Synchrotron Radiation Excited Photoemission Research Articles

The electronic structure of SrCu2O2 studied by synchrotron radiation excited photoemission and hybrid exchange density functional calculations

Photoemission spectra of K-doped SrCu2O2 excited with vacuum ultraviolet synchrotron radiation have been measured over a range of photon energies between 36 eV and 164 eV. Intensity changes in the spectra with changing photon energy show that the states at the top of the valence band are of dominant Cu 3d atomic character, although there is evidence for significant hybridisation between O 2p and Cu 3d states. The experimental results are compared with partial densities of states derived from calculations using a hybrid exchange approximation to density functional theory. � 2007 Elsevier B.V. All rights reserved.

Off-Angle Dependence of Characteristics of 4H-SiC-Oxide Interfaces

SiC-Oxide interfaces on 4H-SiC epitaxial substrates of various off-angles have been characterized by C-V measurements, synchrotron radiation excited photoemission spectroscopy and atomic force microscopy. We found that the interface state density (N it ) and the amount of sub-oxides (S i ) for small off-angle substrates were smaller than those for large off-angle ones. It was also found that there is a good correlation between the amount of the interface states and that of sub-oxides. Step-terrace structures were clearly observed in the surface morphology after the removal of oxide layers for small off-angle substrates, which shows that the interfaces were atomically smooth. These results suggest that the small N it and S i values for small off-angle substrates relate to the smoothness of the interface and that high performance SiC MOS devices are expected by using small off-angle substrates.

Occupied and unoccupied states of the organic infrared emitters Yb- and Er- tris(8-hydroxyquinoline) studied by photoemission and X-ray absorption

The electronic structure of thin films of the organic infrared emitters Yb- and Er- tris(8-hydroxyquinoline), YbQ 3 and ErQ 3, and the related materials GdQ 3 and DyQ 3, on Ag (1 1 1) is investigated using synchrotron radiation excited photoemission, and X-ray absorption at the C and N K edges. Anti-resonant photoemission is used to suppress the 4f emission. We find that, apart from a shift in the oxygen-derived highest occupied molecular orbital (HOMO), and small differences in the X-ray absorption spectra, the results are similar to those of the well-known visible light emitter aluminium tris(8-hydroxyquinoline), AlQ 3. Given the documented differences in the photoluminscence spectra compared to AlQ 3, this implies that the visible light emission from YbQ 3 and ErQ 3 is dominated by triplet states, which are not accessible to the spectroscopies used here. The alignment of the organic HOMO with the Ag Fermi level is shown to be essentially identical to that of AlQ 3 on Ag, suggesting that the wealth of published band alignment studies for AlQ 3 may be directly applicable to these materials. YbQ 3 and ErQ 3 are found to be highly susceptible to radiation damage, particularly at photon energies <100 eV.

Angle Resolved Photoemission Spectroscopy of GaN (101-0): Experiment and Theory

We present synchrotron radiation excited photoemission spectra in off-normal emission from GaN (101-0) surfaces. The samples of GaN single crystals grown by high-pressure high-temperature synthesis were cleaved under ultra high vacuum conditions. The experimental results were compared with calculated spectra within the one-step model. We could identify emissions from surface states and resonances near the upper and lower valence band edge. The detected surface states confirm the assumed surface geometry.

Evidence of two adsorption phases of 5-methyl-2-mercaptobenzoxazole on Cu 2S by synchrotron radiation photoemission

Synchrotron radiation excited photoemission spectroscopy studies have been carried out on 5-methyl-2-mercaptobenzoxazole adsorbed on the surface of Cu 2S. This paper reports one of the first applications of synchrotron radiation excited photoemission spectroscopy in the field of mineral processing investigations. S 2p and O 1s spectra for Cu 2S surface after immersion in 5-methyl-2-mercaptobenzoxazole solutions at different concentrations (10 −6 M and 10 −5 M) and different contact times (1 min and 15 min), together with those for Cu 2S cleaved in an ultra high vacuum are presented and discussed. The use of the synchrotron photon source permits investigation of the real first stage of adsorption, because of the high surface sensitivity of the technique. The presence of two 5-methyl-2-mercaptobenzoxazole species on the Cu 2S surface has been revealed by the fitting procedure. The attribution of the two species and how their structure changes with the experimental parameters are discussed.

