HAXPES of protected ITO surfaces at the buried P3HT/ITO interface

The characterization of buried interfaces is difficult and often has to be performed by a post‐processing method where the interface is exposed. Hard energy X‐ray photoelectron spectroscopy offers the ability to tune the X‐ray energy and thereby change the information depth. In this work, an inorganic/organic interface was evaluated, namely the poly(3‐hexylthiophene) (P3HT) interface with indium tin oxide (ITO), with relevance to organic photovoltaic devices. P3HT/ITO buried interfaces were examined using three X‐ray energies where the ITO surface was prepared under different pretreatment conditions. The P3HT film protected the ITO surface from adventitious adsorbents and allowed for sensitivity to the buried ITO surface. Robust peak fitting parameters were obtained to model the O 1 s and In 3d lineshapes. The deconvolution of these lineshapes allowed for the clear identification of a surface layer on the ITO which is oxidized to a greater extent than the underlying bulk ITO. The surface oxide layer, composed of indium oxide and indium hydroxide, is deficient of oxygen vacancies and would therefore be expected to act as an insulating barrier on the ITO surface. Peak fitting conditions allowed for an estimation of the relative thicknesses of this insulating layer. Copyright © 2012 John Wiley & Sons, Ltd.     

Quantification of hard X-ray photoelectron spectroscopy: Calculating relative sensitivity factors for 1.5- to 10-keV photons in any instrument geometry

A method for the rapid determination of theoretical relative sensitivity factors (RSFs) for hard X-ray photoelectron spectroscopy (HAXPES) instruments of any type and photon energy has been developed. We develop empirical functions to describe discrete theoretically calculated values for photoemission cross sections and asymmetry parameters across the photon energy range from 1.5 to 10 keV for all elements from lithium to californium. The formulae describing these parameters, in conjunction with similar practical estimates for inelastic mean free paths, allow the calculation of a full set of theoretical sensitivity factors for a given X-ray photon energy, X-ray polarisation and instrument geometry. We show that the anticipated errors on these RSFs are less than the typical errors generated by extracting X-ray photoelectron spectroscopy (XPS) intensities from the spectra and thus enable adequate quantification for any XPS/HAXPES experiment up to 10 keV. A spreadsheet implementation of this method is provided in the supporting information, along with example RSFs for existing commercial instruments.     

Nondestructive compositional depth profiling using variable‐kinetic energy hard X‐ray photoelectron spectroscopy and maximum entropy regularization

We discuss the calculation of nondestructive compositional depth profiles from regularization of variable kinetic energy hard X‐ray photoelectron spectroscopy (VKE‐XPS) data, adapting techniques developed for angle‐resolved XPS. Simulated TiO₂/Si film structures are analyzed to demonstrate the applicability of regularization techniques to the VKE‐XPS data and to determine the optimum choice of regularization function and the number of data points. We find that using a maximum entropy‐like method, when the initial model/prior thickness is similar to the simulated film thickness, provides the best results for cases where prior knowledge of the sample exists. For the simple structures analyzed, we find that only five kinetic energy spectra are necessary to provide a good fit to the data, although in general, the number of spectra will depend on the sample structure and noisiness of the data. The maximum entropy‐like algorithm is then applied to two physical films of TiO₂ deposited on Si. Results suggest interfacial intermixing. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.     

Improved depth information from routine analysis of the inelastic background of XPS and HAXPES spectra using optimized two- and three-parameter cross-sections

Determination of the depth distribution of complex nanostructures by X-ray photoelectron spectroscopy (XPS) inelastic background analysis may be complicated if the sample materials have widely different inelastic scattering cross-sections. It was recently demonstrated that this may be solved by using a mixture of cross-sections. This permits retrieval of depth distributions of complex stacks and deeply buried layers with a typical 5% accuracy. This requires however that the cross-sections of the individual sample materials are known which is often not the case and this can complicate practical use for routine analysis. In this paper, we explore to what extent a suitable two- or three-parameter cross-section can be defined independent of prior knowledge of the cross-sections involved but simply defined by fitting the cross-section parameters to the spectrum being analyzed. This paper presents a theoretical study following our recent paper that explored how to make the best choice of inelastic mean free path and inelastic scattering cross-section for the inelastic background analysis with the Quases-Tougaard software. It was previously shown that a rough analysis of the inelastic background could give a good idea of the depth distribution. Here, we demonstrate with model spectra from buried layers created with Quases-Tougaard Generate software that a rather accurate analysis can be performed for very different cases with an average ~5% error. This analysis is easy to apply as it only needs the two- or three-parameter cross-sections generated with the Quases-Tougaard software. This study is aimed to improve routine analysis of the inelastic background of XPS and hard X-ray photoelectron spectroscopy (HAXPES) spectra.     

