Valence Band Photoelectron Spectroscopy Research Articles

C60-terminated Si surfaces: Charge transfer, bonding, and chemical passivation

The interaction of ${\mathrm{C}}_{60}$ with the Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) and Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) surfaces has been investigated using synchrotron radiation core-level and valence-band photoelectron spectroscopy. ${\mathrm{C}}_{60}$ induces distinct spectral changes in the $\mathrm{S}\mathrm{i}\ensuremath{-}2p$ core-level emission from both surfaces, indicative of charge transfer to the adsorbed fullerene molecules. Our results suggest that ${\mathrm{C}}_{60}$ adsorption on Si(111) induces a redistribution of charge within the (7\ifmmode\times\else\texttimes\fi{}7) unit cell involving electron transfer from rest atom to adatom dangling bonds. For a one monolayer coverage [on both Si(111) and Si(100)], broad ${\mathrm{C}}_{60}$-induced chemically shifted components are present in the core-level spectra. Valence-band spectra, however, show no evidence for a high degree of electron occupation of the ${\mathrm{C}}_{60}$ lowest unoccupied molecular orbital. We present core-level data which illustrate that adsorption of a ${\mathrm{C}}_{60}$ monolayer inhibits ambient oxidation of the Si surface.

Low temperature adsorption of Cs on layered TiS 2 studied by photoelectron spectroscopy

The adsorption of Cs on a TiS 2 cleavage plane at 110 K was studied by core level and valence band photoelectron spectroscopy, using synchrotron radiation. The Cs 4d core level spectra, together with LEED observations, reveal that the Cs at low coverage forms a disordered phase on the surface, but condenses to form ordered islands as the coverage is increased. A small amount of Cs was seen to intercalate even at 110 K. As the sample was allowed to warm up, the ordered Cs phase melted to form a disordered phase again, although with the binding energy being different from the disordered low-temperature phase. At this stage the intercalation process accelerated, and before reaching room temperature most of the Cs had intercalated. The S 2p and Ti 3p core level spectra exhibits shifts and broadenings, and the valence band spectra provide clear evidence for charge transfer from both adsorbed and intercalated Cs to the host layers.

Time-resolved valence band photoelectron spectroscopy of liquid AuSn

The solid - liquid - solid-phase transitions of a alloy have been investigated by means of the recently developed method of time-resolved photoelectron spectroscopy, using dye laser pulses to melt the sample surface on a microsecond scale. Starting at a temperature of the liquid sample just above the melting point (740 K), the laser pulse heating allowed us to obtain, for the first time, valence band spectra of an alloy at temperatures of up to 1620 K, distinctly above the vapour pressure limit of steady-state photoelectron spectroscopy. The photoemission results of the liquid at different temperatures are compared with data from the corresponding amorphous alloy, prepared by vapour condensation at liquid-nitrogen temperature. By increasing the temperature of the disordered phase, the Au 5d band is found to shift continuously toward a lower binding energy, showing a linear behaviour over the whole temperature range. Our measurements confirm earlier results obtained with conventional photoemission within a restricted temperature range. The results presented here clearly show the potential of the new technique to investigate the electronic structure of alloys at conditions under which standard photoemission experiments cannot be performed.

Photoemission and x-ray absorption study of superconducting and semiconducting Ba1-xKxBiO3 single crystals.

Semiconducting ${\mathrm{Ba}}_{0.9}$${\mathrm{K}}_{0.1}$Bi${\mathrm{O}}_{3}$ and superconducting ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$Bi${\mathrm{O}}_{3}$ single crystals cleaved in situ have been studied by core level and valence band photoelectron spectroscopy and O $K$ edge x-ray absorption spectroscopy. It was found that the general shape of the valence band spectrum agrees with the shape predicted by band structure calculations, but the intensity near the Fermi level was lower in the experimental spectrum as compared to the calculated. The O $K$ edge spectra showed that the metallic phase is not related to the presence of doping inducted O $2p$ holes. This property of ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}\mathrm{Bi}{\mathrm{O}}_{3}$ shows that the semiconductor-metal transition of this system is of a different nature than that of the hole doped cuprate high-${T}_{c}$ superconductors. The core level photoemission spectra of the cations showed a small asymmetry for ${\mathrm{Ba}}_{0.9}$${\mathrm{K}}_{0.1}$Bi${\mathrm{O}}_{3}$. Corresponding spectra for ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$Bi${\mathrm{O}}_{3}$ showed a larger asymmetry resulting in a resolved high binding energy shoulder in the Bi $4f$ spectrum. The origin of this feature is discussed.