5-methyl-2-mercaptobenzoxazole adsorbed on Cu 2S surface studied by synchrotron radiation excited photoemission

Synchrotron radiation excited photoemission spectroscopy studies have been carried out on 5-methyl-2-mercaptobenzoxazole adsorbed on Cu 2S surface. This paper reports one of the first applications of this technique in the field of mineral processing investigations. The use of synchrotron photon source permits investigation of the real first stage of adsorption, because of the high surface sensitivity. S 2p and O 1s spectra for the Cu 2S surface after immersion in 5-methyl-2-mercaptobenzoxazole solutions at different solution concentrations (10 −6 M and 10 −5 M) and different contact times (1 min and 15 min), together with those for UHV-cleaved Cu 2S at 205 eV and 555 eV photon energies are presented and discussed.

A photoelectron spectroscopic study of the interface formation between chromium and a palladium-intercalated polymer film

Synchrotron radiation excited photoemission was used to investigate the chemical reactions occuring at a metal/polymer interface. Chromium was evaporated onto the surface of a Pd-DEBP [Pd(PBu 3) 2-CCC 6H 4C 6H 4CC −] n film, a π-conjugated palladium-intercalated polymer. The evolution of the core level spectra of the polymer film and the evaporated metal were followed to a nominal thickness of 25 Å of the metal overlayer. The disappearance of the shake-up satellite in the C1s spectra suggests a strong initial interaction between the chronium atoms and the π-system of the DEBP moiety. A new carbide-like C1s signal appears upon increasing the chromium deposition, indicating a charge-transfer reaction from the metal overlayer to the DEBP carbons. An initial formation of a chromium arene π-complex is found; the reactivity of chromium upon further metal deposition leads to a σ-bonded chromium carbide. Moreover, chromium reacts with phosphorus atoms, leading to the formation of phosphide-like species, suggested by the presence of a low BE signal in the P2p spectra.

Combined reflectance anisotropy and photoemission spectroscopies of Cs/GaAs(001) interface formation

We have investigated the adsorption of cesium on GaAs(001) using synchrotron radiation excited photoemission spectroscopy and also by monitoring the optical transitions related to gallium dimers and arsenic dimers using reflectance anisotropy spectroscopy (RAS). Cesium adsorption has little effect on the Ga 3d core level shape. In sharp contrast, we observe modifications of the RA signal related to gallium dimers, induced by the modification of the electronic states involved in the corresponding surface optical transitions. The changes of the RA spectrum reveal two adsorption phases at Cs coverages above and below 0.5 monolayers. The origin for the strong sensitivity of RAS to semiconductor/alkali metal interface formation is discussed.

Adsorption of methyl radicals on diamond C(111) surface studied by synchrotron radiation photoemission

The interactions of methyl radicals and azomethane molecules with the diamond C(111) surface have been studied by synchrotron radiation excited photoemission in both valence band and core level spectral regions. It is found that azomethane is adsorbed intact at low temperatures and completely desorbed above 200 K. Characteristic emission bands are identified in the valence band. Adsorbed methyl radicals exhibit a broad feature centered around 10 eV binding energy. The species is detectable as well in the C(1s) region as a shoulder on the higher binding energy side of the diamond peak. The chemical effects on the surface due to alternate sequential exposures of H and CH 3 are also investigated and briefly discussed.

SYNCHROTRON RADIATION STUDIES OF MBE FORMED SEMICONDUCTOR INTERFACES: Si-GaAs AND GaAs-Si

Studies of silicon on gallium arsenide and gallium arsenide on silicon interface formation will be described and used as examples of how synchrotron radiation excited photoemission is a powerful probe of surface and interface phenomena. The samples and interfaces studied in this work were prepared in-situ using thermal Ga and As MBE sources and a resistive Si evaporation source. The MBE capability has allowed us to explore atomic and electronic structural issues of interface formation during hetero-epitaxy. We have combined these techniques to explore Si and GaAs surfaces, the adsorbate interactions of Ga and As on Si, and interface formation of Si on GaAs and the inverse system, GaAs on Si. Several of these surfaces and interfaces yield model systems which can provide general insights. As an example, studies of arsenic terminated silicon (100) and (111) surfaces will be described. A specific focus of the paper will be the interfacial development of Si on GaAs(100). Si deposited at various coverages below 10 ml onto a room temperature substrate yields a disordered surface. For coverages below two monolayers, subsequent annealing at 450–600°C leads to a single domain 1×2 surface from either the As rich c(2×8) or Ga rich 4×6 reconstructions. While the periodicity is the same for the different starting surfaces, evidence is presented that the atomic arrangement in the unit cell is different.

