Real time investigation of the growth of silicon carbide nanocrystals on Si(1 0 0) using synchrotron X-ray diffraction

The growth of silicon carbide nanocrystals on Si(1 0 0) is studied by synchrotron surface X-ray diffraction (SXRD) during annealing at high temperature. A chemisorbed methanol monolayer is used as carbon source, allowing to have a fixed amount of carbon atoms to feed the growth. At room temperature, minor changes in the 2 × 1 reconstruction of silicon are observed due to the formation of Si-O-CH₃ and Si-H bonds from methanol molecules. When annealed at 500 °C, carbon incorporation into the silicon leads only to local modifications of the surface structure. Above 600 °C, tri-dimensional silicon carbide nanocrystals growth takes place, together with surface roughening and sharp decrease of domain sizes of the 2 × 1 reconstruction. The different processes taking place at each temperature are clearly distinguished and identified during the real time SXRD measurements.     

Photoemission study of cobalt interaction with the oxidized Si(1 0 0)2 × 1 surface

The interaction of cobalt atoms with an oxidized Si(1 0 0)2 × 1 surface was studied by photoelectron spectroscopy with synchrotron radiation at room and elevated temperatures. The SiO_x layer grown in situ on the crystal surface was ∼0.3 nm thick, and the amount of deposited cobalt was varied within eight atomic layers. It was found that Co atoms could penetrate under the SiO_x layer even at room temperature in the initial growth. As the Co amount increased, a ternary Co-O-Si phase was formed at the interface, followed by a Co-Si solid solution. Silicide synthesis associated with the decomposition of these phases started under the SiO_x layer at ∼250 °C, producing cobalt disilicide with a stable CaF₂-type of structure.     

A study of Ga layers on Si(1 0 0)-(2 × 1) by SR-PES: Influence of adsorbed water

Results for deposition and thermal annealing of gallium on the Si(1 0 0)-(2 × 1) surface achieved by synchrotron radiation photoelectron spectroscopy (SR-PES) and low energy electron diffraction (LEED) are presented. In addition to deposition of Ga on a clean surface, the influence of water adsorption on the arrangement of gallium atoms was also studied. The results on Ga deposition at a higher temperature (490 °C) are consistent with a Ga ad-dimer model showing equivalent bond arrangement of all Ga atoms for coverages up to 0.5 ML. The deposition onto a surface with adsorbed water at room temperature led to a disordered gallium growth. In this case gallium atoms bind to silicon dimers already binding fragments of adsorbed water. A subsequent annealing of these layers leads to a surface structure similar to the Ga-(2 × 2), however, it is less ordered, probably due to the presence of silicon oxides formed from water fragments.     

Interaction of copper with sulfur on the sulfur-terminated Si(1 1 1)-(7 × 7) surface

The adsorption of S₂ on the Si(1 1 1)-(7 × 7) surface and the interaction of copper and sulfur on this sulfur-terminated Si(1 1 1) surface have been studied using synchrotron irradiation photoemission spectroscopy and scanning tunneling microscopy. The adsorption of S₂ at room temperature results in the passivation of silicon dangling bonds of Si(1 1 1)-(7 × 7) surface. Excessive sulfur forms S_n species on the surface. Copper atoms deposited at room temperature directly interact with S-adatoms through the formations of Cu-S bonds. Upon annealing the sample at 300 °C, CuS_x nanocrystals were produced on the sulfur-terminated Si(1 1 1) surface.     

Study of the early stages of Cr/6H-SiC(0 0 0 1) interface formation

The early stages of the Cr/6H-SiC(0 0 0 1) interface formation at room temperature were investigated using XPS, LEED and work function (WF) measurements. Upon stepwise Cr evaporation in UHV up to a thickness of 5-10 monolayers (ML) at RT, the binding energy of the XPS Cr 2p_3/2 core level peak shifted from 576.1 eV, at submonolayer coverage, to 574.7 eV (corresponding to metallic Cr) for the final Cr deposit, while the binding energies of the substrate XPS core level peaks remained stable. The WF exhibited a steep decrease of about 0.5 eV from the initial SiC substrate value, upon submonolayer coverage, but then increased gradually to saturation at a value of about 4.8 eV (polycrystalline Cr film with some chemisorbed oxygen). The growth of the ultrathin film was via 3D-cluster formation. The height of the Schottky barrier for the Cr/6H-SiC(0 0 0 1) contact was found by XPS to be 0.5 ± 0.1 eV. The results, generally, indicate the absence of any extended interfacial silicide-like interaction at RT.     

