Abstract
Chemical state analysis in X-ray photoelectron spectroscopy (XPS) relies on assigning well-defined binding energy values to core level electrons originating from atoms in particular bonding ...configurations. Here, we present direct evidence for the violation of this paradigm. It is shown that the C 1s peak due to C–C/C–H bonded atoms from adventitious carbon (AdC) layers accumulating on Al and Au foils splits into two distinctly different contributions, as a result of vacuum level alignment at the AdC/foil interface. The phenomenon is observed while simultaneously recording the spectrum from two metal foils in electric contact with each other. This finding exposes fundamental problems with the reliability of reported XPS data as C 1s peak of AdC is routinely used for binding energy scale referencing. The use of adventitious carbon in XPS should thus be discontinued as it leads to nonsense results. Consequently, ISO and ASTM charge referencing guides need to be rewritten.
With more than 9000 papers published annually, X-ray photoelectron spectroscopy (XPS) is an indispensable technique in modern surface and materials science for the determination of chemical bonding. ...The accuracy of chemical-state determination relies, however, on a trustworthy calibration of the binding energy (BE) scale, which is a nontrivial task due to the lack of an internal BE reference. One approach, proposed in the early days of XPS, employs the C 1s spectra of an adventitious carbon layer, which is present on all surfaces exposed to air. Despite accumulating criticism, pointing to the unknown origin and composition of the adventitious carbon, this is by far the most commonly used method today for all types of samples, not necessarily electrically insulating. Alarmingly, as revealed by our survey of recent XPS literature, the calibration procedure based on the C 1s peak of adventitious carbon is highly arbitrary, which results in incorrect spectral interpretation, contradictory results, and generates a large spread in reported BE values for elements even present in the same chemical state. The purpose of this review is to critically evaluate the status quo of XPS with a historical perspective, provide the technique's operating principles, resolve myths associated with C 1s referencing, and offer a comprehensive account of recent findings. Owing to the huge volume of XPS literature produced each year, the consequences of improper referencing are dramatic. Our intention is to promote awareness within a growing XPS community as to the problems reported over the last six decades and present a guide with best practice for using the C 1s BE referencing method.
•We analyze AdC layers on metals, nitrides, carbides, borides, oxides, and oxynitrides.•BE of C 1s peak EBF varies from 284.08 to 286.74 eV depending on the substrate.•EBF of C 1s peak correlates to ...the sample work function ϕSA: EBF+ϕSA = constant.•Electronic levels of the AdC layer align to the vacuum level.•Complementary measurement of ϕSA is necessary for BE referencing based on C 1s peak.
The accuracy of chemical-state determination by x-ray photoelectron spectroscopy (XPS) used in contemporary advanced materials research relies on a trustworthy binding energy (BE) referencing method. The C 1s peak corresponding to CC/CH bonds of adventitious carbon (AdC), present on a majority of air-exposed samples, is most commonly employed for this purpose, irrespective of whether samples are electrically conducting or not. Contrary to conventional practice, which takes the BE of C 1s peak of AdC as a constant, we find that the C 1s peak position EBF varies over an alarmingly large range, from 284.08 to 286.74 eV, depending on the substrate, for nearly a hundred predominantly thin-film samples comprising metals, nitrides, carbides, borides, oxides, and oxynitrides. Our consistent measurements also show that, independent of materials system, EBF of the C 1s peak is closely correlated to the sample work function ϕSA, such that the sum EBF+ϕSA is constant, indicating that the electronic levels of the AdC layer align to the vacuum level, rather than to the Fermi level as commonly assumed. This phenomenon can be understood given that the AdC layer is not an inherent part of the analyzed sample and that the interaction to the substrate is weak, showing in that a common Fermi level is not established at the interface. Thus, a straightforward complementary measurement of ϕSA enables using the C 1s peak of AdC for the purpose of BE–scale calibration for samples exhibiting decent electrical conductivity. This new practice resolves problems associated with the conventional method and allows for more reliable bonding assignments. It is thus advisable that both ASTM and ISO XPS referencing guides relying on the use of AdC should be reviewed.