Initial stage growth and interface formation of Al on Si(001)2 × 1

Initial stage growth and interface formation of Al on a Si(001)2 × 1 surface has been studied by core-level and valence-band photoelectron spectroscopy using synchrotron radiation for the Al coverages ( Θ Al) up to ∼2 ML at room temperature (RT) and 300°C. The measured Al 2p and Si 2p spectra reveal, for the first time, the existence of interfacial reaction of Al with Si in addition to the rather well-known dimer adsorption of Al. The reaction occurs even at RT for Θ Al ≥ 0.5 ML, which progresses significantly by annealing at 300°C from earlier stage of adsorption. Beyond 0.5 ML, Al 2p core levels indicate formation of Al clusters at both temperatures, which is accompanied by increase of the reacted phase. This interfacial reaction explains the 1 × 1 phase at 300°C found by LEED and the difference in band bending between RT and 300°C above ∼0.5 ML. The evolution of the valence-band surface electronic structures with respect to Θ Al is interpreted in a consistent way to the core-level results.

Imaging of a surface state from clean Cu(001).

First-principles electronic structure calculations and multiple-angle valence-band photoelectron spectroscopy were combined to image a clean surface-related electronic state of the Cu(001) surface. The surface band cresting at 4.30 eV below the Fermi level was computed to have a density of states at the lower end of the bulk Cu {ital d} bands. Based on the results of first-principles calculations for the clean and Cl-covered surfaces, we were able to successfully measure the surface state by taking a ratio of clean versus Cl-covered isoenergetic valence electron angular distributions. We have also carried out first-principles total-energy calculations for the multilayer atomic relaxation of the clean Cu(001) surface, and found that the topmost interlayer separation contracts by 2.9{percent} relative to the bulk value, while the second interlayer is essentially unchanged and the third interlayer contracts by only 0.1{percent}. {copyright} {ital 1996 The American Physical Society.}

Photoelectron spectroscopy of the annealed and deuterium-exposed natural diamond (100) surface

Hydrogen, playing a crucial role in diamond film growth, can be desorbed from the diamond (100) surface simply by annealing at elevated temperatures. The effect of such a treatment on the electronic bulk and surface properties of diamond (100) at different temperatures has been investigated by valence band and core level photoelectron spectroscopy. On annealing, additional electronic states appear in the range from 2.5 to 0.5 eV binding energy below the Fermi level. The intensity of those states show an angular dependency. Exposure of the annealed surfaces to atomic deuterium reverses the effect. The data are discussed in the view of models for surface reconstruction. From a thorough comparison of the observed changes in the photoelectron spectra upon annealing with graphite related features we can clearly exclude surface graphitization of the diamond sample in our experiments.

Valence band spectroscopy of reconstructed (100) and (111) natural diamond

The electronic structures of natural diamond type IIb single crystals of orientations (100) and (111) were investigated by core level and valence band photoelectron spectroscopy. The samples were measured at room temperature, and then heated in situ up to 955 °C. On annealing, a distinct decrease in oxygen contamination at the surface was observed; simultaneously additional electronic states near the valence band maxima appeared. These additional states can be interpreted as surface states of the reconstructed diamond (in both the (100) and (111) orientations). The data obtained are compared with other carbon phases, such as graphite and diamond films prepared by chemical vapour deposition.

The influence of surface treatment on the electronic structure of CVD diamond films

The electronic structure of CVD diamond surfaces has been studied by core level and valence band photoelectron spectroscopy. Upon annealing at 870 °C, a band of surface states appears within the top 6 eV of the valence bands which is accompanied by a decrease in spectral intensity around 9 eV and a band bending of 0.4 eV. Similar changes could be induced by Ar ion bombardment. Exposure to atomic hydrogen largely restored the original spectra. These observations are compared with corresponding data on single-crystalline diamond surfaces. Similarities and differences are discussed in terms of structural models of the CVD diamond surface.

