Shift Of Valence Band Research Articles

Photophysical properties of nano Si/SiO x composites in Al/composite/mono Si structures for green light emitting and photodetector-Schottky diodes

The concept of the self-formation of a nanocrystallite (nc-) Si/SiO x : Si z O y Al nanocomposite at the Al/oxidized porous silicon interface in the result of solid-phase processes between Al and oxidized porous Si (PS) and the influence of its composition on photophysical properties were developed and experimentally confirmed for the Si chip with optical intra-chip interconnect consisting of light emitting and photodetector diodes and alumina waveguide on oxidized PS surface with aluminum electrodes. The peculiarities of nanocomposite photophysical properties (the refractive index, photoluminiscence (PL) peak situation, PL spectrum shape in the green range) have been shown to be due to the quantum confinement effects (revealed by XPS, Raman spectroscopy) and depend on the Al presence in the nanocomposite (obtained by XPS, IR spectroscopy). The experimental confirmation of this concept is (i) the shift of the nc-Si valence band relatively to that of monocrystalline Si (c-Si) on 0.2–0.7 eV for nc-Si size in 2.5–6.5 nm range; (ii) the decrease of Si nanocrystallite size in the Al presence; (iii) the approach of the value of the refractive index of nc-Si : SiO : Si 2O 3 : Si z O y Al nanocomposite at λ=236 nm to that of porous Si with 45% porosity and (iv) the stable green PL spectra in the Si z O y Al presence in the nanocomposite.

Charging Effect in Amorphous Silicon Quantum Dots Embedded in Silicon Nitride

AbstractCapacitance-voltage was investigated for amorphous silicon quantum dots (a-Si QDs) embedded in a silicon nitride as a function of dot size and nitride thickness. a-Sci QDs were grown by plasma enhanced chemical vapor deposition. The electron charging was decreased as the dot size was decreased. These results showed that the conduction band shift is larger than the valence band shift as the dot size decreased and, as a result, electrons are easily discharged in a-Si QDs due to the lower barrier height. For high dot-density-sample, the capacitance-voltage curves were also shifted toward the negative voltage direction when a higher forward bias was applied at forward condition due to the transfer of electrons trapped in the a-Sci QDs from the a-Sci QDs near Si substrate to those near the top metal.

Why changes in bond lengths and cohesion lead to core-level shifts in metals, and consequences for the spatial difference method

In metals, the chemical shifts of core-level energy loss spectra are largely determined, not by charge transfers, but instead by valence band shifts. The valence band shifts in turn are determined by changes in bandwidth, which result from changes in the type, number and distance to neighboring atoms. The core-level shifts tracks the valence-band shifts to within 0.1 eV, thus providing information on the occupied electronic states. As a consequence, core-level shifts are almost unavoidable at interfaces and cannot be ignored when analyzing data obtained by the `spatial’ difference method. Core-level shifts introduce first-derivative-like features in spatial difference spectra, that under typical conditions will be larger than the changes in energy-loss fine structure. Fortunately, it is far simpler to connect the core-level shifts to changes in cohesive energy than parameterizations of the fine structure, such as charge transfers. Reinterpreting spatial difference measurements of Cu : Bi, Fe : B and Fe : P grain boundaries as arising from core-level shifts may reconcile the experimental measurements with existing electronic structure calculations.

Electronic structure of Au-Ag bimetallics: Surface alloying on Ru(001)

We report a study of the electronic structure of Au-Ag overlayers (total coverage varies from 2 to 9 ML) supported on Ru(001). Photoemission spectroscopy with synchrotron radiation and a Mg $K\ensuremath{\alpha}$ x-ray source has been used to study the valence band and the Au $4f$ and Ag $3d$ core levels of these systems. It is found that, in most cases, two-dimensional alloy formation occurs upon deposition at room temperature. The extent of surface alloying depends on the order of deposition, composition, thickness, and annealing temperature. By comparing the valence band and binding energy shifts for bulk and surface alloys, we found that both systems exhibit the same trend: that is, that relative to the d band of pure Au, the alloy d band narrows and the Au $4f$ shifts to higher binding energy upon decreasing Au concentration. However, the surface alloy d band is consistently narrower than that of the bulk with the same stoichiometry. Similar surface bulk behavior is also seen in the Au $4f$ binding energy shift and the Au d-band doublets. This behavior is attributed to the two-dimensional nature (reduction in coordination number of like nearest neighbors on average) of the surface alloy.