•We present first self-consistent model of TiN core level spectra with a cross-peak qualitative and quantitative agreement.•Model is tested for a series of TiN thin films oxidized to different extent ...by varying the venting temperature.•Conventional deconvolution process relies on reference binding energies that typically show large spread introducing ambiguity.•By imposing requirement of quantitative cross-peak self-consistency reliability of extracted chemical information is enhanced.•We propose that the cross-peak self-consistency should be a prerequisite for reliable XPS peak modelling.
We present first self-consistent modelling of x-ray photoelectron spectroscopy (XPS) Ti 2p, N 1s, O 1s, and C 1s core level spectra with a cross-peak quantitative agreement for a series of TiN thin films grown by dc magnetron sputtering and oxidized to different extent by varying the venting temperature Tv of the vacuum chamber before removing the deposited samples. So-obtained film series constitute a model case for XPS application studies, where certain degree of atmosphere exposure during sample transfer to the XPS instrument is unavoidable. The challenge is to extract information about surface chemistry without invoking destructive pre-cleaning with noble gas ions. All TiN surfaces are thus analyzed in the as-received state by XPS using monochromatic Al Kα radiation (hν=1486.6eV). Details of line shapes and relative peak areas obtained from deconvolution of the reference Ti 2p and N 1s spectra representative of a native TiN surface serve as an input to model complex core level signals from air-exposed surfaces, where contributions from oxides and oxynitrides make the task very challenging considering the influence of the whole deposition process at hand. The essential part of the presented approach is that the deconvolution process is not only guided by the comparison to the reference binding energy values that often show large spread, but in order to increase reliability of the extracted chemical information the requirement for both qualitative and quantitative self-consistency between component peaks belonging to the same chemical species is imposed across all core-level spectra (including often neglected O 1s and C 1s signals). The relative ratios between contributions from different chemical species vary as a function of Tv presenting a self-consistency check for our model. We propose that the cross-peak self-consistency should be a prerequisite for reliable XPS peak modelling as it enhances credibility of obtained chemical information, while relying entirely on reference binding energy values introduces large ambiguity.
The resistance to high-temperature oxidation of Ti1-xAlxN films determines performance in numerous applications including coated cutting tools. Here, we present a comprehensive study covering ...Ti1-xAlxN films with 0 ≤ x ≤ 0.83 annealed in air for 1 h at temperatures Ta ranging from 500 to 800 °C. Layers are grown by the combination of high-power impulse and dc magnetron sputtering (HiPIMS/DCMS) in Ar/N2 atmospheres. We use X-ray photoelectron spectroscopy to study the evolution of surface chemistry and to reconstruct elemental distribution profiles. No dependence of oxidation process on the phase content, average grain size, or preferred orientation could be confirmed, to the accuracy offered by the employed X-ray diffraction techniques. Instead, our results show that, under the applied test conditions, the Ti1-xAlxN oxidation scenario depends on both x and Ta. The common notion of double-layer Al2O3/TiO2 oxide formation is valid only in a limited region of the x-Ta parameter space (Type-1 oxidation). Outside this range, a mixed and non-conformal Al2O3-TiO2 layer forms, characterized by larger oxide thickness (Type-2 oxidation). The clear distinction between different Ti1-xAlxN oxidation scenarios revealed here is essential for numerous applications that can benefit from optimizing the Al content, while targeting a given operational temperature range.
•Oxidation resistance of Ti1-xAlxN films with 0 ≤ x ≤ 0.83 is studied by XPS.•Al content has a crucial influence on the Ti1-xAlxN oxidation scenario.•Double-layer stoichiometric Al2O3/TiO2 oxide forms only in a limited range of x and anneal temperatures.•Outside this region, non-conformal Al2O3-TiO2 layer grows, characterized by larger oxide thickness.•This finding is essential for applications that can benefit from optimizing the Al content.
