XPS surface analysis of monocrystalline silicon solar cells for manufacturing control

X-ray photoelectron spectroscopy (XPS) has been applied to surfaces of silicon wafers in the different stages of the assembly line for large-scale monocrystalline silicon solar cell manufacturing (ISOFOTON, Malaga, Spain). XPS results have shown that a considerable amount of carbon is present on the pyramidal-textured monocrystalline silicon surface. This amount decreases slightly but is still present after the process of phosphor diffusion (p-n junction), as well as after subsequent calcination in humid air for SiO₂ film formation (passivation). This amount of carbon may be buried during the process of CVD coating an anti-reflection TiO₂ film. After calcination of the film in order to obtain the TiO₂ rutile phase, an even higher amount of carbon is detected on the TiO₂ anti-reflection coating surface. This indicates that not all organics from the tetra-isopropile ortho-titanate (TPT) precursor were released from the film. Furthermore, in this case phosphor is found in excess on the SiO₂ wafer surface (dead layer) and also on the rutile TiO₂ surface, indicating that an extra phosphor diffusion from the bulk silicon through the TiO₂ film has taken place during calcination. These results demonstrate how thermal treatments applied in the solar cell manufacturing assembly line can influence and may change the intended compositional distribution. These treatments may also introduce defects that act as recombination centres for charge carriers in the solar cell device. Received: 13 September 2000 / Accepted: 10 January 2001 / Published online: 3 May 2001     

Surface modification by silver coating for improving electrochemical properties of LiFePO4

LiFePO₄ is a potential candidate for the cathode material of the lithium secondary battery. Fine particle LiFePO₄ was synthesized by the simple co-precipitation method, and aqueous coating on the LiFePO₄ was tried using silver nitrate solution in order to increase electronic conductivity. Highly dispersed silver on the particles enhances the electronic conductivity and increases the capacity. The electrochemical properties of the silver coated LiFePO₄ with the various current densities are analogous to those of highly conductive LiFePO₄. The silver coating can be a promising tool to preserve the capacity even at the high current densities.     

Quantitative XPS imaging—new possibilities with the delay-line detector

X-ray photoelectron spectroscopy (XPS, ESCA) is an ideal tool for identifying differences in surface chemistry. In the past, it has lacked the spatial resolution as well as the performance of elemental or even chemical state imaging, to be of significant use in detecting most microscopic surface phenomena. The recent development of improved micro- or small spot-XPS systems with near-micron spatial resolution as well as outstanding chemical state image performance has opened up a significant opportunity to undertake more detailed studies of micro-structured or micro-patterned surfaces or technical samples with locally distributed impurities. The introduction of a new detector type, the delay-line detector (DLD), to XPS-equipments allows for the first time the acquisition of quantifiable XPS images. This study is intended primarily to explore the capabilities of quantitative ESCA-imaging with respect to the possibilities and limits.     

Surface characterisation and interface studies of high-k materials by XPS and TOF-SIMS

High-k dielectric LaAlO₃ (LAO) films on Si(100) were studied by TOF-SIMS and XPS to look for diffusion processes during deposition and additional thermal treatment and for the formation and composition of possible interfacial layers. The measurements reveal the existence of SiO₂ at the LAO/Si interface. Thermal treatment strengthens this effect indicating a segregation of Si. However, thin LAO layers show no interfacial SiO₂ but the formation of a La-Al-Si-O compound. In addition, Pt diffusion from the top coating into the LAO layers occurs. Within the LAO layer C is the most abundant contamination (10²¹ at/cm³). Its relatively high concentration could influence electric characteristics. XPS shows that CO₃²⁻ is intrinsic to the LAO layer and is due to the adsorption of CO₂ of the residual gas in the deposition chamber.     

Prerequisites for high-quality magnetic tunnel junctions: XPS and NMR study of Co/Al bilayers

Aluminum films with thicknesses ranging from 1 nm to 12 nm have been sputtered on 20 nm thick Co layers. The properties of the Co/Al bilayers were studied by X-ray photoemission spectroscopy (XPS) and spin-echo nuclear magnetic resonance (NMR). Both methods show independently that a 1 nm Al film covers the Co surface completely. XPS and NMR also showed that layers thicker than 1 nm Al are not oxidized completely in ambient air. Similarities to and deviations from niobium with Al overlayers (Nb/Al) are described. Prerequisites for the fabrication of tunneling magnetoresistance devices based on Co or NiFe ferromagnets and an aluminum oxide barrier are discussed. Received: 7 July 1999 / Accepted: 11 November 1999 / Published online: 8 March 2000     