The growth of Au/Pd/alumina/Cu-Al system studied by SRPES

An ultra-thin alumina layer grown on Cu-9at.%Al (1 1 1) surface was studied using synchrotron radiation photoelectron spectroscopy (SRPES), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). By deconvolving SRPES spectra of the Al 2p doublet, four components belonging to metallic as well as oxide phases were recognized. Pd-Au alloy formation was confirmed by SRPES measurement during Pd and Au deposition. The study of the system's thermal stability reveals diffusion of Pd and Au atoms through the alumina layer. While Au atoms start to diffuse under the alumina layer at 670 K, Pd atoms are forming Pd-Al surface alloy at this temperature. The diffusion of Pd atoms through alumina occurs when sample was heated over 770 K. Alumina layer was stable even after heating the sample at 870 K, but its structure was corrupted probably due to the diffusion of metal atoms.     

Oxygen adsorption on a Mo2C(0 0 0 1) surface: Angle-resolved photoemission study

Oxygen adsorption on a C-terminated α-Mo₂C(0 0 0 1) surface has been investigated with Auger electron spectroscopy, low-energy electron diffraction, and angle-resolved photoemission spectroscopy utilizing synchrotron radiation. It is found that the oxygen atoms adsorb on the Mo atoms in the second layer forming a (1 × 1) orthorhombic periodicity. The oxygen adsorption induces a peculiar state around the Fermi level, which is observed at 0.4 eV in the normal-emission spectra. ARPES measurements show that the state is a partially occupied metallic state. The photoionization cross section of the state shows a maximum at the photon energy of 56 eV, which is assigned as originating from the resonance of the Mo 4d photoemission involving Mo 4p → 4d photoexcitation.     

Electronic structure of oxidized SiC(0 0 0 1) studied by inverse photoemission spectroscopy

Starting from the silicon rich (3 × 3) reconstruction of SiC(0 0 0 1) we prepared oxidized surfaces by hydrogen etching as well as by exposure to molecular oxygen. LEED pictures show a (1 × 1)-reconstructed surface with a faint structure being more pronounced for the hydrogen-etched surfaces. Auger spectra reveal a distinct change in the shape of the Si_LVV peak indicating the existence of Si-O bonds on the surface. Inverse photoemission (IPE) is employed to study the electronic structure above the Fermi level of the oxidized samples. On the hydrogen-etched surface difference spectra reveal a surface feature at 1 eV above the Fermi level that presumably originates from an isolated dangling bond on ordered patches of the oxidized surface.     

XPS analysis of surface chemistry of near surface region of epiready GaAs(1 0 0) surface treated with (NH4)2Sx solution

In this work we analyze the effect of (NH)₂S_x wet treatment on the GaAs(1 0 0) covered with “epiready” oxide layer without any pretreatment in order to check the removal of oxides and carbon-related contamination, and the formation of sulfur species. The sulfidation procedure consisted of epiready sample dipping (at room and 40 °C temperatures) in an ammonium polysulfide solution combined with a UHV flash annealing up to 500 °C.The inspection of the XPS As 2p_3/2 and Ga 2p_3/2 spectra taken at surface sensitive mode revealed: (i) the temperature-dependent reduction of the amount of GaAs oxides and carbon contamination after sulfidation, and almost their complete removal after subsequent annealing, (ii) the creation of sulfur bonds with both Ga and As, with more thermally stable Ga-S bonds, and (iii) the slight reduction in elemental arsenic amount.     