Theoretical and experimental studies of the interaction between sodium and oligothiophenes.

Quantum-chemical calculations and ultraviolet photoelectron spectroscopy (UPS) measurements have been performed in order to study the interaction between sodium and oligothiophenes, with a focus on the origin of experimentally observed relaxation energy effects in alkali-metal-doped conjugated molecules. Upon doping of \ensuremath{\alpha}-sexithienylene (\ensuremath{\alpha}-6T) with sodium atoms, (1) a broad feature appears in the valence band, in an energy region corresponding to the band gap in pristine \ensuremath{\alpha}-6T, and (2) certain structural features in the valence band shift towards lower binding energies in the doped material. In particular, upon doping, a structural peak related to electronic levels mainly localized to the sulfur and \ensuremath{\beta}-carbon atoms destabilizes to an energy corresponding to that of the valence-band edge in pristine \ensuremath{\alpha}-6T. The results of ab initio Hartree-Fock and local-spin-density calculations on \ensuremath{\alpha}-trithienylene and bithiophene are consistent with the experimental data, and allow for an assignment of these destabilization effects in terms of initial-state relaxations. We stress that similar destabilization effects, reported for other alkali-metal-doped conjugated systems, had previously been proposed to be associated with final-state electronic screening, i.e., a dynamic artifact within the UPS measurements; this is in contradiction to the results of our ab initio theoretical studies. Our present results show that all structural features in the UPS data are contained in the results of sufficiently complete quantum chemical calculations. \textcopyright{} 1996 The American Physical Society.

Quantum confined luminescence in Si/SiO2 superlattices.

Superlattices of $\mathrm{Si}/{\mathrm{SiO}}_{2}$ have been grown at room temperature with atomic layer precision using state of the art molecular beam epitaxy and ultraviolet ozone treatment. Photoluminescence was observed at wavelengths across the visible range for Si layer thicknesses $1<d<3\phantom{\rule{0ex}{0ex}}\mathrm{nm}$. The fitted peak emission energy $E(\mathrm{eV}){\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.60+0.72d}^{\ensuremath{-}2}$ is in accordance with effective mass theory for quantum confinement by the wide-gap ${\mathrm{SiO}}_{2}$ barriers and also with the bulk amorphous Si band gap. Measurements of the conduction and valence band shifts by x-ray techniques correlate with $E(d)$, confirming the role of quantum confinement and indicating a direct band-to-band recombination mechanism.

A Molecular Orbital Study of CO Chemisorption on the Pd/W(110) System

The electronic influence of a W substrate on the CO chemisorptive properties of a Pd overlayer has been studied using a semiempirical molecular orbital formalism. The CO/Pd/W(110) system has been examined in relation with CO/Pd(100) and CO/W(110). Our results support previous experimental evidence of CO chemisorption inhibition in Pd/W systems. This behavior is explained in terms of the CO molecule electronic structure and is correlated with a shift of Pd valence band to greater binding energies.

Microstructural Imaging of Localized Chemical Reactions using Valence Photoelectrons

ABSTRACTWe have utilized scanning soft X-ray photoelectron spectromicroscopy-MAXIMUM to investigate the microstructural evolution induced by localized corrosion in the Al-Cu-Si alloy thin films. We present energy-specific photoelectron micrographs showing the distribution of Cu-rich precipitates and corrosion products for the thin films after corrosion. Microspectroscopy performed across a corrosion site reveals that the O 2p valence band shifts in energy with location and the amount of shift can be related to the degree of corrosion. The photoelectron micrographs also show that the Cu-rich phase precipitates near the surface region and grows with annealing temperature.

Plasma enhanced chemical vapor deposition silicon oxides as studied by x-ray photoelectron spectroscopy, Rutherford backscattering, electron recoil detection analysis, infrared spectroscopy, and electron spin resonance

SiO2 thin films were obtained by microwave plasma enhanced chemical vapor deposition. Plasma with two different gas mixtures were studied: Ar/O2/SiH4 and O2/SiH4, the gas ratio R=O2/SiH4 varying from 0.5 to 10 with two radio frequency bias powers (0 W and 100 W). Seventeen dielectric layers with a thickness of around 500 nm were studied to determine their chemical composition by x-ray photoelectron spectroscopy and valence band studies, Rutherford backscattering spectroscopy and elastic recoil detection analysis, infrared spectroscopy, and electronic spin resonance. These samples could then be classified into three groups corresponding to different SiOH and SiH or only SiOH concentrations. Their stoichiometries were also determined. An explanation of the valence band shift and its modifications is proposed.