•First non-destructive measurements of XPS core level binding energies for group IVb-VIb transition metal nitrides are presented.•All films are grown under the same conditions and analyzed in the ...same instrument, providing a useful reference for future XPS studies.•Extracted core level BE values are more reliable than those obtained from sputter-cleaned N-deficient surfaces.•Comparison to Ar+-etched surfaces reveals that even mild etching conditions result in the formation of a nitrogen-deficient surface layer.•The N/metal concentration ratios from capped samples are found to be 25-90% higher than those from the corresponding ion-etched surfaces.
We present the first measurements of x-ray photoelectron spectroscopy (XPS) core level binding energies (BE:s) for the widely-applicable group IVb-VIb polycrystalline transition metal nitrides (TMN’s) TiN, VN, CrN, ZrN, NbN, MoN, HfN, TaN, and WN as well as AlN and SiN, which are common components in the TMN-based alloy systems. Nitride thin film samples were grown at 400°C by reactive dc magnetron sputtering from elemental targets in Ar/N2 atmosphere. For XPS measurements, layers are either (i) Ar+ ion-etched to remove surface oxides resulting from the air exposure during sample transfer from the growth chamber into the XPS system, or (ii) in situ capped with a few nm thick Cr or W overlayers in the deposition system prior to air-exposure and loading into the XPS instrument. Film elemental composition and phase content is thoroughly characterized with time-of-flight elastic recoil detection analysis (ToF-E ERDA), Rutherford backscattering spectrometry (RBS), and x-ray diffraction. High energy resolution core level XPS spectra acquired with monochromatic Al Kα radiation on the ISO-calibrated instrument reveal that even mild etching conditions result in the formation of a nitrogen-deficient surface layer that substantially affects the extracted binding energy values. These spectra-modifying effects of Ar+ ion bombardment increase with increasing the metal atom mass due to an increasing nitrogen-to-metal sputter yield ratio.
The superior quality of the XPS spectra obtained in a non-destructive way from capped TMN films is evident from that numerous metal peaks, including Ti 2p, V 2p, Zr 3d, and Hf 4f, exhibit pronounced satellite features, in agreement with previously published spectra from layers grown and analyzed in situ. In addition, the N/metal concentration ratios are found to be 25–90% higher than those obtained from the corresponding ion-etched surfaces, and in most cases agree very well with the RBS and ToF-E ERDA values. The N 1s BE:s extracted from capped TMN films, thus characteristic of a native surface, show a systematic trend, which contrasts with the large BE spread of literature “reference” values.
Hence, non-destructive core level XPS employing capping layers provides an opportunity to obtain high-quality spectra, characteristic of virgin in situ grown and analyzed TMN films, although with larger versatility, and allows for extracting core level BE values that are more reliable than those obtained from sputter-cleaned N-deficient surfaces. Results presented here, recorded from a consistent set of binary TMN’s grown under the same conditions and analyzed in the same instrument, provide a useful reference for future XPS studies of multinary materials systems allowing for true deconvolution of complex core level spectra.