Observation of a local magnetic moment in face-centred cubic Fe

Fe can be obtained at room temperature in its metastable fcc phase by evaporation of thin epitaxial films onto a Cu(001) surface. Valence-band photoemission of fcc Fe shows no exchange splitting, suggesting a loss of long-range ferromagnetic ordering in this phase. To investigate the local magnetic ordering in fcc Fe we have carried out synchrotron radiation excited photoemission from the 3s core level of Fe. In contrast to the valence-band results, the exchange splitting of the 3s level is maintained in the fcc phase, indicating that the local magnetic moment persists.

Observation of a local magnetic moment in face-centred cubic Fe

Abstract Fe can be obtained at room temperature in its metastable fcc phase by evaporation of thin epitaxial films onto a Cu(001) surface. Valence-band photoemission of fcc Fe shows no exchange splitting, suggesting a loss of long-range ferromagnetic ordering in this phase. To investigate the local magnetic ordering in fcc Fe we have carried out synchrotron radiation excited photoemission from the 3s core level of Fe. In contrast to the valence-band results, the exchange splitting of the 3s level is maintained in the fcc phase, indicating that the local magnetic moment persists.

Metal-derived band gap states: Ti on GaAs(110)

The emergence of filled and empty Ti-derived interface states in the band gap of GaAs has been observed for coverages as low as 0.03 monolayer with synchrotron radiation excited photoemission (XPS) and inverse photoemission (IPS) spectroscopies. The onset of the photoelectron threshold for filled states in the valence band spectra and of the IPE photon threshold for the empty states follow directly the position of the surface Fermi level in the band gap. The origin of these states is associated with the rehybridization of the Ti-3d electrons with the valence electrons of the Ga and As atoms of the interface. This notion is supported by XPS core level studies, which show the occurrence of exchange reactions between the Ti and Ga, and the out-diffusion of As and its reaction with Ti. Chemical rebonding at the interface, which produces bonding (filled) and nonbonding (empty) states, is thus the first demonstrable mechanism for Fermi level pinning and Schottky barrier formation.

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

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.

Metal Atom Reactions with Polymer Films

AbstractThe chemical nature of the interface formed when an electropositive metal (chromium) is evaporated on the surface of oxygen-containing polymers has been investigated by synchrotron-radiation excited photoemission spectroscopy and by near-edge X-ray absorption fine stnrcture spectroscopy (NEXAFS). PMDA-ODA polyimide was studied along with two polymers intended to more simply model the complex chemical functionality of polyimide. Core-level spectra were resolved into distinct peaks, which were assigned to specific atoms in the polymer units, and changes in the spectra were monitored as chromium metal atoms were evaporated onto the surface. Similarly, NEXAFS spectra were taken as a function of chromium deposition. Results of these experiments show that chromium initially reacts very rapidly with surface carbonyl groups, but less strongly with ether oxygens or arene rings. In all polymers studied, formation of an electron-rich, carbide-like species was observed.

Orientation dependent adsorption on a cylindrical silicon crystal: I. Water

The full orientation dependence of water adsorption at temperatures between 350 and 870 K was investigated on a cylindrical Si sample using Auger electron spectroscopy and synchrotron radiation excited photoemission. Below 450 K, water adsorbs quickly on (001) and all orientations containing (001) terraces or facets as well as on defects. On (001), its sticking coefficient is high and constant up to saturation at 1 2 monolayer, suggesting a mobile precursor state. This species was identified as OH + H by Ibach et al. and Chabal and Christman. The O 2s level position and the adsorbate induced Si 2p core level shifts were analysed and it is shown that the OH is converted thermally to bridge bonded O ( T ≳ 550 K) which finally penetrates and forms SiO x ( T ≳ 750 K.). In the second, slow adsorption step, depending on orientation, different fully dissociated species are found at 350 K. On (110), atomically adsorbed oxygen is observed, whereas on (111), the Si 2p level shifts indicate penetrated oxygen which probably has a strongly disturbed bond configuration. At 350 and 670 K, the slowly adsorbed total amount is nearly identical for all orientations. This could be explained assuming the same temperature and orientation independent precursor dominating the sticking behaviour.

