Conventional X-ray Photoelectron Spectroscopy Research Articles

Hard x-ray photoemission study of near-Heusler FexSi1−xalloys

The structural and electronic properties of epitaxial and amorphous Fe${}_{x}$Si${}_{1\ensuremath{-}x}$ alloys with $x$ $=$ 0.72 and 0.67 near the binary Heusler composition of $x$ $=$ 0.75 were determined using hard x-ray photoelectron spectroscopy (HXPS). By performing the measurements at a photon energy of 5950.3 eV, the bulk-sensitivity of the measurement is enhanced by a factor of 4--7 compared to conventional soft x-ray photoelectron spectroscopy at about 1000 keV. HXPS probes, on average, as far as 76 \AA{} into the Fe${}_{x}$Si${}_{1\ensuremath{-}x}$ samples. Via core-level spectra, it is found in the amorphous alloy that, in spite of the disordered structure that could lead to a broad distribution of chemical environments, the Si environment is mostly unique. Valence-band spectra reveal a clear distinction between the contributions of the two inequivalent Fe sites of the most highly ordered ($x$ $=$ 0.72, $D$0${}_{3}$) epitaxial sample. The valence-band spectra are compared to results of fully relativistic coherent potential approximation calculations performed in the framework of the one-step model of photoemission, which reveal details of the atomic-orbital makeup of various features, and generally exhibit good agreement with experiment.

Effects of photoirradiation in UV and VUV regions during plasma exposure to polymers

Interactions between photons irradiated from Ar–O 2 mixture plasmas and polymer surfaces were investigated on the basis of depth analyses of chemical bonding states in the nano-surface layer of polyethylene terephthalate (PET) films via hard X-ray photoelectron spectroscopy (HXPES) and conventional X-ray photoelectron spectroscopy (XPS). The PET films were exposed to photons from the Ar–O 2 mixture plasmas by covering the PET samples with MgF 2 and quartz windows as optical filters for evaluation of photoirradiation effects in ultraviolet (UV) and vacuum ultraviolet (VUV) regions. The HXPES results indicated that the degradation of the chemical bonding states due to photoirradiation in regions was insignificant in deeper regions up to about 50 nm from the surface. Whereas, conventional XPS analysis showed that C O bond, O C O bond and C O bond increased after photoirradiation in UV and VUV regions. These results suggest that the increase in oxygen functionalities (C O bond, O C O bond and C O bond) may be attributed to chemical reactions and/or terminations of scissed bonds via photodecompositions of the polymer with oxygen and/or OH species (oxygen molecules and radicals during plasma exposure and/or oxygen molecules and moisture after taking the PET samples out of the plasma reactor to the ambient air) in the vicinity of the sample surface.

Plasma processing of soft materials for development of flexible devices

Plasma-polymer interactions have been studied as a basis for development of next-generation processing of flexible devices with soft materials by means of low-damage plasma technologies (soft materials processing technologies). In the present article, interactions between argon plasmas and polyethylene terephthalate (PET) films have been examined for investigations of physical damages induced by plasma exposures to the organic material via chemical bonding-structure analyses using hard X-ray photoelectron spectroscopy (HXPES) together with conventional X-ray photoelectron spectroscopy (XPS). The PET film has been selected as a test material for investigations in the present study not merely because of its specific applications, such as a substrate material, but because PET is one of the well defined organic materials containing major components in a variety of functional soft materials; C–C main chain, CH bond, oxygen functionalities (O=C–O bond and C–O bond) and phenyl group. Especially, variations of the phenyl group due to argon plasma exposures have been investigated in the present article in order to examine plasma interactions with π-conjugated system, which is in charge of electronic functions in many of the π-conjugated electronic organic materials to be utilized as functional layer for advanced flexible device formations. The PET films have been exposed to argon plasmas sustained via inductive coupling of RF power with low-inductance antenna modules. The HXPES analyses exhibited that the degradations of the oxygen functionalities and the phenyl group in the deeper regions up to 50 nm from the surface of the samples were insignificant indicating that the bond scission and/or the degradations of the chemical bonding structures due to photoirradiation from the plasma and/or surface heating via plasma exposure were relatively insignificant as compared with damages in the vicinity of the surface layers.