Valence band photoelectron spectroscopy and structural morphology of nitrogen- and boron-containing MPACVD diamond films

Diamond thin films with various concentrations of nitrogen and boron were prepared by microwave-assisted chemical vapour deposition in a standard downstream microwave system. For the nitrogen-containing films, nitrogen was added to the feed gas, while the boron-containing films were produced by placing a piece of boron carbide (B 4C) in the plasma. The films were characterized by photoelectron spectroscopy and scanning electron microscopy (SEM). One boron-containing sample has been examined by secondary ion mass spectrometry and a homogeneous boron concentration depth profile throughout the diamond film has been observed. SEM images show that nitrogen in the feed gas induces a more ballas-like crystal structure, whereas boron incorporation has only a small influence on the crystal size and structure in the films. For these films the valence band structure is measured by UV photoelectron spectroscopy. Raman spectroscopy has been used in order to characterize the film quality.

Coadsorption and reaction of NH 3 with NO, O and OH on Ge surfaces

The coadsorption and reaction of NH 3 and NO on the (001) and (111) surfaces of Ge has been studied by valence-band and core-level photoelectron spectroscopy. On Ge(001), NH 3 alone adsorbs molecularly at 100–130 K, first on the dimer-down atoms with the 2a1, 1e and 3a1 orbitals at −23.0, −11.9 and −5.6 eV below E v, then also on the up-atoms. Coadsorbed NO binds molecularly to the up-atoms and causes displacement of NH 3 from these sites. A strongly polarized NH 3-NO complex, stabilized by hydrogen bonding and the antiparallel orientation of the molecular dipoles, is formed with the NH 3 orbitals shifted to −19.9, −10.0 and −5.4 eV. Upon annealing to 270–300 K, this complex reacts by splitting off the N atom from NO. Coadsorbed OH and NH 2 are formed, probably again stabilized by hydrogen bonding. The NH 2-derived orbitals appear at −19.6, −9.6, −4.1 and −2.0 eV. Above about 370 K, further decomposition occurs and at 490 K oxygen in the form of GeO x and some atomic N are visible in photoemission. If the surface is exposed first to NO at 100–130 K, partial decomposition occurs and NO and O are observed. Coexposure to NH 3 leads first to NH 3-NO-complex formation. Further NH 3 adsorbs molecularly and is polarized by the O atoms. Coadsorption of NH 3 to O or OH + H on the surface yields polarized NH 3 with peak shifts corresponding to the strength of the interaction. On Ge(111), coadsorbed NO dissociates partly and forms NO and O. A similar NH 3-NO complex is formed. Because of the overlap with O ad-derived features, the formation of OH + NH 2 upon annealing cannot unambiguously be deduced from the spectra.

Direct observation of Coulomb interactions in highly conducting

Valence-band photoelectron spectroscopy and scanning-tunneling microscopy demonstrate the significance of Coulomb interactions for the semiconducting state of single crystalline [2,5-dimethyl-dicyanoquinonediimine${]}_{2}$Ag. Direct evidence for the electron-electron interactions is deduced from the spectroscopic observation of a pseudogap in the density of states near the Fermi energy and from the dimerization in real space observed by tunneling microscopy.

Atomic and electronic structure of B/Si(100)

The atomic and electronic structures of the B/Si(100) surface have been studied using high-resolution core level and valence band photoelectron spectroscopy as well as low energy electron diffraction. The structure models for such a surface have been proposed, which consist of boron atoms residing on the top surface adatom sites. Experimental evidence shows that this surface is not a dangling bond-free surface. Epitaxial growth of Si and Ge on this structure has been achieved at relatively low temperature, and our results indicate that boron atoms underneath the epitaxial layer act as acceptors. As a result, conversion of an n-type to p-type region takes place near the surface. Comparison of this surface with the other group-III metals on Si(100) surface has also been conducted.

Characterization of the B/Si surface electronic structures

High-resolution angle-resolved core-level and valence-band photoelectron spectroscopy have been used to characterize the electronic structures of the B/Si (111)-(√3×√3) surfaces. The results have been compared with theoretic calculations and other group-III metals and Si-terminated Si (111) surfaces that share the same type of surface reconstruction. We have observed a structure evolution from B-T4 to B-S5 and finally to Si-T4 as deposited boron atoms diffuse into the substrate with increasing annealing temperature. The chemically shifted component appearing in the Si 2p core-level spectrum is attributed to charge transfer from the top layer Si and Si adatoms to the sublayer B-S5 atoms. For the Si/Si (111)-(√3×√3) surface, a newly discovered chemically shifted component is associated with back bond formation between the Si adatoms and the underneath Si atoms. A new emission feature has been observed in the valence-band spectra unique to the B/Si (111)-(√3×√3) surface with B-S5 configuration. Thin Ge layer growth on this structure has also been performed, and we found that no epitaxial growth could be achieved and the underneath structure was little disturbed.