PECVD silicon oxides as studied by XPS, RBS, ERDA, IRS and ESR

SiO 2 thin films were obtained by microwave plasma-enhanced chemical vapour deposition. Plasmas with two different gas mixtures were studied: Ar/O 2/SiH 4 and O 2/SiH 4, the gas ratio R = O 2 SiH 4 varying from 0.5 to 10 with two radio frequency (RF) bias powers (0 and 100 W). Seventeen dielectric layers with a thickness around 500 nm were studied to determine their chemical composition by X-ray photoelectron spectroscopy (XPS) and valence-band studies, Rutherford backscattering spectroscopy (RBS) and elastic recoil detection analysis (ERDA), infrared spectroscopy (IRS) and electronic spin resonance (ESR). These samples could then be classified into three groups corresponding to different SiOH and SiH or only SiOH concentrations. Their stoichiometries were also determined. An explanation of the valence-band shift and its modifications is proposed.

Photoemission study of the metal to insulator transition of Bi2Sr2Ca1−xYxCu2O8+δ

Of particular interest for the mechanism of high-Tc superconductivity are the electronic states near the Fermi energy, for which various models are discussed in view of p- or n-type doping at intermediate doping levels. Here we present photoemission results on Bi2Sr2Ca1−xYxCu2O8+δ polycrystals with varying Y contents. With increasing x a decrease of density of states (DOS) at EF and also at 1.5 eV binding energy has been observed as well as the occurrence of new DOS around 2.5 eV binding energy for the insulating samples. In addition an almost continuous shift of the entire valence band to higher binding energies appears with increasing x. The photoemission results will be discussed together with transmission electron energy-loss data of the O 1s absorption edge within the framework of existing models.

Photoemission study on Ba 1−xK xBiO 3−δ and BaPb xBi 1−δO 3−δ

The well characterized surfaces of Ba 1−xK xBiO 3−δ and BaPb xBi 1−xO 3−δ are investigated by x-ray and ultraviolet photoelectron spectroscopies (XPS and UPS). The clear Fermi edge is found for the fractured surfaces of Ba 0.5K 0.5BiO 3−δ and BaPb 0.75BiO 0.25O 3−δ using XPS and He l UPS. The valence band shift toward the Fermi level (E F) by about 0.4 eV with doping K and Pb is observed. The results are compared with the band calculation.

Investigation on electronic structure of Cu clusters on graphite by EELS and XPS studies

We have applied electron energy loss (EELS) and X-ray photoemission (XPS) spectroscopies to study the electronic structure of copper clusters deposited in situ on clean graphite. The XPS results show an increase in the binding energy of both valence band and core levels when the cluster size decreases. Furthermore the EELS results show a sizeable increase in energy loss for the feature located at about 4 eV involving 3 d → E f states when the cluster size decreases. The observed shifts in the EELS spectrum are the same as those measured for the 3 d valence band shifts in the XPS spectra. Since the electrostatic charging effects, suggested in the past as the explanation for the variation in electronic properties occuring in the XPS of small clusters, cannot be invoked in EELS measurements we interpret the reported shifts as a genuine effect caused by band structure variation due to a redistribution of the states within the cluster d-band.

Photoemission study of GaAs1−xPx and Ga1−xAlxP mixed crystals

The different kinds of mixed crystal systems for GaAs1–xPx and Ga1–xAlxP have been investigated by x-ray photoemission spectroscopy (XPS). The core level binding energies of Ga in GaAs1−xPx increase, whereas those of P in GaAs1−xAlPxP decrease with composition x, which are analyzed using Pauling’s ionicity and the ionicity scale of Kowalczyk et al. For two kinds of mixed crystals, we found that the energies of plasmon loss peaks increase, all major peaks in the valence band shift the band edge, and the top of the valence band recedes linearly with composition x. The composition variation of the energy gap for mixed crystals are caused basically by the top of the valence band receding.

Noble- and transition-metal clusters: The d bands of silver and palladium.

