Method of observation of low density interface states by means of X-ray photoelectron spectroscopy under bias and passivation by cyanide ions

X-ray photoelectron spectroscopy (XPS) measurements under bias can observe low density interface states for metal-oxide-semiconductor (MOS) diodes with low densities. This method can give energy distribution of interface states for ultrathin insulating layers for which electrical measurements cannot be performed due to a high density leakage current. During the XPS measurements, a bias voltage is applied to the rear semiconductor surface with respect to the ∼3 nm-thick front platinum layer connected to the ground, and the bias voltage changes the occupation of interface states. Charges accumulated in the interface states shift semiconductor core levels at the interface, and thus the analysis of the bias-induced shifts of the semiconductor core levels measured as a function of the bias voltage gives energy distribution of interface states. In the case of Si-based MOS diodes, the energy distribution and density of interface states strongly depend on the atomic density of silicon dioxide (SiO₂) layers and the interfacial roughness, respectively. All the observed interface state spectra possess peaked-structures, indicating that they are due to defect states. An interface state peak near the Si midgap is attributable to isolated Si dangling bonds at the interface, while those above and below the midgap to Si dangling bonds interacting weakly with Si or oxygen atoms in the SiO₂ layers. A method of the elimination of interface states and defect states in Si using cyanide solutions has been developed. The cyanide method simply involves the immersion of Si in KCN solutions. Due to the high Si-CN bond energy of ∼4.5 eV, the bonds are not ruptured at 800 °C and upon irradiation. The cyanide treatment results in the improvement of the electrical characteristics of MOS diodes and solar cells.     

On the electronic structure of the light rare earth hydrides

Photoemission (XPS, HeI) and inverse photoemission (hv=9.7 eV) data on Y, La, Ce, Pr and their hydrides are presented. The discussion focusses on the ground state electronic properties of the hydrides and in particular on the empty density of states. In addition the correlation effects which appear in these experimental techniques in the presentce of well localized, partly filled 4f states are stressed. Hybridization is displayed in three typical situations:a) The very existence of a weak 4f-conduction band mixing allows electronic transitions into 4f derived states to be observed by low energy BIS, although the cross section is exceedingly small at this energy-b) 3d core excitation pulls the empty 4f levels close to the hydrogen ligand levels such that they become strongly mixed and the screening level is markedly split-c) A schematic relation is found between core level shifts upon hydriding, charge transfer and the position of the hydrogen induced band, simply by considering the mixing of hydrogen ligand states with metald states.     

Nonideality of Au/Si and Au/GaAs Schottky barriers due to process-induced defects

A mechanism of local lowering of the Schottky barrier height (SBH) is proposed, which causes nonideality in nearly ideal Au/n-Si and Au/n-GaAs Schottky barriers. Positively ionized defects generated by the process very close to the interface induce electrons in the metal-induced gap states (MIGS) and lower the SBH locally. The spatial density distribution of the ionized defects obtained from the SBH distribution is determined by the unique interaction with the MIGS. The defects are considered to have the negative-U property and are neutralized at very close positions to the MIGS. The potential distributions close to the interface have a considerable potential drop due to the large defect density. These inhomogeneous potentials are coincident with the energy level scheme of the defect identified as the defect causing the nonideality. This defect is Si self-interstitial in Au/Si SB, and As antisite in Au/n-GaAs SB. This MIGS with process-induced defect model supersedes the previously proposed two major Fermi level pinning models. The mystery of the T₀ effect is solved. The thermionic-field emission current taking place in the strong electric field has influence on the I-V characteristics at low temperatures. Regarding the C-V characteristics of Au/Si SB, the observed extra capacitance under the forward bias is an experimental evidence in accordance with the proposed model.     

An XPS investigation of the electronic structure in AgZn

We have calculated the densities of states in ordered (CsCl) and disordered (bcc) AgZn using first-principles SCF-LMTO-ASA and KKRCPA techniques. We obtain good agreement between calculated XPS photocurrent spectra and experimental measurements in the valence band region. As far as we can determine there are no shifts of the Ag nor Zn core levels on alloying.     

