Applied Surface Science (v.173, #1-2)

The diffusion process from water into and out of the bulk of an injection-molded bisphenol-A-polycarbonate sample has been characterized by weighing. In the equilibrium state, the amount of water inside the polycarbonate depends, therefore, on the environmental humidity. Transferring the experiments into UHV showed that the amount of water could be determined by the time-integrated H2O-signal of a mass-spectrometer as depending on the storage time in vacuum before measurement.After Ar-plasma etching; samples showed a reduced H2O-signal, which is the consequence of a hydrophobic barrier on the sample’s surface created by the plasma. The thesis of a barrier was supported by XPS-measurements, which showed a decrease of the polar carboxylic group of the polycarbonate.
Keywords: Polycarbonate; Ar- and He-plasma; Mass spectroscopy; XPS; ESCA; Metallization; Tape-pull-test;

The overlayers of Cs2O, Cs2O2 and CsO2/Cs2O2 were prepared by simultaneous oxygen-cesium adsorption on Ge(1 1 1) at room temperature. Electron spectroscopy has been used to investigate the surfaces of Cs2O/Ge, Cs2O2/Ge and CsO2/Cs2O2/Ge as a function of annealing temperature. The interaction of Cs2O with Ge(1 1 1) is weak and the surface has a work-function value of 0.8–0.9 eV before the decomposition of Cs2O at 660 K. The work function of the Cs2O2/Ge surface is 1.3 eV and decreases to a minimum value of 0.95 eV after annealing at 440 K. Both CsO and GeO bonds are formed before the Cs2O2 species decomposes at 550 K. The results show that the CsO2/Cs2O2/Ge system is reactive as some Ge atoms are bonded to oxygen after coadsorption. The work function of the surface remains 2.15 eV before the decomposition of CsO2. For all surfaces, when the cesium oxides decompose, both CsO and GeO bonds are dominant with a work function of about 1.2 eV. Further annealing gives rise to the increases of the work function.
Keywords: Alkali metal; Oxygen; Germanium; Photoelectron spectroscopy;

Phase transformations in hafnium dioxide thin films grown by atomic layer deposition at high temperatures by Jaan Aarik; Aleks Aidla; Hugo Mändar; Teet Uustare; Kaupo Kukli; Mikael Schuisky (15-21).
High-temperature cubic phase of HfO2 was observed by reflection high-energy electron diffraction in nanocrystalline thin films grown by atomic layer deposition from HfCl4 and H2O at substrate temperatures of 880–940°C. The phase was formed at properly chosen precursor doses and it was observed on the surface of films, which according to X-ray diffraction data consisted of monoclinic HfO2. The thickness of the surface layer, in which the cubic phase appeared, was estimated to be 5–10 nm. According to Auger electron spectroscopy data, formation of the cubic phase was accompanied with an increase in the ionicity of O–Hf bonds.
Keywords: Hafnium dioxide; Atomic layer deposition; Structure; Auger electron spectroscopy;

Photoemission study of the stainless steel surface interacting with oxygen by Boklae Cho; Sukmin Chung; Kijeong Kim; Taihee Kang; Chongdo Park; Bongsoo Kim (22-29).
Oxidation behavior of stainless steel surface was investigated at 450°C in oxygen partial pressure of 1×10−9  Torr using surface-sensitive photoemission spectroscopy (PES). At the initial stage of oxidation, a layer mixed with metallic chromium and oxygen existed on top of Cr2O3 layer, showing relatively high oxygen concentration compared to the Cr2O3 layer. The valence-band electronic structure of the oxide film was hardly affected by the presence of impurities such as Mn and Fe. Attenuation of chromium satellite peaks was observed when the oxide film contained metallic chromium. The authors propose that the attenuation might stem from the screening of charge excitations by free carriers from the metallic chromium.
Keywords: Synchrotron radiation photoelectron spectroscopy; Oxidation; Stainless steel;

Cyclotene 3022, a low permittivity insulator was subjected to Ar+ surface treatment in an effort to increase the adhesion of subsequently evaporated Cu. Photoacoustic FTIR shows the loss of aromaticity and the formation of SiH n , where n=2, 3. XPS evidence suggests that beam damage causes the formation of SiO• and Si•, as well as C• fragments; both SiO• and C• react with evaporated Cu0 to form SiOCu and CCu; these bonds to Cu are too weak to prevent lateral surface diffusion of the Cu to form clusters.
Keywords: Cyclotene 3022; Photoacoustic FTIR; Argon ion; Sputtering; Copper evaporation; Copper adhesion;