A comparison of x-ray photoemission from Ag and Pd clusters grown on amorphous carbon substrates highlights the importance of the unfilled 4d band in Pd clusters. In both cases, the valence-band spectra show the d-band narrowing with decreasing cluster size, as expected. In both cases, also, there is a positive shift of the binding energies of the d-band centroids and of the core levels, primarily due to the unit positive charge that remains on the cluster in the photoemission final state, as occurs for other metal clusters on amorphous carbon. In Ag clusters the core-level shift is smaller than the valence-band shift because in small clusters the Coulomb energy of the charged cluster suppresses the conduction electron screening of the core hole. By contrast, in Pd clusters the increased localization causes a reduction in the d-electron density of states at ${E}_{F}$, resulting in a transition to s-electron screening and hence a core-level shift that is larger than the valence-band shift.

Why carbon monoxide is stable lying down on a negatively charged Ru(001) surface but not on Pt(111)

Recently, an electron energy loss vibrational study has shown that CO lies down at lower than saturation coverages on a Ru(001) surface which has been promoted by 0.1 monolayer of potassium. We have examined the bonding of CO to Ru(001) using molecular orbital theory and find a much smaller difference between adsorption energies for upright and lying-down orientations than for the similar Pt(111) surface. This is explained in terms of donation bonds from the CO √ orbitals to the surfaces. These stabilizations are greater for the lying-down positions, and in the case of Ru the antibonding counterpart orbitals are half-filled due to the smaller number of d electrons than are present for platinum where these orbitals are doubly occupied. With cathodic charging due to adsorbed K, the metal valence band shifts and this leads to a definite preference for the lying-dowm μ/π site compared to upright orientations on the Ru surface, but not on the Pt surface. Thus the number of d electrons determines the favored orientations of CO on these transition metal surfaces.

Pressure induced conduction and valence band shifts in InP and GaAs from measurements at semiconductor–electrolyte interfaces

The effect of pressure on the band gap and flatband potential of n- and p-type InP and GaAs was studied to 10 kbar at 300 K. For both InP and GaAs it was determined that the increase of the energy gap of the direct transition is to first order due to an increase in energy of the conduction band edge.

Energy-band distortion in highly doped silicon

A plausible model for the doping dependences of the conduction-and valence-band shifts, and of the concomitant narrowings of the optical and electrical energy gaps, caused by high doping concentrations in silicon is developed by refining, extending, and combining previously described theories. A meaningful link is thereby established between the fundamental solid-state physics that underlies the energy-band distortion and the silicon device physics that describes the pragmatic ramifications of it. The important many-body effects, which produce actual rigid shifts in the conduction and valance bands, are identified and characterized, and are shown to exclusively narrow the optical energy gap. The electrical energy-gap narrowing comprises additional effective shifts defined by the band tails due to the randomness of the dopant distribution, which are described. Predictions of the optical and electrical energy-gap narrowings, the difference between which is explained physically for the first time, are shown to agree well with measurements from several independent experiments.

Observation of nonuniform energy band shifts for Ge on GaAs(110)

Fractional monolayer studies of Ge on GaAs(110) were performed with angle-resolved photoemission at the Wisconsin Synchrotron Radiation Center. Normal emission data for 14 < hν <27 eV were analyzed. The most significant effects of the Ge adatoms are shifts in photoemission peaks known to be due to GaAs interband transitions. The shifts, which are nonuniform, are due to specific nonuniform shifts in interband transitions. Valence band (VB) shifts relative to the VB maximum can be determined from some of the transitions. The top VB shifts downward and the next deeper VB shifts upward in the neighborhood of one-third the distance from Γ to X on the Σ line (0.57 Å −1). We suggest the shifts are the result of chemisorption-induced charge transfers whose resonance-like states decay over several atomic layers below the surface.

Ultraviolet photoelectron spectroscopy of the valence bands of solid NH3, H2O, CO2, SO2and N2O4

A photoelectron study is presented for the outermost bands of the molecular solid phase of: NH3, H2O, CO2, SO2 and N2O4. Both gaseous and solid phase spectra were acquired with 40.81 eV ultraviolet photoelectron spectroscopy (UPS) techniques (except for gaseous N2O4). For the solids, charging effects were measured systematically and appropriate corrections made. Some valence band shifts were observed, as well as significant changes in valence band widths, between the gas and solid phase spectra, but the shifts were in general much less than those measured by previous workers. These results are interpreted in terms of molecular solid bonding and relaxation effects.