Alloy formation of Co/Ni/Pt(1 1 1)

The initial growth and alloy formation of ultrathin Co films deposited on 1 ML Ni/Pt(1 1 1) were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS). A sequence of samples of d_Co Co/1 ML Ni/Pt(1 1 1) (d_Co = 1, 2, and 3 ML) were prepared at room temperature, and then heated up to investigate the diffusion process. The Co and Ni atoms intermix at lower annealing temperature, and Co-Ni intermixing layer diffuses into the Pt substrate to form Ni-Co-Pt alloys at higher annealing temperature. The diffusion temperatures are Co coverage dependent. The evolution of UPS with annealing temperatures also shows the formation of surface alloys. Some interesting LEED patterns of 1 ML Co/1 ML Ni/Pt(1 1 1) show the formation of ordered alloys at different annealing temperature ranges. Further studies in the Curie temperature and concentration analysis, show that the ordered alloys corresponding to different LEED patterns are Ni_xCo_1−xPt and Ni_xCo_1−xPt₃. The relationship between the interface structure and magnetic properties was investigated.     

Adsorption and reaction of methanethiol on the Ru(0 0 0 1)-p(2 × 2)O surface: A TPD and XPS study

Methanethiol adsorbed on Ru(0 0 0 1)-p(2 × 2)O has been studied by TPD and XPS. The dissociation of methanethiol to methylthiolate and hydrogen at 90 K is evidenced by the observation of hydroxyl and water. The saturation coverage of methylthiolate is ∼0.15 ML, measured by both XPS and TPD. A detailed analysis suggests that only the hcp-hollow sites have been occupied. Upon annealing the surface, water and hydroxyl desorb from the surface at ∼210 K. Methylthiolate decomposes to methyl radical and atomic sulphur via C-S cleavage between 350 and 450 K. Some methyl radicals (0.05 ML) have been transferred to Ru atoms before they decompose to carbon and hydrogen. The rest of methyl radicals desorb as gaseous phase. No evidence for the transfer of methyl radical to surface oxygen has been found.     

On the short-range order of the SiOx (0 ≤ x ≤ 2) surface

Fine (oscillating) structure (FS) in the elastically scattered electron spectra (ESES) [O. Bondarchuk, S. Goysa, I. Koval, P. Melnik, M. Nakhodkin, Surf. Sci. 258 (1991) 239; O. Bondarchuk, S. Goysa, I. Koval, P. Melnik, M. Nakhodkin, Surf. Rev. Lett. 4 (1997) 965] was used to investigate surface structure of the SiO_x (0 ≤ x ≤ 2). SiO_x surface with different stoichiometry was prepared by implantation of 500 eV oxygen ions into a silicon wafer. Fourier transformation of the FS ESES contains one peak at 2.32 Å for Si, two peaks at 1.62 Å and 2.65 Å for a-SiO₂ and three peaks centered at 1.6-1.7 Å, 2.1-2.2 Å and 2.65-3.04 Å for SiO_x. Peaks at 1.62 Å and 2.65 Å are assigned to Si-O and O-O nearest distances correspondently. Ratio of the area under the peak at 2.65 Å to the area under the peak at 1.62 Å turned out to be not constant but grows linearly with the composition parameter x. The latter is considered to prove validity of the Random Bond Model to describe short-range order on the surface of non-stoichiometric silicon oxide.     

The plasma oxidation of Pd(1 0 0)

The oxidation of Pd(1 0 0) by an oxygen plasma was characterized using X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (ISS), temperature programmed desorption (TPD), and low energy electron diffraction (LEED). The oxygen uptake followed a typical parabolic profile with oxygen coverages reaching 32 ML after 1 h in the plasma; a factor of 40 higher than could be achieved by dosing molecular oxidants in ultra high vacuum. Even after adsorbing 32 ML of oxygen, XPS revealed both metallic Pd and PdO in the surface region. The R27^o LEED pattern previously attributed to a surface oxide monolayer, slowly attenuated with oxygen coverage indicating that the PdO formed poorly ordered three dimensional clusters that slowly covered the ordered surface oxide. While XPS revealed the formation of bulk PdO, only small changes in the ISS spectra were observed once the surface oxide layer was completed. The leading edges of the O₂ TPD curves showed only small shifts with increasing oxygen coverage that could be explained in terms of the lower thermodynamic stability of small oxide clusters. The desorption curves, however, could not be adequately described as simple zero order decomposition of PdO. There has been an ongoing debate in the literature about the relative catalytic activities of PdO and oxygen phases on Pd, the results indicate that any differences in the reactivity between bulk PdO and surface oxides are not associated with differences in the density of exposed Pd atoms or the decomposition kinetics of these two phases.     