Thickness dependence of the interfacial interaction for the Fe/ZnO (0001) system studied by photoemission

Synchrotron radiation photoemission spectroscopy (SRPES) and conventional X-ray photoelectron spectroscopy (XPS) were used to study the Fe/ZnO(0001 ) interface formation at room temperature. The interaction extent of Fe overlayer during the ZnO substrate was carefully monitored during the increase of Fe coverage, the results showed obvious Fe2+ formation at the initial stage of Fe deposition. Based on the photoemission spectra (PES) changes observed during the deposition of Fe on ZnO up to 3 nm, three meaningful and critical thicknesses have been observed which may be related to the surface charge transport, chemical reaction, and magnetic property, respectively. The new finding may be helpful to the design of related devices based on Fe/ZnO interface.

Study of Water Adsorption on Organics Crystal Surfaces Using a Modified X-ray Photoelectron Spectroscopy Instrument

The coupling of a high partial pressure water vapor system to a conventional X-ray photoelectron spectroscopy instrument for use with both organic powders and single crystals is described. The modified chamber serves a dual purpose as both a treatment and sample preparation apparatus. The design utilizes the high- and cryo-temperature capabilities of the existing system to allow investigation of various states of surface water, as well as the interplay between temperature, water, and surface structure of the substrate. Sample handling and methods for compatibility testing are detailed, and data illustrating the application of the modified and integrated capabilities are summarized, detailing novel studies on the state of water on the surface of model organic pharmaceutical salts. In addition to facilitating investigation of the presence of various states of water at the surface of organic material, namely, dissociated versus molecular water, the setup allows the investigation of structural effects of the surface on the state of water.

High resolution photoemission study of the formation and thermal stability of Mg silicide on silicon

The focus of this study is to use high resolution synchrotron based photoemission to investigate the initial growth mode of magnesium silicide which has been formed by both stepwise and continuous deposition of metallic Mg onto a thermally grown ultra-thin Si oxide surface. The findings suggest that stepwise deposition of Mg initially results in the growth of Mg silicide islands on the surface. Further magnesium deposition leads to the growth of metallic Mg on the surface of these silicide islands, along with the continued growth of silicide species on the uncovered oxide surface. However, it has been shown that continuous deposition of Mg results in considerably less silicide growth. The thermal stability of Mg silicide and a mechanism for high temperature silicide growth have also been studied using conventional X-ray photoelectron spectroscopy. The results suggest that the presence of oxidised Si acts as a barrier to Si diffusion during vacuum annealing, hence preventing the growth of further Mg silicide. It has also been shown that metallic Mg desorbs from the surface below 300°C, while Mg silicide is not stable at temperatures above 500°C in contrast to other metal silicides.

X-ray photoelectron spectroscopy for analysis of plasma–polymer interactions in Ar plasmas sustained via RF inductive coupling with low-inductance antenna units

Plasma–polymer interactions have been investigated on the basis of hard X-ray photoelectron spectroscopy (HXPES) together with conventional X-ray photoelectron spectroscopy (XPS) for analysis of chemical bonding states in the surface nano-layers of polymethylmethacrylate (PMMA) films, which were exposed to argon plasmas sustained via RF inductive coupling with multiple low-inductance antenna units. The PMMA films were exposed to argon plasmas on a water-cooled substrate holder. Average ion energies bombarding onto PMMA films were varied in the range of 6–16 eV, which were evaluated as ion kinetic energies at the sheath edge to the ground potential using a mass-separated ion-energy analyzer. The etching of PMMA surface after Ar plasma exposure with an ion dose of 3.4 × 10 18 ions/cm 2 was measured to be insignificant (less than 20 nm). Surface roughness of PMMA slightly increased from 0.3 nm to 0.4 nm with increasing ion bombardment energy from 6 eV to 16 eV. HXPES was carried out together with conventional XPS to examine chemical bonding states of the PMMA surface in deeper regions (about 54 nm) with HXPES as compared to those observed in shallower regions (8 nm) with the conventional XPS.