An expanding thermal plasma for deposition of surface textured ZnO:Al with focus on thin film solar cell applications by R. Groenen; J.L. Linden; H.R.M. van Lierop; D.C. Schram; A.D. Kuypers; M.C.M. van de Sanden (40-43).
A new method for low temperature deposition of surface textured ZnO:Al is presented utilizing an expanding thermal plasma created by a cascaded arc. Films are deposited at 200°C at the rate of 0.65–0.75 nm s−1 exhibiting low resistivity (<10−3  Ω cm), high visible transmittance (>80%) and a rough surface texture. First application in p-i-n a-Si:H solar cells indicates promising light trapping properties.
Keywords: Zinc oxide; Expanding thermal plasma; Surface texture; Transparent conducting oxide; Solar cells;

The interactions of mercury on cleaved and polished galena PbS(0 0 1) were studied at room temperature. The surface of the samples was characterised by X-ray photoelectron spectroscopy (XPS), RHEED and X-ray absorption spectroscopy (EXAFS) before and after interaction with a pressure of 2.5×10−3   mbar of mercury vapours. Sorbed mercury was oxidised to Hg(II), as revealed by the chemical shift observed on the Hg 4f7/2 levels. Since the adsorption of mercury is completely inhibited when the oxidation products of galena cover the surface, the formation of Hg–S bonding is most likely to occur. The amount of adsorbed mercury at saturation was estimated to be about one monolayer, i.e. one atom of mercury per surface atom of sulphur. A (1×1) structure was observed by RHEED for the longest exposures. The analysis of the EXAFS oscillations at the LIII edge of mercury allowed an estimate of the Hg–S bond length of 2.62 Å and a co-ordination number of 3. A model is proposed for the structure of the chemisorbed layer of mercury.
Keywords: Adsorption of mercury on galena; Oxidation of galena; XPS; RHEED; EXAFS;

This study deals with the characterization of the corrosion films formed on the surfaces of amorphous Mg82−x Ni18Nd x (x=0, 5, 15) alloys in 3% NaCl solution saturated with Mg(OH)2. Hydrogen evolution test revealed that the addition of Nd has a beneficial effect to reduce the dissolution rate of Mg82−x Ni18Nd x alloys. SEM showed that the least severe corrosion occurred for the Mg67Ni18Nd15 alloy which has the highest Nd concentration. XPS revealed that the corrosion film formed on Mg67Ni18Nd15 is the least hydroxidized among the amorphous Mg82−x Ni18Nd x (x=0, 5, 15) alloys. The improved corrosion resistance of Mg67Ni18Nd15 is attributed to the high content of oxidized Nd in the corrosion film preventing the detrimental Cl and CO3 2− anions from penetrating.
Keywords: Corrosion film; Mg82−x Ni18Nd x ; XPS; Amorphous; Oxidized Nd;

Thin film photovoltaic devices based on chalcopyrite absorber layers are important candidates for the large-scale production of economically viable solar cells and modules. The material quality of the polycrystalline absorber layers is critically influenced by the bulk compositional uniformity of these films. In this regard, it has been generally reported that the structural and electronic properties of two-step processed chalcopyrite thin films are dominated by metal losses during the high temperature selenization steps. In this study, precursor films of copper and indium and/or gallium were selenized by different techniques at varying temperatures. The samples were analyzed by X-ray fluorescence (XRF) before and after each reaction step. The generally reported loss of indium and/or gallium during selenization at elevated temperatures was contradicted by XRF measurements (i.e. the amount of the metals remained constant at typical process temperatures). The apparent material losses are explained in terms of a measurement artifact of the widely used electron probe microanalysis (EPMA or EDX) due to metal segregation.
Keywords: X-ray fluorescence; CuInSe2; Semiconductors; Thin films; Photovoltaics;

The growth of vapor-deposited Au on the stepped [5(0 0 1)×(1 1 1)] nickel surface at room temperature was studied by X-ray photoelectron and Auger electron spectroscopy (XPS/XAES) and ultraviolet photoelectron spectroscopy (UPS) for Au coverages ranging from about 0.2 up to 13 Ml. The Au 4f7/2 binding energy is initially lower than that of metallic gold and reaches its bulk value above 4 Ml. Helium II UPS exhibits an initial Au 5d doublet splitting of 1.5 eV, which increases up to the bulk value of 2.7 eV after 13 Ml of gold deposition, as well as an abnormally strong attenuation of the Ni 3d state up to a Au coverage of about 0.5 Ml. The results suggest charge transfer from Ni to Au and AuNi surface alloying in the submonolayer regime.
Keywords: X-ray photoelectron spectroscopy; Ultra-violet photoelectron spectroscopy; Gold; Nickel; Stepped surface; Surface alloying;