Transport properties and electronic structure of epitaxial tunnel junctions

We present ab initio calculations for the electronic ground-state and transport properties of epitaxial Fe/semiconductor/Fe (0 0 1) tunnel junctions. The ground state properties are determined by the ab initio Screened KKR Green's function method and the transport properties by a Green's function formulation of the Landauer–Büttiker formalism. We focus on tunnel junctions with a semiconducting ZnSe barrier and compare them to results for junctions with Si and GaAs barriers. We comment on the presence of metal-induced gap states (MIGS) in the semiconductor, the spin polarization of which strongly depends on the nature of the barrier. We investigate furthermore the influence of one atomic layer at the interface of a non-magnetic metal (Cu, Ag, Al) and of a magnetic 3d transition metal.     

The specific oxidation mechanism in the initial stages of O2 adsorption on submonolayer Ba covered W(1 1 0) surface

The initial oxidation of a polarized 0.35-ML Ba-covered W(1 1 0) surface has been investigated in detail by high-resolution photoemission spectroscopy using synchrotron radiation. Upon small exposures (?0.15 L) to O₂, both interfacial W 4f (Ba-coordinated W) and Ba 4d core levels shift simultaneously toward smaller binding energies and then fix during 0.15-0.25 L, finally disappear at 0.6 L. The concurrent negative shifts of both interfacial W 4f and Ba 4d peaks can be attributed to be a consequence of the increased occupancy of 5d states. This explanation is supported by the variations of 5d stats in valence band spectra upon the initial oxidation. Our results clearly demonstrate that charge rearrangement is the dominant effect in the surface reactivity of our oxidized surface. We also found that this charge redistribution is associated with a structural change during oxidation, which is in contrary to previous studies of alkali-metal promotion. That is, oxygen is first chemisorbed atop the polarized Ba adlayer, then incorporates beneath Ba adatoms to form a covalent Ba-O-W complex upon further dosage, and reacts strongly with the Ba overlayer to become an ionic Ba⁺²O⁻² surface dipole layer at 0.6 L.     

X-ray Photoelectron Spectroscopy of Sb2S3 Crystals

The paper presents the X-ray photoelectron spectra (XPS) of the valence band and core levels of semiconductor ferroelectric Sb₂S₃ single crystals, which show weak phase transitions and anomalies of various physical properties. The XPS were measured with monochromatized Al K _α radiation in the energy range 0-1450 eV and the temperature range 160-450 K. The valence band is located 0.8-7.5 eV below the Fermi level. Experimental results of the valence band and core levels are compared with the results of theoretical ab initio calculations of the molecular model of Sb₂S₃ crystal. The chemical shifts in Sb₂S₃ crystal for the Sb and S states are obtained. Results revealed that the small structural rearrangements at the phase transition T _c1 = 300 K shift the Fermi level and all electronic spectrum. Also, temperature dependence of a spontaneous polarisation shifts the electronic spectra of the valence band and core levels. Specific temperature-dependent excitations in Sb 3d core levels are also revealed.     

XPS and ARXPS investigations of ultra thin TaN films deposited on SiO2 and Si

TaN films were deposited by reactive DC magnetron sputtering onto Si and SiO₂ with thicknesses of less than one monolayer up to 10 nm. After this, the samples were transferred into the analysis chamber without breaking the vacuum and analysed by means of X-ray photoelectron spectroscopy (XPS) and angular resolved XPS (ARXPS).XPS measurements as a sensitive method to characterise chemical states showed a silicon nitride formation at the interface at deposition on Si. On the SiO₂ no reaction was found at the interface.Based on these observations layer models for quantification of ARXPS measurements by means of model calculations were derived, so it was possible to obtain information on the in-depth element distribution in a non-destructive manner. For comparison to the ARXPS investigations analyses of the inelastic background of the Ta4d peak are shown and discussed.     