Interaction of cuprite with dialkyl dithiophosphates

The interaction of freshly abraded surfaces of cuprite, Cu 2O, with neutral or mildly alkaline aqueous solutions of diethyl or di- n-butyl dithiophosphate (DTP) has been investigated by means of conventional and synchrotron X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. It was confirmed that DTP adsorbs readily on Cu atoms in the surface layer of the mineral treated with solutions of the collector at pH values near 7 and 9 in the presence of air, and renders the surface hydrophobic. When cuprite is treated with relatively high concentrations of DTP for sufficiently long periods, collector can also be adsorbed as CuDTP, but the coverage does not exceed a thin layer of CuDTP on the adsorbed DTP monolayer, unlike the situation with Cu metal or chalcocite where a thick multilayer can be formed.

Photoemission studies of the initial interface formation of ultrathin MgO dielectric layers on the Si(111) surface

This study investigates the interface formation between a magnesium oxide dielectric overlayer and an ultrathin SiO 2 layer (∼ 0.3 nm) grown on the atomically clean p-type Si(111) surface in ultra high vacuum. Both soft X-ray synchrotron radiation based photoemission and conventional X-ray photoelectron spectroscopy have been used to characterise the evolution of the interface and monitor the change in the interfacial oxide thickness. As the MgO film grows, there is an increase in the intensity of the silicon oxide features indicating the growth of the interfacial oxide which saturates at a thickness of approximately 0.7 nm. Spectra acquired at the surface sensitive 130 eV photon energy, reveal the emergence of a chemically shifted component on the low binding energy side of the substrate peak which is attributed to atomic displacement of silicon atoms from the substrate to the interfacial oxide at room temperature. This evidence of atomic disruption at the high dielectric constant material (high-κ) and silicon interface would be expected to contribute to charge carrier scattering mechanisms in the silicon and could account for the generally observed mobility degradation in high-κ stacks. Thermal annealing studies of deposited MgO films show that dissociation begins to occur above 600 °C with desorption of Mg and the growth of a silicon oxide.

Probing the interface ofFe3O4/GaAsthin films by hard x-ray photoelectron spectroscopy

Magnetite (Fe3O4) thin films on GaAs have been studied with HArd X-ray PhotoElectron Spectroscopy (HAXPES) and low-energy electron diffraction. Films prepared under different growth conditions are compared with respect to stoichiometry, oxidation, and chemical nature. Employing the considerably enhanced probing depth of HAXPES as compared to conventional x-ray photoelectron spectroscopy (XPS) allows us to investigate the chemical state of the film-substrate interfaces. The degree of oxidation and intermixing at the interface are dependent on the applied growth conditions; in particular, we found that metallic Fe, As2O3, and Ga2O3 exist at the interface. These interface phases might be detrimental for spin injection from magnetite into GaAs.

Electronic environments in carrollite, CuCo 2S 4, determined by soft X-ray photoelectron and absorption spectroscopy

Fracture surfaces of a natural carrollite specimen have been characterised by synchrotron and conventional X-ray photoelectron spectroscopy and near-edge X-ray absorption spectroscopy. For the synchrotron X-ray measurements, the mineral surfaces were prepared under clean ultra high vacuum and were unoxidised. The characterisation was undertaken primarily to establish unequivocally the oxidation state of the Cu in the mineral, but also to obtain information on the electronic environments of the Co and S, and on the surface species. Experimental and simulated Cu L 2,3-edge absorption spectra confirmed an oxidation state of Cu I, while Co 2 p photoelectron and Co L 2,3 absorption spectra were largely consistent with the Co III established previously by nuclear magnetic resonance spectroscopy. S 2 p photoelectron spectra provided no evidence for S to be present in the bulk in more than one state, and were consistent with an oxidation state slightly less negative than S –II. Therefore it was concluded that carrollite can be best represented by Cu ICo III 2(S 4) –VII. The Cu I oxidation state is in agreement with that expected for Cu tetrahedrally coordinated by S, but is in disagreement with the Cu II deduced previously from some magnetic, magnetic resonance and Cu L-edge X-ray absorption spectroscopic measurements. A significant concentration of S species with core electron binding energies both lower and higher than the bulk value were formed at fracture surfaces, and these entities were assigned to monomeric and oligomeric surface S species. The density of Cu d states calculated for carrollite differed from that previously reported but was consistent with the observed Cu L 3 X-ray absorption spectrum. The initial oxidation of carrollite in air under ambient conditions was confirmed to be congruent, unlike the incongruent reaction undergone by a number of non-thiospinel sulfide minerals.