The response of tape-casted polycrystalline dense sintered AlN wafers and uniaxial densified AlN powder pellets to pulsed laser radiation (pulse length 7 ns FWHM) is studied at energy fluences over the ablation threshold. The pulse energy is varied between 64 and 1400 mJ/cm2 at wavelengths between 250 and 1000 nm. The ablation yield is quantified by a quadruple mass spectrometer. Dense sintered AlN samples show a linear response upto 400 mJ/cm2 while at fluences over 400 mJ/cm2 a saturation of the ablation yield is noticeable. AlN powder samples, on the other hand, do not show this saturation. Moreover, the total ablation yield on powder samples is significantly higher. These observations supplemented by secondary electron microscopy images have been discussed in terms of sample composition, surface metallization, and ablation plume formation.
Keywords: AlN; Mass spectroscopy; Laser; Sintering;

Influence of sputtering pressure on the structure and properties of ZrO2 films prepared by rf reactive sputtering by Pengtao Gao; L.J. Meng; M.P. dos Santos; V. Teixeira; M. Andritschky (84-90).
Zirconium oxide (ZrO2) films have been prepared by rf reactive magnetron sputtering at different sputtering pressures. It is found that the monoclinic phase is the dominant phase in the films and there is a small fraction of tetragonal phase in the films prepared in the low pressure region. The influence of sputtering pressure on the microstructure, residual stress and optical properties of the films has been studied. The films have been characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and optical spectroscopy.
Keywords: Thin films; ZrO2; Sputtering;

We study the effect of a rare earth (RE) catalyst on the kinetics of gas carburizing of 08F and 8620 steels. Carburizing was conducted in a sealed-quench furnace employing an endothermic atmosphere with and without RE addition at 900°C for different times. These experimental results show that compared with conventional gas carburizing the incorporation of RE leads to an obvious increase in layer depth and carbon (C) concentration profile under the same carburizing condition. Cerium (Ce) existing in the carburizing atmosphere and diffusing into the surface layer does not change the kinetics law of carburized layer growth. In addition, the calculated results show that the reaction constant and the diffusion coefficient of carbon between the gas–solid interface can also be increased during RE carburizing.
Keywords: Rare earth; Catalysis; Endothermic atmosphere; Carburizing; Kinetics;

Adsorption and reaction of NH3 on Ti/Si(1 0 0) by H.L. Siew; M.H. Qiao; C.H. Chew; K.F. Mok; L. Chan; G.Q. Xu (95-102).
The nitridation of ultra-thin Ti films on Si(1 0 0) have been studied using X-ray photoelectron spectroscopy (XPS) in a temperature range of 120–1000 K. Upon ammonia exposure to the multilayer Ti thin films at 120 K, three N 1s peaks at 397.8–398.1, 400.5–400.8 and 402.2–402.6 eV were observed, attributable to NH x (x=1 or 2), molecular NH3 and NH4 δ+, respectively. Annealing of the NH3 saturated Ti/Si(1 0 0) surfaces results in the conversion of the NH x species. This species undergo two different pathways between 300 and 800 K, i.e. further dissociation to N(a) and H(a), and recombing with H(a) to form NH3(g). The atomic N reacts with Ti to yield a stable TiN film that retards significantly the interdiffusion at the Ti/Si interface.
Keywords: Titanium; Si(1 0 0); X-ray photoelectron spectroscopy; Titanium nitride; Ammonia; Nitridation;

Ag33Sb31Se36 thin films were prepared by thermal evaporation technique on glass substrates. X-ray diffraction analysis for the as-deposited films showed that they have amorphous structure. On annealing at 200°C films have a polycrystalline structure. The optical constants n and k of the as-deposited and annealed films have been calculated from optical transmittance and reflectance data in the wavelength range 400–1500 nm using Murmann’s equations. Both n and k are practically independent on the film thickness in the range 182–478 nm. Analysis of the refractive index n yields a low frequency dielectric constant. The optical transitions are found to be allowed indirect for the as-deposited and annealed films, and the corresponding energy gaps increase with increasing annealing temperature.
Keywords: Ag33Sb31Se36 films; As-deposited films; Annealed films;