We review results on the growth of metastable Ti1−xAlxN alloy films by hybrid high-power pulsed and dc magnetron co-sputtering (HIPIMS/DCMS) using the time domain to apply substrate bias either in ...synchronous with the entire HIPIMS pulse or just the metal-rich portion of the pulse in mixed Ar/N2 discharges. Depending upon which elemental target, Ti or Al, is powered by HIPIMS, distinctly different film-growth kinetic pathways are observed due to charge and mass differences in the metal-ion fluxes incident at the growth surface. Al+ ion irradiation during Al–HIPIMS/Ti–DCMS at 500°C, with a negative substrate bias Vs=60V synchronized to the HIPIMS pulse (thus suppressing Ar+ ion irradiation due to DCMS), leads to single-phase NaCl-structure Ti1−xAlxN films (x≤0.60) with high hardness (>30GPa with x>0.55) and low stress (0.2–0.8GPa compressive). Ar+ ion bombardment can be further suppressed in favor of predominantly Al+ ion irradiation by synchronizing the substrate bias to only the metal-ion-rich portion of the Al–HIPIMS pulse. In distinct contrast, Ti–HIPIMS/Al–DCMS Ti1−xAlxN layers grown with Ti+/Ti2+ metal ion irradiation and the same HIPIMS-synchronized Vs value, are two-phase mixtures, NaCl-structure Ti1−xAlxN plus wurtzite AlN, exhibiting low hardness (≃18GPa) with high compressive stresses, up to −3.5GPa. In both cases, film properties are controlled by the average metal-ion momentum per deposited atom transferred to the film surface. During Ti–HIPIMS, the growing film is subjected to an intense flux of doubly-ionized Ti2+, while Al2+ irradiation is insignificant during Al–HIPIMS. This asymmetry is decisive since the critical limit for precipitation of w-AlN, 135 eV-amu1/2, is easily exceeded during Ti–HIPIMS, even with no intentional bias. The high Ti2+ ion flux is primarily due to the second ionization potential (IP2) of Ti being lower than the first IP (IP1) of Ar. New results involving the HIPIMS growth of metastable Ti1−xAlxN alloy films from segmented TiAl targets are consistent with the above conclusions.
•Ti1−xAlxN alloys with high hardness and low residual stress are demonstrated.•Hybrid HIPIMS/DCMS approach with opposing metal targets is used.•Film growth pathways depend upon which metal target is powered by HIPIMS.•Al–HIPIMS/Ti–DCMS alloys have a much higher solid-solubility limit, xmax=0.64.•Low mass and single charge of the Al+ ion permit tuning properties of metastable cubic Ti0.38Al0.62N by adjusting Vs.
The high-temperature oxidation resistance of Ti1-xSixN films with Si content varying in wide range, 0.13 ≤ x ≤ 0.91, is evaluated. Films are grown in Ar/N2 atmospheres using a hybrid high-power ...impulse and dc magnetron sputtering (HiPIMS/DCMS) configuration with Si target powered by HiPIMS and Ti target operated in DCMS mode. The substrate bias is synchronized to the Si+-rich portions of the HiPIMS pulses in order to promote solid solution formation. A combination of X-ray photoelectron spectroscopy, elastic recoil detection analysis, and cross-sectional scanning electron microscopy reveals a sharp increase in the oxidation resistance for layers with x > 0.50. The thickness of the oxide layer, following 1 h anneal at 800 °C in air, is in the range 150–200 nm for 0.13 ≤ x ≤ 0.50 and decreases to only 4 nm with x = 0.91, which is ~30 times lower than for the best performing Ti1-xAlxN film (x = 0.64) tested under the same conditions. The oxide forming on top of Ti1-xSixN films with x ≥ 0.41 consists of a SiO2-TiO2 two-phase mixture with a molar ratio given by Si/Ti ratio. In Ti1-xSixN layers with x ≤ 0.31, the presence of grain boundaries, which act as diffusion paths facilitates Si diffusion towards the bulk of the film resulting in that TiO2, the thermodynamically more stable oxide, terminates the surface. Ti0.09Si0.91N films, are essentially unaffected by the anneal and exhibit a hardness of 23 GPa, which is ~30% higher than for the reference SiNz film. Moreover, we demonstrate that 25 nm thick Ti0.09Si0.91N capping layers successfully prevent Ti0.36Al0.64N oxidation at 800 °C. Such approach provides superior oxidation protection compared to alloying TiAlN with Si. Our results suggest that multilayers including nm thin layers of high Si-content TiSiN is a most effective approach to improve high-temperature oxidation resistance of functional ceramic thin films.
•Oxidation resistance of Ti1-xSixN films with 0.13 ≤ x ≤ 0.91 is studied.•Films are annealed for 1 h at 800 °C in air.•The oxide layer thickness dO decreases rapidly for x > 0.50.•dO = 4 nm with x = 0.91, which is ~30 times lower than for TiAlN reference•25 nm thick Ti0.09Si0.91N capping layers prevent Ti0.36Al0.64N oxidation at 800 °C.
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