Effect of Oxide Charge Trapping on X-ray Photoelectron Spectroscopy of HfO2/SiO2/Si Structures

We examined the effects of interfacial SiO2 layers and a surface metal layer on the photoelectron spectra of HfO2/SiO2/Si structures by hard X-ray photoemission spectroscopy with synchrotron radiation as well as conventional X-ray photoelectron spectroscopy (XPS). The Hf 4f and Hf 3d photoelectron peaks broadened and shifted toward a higher binding energy with increasing thickness of the interfacial SiO2 layer, even though photoelectrons may have been emitted from the HfO2 layer with the same chemical composition. Thinning the interfacial Si oxide layer to approximately one monolayer and depositing a metal layer on the HfO2 surface suppressed these phenomena. The O 1s photoelectron spectra revealed marked differences between the metal- and nonmetal-deposited HfO2/SiO2/Si structures; HfO2 and SiO2 components in the O 1s photoelectron spectra for the metal-deposited structures were observed at reasonably separated binding energies, but those for the nonmetal-deposited structures were not separated clearly. From this behavior concerning the effects of interfacial SiO2 and surface metal layers, we concluded that the Hf 4f, Hf 3d, and O 1s spectra measured from the HfO2/SiO2/Si structures did not reflect actual chemical bonding states. We consider that potential variations in the HfO2 film owing to charge trapping strongly affect the measured photoelectron spectra. On the basis of angle-resolved XPS measurements, we propose that positive charges are trapped at the HfO2 surface and negative charges are trapped inside the HfO2 layer.

Practical expressions for the mean escape depth, the information depth, and the effective attenuation length in Auger-electron spectroscopy and x-ray photoelectron spectroscopy

The authors report new calculations of mean escape depths (MEDs), information depths (IDs), and effective attenuation lengths (EALs) for 16 photoelectron lines and 9 Auger-electron lines of five elemental solids (Si, Cu, Ag, W, and Au) and four inorganic compounds (ZrO2, ZrSiO4, HfO2, and HfSiO4). These calculations were made to update similar previous calculations with improved data for the transport mean free path (TMFP) that are now available. Ratios of averages of the new MEDs, IDs, and EALs to the inelastic mean free path for electron emission angles between 0° and 50° varied linearly with the single-scattering albedo, a simple function of the inelastic mean free path and TMFP. The slopes of the linear relations depend only weakly on the atomic potential used in calculations of differential elastic-scattering cross sections (from which TMFPs are derived). The new linear relations are simple practical expressions for determining the MED, ID, and EAL for any solid in conventional Auger electron spectroscopy and x-ray photoelectron spectroscopy.

Infrared Surface Analysis Using a Newly Developed Thin-Sample Preparation System

We developed a new sampling system, the Nano Catcher, for measuring the surface chemical structure of polymers or industrial products and we evaluated the performance of the system. The system can directly pick up surface species whose depth is on the order of approximately 100 nm and can easily provide a sample for a Fourier transform infrared (FT-IR) system without the necessity of passing it over to a measurement plate. The FT-IR reflection data obtained from the Nano Catcher were compared with those obtained using the attenuated total reflection (ATR) method and sampling by hand. Chemical structural analysis of a depth region from a few tens of nanometers to a few hundred nanometers can be directly performed using this system. Such depths are beyond the scope of conventional X-ray photoelectron spectroscopy (XPS) and ATR methods. We can expect the use of the Nano Catcher system to lead to a great improvement in the detection of signals of surface species in these depth regions.

Cerium oxide stoichiometry alteration via Sn deposition: Influence of temperature

Cerium oxide layers grown on Cu(1 1 1) were studied by conventional X-ray and resonant photoelectron spectroscopy with synchrotron radiation. A quantitative method of determining the cerium chemical state from the Ce 3d photoelectron spectra is described in detail. After the preparation of the ceria layer, Sn films of different thickness were evaporated onto the surface at temperatures of 120, 300 and 520 K. In all three cases, the deposited Sn was oxidized, CeO 2 was partially reduced, and a mixed Sn–Ce–O oxide was formed. The quantitative extent of these reactions was found to be determined by limited diffusion of the deposited Sn atoms into the ceria layer at low temperature. The excess of tin formed a metallic overlayer on the sample surface.