Information depth determination for hard X‐ray photoelectron spectroscopy up to 15 keV photoelectron kinetic energy

In the present work, we have determined the information depth in a solid for hard X‐ray photoelectron spectroscopy (HAXPES) up to a photoelectron kinetic energy of 15 keV. For that, we have followed the evolution of the photoemission signal from different core levels of a gold overlayer grown in situ on a polycrystalline copper substrate as a function of the photoelectron kinetic energy. We demonstrate that in the case of gold, an information depth of 57 nm can be achieved by detecting photoelectrons with 15‐keV kinetic energy. The photoemission signal produced at this depth corresponds to 0.2% of the signal coming from a semi‐infinite solid bulk. Such a high sensitivity can only be reached with the combination of a third‐generation synchrotron radiation beam with a high‐transmission electron analyzer. Copyright © 2008 John Wiley & Sons, Ltd.     

HAXPES beamline PES‐BL14 at the Indus‐2 synchrotron radiation source

The Hard X‐ray Photo‐Electron Spectroscopy (HAXPES) beamline (PES‐BL14), installed at the 1.5 T bending‐magnet port at the Indian synchrotron (Indus‐2), is now available to users. The beamline can be used for X‐ray photo‐emission electron spectroscopy measurements on solid samples. The PES beamline has an excitation energy range from 3 keV to 15 keV for increased bulk sensitivity. An in‐house‐developed double‐crystal monochromator [Si (111)] and a platinum‐coated X‐ray mirror are used for the beam monochromatization and manipulation, respectively. This beamline is equipped with a high‐energy (up to 15 keV) high‐resolution (meV) hemispherical analyzer with a microchannel plate and CCD detector system with SpecsLab Prodigy and CasaXPS software. Additional user facilities include a thin‐film laboratory for sample preparation and a workstation for on‐site data processing. In this article, the design details of the beamline, other facilities and some recent scientific results are described.     

Theoretical study toward rationalizing inelastic background analysis of buried layers in XPS and HAXPES

The approach of inelastic background analysis was previously demonstrated to be a useful tool for retrieving the depth distribution of buried layers with an accuracy, which is better than 5% even for some complex samples. This paper presents a study that attempt at rationalizing the approach by exploring how to make the best choice of the inelastic mean free path and the inelastic scattering cross section, which are the two main input parameters needed in the analysis. To this end, spectra from buried layers were created with Quases-Generate software. The layers consisted of Si 1s recorded at 6099 eV and Au 4d recorded at 1150 eV kinetic energy buried under overlayers of Si, Au, Al, polymer, or Ta. Spectra from samples with a wide range of buried layer thickness and overlayer thickness were created. Subsequently, these spectra were analyzed with Quases-Analyze software and for each case the analysis was done with different combinations of the input parameters. Among these, the best choice for all cases was to use an effective IMFP and effective inelastic scattering cross section with relative weights being half the thickness of the buried layer and the full thickness of the overlayer. This general formula together with a new version of the software makes the inelastic background analysis of buried layers faster and easier to apply even for nonspecialists.     

Expanding the view into complex material systems: From micro-ARPES to nanoscale HAXPES

The analysis of chemical and electronic states in complex and nanostructured material systems requires electron spectroscopy to be carried out with nanometer lateral resolution, i.e. nanospectroscopy. This goal can be achieved by combining a parallel imaging photoelectron emission microscope with a bandpass energy filter. In this contribution we describe selected experiments employing a dedicated spectromicroscope – the NanoESCA. This instrument has a particular emphasis on the spectroscopic aspects and enables laterally resolved photoelectron spectroscopy from the VUV up into the hard X-ray regime.     

Relative sensitivity factors for hard X-ray photoelectron spectroscopy with photon energies of 3.0, 5.9, 7.9, and 9.9 keV

Empirical relative sensitivity factors (RSFs) for the 1s, 2p_3/2, and 3d_5/2 levels relative to O1s were derived from the hard X-ray photoelectron spectroscopy measurements with photon energies of 3.00, 5.95, 7.94, and 9.92 keV. The data for 5.95, 7.94, and 9.92 keV were obtained at BL46XU in SPring-8, and those for 3.00 keV were obtained at BL6N1 in AichiSR (note that the measurement conditions, i.e., electron spectrometer and measurement geometry, at the two beamlines were different; for details, see Section >3). It was found that the empirical RSFs showed the linear behaviors as a function of the binding energy on a log–log plot. Also, from the comparison of the empirical RSFs and the theoretical RSFs calculated from Hartree–Slater cross sections reported by Scofield combined with the energy dependence on the spectrometer function and the inelastic mean free path, it is observed that there is an agreement between the empirical and the theoretical RSF values with several exceptions.