R. Larciprete

XPS study of the L-CVD deposited SnO2 thin films exposed to oxygen and hydrogen

In this paper, the XPS study of SnO2 thin films deposited by the L-CVD technique are presented. The influence of exposition of the as-deposited samples to oxygen O2 and hydrogen H2 on their stoichiometry was determined. Moreover, on the basis of detailed shape analysis of the Sn3d5/2 and O1s XPS peaks, the chemical shift of binding energy corresponding to the change of sample stoichiometry was separated from a shift of the binding energy corresponding to the change of interface Fermi level position EF–Ev in the band gap, using a new procedure of deconvolution of the core level XPS peaks. The shift of the Sn3d5/2 peak by approximately 0.5 eV towards the lower binding energy after highest H2 exposure was interpreted as a true chemical shift due to an increase of Sn2+ component, whereas the shift of Sn3d5/2 peak and O1s peak after highest O2 exposure by approximately 0.5 eV towards the lower binding energy was interpreted as a result of the shift of the interface Fermi level position in the band gap towards the top of valence band at the surface, which corresponds to a deep accumulation layer typical for SnO2 thin films.

A synchrotron radiation photoemission study of the oxidation of tin

Abstract The room temperature oxidation of a tin metal foil up to O2 exposures of 8 × 1011 L (1 L = 10−6 Torr · s) has been studied by synchrotron radiation photoemission spectroscopy. Valence band (VB) and Sn 4d core level energy distribution curves (EDC have been measured at photon energies of 50 and 90 eV. It resulted that the oxide film formed on tin after oxygen chemisorbtion contained Sn2+ and Sn4+ with relative concentrations determined by the stage of the oxidation. The VB, that at 500 L of O2 had the characteristic profile of the SnO VB, showed at higher exposures the appearance of spectral features due to SnO2. The analysis of the Sn4d core levels allowed us to identify the presence of a chemical shift of 0.73 ± 0.05 eV between the Sn4+ and Sn2+ peaks. In fact the curves obtained by subtracting the metallic contribution due to the substrate from the Sn 4d peaks measured at increasing O2 exposure, appeared progressively shifted towards higher binding energies, because of the change in the composition of the oxide layer. Best fit curves of the previous peaks, deconvoluted using doublets of Gaussians for the Sn2+ and Sn4+ 4d 3 2 and 4d 5 2 spin orbit components, were used to evaluate the concentration of the two phases. Only the Sn2+ component was found by the fitting program in the peak observed at 500 L, whereas increasing concentrations of Sn4+ (up to 71% at 8 × 1011 L) showed up at heavier oxidation. Photoemission spectra, taken at different depths in the oxide layer, etched by low energy (200 eV) Ar+ sputtering, showed the presence of a composition gradient; SnO2 being an overlayer lying above a film containing mostly SnO. The thickness of the oxide film present on the tin foil dosed with 8 × 1011 L of oxygen was evaluated to be about 17 A.

Organotin films deposited by laser-induced CVD as active layers in chemical gas sensors

Abstract In this work, an organotin film obtained by L-CVD at 193 nm from tetramethyltin was used as active layer in a conductometric gas sensor. As it was stated by in situ surface analysis, the `as deposited material contained Sn and C. However, exposure to a large dose of O 2 or air determined the formation of a thin SnO 2 layer covering the remaining film, which was less oxidised and had an oxygen concentration gradient in its depth. Electrical tests of the sensors showed high sensitivity and short response time for the detection of NO 2 . Satisfactory performances were observed also with reducing gases. In this case, the high sensitivity measured for H 2 and ethanol, in comparison to CO and hydrocarbons, indicated the possibility to use the sensor for selective revelation.

Electron accumulation layer on clean In-terminated InAs(001)(4×2)-c(8×2) surface

Abstract We have investigated clean In-terminated InAs(0xa00xa01)(4×2)-c(8×2) surface by LEED, STM and high-resolution core-level and valence band (VB) spectroscopies. The In4d and As3d core levels showed the presence of chemically shifted components. The decomposition of In4d core level exhibited a new surface component located close to that of free indium metal. This indicates that the origin of the electron emission at the Fermi level measured in the VB spectra is probably due to the free In charge. STM measurements showed a uniform distribution of charge on the In-rows, which are highly ordered over large areas of the surface.

SURFACE ANALYSIS STUDY OF THE OXIDATION OF ORGANOTIN FILMS DEPOSITED BY ARF EXCIMER LASER CHEMICAL VAPOR DEPOSITION

ArF excimer laser photolysis of gas phase tetramethyltin was used to deposit thin organotin films on Si. In situ Auger electron spectroscopy and x-ray photoelectron spectroscopy (XPS) film analysis, showed the presence of C and Sn, which, according to Cu20091s and Snu20093d5/2 peak deconvolution, were organized in highly branched polymeric chains likely containing H as well. The film was oxidized at room temperature by exposure to controlled quantities of O2 up to 5.4×1012L. Snu20093d5/2, Cu20091s, and Ou20091s XPS peaks monitoring allowed us to follow the sequence of the oxidative reactions. It resulted that oxygen attacks first the Sn–H and Sn–C sites leading to the elimination of H2O and C containing volatile compounds, which determines a rearrangement of the cleaved bonds and an overall decrease of the measured C content. After this fast phase, oxygen is inserted in the Sn–Sn and Sn–C bonds, with formation of Sn–O–Sn, Sn–O–C, and Sn–C–O species. At the highest oxygen doses the dominating SnO2 component shows that almost ...