Fabrication of solid-state fuel cell based on DNA film

We have fabricated a fuel cell based on the DNA film (DNAFC) and examined its properties under various humidity conditions at room temperature. The open-circuit voltage of a DNAFC is generated by supplying H₂ gas to the anode. The open-circuit voltage strongly depends on the humidity conditions, and in a DNA film, the optimum condition in which the open-circuit voltage attains a value as high as 0.55 V is achieved under the relative humidity condition of 55%. Furthermore, the cell voltage of the DNAFC decreases with an increase in current density, as observed in fuel cells such as proton exchange membrane fuel cell, solid oxide fuel cell, and several others. These results indicate that DNA film can be used as the fuel cell electrolyte under approximately 55% humidity condition.     

The acid-base properties of the surface of native zinc oxide layers: An XPS study of adsorption of 1,2-diaminoethane

The acid-base properties of native zinc oxide surfaces have been studied using X-ray photoelectron spectroscopy (XPS). The native layers of zinc oxide have been obtained by ageing mechanically polished pure zinc disks in a glass dryer for 1 month. Such a treatment lead to the formation of an unstable oxide layer and dehydroxylation has been observed during storage in vacuum. By following adsorption in ultrahigh vacuum of 1,2-diaminoethane (DAE) several types of active sites have been evidenced. Zinc cations react with the probe molecule following a Lewis acid/base interaction, while the hydroxyl and the carbonate-like species react following a Brønsted acid/base reaction. Although initial interaction via the Brønsted-like mechanisms is favoured, it has been shown that the resulting complexes are not stable. Under vacuum conditions, the adsorbed DAE molecules either partly desorb or modify their interaction mode with the surface to form additional Lewis-like bonded stable complexes. In addition, a cleaning effect of the molecule has been observed which lead to partial removal of the carbonate-like contamination.     

Surface of Zn-Mn-O and its role in room temperature ferromagnetism: An XPS analysis

The existence of ferromagnetism in Zn-Mn-O semiconductor samples and dependence on the preparation condition were investigated. We systematically examined the samples with manganese concentration ranging from 0 to 10 at.%, prepared by a solid state reaction route using (ZnC₂O₄·2H₂O)_1−x and (MnC₂O₄·2H₂O)_x as precursors. Thermal treatment was carried out in air at temperatures ranging from 400 to 900 °C. The samples were investigated by X-ray diffraction, transmission electron microscopy, magnetization measurements and XPS spectroscopy. XPS surface composition, chemical analysis and depth profiling were successfully employed on powder revealing the chemical composition at the surface of the grains and underneath. The present investigation suggests that physical properties and observed room temperature ferromagnetism might be due to grain surface effects. It seems that the ferromagnetic phase is correlated with oxygen build up at the surface.     

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.     

XPS and ToF-SIMS analysis of natural rubies and sapphires heated in an inert (N2) atmosphere

Advanced surface analysis techniques: X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, have been employed in the study of heat treatment of natural corundum as ruby and sapphire. The stones were heat treated in an inert (N₂) atmosphere. The setting temperatures were: 1000, 1100, 1200, 1300, 1400, 1500 and 1600 °C. The XPS studies and the parallel ToF-SIMS experiments revealed diffusion behavior of Fe and Ti in the as-mined stones as evidenced by surface observations. Both metals exhibited broad maxima in surface concentration near 1300 °C. Owing to its superlative detection limit, ToF-SIMS spectra are able to provide the temperature-dependent concentration profiles of trace transition metals such as Cr, Cu and V at a level not detectable by XPS. Visible appearance of the stones is clearly affected by heat treatment. Interestingly, the ruby stones did not exhibit cloudy inclusion (“silk”) on heating, contrary to previous experiments under atmospheric conditions.     

An XPS study of CrOx on a thin alumina film and in alumina supported catalysts

We have investigated chromium layers evaporated onto a thin alumina film at room temperature. The oxidation and reduction behavior of this model catalyst was compared to atomic layer deposition (ALD) and impregnated alumina supported catalysts using X-ray photoelectron spectroscopy (XPS) with a detailed analysis method utilizing asymmetric peak shapes to represent both metallic and oxidic states. The ALD and impregnated catalysts were measured after calcination in air and after reduction with several gases at 850 K. Both catalysts show Cr³⁺ and Cr⁶⁺ species after calcination and mostly Cr³⁺ after reduction. The chromium layers deposited in vacuum show initially small partial oxidation due to the interaction with the oxygen terminated alumina film. These model catalysts can be oxidized in vacuum to Cr³⁺ species but not to higher oxidation states. The model catalysts were also subjected to calcination and reduction treatments after deposition in vacuum. Under these conditions the model systems exhibit similar oxidation/reduction behavior as the supported catalysts. Photoreduction of Cr⁶⁺ during the measurements was also studied and found to be very slow having a negligible effect on the results.     