Mass density determination of thin organosilicon films by X-ray reflectometry by A. van der Lee; S. Roualdes; R. Berjoan; J. Durand (115-121).
The mass density of thin organosilicon films deposited by chemical vapour deposition on silicon (0 0 1) surfaces has been determined by X-ray reflectometry. This method does not give a unique mass density value, but instead a bandwidth of possible values and compositions that are compatible with the experimental data if the layer thickness is not larger than about 350 nm. For thicker films thickness values obtained from ellipsometry data are used. The possible composition ranges are compared with data obtained by X-ray photo electron spectroscopy.
Keywords: Reflectometry; Mass density;

The amount of hydrogen adsorbed on palladium during water formation has been studied by measurements of the rate at which hydrogen permeates a palladium membrane. The water formation and hydrogen permeation rates were measured simultaneously for palladium membranes exposed to a hydrogen–oxygen–argon mixture on one side and to pure argon on the other side. Investigations were carried out for the temperatures 100, 150 and 200°C at a total pressure of 760 Torr. If the rates are plotted as a function of α, where α=p H2 /(p H2 +p O2 ), p H2 and p O2 are the pressures of hydrogen and oxygen at the palladium surface, respectively, it is found that, at 100°C, a peak occurs in the water formation rate at α max≈0.2. The palladium surface is dominated by hydrogen for α>α max and is hydrogen deficient for α<α max. This is consistent with a rate limiting step for the water forming reaction where adsorbed hydrogen is one of the reactants. It is also concluded that the heat of adsorption for hydrogen on the palladium surface is significantly lower than previously derived from experiments made under ultrahigh vacuum conditions.
Keywords: Palladium; Hydrogen; Water formation; Hydrogen permeation;

We propose a method for the calculation of the average size of spherical clusters on a surface, using XPS emission intensity ratios. The method is applied to the case of Cu clusters on Dow Cyclotene 3022, a low permittivity polymer. It gives reasonable particle sizes and number densities, and shows that the clusters coalesce on annealing. TEM photomicrographs confirm the size and sphericity of the clusters formed.
Keywords: Cluster formation; Metallization;

Amorphous oxysulfide thin films MO y S z (M=W, Mo, Ti) XPS characterization: structural and electronic pecularities by J.C Dupin; D Gonbeau; I Martin-Litas; Ph Vinatier; A Levasseur (140-150).
The present paper reports the XPS study of different amorphous oxysulfides thin films MO y S z (M=W, Ti, Mo), prepared by radio frequency magnetron sputtering. It has been shown the coexistence of various environments and formal oxidation numbers for metal atoms. In addition, the observation of several types of sulfur ions has revealed the specific character of such amorphous layers. In order to precise the common features and the differences as a function of the nature of the metal atom, a comparison of the data for the three kinds of thin films has been done.
Keywords: Thin films; Amorphous; Sputtering; X-ray photoelectron spectroscopy;

The coadsorption of methanol and oxygen on Cu(100) has been examined using secondary ion mass spectrometry (SIMS). The study provides a mass-spectrometric observation of the surface intermediates present during adsorption at 130 K, and after heating to 320 K. Isotopically-labelled reagents ( 18 O 2 , CD3OD, CH3OD) have been used to facilitate the interpretation of spectra. The SIMS data indicate that MeOD dissociates at 130 K in the presence of pre-adsorbed 18 O 2 to yield a physisorbed D2O layer, in addition to a methoxy (MeO) intermediate and other products. Spectra obtained at 320 K reveal an enhanced concentration of formate species (predominantly as HC 18 O 16 O ). The principal positive secondary ion associated with the methoxy intermediate was found to be CuCH2O+, rather than the expected CuCH3O+, which was not detected. The observed behaviour is consistent with reaction mechanisms proposed by Wachs and Madix to account for methanol oxidation on O/Cu surfaces [J. Catal. 31 (1982) 243].
Keywords: Secondary ion mass spectrometry; Catalysis; Copper; Oxygen; Methanol;

Recently, it was shown that the as-prepared surfaces of Nd1Ba2Cu3O y (Nd123) single crystals are highly stable in air. In this letter, we report our preliminary results of nanostructure fabrication of pyramids and convex lines through patterning on the surfaces of Nd123 single crystals using an ultrahigh vacuum scanning tunnelling microscope system. This opens up a new door for nanostructure engineering and device fabrications on the surfaces of high temperature superconductors.