The photochemistry of CH4 adsorbed on Pt(111) studied by high resolution fast XPS

Abstract The photochemistry induced at 40 K by synchrotron radiation at 400 eV in CH 4 molecules adsorbed on Pt(1xa01xa01) was studied by high resolution fast XPS. The photoreactions were monitored by acquiring the C1s core level spectrum as a function of time for methane adlayers with coverage ranging from submonolayer to multilayer regime. We have observed molecular desorption of the multilayer and effective dissociation in the layer in contact with the metal. Differently from UV photons and low energy e-beam induced dissociation of CH 4 adsorbed on Pt(1xa01xa01) – which only produced adsorbed CH 3 and H fragments – the C1s core level measured on methane photodissociated at 400 eV shows that other species are formed, which start to thermally dissociate at about 100 K, producing CH and possibly H. Probable candidates are CH 2 , formed by multiple C–H bond cleavage together with CH 3 groups during the initial methane photodissociation, or a C–C containing intermediate, resulting from methyl surface chemistry.

Synchrotron radiation photoemission analysis of ArF laser deposited tin oxide

The composition of films deposited by dissociating pure tetramethyltin (TMT) or TMT/O2 mixtures at 193 nm, was monitored by ‘‘in situ’’ Auger spectroscopy. The presence of C contamination, which was considerable in films obtained from the pure organometallic, was greatly reduced by the occurrence of oxidation and carbon‐free films were achieved when the gas phase concentration of TMT was sufficiently small. The dependence of the O/Sn Auger electron spectroscopy peak‐to‐peak height ratio on the p(O2)/p(TMT) ratio in the precursor mixture showed a saturation for large concentration of oxygen. The surface properties of the tin oxide films were investigated by synchrotron radiation ultraviolet photoemission spectroscopy (UPS). To this aim samples were transferred to a synchrotron radiation facility, without any exposure to air and kept in dynamical high vacuum. The UPS demonstrated that the films were essentially made of SnO2: in fact the valence band exhibited the typical features of SnO2 and the analysis of...

NO2 decomposition on Rh clusters supported on single-walled carbon nanotubes

High-resolution photoemission spectroscopy was used to study Rh decorated single-walled C nanotubes before and after the exposure to NO2. We found that the metal adatoms form ordered nanocrystallites exhibiting a surface termination close to the (111) oriented crystal. After NO2 uptake at 200K, the thermal evolution of the NOx adspecies indicates for the Rh nanocrystallites a catalytic behavior similar to the low index Rh surfaces. The Rh 3d5∕2 line shape discloses the bonding of Rh atoms to NO2 and its fragments, whereas the oxidation of the metal/nanotubes contact is monitored by the energy shift of the C 1s peak.

ArF excimer laser deposited tin oxide films studied by “in situ” surface diagnostics and by synchrotron radiation induced UV photoemission

Abstract Tin oxide films were deposited by ArF excimer laser induced photolysis of Sn(CH3)4/O2 mixtures. The “in situ” Auger analysis was used to determine the effect of the gas-phase oxygen concentration on the film composition. The surface properties of the films were investigated by synchrotron radiation UV photoemission spectroscopy. To this aim samples were transferred to a synchrotron radiation facility, without any exposure to air and kept in dynamical high vacuum. The results obtained showed that the valence-band spectrum was characteristic for SnO2 and that the Sn 4d core peak did not have any contribution due to metallic tin. Furthermore, by tuning the synchrotron radiation photon energy, in order to vary the photoelectron escape depth, we got the indication that the oxygen vacancies responsible for the weak emission in the band-gap region were localized at the sample surface.

X-ray diffraction and x-ray photoelectron spectroscopy study of partially strained SiGe layers produced via excimer laser processing

Structural properties of graded Si(1−x)Gex layers obtained on Si(100) by pulsed laser induced epitaxy were investigated by means of conventional powder x-ray diffraction and x-ray photoelectron spectroscopy. The Si(1−x)Gex epitaxial layers were formed by pulsed KrF-laser driven rapid melting and crystallization of thin amorphous Ge layers deposited onto the Si(100). The experimental results showed that, by increasing the number of laser pulses, good quality and partially strained epitaxial layers could be attained. A Monte Carlo data evaluation algorithm is proposed, which is capable to determine, by the simultaneous fit of data obtained by x-ray diffraction and x-ray photoelectron spectroscopy, the strain level as a function of Ge concentration.