Constant final state spectroscopy of the interlayer state in graphite

The recently predicted interlayer state in graphite is observed, using synchrotron radiation excited photoemission constant-final-state spectroscopy on a single crystal of natural graphite, to lie where calculated in a recent theoretical band structure already known to give good agreement with the experimental band structure determined by angle-resolved photoemission. This result highlights discrepancies between photoemission and inverse photoemission over the position of conduction band features in graphite.

THE INTERACTION OF HYDROGEN WITH GaAs SURFACES

Aspects of the interaction of hydrogen with MBE grown GaAs surfaces have been studied in situ with synchrotron radiation excited photoemission core level spectroscopy. We show how this characterization technique is a sensitive probe of the surface chemical composition. Using this technique, the GaAs(100) and (111) surface phase diagram at room temperature has been obtained as well as aspects of the AlAs surface. The measurements showed that for GaAs(100) the surface reconstructions proceed through a series of centered [c(4x4), c(2x8), c(8x2)] and primitive structures [(1x6) ,(4x6)] and for the (111) surface (2x2), (√ 3x √ 3)R(30°), (√ l9x √ l9)R(23.4°) as the surface As to Ga ratio is decreased. The study of the clean surfaces has been extended with an investigation of the interaction with atomic and molecular hydrogen. This is of interest both because hydrogen is a common growth ambient and because there is some indication that MBE growth in the presence of hydrogen improves crystal electrical quality. In all cases, the experiments showed a saturation hydrogen coverage was achieved at an exposure of 106L of hydrogen when a cracking filament is used. Core level spectroscopy shows that the surface composition is modified by the presence of hydrogen and we have deduced a mechanism whereby the hydrogen forms a surface gallium hydride which sublimes. The hydrogen converts the clean surface reconstructions to a distinct semiconducting reconstruction with a specific composition independent of the starting reconstruction. These results indicate that hydrogen could modify the MBE growth surface thereby effect impurity incorporation.

4f-surface binding energy shift for Eu and Gd metals

Synchrotron radiation excited photoemission spectra of Eu and Gd are presented. For Eu a 4 f-surface component is resolved with 0.63 eV binding energy shift. For Gd the comparison with XPS data gives strong evidence for a surface shifted 4 f-component ( ΔE=0.48 eV). The mean free path of photoelectrons is determined.

Dissociative surface reactions at Schottky and heterojunction interfaces with AlAs and GaAs

We have conducted synchrotron radiation excited photoemission measurements of the initial stages of Au, Ge, and Ga deposition on AlAs(110) and (100) surfaces grown in situ by molecular beam epitaxy (MBE). Au evaporated onto room temperature AlAs produces a larger dissociation of the substrate surface then reported for Au on GaAs. However, the net Au–AlAs interdiffusion is smaller than for GaAs as seen by the more rapid approach of the Au(5d) and (4f) spin-orbit splittings to the bulk metallic values. Ge depositied onto AlAs(110) produces a dissociative outdiffusion of As at growth temperatures where abrupt, epitaxial Ge–GaAs(110) interfaces are produced. Since the bulk heat of formation of AlAs is over 50% greater than that of GaAs, the bulk compound heat of reaction incorrectly predicts the behavior of both Schottky and heterojunction interfaces. We conclude that it is the surface adatom bonding to the individual substrate constituents which determines the ultimate interface composition and extent. When the AlAs surface dissociates during interface formation, the Al does not substantially diffuse through the growing Au or Ge overlayers, in marked contrast to Ga from GaAs. Therefore, if metal adlayers are evaporated onto surfaces to influence the subsequent formation of Schottky interfaces, their mobility at the nominal interface is controlled by the bonding of the individual atomic species.