Study of thermal oxidation of LaSix/Si(1 0 0) by grazing incidence electron-induced X-ray emission spectroscopy

A new X-ray emission spectroscopy (XES) apparatus for analyzing nondestructively the chemical states of the inner regions of layered materials was constructed. Using electron excitation at grazing incidence, it also enables the analyzing depth to be confined to the surfaces of the materials. The apparatus was employed to the study of oxidation of a La silicide layer formed on a Si substrate. By comparing the Si Kβ emissions with results by XPS analysis, it was concluded that the Si Kβ emission band of the oxidized sample at the excitation energy of 3.0 keV represents the Si 3p density of states (DOS) of a LaSiO mixed oxide. The variations of the spectra at increasing electron beam energies were compared with probing depths calculated by an empirical model. This result indicates that we can analyze the chemical states of a mixed oxide layer of one to two tens angstroms in thickness, and nondestructively probe into the depth of about several tens angstroms.     

Ge/Si混合物纳米薄膜中Ge钠米颗粒氧化态的XPS研究(英文)

pacc:7300,3320R TheoxidationstatesofGenanoparticlesin thesurfacelayerofGe/SifilmwerestudiedbyX-rayphotoelectronspectroscopy.Newfeatures appearedatthehighbindingenergysideofthe XPSGe3dpeakwhensampleswereannealedin atmosphere,whichwerecausedbythelargein ter…     

On the origin of the variation of the binding energy shifts of core levels between surface and bulk atoms in transition metals

We present here a model in which the change of the binding energy of core levels at the surface is due to the displacement of the d dand which arises from the narrowing of the surface density of d states. We predict a change of sign of the surface modification of the core level energy shifts near the middle of the d series (i.e. between Ta and W in the 5d series). Finally, chemisorption effects are discussed.     

Photoelectron spectroscopic studies of adsorption of CO and oxygen on Ru(001)

XPS and UPS have been used for a detailed study of the adsorption and coadsorption of CO and oxygen on a clean Ru(001) single crystal. The measured substrate and adsorbate core level binding energies and valence levels are discussed. The O 1s XPS peak intensity has been used for kinetic studies of adsorption and coadsorption. Some studies of the angular dependence of adsorbate and substrate peak intensity ratios are presented. We also present data on the shifts of XPS peaks and changes in UPS spectra as a function of adsorbate coverage. The data are correlated with the results of earlier measurements with other methods.     

Photoemission spectroscopy and electronic energy levels of CoS

Photoemission (XPS and UPS) and inverse photoemission spectra (BIS) of CoS have been measured. From these data the occupied and empty density of states of this material are deduced. The metallic nature of CoS is not caused by a breakdown of Mott-insulation but rather because of Co(d) - S(p) hybridization.     

Trap states in oxidation layer of nanocrystal Si

The photoluminescence （PL） of nanocrystal present in porous silicon shifts from the near infrared to the ultraviolet depending on the size when the surface is passivated with Si-H bonds. After oxidation, the centre wavelength of PL band is pinned in a region of 700-750 nm and its intensity increases obviously. Calculation shows that trap electronic states appear in the band gap of a smaller nanocrystal when Si = O bonds or Si-O-Si bonds are formed. The changes in PL intensity and wavelength can be explained by both quantum confinement and trap states in an oxidation layer of nanocrystal. In the theoretical model, the most important factor in the enhancement and the pinning effects of PL emission is the relative position between the level of the trap states and the level of the photoexcitation in the silicon nanocrystal.     

Metal-induced gap states at well defined alkali-halide/metal interfaces

In order to search for states specific to insulator/metal interfaces, we have studied epitaxially grown interfaces with element-selective near edge x-ray absorption fine structure. An extra peak is observed below the bulk edge onset for LiCl films on Cu and Ag substrates. The nature of chemical bonds as probed by x-ray photoemission spectroscopy and Auger electron spectroscopy remains unchanged, so we regard this as evidence for metal-induced gap states (MIGS) formed by the proximity to a metal, rather than local bonds at the interface. The dependence on the film thickness shows that the MIGS are as thin as one monolayer. An ab initio electronic structure calculation supports the existence of the MIGS that are strongly localized at the interface.     