In situ x-ray photoelectron spectroscopy studies of water on metals and oxides at ambientconditions

X-ray photoelectron spectroscopy (XPS) is a powerful tool for surface and interfaceanalysis, providing the elemental composition of surfaces and the local chemicalenvironment of adsorbed species. Conventional XPS experiments have beenlimited to ultrahigh vacuum (UHV) conditions due to a short mean free path ofelectrons in a gas phase. The recent advances in instrumentation coupled withthird-generation synchrotron radiation sources enables in situ XPS measurements atpressures above 5 Torr. In this paper, we describe the basic design of the ambientpressure XPS setup that combines differential pumping with an electrostaticfocusing. We present examples of the application of in situ XPS to studies ofwater adsorption on the surface of metals and oxides including Cu(110), Cu(111),TiO2(110) under environmental conditions of water vapor pressure. On all these surfaces we observe ageneral trend where hydroxyl groups form first, followed by molecular water adsorption.The importance of surface OH groups and their hydrogen bonding to water molecules inwater adsorption on surfaces is discussed in detail.

Photoemission studies of the interface formation of ultrathin MgO dielectric layers on the oxidised Si(111) surface

Magnesium oxide (MgO) has been proposed as a medium-k dielectric material for integration into advanced transistor fabrication. In this study, soft x-ray synchrotron radiation based photoemission and conventional x-ray photoelectron spectroscopy have been used to characterise the evolution of the MgO/Si(111) interface as an ultrathin film is been grown. The MgO film was grown by thermally depositing magnesium in an oxygen partial pressure at room temperature. The Mg2p peak profile indicates that the magnesium exists in a single chemical state throughout the deposition sequence with a binding energy indicative of MgO. The initial submonolayer deposition resulted in the Si2p peak shifting 0.3eV towards higher binding energy consistent with electron transfer between the deposited magnesium and the substrate. This is accompanied by an increase in the intensity of the Si oxidation states indicating growth of the interfacial oxide thickness. As the MgO film thickness increases, the Fermi level moves back to within 0.1eV of its original midgap position. Subsequent XPS measurements show that the saturation thickness of the interfacial silicon oxide layer is less than 0.7nm. The valence band offset has been measured and the conduction band offset has been deduced by estimating the bandgap of the thin dielectric film. The valence band offset between the thin MgO layer and the silicon substrate is 4.1eV. The MgO workfunction was 2.8eV and assuming an electron affinity value of 0.85eV, the bandgap of this film is 6.72eV, smaller than that reported for bulk MgO. The calculated conduction band offset is 1.5eV which should be a sufficiently high barrier to minimise leakage currents.

Surface depth analysis for fluorinated block copolymer films by X-ray photoelectron spectroscopy using C 60 cluster ion beam

X-ray photoelectron spectroscopy (XPS) using fullerene (C 60) cluster ion bombardment was applied to films of a fluorinated block copolymer. Spectra so obtained were essentially different from those using Ar ion beam. Structure in the surface region with the depth down to 60 nm drawn on the basis of XPS with C 60 beam was essentially the same as the one drawn by the result using dynamic secondary ion mass spectrometry, which is a well-established method for the depth analysis of polymers. This implies that XPS using C 60 beam enables one to gain access to the depth analysis of structure in polymer films with the depth range over the analytical depth of conventional XPS, that is, three times inelastic mean-free path of photoelectrons.