Effects of UV photon irradiation on SiOx (0 < x < 2) structural properties

Thin films of a-SiO_x (0 < x < 2) were prepared by reactive r.f. magnetron sputtering from a polycrystalline-silicon target in an Ar/O₂ gas mixture. The oxygen partial pressure in the deposition chamber was varied so as to obtain films with different values of x. The plasma was monitored, during depositions, by optical emission spectroscopy (OES) system. Energy dispersive X-ray (EDX) measurements and infra-red (IR) spectroscopy were used to study the compositional and structural properties of the deposited layers.Structural modifications of SiO_x thin films have been induced by UV photons’ bombardment (wavelength of 248 nm) using a pulsed laser. IR spectroscopy and X-ray photoemission spectroscopy (XPS) were used to investigate the structural changes as a function of x value and incident energy. SiO_x phase separation by spinodal decomposition was revealed. The IR peak position shifted towards high wavenumber values when the laser energy is increased. Values corresponding to the SiO₂ material (only Si⁴⁺) have been found for laser irradiated samples, independently on the original x value. The phase separation process has a threshold energy that is in agreement with theoretical values calculated for the dissociation energy of the investigated material.For high values of the laser energy, crystalline silicon embedded in oxygen-rich silicon oxide was revealed by Raman spectroscopy.     

Hydroxylation-induced modifications of the Al2O3/NiAl(0 0 1) surface at low water vapour pressure

STM, STS, LEED and XPS data for crystalline θ-Al₂O₃ and non-crystalline Al₂O₃ ultra-thin films grown on NiAl(0 0 1) at 1025 K and exposed to water vapour at low pressure (1 × 10⁻⁷-1 × 10⁻⁵ mbar) and room temperature are reported. Water dissociation is observed at low pressure. This reactivity is assigned to the presence of a high density of coordinatively unsaturated cationic sites at the surface of the oxide film. The hydroxyl/hydroxide groups cannot be directly identify by their XPS binding energy, which is interpreted as resulting from the high BE positions of the oxide anions (O_1s signal at 532.5-532.8 eV). However the XPS intensities give evidence of an uptake of oxygen accompanied by an increase of the surface coverage by Al³⁺ cations, and a decrease of the concentration in metallic Al at the alloy interface. A value of ∼2 for the oxygen to aluminium ions surface concentration ratio indicates the formation of an oxy-hydroxide (AlO_xOH_y with x + y ∼ 2) hydroxylation product. STM and LEED show the amorphisation and roughening of the oxide film. At P(H₂O) = 1 × 10⁻⁷ mbar, only the surface of the oxide film is modified, with formation of nodules of ∼2 nm lateral size covering homogeneously the surface. STS shows that essentially the valence band is modified with an increase of the density of states at the band edge. With increasing pressure, hydroxylation is amplified, leading to an increased coverage of the alloy by oxy-hydroxide products and to the formation of larger nodules (∼7 nm) of amorphous oxy-hydroxide. Roughening and loss of the nanostructure indicate a propagation of the reaction that modifies the bulk structure of the oxide film. Amorphisation can be reverted to crystallization by annealing under UHV at 1025 K when the surface of the oxide film has been modified, but not when the bulk structure has been modified.     

Photoelectron spectroscopy study of oxygen adsorption on Mo2C(0 0 0 1)

Oxygen adsorption on the α-Mo₂C(0 0 0 1) surface has been investigated with X-ray photoelectron spectroscopy and valence photoelectron spectroscopy utilizing synchrotron radiation. It is found that oxygen adsorbs dissociatively at room temperature, and the adsorbed oxygen atoms interact with both Mo and C atoms to form an oxycarbide layer. As the O-adsorbed surface is heated at ≧800 K, the C-O bonds are broken and the adsorbed oxygen atoms are bound only to Mo atoms. Valence PES study shows that the oxygen adsorption induces a peculiar state around the Fermi level, which enhances the emission intensity at the Fermi edge in PES spectra.     