XPS depth profiling studies of L-CVD SnO2 thin films

In this paper we present the results of the XPS atomic depth profile analysis, using ion beam sputtering, of L-CVD SnO₂ thin films grown on an atomically clean SiO₂ substrate after annealing at 400 °C in dry atmospheric air. From the evolution of the Sn 3d_5/2, O 1s, Si 2p and C 1s core level peaks our experiments allowed the determination of the in depth atomic concentration of the main components of the SnO₂/SiO₂ interface. Thin (few nm) nearly stoichiometric SnO₂ films are present at the topmost layer of the thin films, and progressive intermixing with SnO and silicon oxide is observed at deeper layer. The interface between the Sn and the Si oxide layers (i.e. the effective Sn oxide thickness) is measured at 13 nm.     

Characterization of thin Ta-Si-Nx layers of different nitrogen content using XPS, UPS and STM

Reactively sputtered Ta-Si-N_x barrier systems of different nitrogen content on copper were investigated by photoelectron spectroscopy (XPS, UPS) and scanning tunnelling microscopy (STM). The measured photoelectron spectra (excitation He-I) showed a clear dependence of the electron state density near the Fermi edge on the content of nitrogen. These results correlate with the I(U) characteristics of the STM measurements and the electrical conductivity of these layers.     

Intermetallic compounds as negative electrodes of Ni/MH batteries

This review is devoted to the main families of thermodynamically stable intermetallic compounds (AB₅-, AB₂- and AB-type alloys) that have been researched in the last thirty years as materials for negative electrodes in nickel–metal hydride batteries. The crystal structure of these compounds and their hydrides is widely described. Their solid–gas hydrogenation properties and, particularly, the related desorption isotherm curves are examined as a useful criterion for the selection of suitable battery materials. The electrochemical performances obtained with these alloys are reported and the given solutions to common problems such as corrosion, passivation, decrepitation and short cycle life are discussed. Only AB₅-based compounds have achieved, up to now, enough development for being widely present on the market, and exhibit improved battery performances in comparison with the polluting Ni/Cd batteries. The high capacity of AB₂-based compounds and the remarkable electrochemical activity of some AB-based alloys make, however, further research on all the reviewed families still valuable. Received: 16 August 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

XPS and AES investigations of hard magnetic Nd-Fe-B films

Nd-Fe-B is a promising material system for the preparation of thin films with good hard magnetic properties. One problem of this material class is the sensitivity against oxidation, resulting in a degradation of the magnetic properties. Using XPS depth profiling in combination with peak-shape analysis it is shown that already after several hours oxygen can diffuse deep into the thin laser-deposited films and that Nd is mainly responsible for the oxidation. Local element analysis with AES revealed boron inhomogeneities from droplet formation during laser deposition. These problems can be solved by using a capping Cr layer and an FeB target for thin film preparation, respectively.     

Role of pH and calcium ions in the adsorption of an alkyl N-aminodimethylphosphonate on steel: An XPS study

The role of pH and calcium ions in the adsorption of an alkyl N-aminodimethylphosphonate on mild steel (E24) surfaces was investigated by XPS. Fe 2p_3/2 and O 1s spectra show that the oxide/hydroxide layer developed on the steel surface, immersed in the diphosphonate solution (7 ≤ pH ≤ 13, without Ca²⁺) or in a filtered cement solution (pH 13, 15.38 mmol l⁻¹ of Ca²⁺), consists of Fe₂O₃, covered by a very thin layer of FeOOH (goethite). The total thickness of the oxide/hydroxide layer is ∼3 nm and is independent of the pH and the presence/absence of Ca²⁺. In the absence of Ca²⁺ ions, the N 1s and P 2p spectra reveal that the adsorption of the diphosphonate on the outer layer of FeOOH takes place only for pH lower than the zero charge pH of goethite (7.55). At pH 7, the adsorbed diphosphonate layer is continuous and its equivalent thickness is ∼24 Å (monolayer). In the presence of Ca²⁺ ions, the C 1s and Ca 2p signals indicate that calcium is present on the steel surface as calcium phosphonate (and Ca(OH)₂, in very small amount). The adsorption of the diphosphonate molecules on the steel surface is promoted in alkaline solution (pH > 7.55) by the doubly charged Ca²⁺ ions that bridge the O⁻ of goethite and the P-O⁻ groups of the diphosphonate molecules. The measured values for the Ca/P intensity ratio are in the range 0.75-1, which suggests that the diphosphonate molecules are adsorbed on steel forming a polymer cross-linked by calcium ions through their phosphono groups. In the presence of Ca²⁺ ions in alkaline solution, the adsorbed diphosphonate layer is discontinuous and the surface coverage is found to be ∼34%.     