Interface reaction of Pt on p-WSe 2(0001) surfaces

The Pt p-WSe 2 interface has been studied by synchrotron induced core-level and He I excited valence band photoelectron spectroscopy. Platinum forms a continuous metallic overlayer. The deposition of platinum creates selenium defects at the surface leading to an initial band bending of 540 meV which is reduced by subsequent deposition of platinum. The origin of the reactivity is attributed to the high condensation energy of the deposited metal rather than to a possible metal exchange reaction.

Dissociative H 2O adsorption on the Si (100) 2× 1 and Ge (100) 2× 1 surfaces

Core-level spectroscopy and valence band photoelectron spectroscopy were used to study H 2O adsorption on the Si(100) and Ge(100) surfaces. We find that H 2O dissociates into H and OH on both surfaces at 300 K. The H and OH are adsorbed in on top positions on the surface. The OH group is tilted with respect to the surface normal on the Si(100) surface. We consider two possible interpretations for the results from the H 2O dosed Ge(100) surface at 300 K. Either a similar model as for the Si(100) surface or a model based on two adsorption sites.

Local order, epitaxy, and electronic structure of the Bi/III–V semiconductor interfaces

The growth morphology and electronic structure of the Bi/InP(110) interface is investigated and compared to the Bi/GaAs(110) system. High-resolution synchrotron radiation core level and valence band photoelectron spectroscopy is employed in the determination of the local order, epitaxy, and electronic structure development for these systems. Valence band spectra show the development of a density of states between the InP valence band maximum and the Fermi level in the coverage range 0–1 monolayer. These Bi-derived states show a 450 meV gap between the onset of emission and the Fermi level indicating the first monolayer is semiconducting. At all coverages beyond 1 monolayer, the Bi-derived states show emission at the Fermi level. Core-level intensity attenuation profiles indicate a Stranski–Krastanov growth mode. Computer fitting of the Bi 5d core level shows two bonding sites for Bi on the surface, with a third Bi 5d component at all coverages above 1 monolayer. A zigzag chain model used for the Bi/GaAs(110) system is proposed as an appropriate model for the Bi/InP(110) interface.

Core-level shifts on the H2O exposed Ge(100)2×1 surface

Core-level spectroscopy and valence-band photoelectron spectroscopy were used to study the Ge(100)2×1 surface dosed with 0.5–100 L H2O at 160 K. It is determined that H2O adsorbs molecularly at 160 K and forms ice. The H2O molecules dissociate into H and OH radicals on the Ge(100)2×1 surface when the sample is heated to 300 K. A simple adsorption model that accounts for the calculated H and OH coverages is proposed.

Magnetic and electronic properties of M-Ba–Cu–O (M = Y, Er, Eu)

Various high-Tc superconductors of the La–(Ba,Sr)–Cu–O and the M–Ba–Cu–O systems with M = Y, Er, and Eu have been prepared by the solid-state reaction method. Single-phase samples with no additional diffraction peaks as verified by x-ray diffraction (XRD) measurements have been obtained. Measurements of the electrical resistivity and of the magnetization showed sharp superconducting transitions with a width of 1 K. The measurements of the magnetic susceptibility have been extended above room temperature up to 770 K. There is clear evidence for the formation of a magnetic moment in all M–Ba–Cu–O samples. Monochromated x-ray photoelectron spectroscopy (MXPS) valence band and x-ray photoelectron spectroscopy (XPS) core level spectra have been measured on various samples at room temperature and at liquid nitrogen temperature.

Orientation and temperature dependent adsorption of H 2S on GaAs: Valence band photoemission

A cylindrically shaped GaAs single crystal was used to study the adsorption of H 2 S on the six inequivalent orientations (001), (113), (111), (110), (111) and (113) by angle resolved valence band photoelectron spectroscopy and surface dipole measurements. Adsorption at 150 K on the surface prepared by molecular beam epitaxy (MBE) yields similar adsorbate induced emission on all orientations which were ascribed to SH radicals. On (110), where preferential adsorption occurs additional features from molecular H 2 S are observed. The adsorbate spectra at 720 K are ascribed to atomic sulphur. On the surface prepared by ion bombardment and annealing, defect enhanced adsorption occurs in the range (111)–(113). The adsorbate spectra are very similar to those on the MBE surface at 720 K. Thus, no new species are adsorbed on defects but only sticking probability and penetration capability are increased.