Behavior of N atoms in atomic-order nitrided Si0.5Ge0.5(1 0 0)

Behavior of N atoms in atomic-order nitrided Si_0.5Ge_0.5(1 0 0) by heat treatment in Ar at 600 °C was investigated by X-ray photoelectron spectroscopy (XPS). For thermal nitridation by NH₃ at 400 °C, nitridation of surface Si atoms tends to proceed preferentially over nitridation of surface Ge atoms. It is also clear that, with the heat treatment, nitridation of Si atoms proceeds by transfer of N atoms from Ge atoms. Angle-resolved XPS results show that Ge fraction beneath the surface nitrided layer increases significantly at 600 °C compared to the initial surface. These results indicate that preferential nitridation of Si atoms at surface over Ge atoms induces Ge segregation beneath the surface nitrided layer at higher temperatures above 400 °C.     

Effect of annealing temperature and annealing atmosphere on the structure and optical properties of ZnO thin films on sapphire (0 0 0 1) substrates by magnetron sputtering

ZnO thin films were epitaxially grown on sapphire (0 0 0 1) substrates by radio frequency magnetron sputtering. ZnO thin films were then annealed at different temperatures in air and in various atmospheres at 800 °C, respectively. The effect of the annealing temperature and annealing atmosphere on the structure and optical properties of ZnO thin films are investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL). A strong (0 0 2) diffraction peak of all ZnO thin films shows a polycrystalline hexagonal wurtzite structure and high preferential c-axis orientation. XRD and AFM results reveal that the better structural quality, relatively smaller tensile stress, smooth, uniform of ZnO thin films were obtained when annealed at 800 °C in N₂. Room temperature PL spectrum can be divided into the UV emission and the Visible broad band emission. The UV emission can be attributed to the near band edge emission (NBE) and the Visible broad band emission can be ascribed to the deep level emissions (DLE). By analyzing our experimental results, we recommend that the deep-level emission correspond to oxygen vacancy (V_O) and interstitial oxygen (O_i). The biggest ratio of the PL intensity of UV emission to that of visible emission (I_NBE/I_DLE) is observed from ZnO thin films annealed at 800 °C in N₂. Therefore, we suggest that annealing temperature of 800 °C and annealing atmosphere of N₂ are the most suitable annealing conditions for obtaining high quality ZnO thin films with good luminescence performance.     

Sn interaction with the CeO2(1 1 1) system: Bimetallic bonding and ceria reduction

A tin layer 0.8 nm thick was deposited onto the CeO₂(1 1 1) surface by molecular beam deposition at a temperature of 520 K. The interaction of tin with cerium oxide (ceria) was investigated by X-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS) and resonant photoelectron spectroscopy (RPES). The strong tin-ceria interaction led to the formation of a homogeneous bulk Ce-Sn-O mixed oxide system. The bulk compound formation is accompanied by partial Ce⁴⁺ → Ce³⁺ reduction, observed as a giant 4f resonance enhancement of the Ce³⁺ species. CeO₂ and SnO₂ oxides were formed after oxygen treatment at 520 K. The study proved the existence of strong Ce-Sn interaction and charge transfer from Sn to the Ce-O complex that lead to a weakening of the cerium-oxygen bond, and consequently, to the formation of oxygen deficient active sites on the ceria surface. This behavior can be a key for understanding the higher catalytic activity of the SnO_x/CeO_x mixed oxide catalysts as compared with the individual pure oxides.     

Combined EELS, LEED and SR-XPS study of ultra-thin crystalline layers of indium nitride on InP(1 0 0)—Effect of annealing at 450 °C

In this study, InP(1 0 0) surfaces were bombarded by argon ions in ultra high vacuum. Indium metallic droplets were created in well controlled quantities and played the role of precursors for the nitridation process. A glow discharge cell was used to produce a continuous plasma with a majority of N atomic species. X-ray photoelectron spectroscopy (XPS) studies indicated that the nitrogen combined with indium surface atoms to create InN thin films (two monolayers) on an In rich-InP(1 0 0) surface. This process occurred at low temperature: 250 °C. Synchrotron radiation photoemission (SR-XPS) studies of the valence band spectra, LEED and EELS measurements show an evolution of surface species and the effect of a 450 °C annealing of the InN/InP structures. The results reveal that annealing allows the crystallization of the thin InN layers, while the LEED pattern shows a (4 × 1) reconstruction. As a consequence, InN related structures in EELS and valence bands spectra are different before and after the annealing. According to SR-XPS measurements, the Fermi level is found to be pinned at 1.6 eV above the valence band maximum (VBM).