A comparative study of the microstructures and optical properties of Cu- and Ag-doped ZnO thin films

Cu- and Ag-doped ZnO films were deposited by direct current co-reactive magnetron sputtering technique. The microstructure, the chemical states of the oxygen, zinc, copper and silver and the optical properties in doped ZnO films were investigated by X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS) and UV-Visible spectroscopy. XRD analysis revealed that both of Cu- and Ag-doped ZnO films consist of single phase ZnO with zincite structure while the doping elements had an evident effect on the (0 0 2) preferential orientation. The XPS spectra showed that the chemical states of oxygen were different in Cu- and Ag-doped ZnO thin films, which may lead to the shift of the band gap as can be observed in the transmittance and absorption spectra. Meanwhile, the widths of band tails of ZnO films became larger after Cu and Ag doping.     

XPS characterisation of plasma treated and zinc oxide coated PET

At first, X-ray photoelectron spectroscopy (XPS) analyses of reference and carbon dioxide plasma treated polyethylene terephthalate (PET) were carried out. Significant chemical modifications were outlined in the treated PET surface in comparison with the reference one. The formation of new oxygenated groups was evidenced. These modifications heighten the level of interactions between the polymer substrate and the deposited coating.In a second stage, zinc oxide thin films were elaborated by r.f. magnetron sputtering from a ceramic target and with a reactive gas (mixture of argon-1% oxygen) under optimised conditions on CO₂ plasma treated PET. The interfacial chemistry between the plasma treated PET and the zinc oxide was also studied by XPS. The line shape changes in the high-resolution core level spectra of carbon C1s, oxygen O1s, and zinc (Zn2p3/2, Zn3p), with the progressive deposition of zinc oxide coatings being recorded. The obtained spectra were fitted to mixed Gaussian-Lorentzian components using XPS CASA software.An interaction scheme between the zinc oxide thin layer and its polymer substrate, in the first stage of deposition, was proposed and checked by corroborating the findings of the different XPS spectra and their decompositions. It suggests the formation of ZnOC complexes at the interface, which are promoted by an electron transfer from zinc to oxygen in oxygenated species, mainly alcohol groups, generated by the CO₂ plasma treatment of PET.     

Study of the surface contamination of copper with the improved positron annihilation-induced Auger electron spectrometer at NEPOMUC

The high intensity positron source NEPOMUC at the FRM-II in Munich enables measurement times for positron annihilation-induced Auger electron spectroscopy (PAES) of only 2.4 h/spectrum, in contrast to usual lab beams with measurement times up to several days. The high electron background due to surrounding experiments in the experimental hall of the FRM-II has been eliminated and hence background free experiments have become possible. Due to this, the signal to noise ratio has been enhanced to 4.5:1, compared to 1:3 with EAES. In addition, a long-term measurement has been performed in order to observe the contamination of a polycrystalline copper foil at 150 °C.     

Effects of non-stoichiometric RuO x thin films on the dielectric properties of BaTiO3 thin films

The deconvolution process of X-ray photoemission spectra for O 1s and Ru 3d, X-ray diffraction and Rutherford backscattering spectrometry reveal that the RuO _x films (x = 2.0 – 2.2) deposited at a O₂ partial pressure less than 30% show (110)-oriented grains, whereas the RuO _x films (x = 2.3 – 2.4) deposited at a 40–50% O₂ partial pressure show amorphous and (101)-oriented grains due to the excess O interstitials and RuO₃ or RuO₄. These differences in the crystal phases of RuO _x influence the crystal structure of BaTiO₃ deposited on these RuO _x bottom electrodes, resulting in a higher dielectric constant and a lower dissipation factor for tetragonal BaTiO₃/RuO _x (x = 2.1) than amorphous BaTiO₃/RuO _x (x = 2.4).