Impact of O2 exposure on surface crystallinity of clean and Ba terminated Ge(1 0 0) surfaces

In analogy with the case of Sr on Si [Y. Liang, S. Gan, M. Engelhard, Appl. Phys. Lett. 79 (2001) 3591], we studied surface crystallinity and oxidation behaviour of clean and Ba terminated Ge(1 0 0) surfaces as a function of oxygen pressure and temperature. The structural and chemical changes in the Ge surface layer were monitored by LEED, XPS and real-time RHEED. In contrast to the oxidation retarding effect, observed for 1/2 monolayer of Sr on Si, the presence of a Ba termination layer leads to a pronounced increase in Ge oxidation rate with respect to clean Ge. In fact, while the Ge(1 0 0) surface terminated with 1/2 ML Ba amorphizes for a pO₂ of 10⁻² Torr, LEED indicates that clean Ge forms a thin (4.5 Å), 1 × 1 ordered oxide upon aggressive O₂ exposure (150 Torr, 200 °C, 30 min). We briefly discuss the origins for the difference in behaviour between Ba on Ge and Sr on Si.     

XPS analysis of sonochemically prepared SbSI ethanogel

This paper, for the first time, presents the results of the X-ray photoelectron spectroscopy (XPS) of the valence band and core levels in sonochemically prepared SbSI ethanogel consisting of nanowires. The XPS spectra were measured with monochromatized Al K_α radiation in the energy range of 0-1400 eV at room temperature. It was found that the sonochemically prepared SbSI ethanogel is a p-type semiconductor. The XPS determined composition of this material suggests the existence of antimony subiodide at surfaces of SbSI nanowires. The chemical shifts in SbSI ethanogel for the Sb, I and S states are reported. Experimentally obtained binding energies are compared with the results reported for single crystals of SbSI.     

Influence of elastic scattering on the measurement of core-level binding energy dispersion in X-ray photoemission spectroscopy

We explore the interplay between the elastic scattering of photoelectrons and the surface core level shifts with regard to the determination of core level binding energies in Au(111) and Cu₃Au(100). We find that an artificial shift is created in the binding energies of the Au 4f core levels, that exhibits a dependence on the emission angle, as well as on the spectral intensity of the core level emission itself. Using a simple model, we are able to reproduce the angular dependence of the shift and relate it to the anisotropy in the electron emission from the bulk layers. Our results demonstrate that interpretation of variation of the binding energy of core-levels should be conducted with great care and must take into account the possible influence of artificial shifts induced by elastic scattering.     

The Ba/Si(100)-2 × 1 interface I. XPS and XAES studies of silicide formation

In two complementary papers we present the results and analysis of an extended study of the Ba/Si(100)−2 × 1 interface. In this paper, we will discuss X-ray excited Auger electron spectroscopy- and X-ray photoelectron spectroscopy chemical shifts, as well as plasmon losses, which have been studied to answer the question whether silicide formation occurs at this interface. It is found that no silicide formation takes place at room temperature. Two Ba-Si phases are detected as reaction products upon annealing the Ba/Si(100) system at ˜ 550 K.     

Investigation of Fe/Al interface as a function of annealing temperature using XPS

This article describes the systemic investigation of the interface chemical and electronic properties of ultrathin Fe/Al multilayer structure (MLS) as a function of annealing temperature. For this purpose electron beam evaporated [Fe/Al]_×15 ML samples have been prepared under ultrahigh vacuum conditions. The chemical and electronic information of the interfaces at different depth has been obtained from XPS technique.The core level study show a gradual change in the nature of the electronic bonding at the interface as a result of annealing. In particular, the MLS annealed at 200 °C and 400 °C clearly show shifts in the binding energy position of Fe-2p_3/2 core line towards higher energy and Al-2p_3/2 core line towards lower energy side as compared to as-deposited sample, suggesting the formation of FeAl alloy phase at the interface. Another important finding with annealing is that the intensity of peak corresponding to pure Al-2p increases and that of Fe-2p decreases as compared to as-deposited case. The increase in intensity of Al-2p core line suggests the migration of Al atoms towards the surface owing to annealing induced inter-diffusion. The corresponding valence band spectra show appreciable changes in the Fe-3d as well as Al-3s density of states due to strong hybridization of sp-d states at the Fermi level as a result of charge transfer and also provide strong evidence for FeAl alloy formation.