Enargite oxidation: A review

Enargite, Cu 3AsS 4, is common in some deposit types, e.g. porphyry systems and high sulphidation epithermal deposits. It is of environmental concern as a potential source of arsenic. In this communication, we review the current knowledge of enargite oxidation, based on the existing literature and our own original data. Explicit descriptions of enargite oxidation in natural environments are scarce. The most common oxidized alteration mineral of enargite is probably scorodite, FeAsO 4.2H 2O, with iron provided most likely by pyrite, a phase almost ubiquitously associated with enargite. Other secondary minerals after enargite include arsenates such as chenevixite, Cu 2Fe 2(AsO 4) 2(OH) 4.H 2O, and ceruleite, Cu 2Al 7(AsO 4) 4.11.5H 2O, and sulphates such as brochantite, Cu 4(SO 4)(OH) 6, and posnjakite, Cu 4(SO 4)(OH) 6·H 2O. Detailed studies of enargite field alteration at Furtei, Sardinia, suggest that most alteration occurs through dissolution, as testified by the appearance of etch pits at the surface of enargite crystals. However, apparent replacement by scorodite and cuprian melanterite was observed. Bulk oxidation of enargite in air is a very slow process. However, X-ray photoelectron spectroscopy (XPS) reveals subtle surface changes. From synchrotron-based XPS it was suggested that surface As atoms react very fast, presumably by forming bonds with oxygen. Conventional XPS shows the formation, on aged samples, of a nanometer-size alteration layer with an appreciably distinct composition with respect to the bulk. Mechanical activation considerably increases enargite reactivity. In laboratory experiments at acidic to neutral pH, enargite oxidation/dissolution is slow, although it is accelerated by the presence of ferric iron and/or bacteria such as Acidithiobacillus ferrooxidans and Sulfolobus BC. In the presence of sulphuric acid and ferric iron, the reaction involves dissolution of Cu and formation of native sulphur, subsequently partly oxidized to sulphate. At alkaline pH, the reactivity of enargite is apparently slightly greater. XPS spectra of surfaces conditioned at pH 11 have been interpreted as evidence of formation of a number of surface species, including cupric oxide and arsenic oxide. Treatment with hypochlorite solutions at pH 12.5 quickly produces a coating of cupric oxide. Electrochemical oxidation of enargite typically involves low current densities, confirming that the oxidation process is slow. Important surface changes occur only at high applied potentials, e.g. + 0.74 V vs. SHE. It is confirmed that, at acidic pH, the dominant process is Cu dissolution, accompanied (at + 0.56 V vs. SHE, pH = 1) by formation of native sulphur. At alkaline pH, a number of surface products have been suggested, including copper and arsenic oxides, and copper arsenates. XPS studies of the reacted surfaces demonstrate the evolution of Cu from the monovalent to the divalent state, the formation of As–O bonds, and the oxidation of sulphur to polysulphide, sulphite and eventually sulphate. In most natural and quasi-natural (mining) situations, it is expected that enargite reactivity will be slow. Moreover, it is likely that the release of arsenic will be further slowed down by at least temporary trapping in secondary phases. Therefore, an adequate management of exposed surfaces and wastes should minimize the environmental impact of enargite-bearing deposits. In spite of an increasing body of data, there are several gaps in our knowledge of enargite oxidation. The exact nature of most mechanisms and products remains poorly constrained, and there is a lack of quantitative data on the dependence on parameters such as pH and dissolved oxygen.

Module to guide the expert use of x-ray photoelectron spectroscopy by corrosion scientists

This contribution, to the potential development of data systems having some degree of “expert” character for use in x-ray photoelectron spectroscopy (XPS), illustrates the manner in which models of “Rules” might be developed by the user community. The field of corrosion science is taken as an example of one community of researchers who make regular use of XPS for well defined needs. These “needs” are redefined as a series of Goals that have to be reached in order to characterize the surface in terms of layer sequences and the enrichment of given elements within them. Rules are written to allow a structured approach to achieve each Goal. A feature of this set of Rules is that they are designed expressly to allow automated interpretation of the survey scan. This approach is facilitated by the use of a recommendation that the survey spectrum be acquired as a series of accumulated scans instead of the usual approach of making a single scan through the spectrum. Repeat scans enable the information extracted by the operation of the Rules to be processed and displayed for information during the period that is normally used for the survey scan. It is intended that this information will inform the setting up of any subsequent high resolution scans and their interactive interpretation. It will also inform any future operations such as ion etching or angle-resolved measurements. In some cases, the information made available may be all that is required by the user and in this case the “expert module” approach becomes particularly cost effective. The operation of the rules is illustrated throughout by an examination of data obtained for passivated stainless steel, giving a data set of measurements, typical of those made by corrosion scientists, that can be compared with the literature values obtained by more conventional XPS interpretation.