) surfaces at a Au coverage of about 25 monolayers. It is suggested that this effect is due to the movement of Au into the semiconductor; at room temperature it creates acceptor states near the valence band maximum (VBM) that cause the Fermi level at the surface (Efs ) to move close to the VBM. We found that heating of the GaAs(110) surface (above 100 °C) covered with a small amount of Au (0.2 monolayer) causes Efs to move back to its original position (on the clean surface before Au deposition). The heating process is found to greatly inhibit the formation of large barrier heights due to the removal of defect states from the surface region.

Column V elements adsorbed on GaAs (110) exhibit strikingly different behavior than adsorbed column III elements in terms of the overlayers bonding to the semiconductor, the effect on the semiconductor surface lattice, and long‐range order. A strong interaction between adatoms is found with submonolayer coverings of column III metals which leads to the formation of flat raftlike metallic patches. Unlike the column III metals, Sb adsorption produces large changes in the electronic states and atomic arrangement of the semiconductor surface lattice. An elementary view of the factors responsible for the different characteristics and effects of these overlayers on GaAs (110) is given, but its extension to other adatoms is shown to be limited. These results have strong relevance to current theoretical models of Al and Ga overlayers on GaAs (110), as well as to molecular beam epitaxy and Schottky barrier formation.

We report the first photoemission studies of the electronic structure at the interface between Ge(111) and d-metal overlayers at monolayer coverages. Two systems are discussed, Ge(111)/Pd and Ge(111)/Ni. In the case of Ge(111)/Pd a broad, reacted germanide-like phase is formed with similar electronic features to the Si(111)/Pd interface. In the Ge(111) Ni case an evolution of the valence band structure versus coverage from a reacted situation to a metal rich situation is found, indicating a strong gradient of concentration. The interface is narrower than for Ge/Pd. Photoemission spectra of the valence band at hv =80 eV are presented as well as core line data of the Ge 3d which show the trends in binding energy shifts and the changes in the relative intensities as a function of metal coverage. The discussion is carried out on the differences in the growth of these interfaces and comparisons are made to the correspondent silicide/metal systems.

In conjunction with Auger electron spectroscopy measurements, we have developed a procedure for obtaining a clean Nb surface using an electron bombardment heating technique. We have then studied the valence band photoemission as a function of photon energy. We find that calculations for the bulk theoretical density of states are in basic agreement with the ultraviolet photoemission spectroscopy data. We have also studied the initial oxidation of the clean Nb surface. Structure in the energy distribution curves suggests three stages in the oxidation process. Furthermore, the valence band energy distribution curves for clean Nb3Sn prepared ’’in situ’’ were measured and showed a strong resemblance to pure Nb as has been observed previously in x‐ray photoemission. Evidence is found for the existence of Nb2O5 and SnO2 in the surface oxides of Nb3Sn.

We give the first photoemission results on the enhancement of Si reactivity to oxygen when a noble metal (Ag) is present. The tunability of synchrotron radiation (SR) has been used to get high surface sensitivity and to take advantage of cross section energy dependence. We show that when one monolayer of Ag is deposited onto Si(111), the exposure to oxygen (30 × 106L) originates the overgrowth of an oxide phase which is basically SiO2. This indicates that Ag breaks the sp3 configuration of Si atoms with a consequent dramatic increase in the Si reactivity. This behaviour rules out the model of Ag adsorbed on top of Si with an atomically abrupt interface.

The room‐temperature interface between Si and Hg1−xCdxTe (x=0, 0.30, 0.39, 1.0) was investigated using the surface sensitive techniques of photoemission spectroscopy and low‐energy electron spectroscopy (LEED). Elemental Si was electron beam evaporated onto atomically clean cleaved (110) surfaces in ultrahigh vacuum and the chemistry and morphology of the interface was monitored as a function of Si overlayer growth. The LEED pattern disappeared at ∼1 monolayer (ML) coverage Si for all substrates, indicating that the overlayer coverage was fairly uniform. At submonolayer Si coverages on the Hg containing alloys, the substrates exhibited Hg loss with Te coming out of the substrate into the overlayer. At moderate coverages (