| AVS 64th International Symposium & Exhibition | |
| Applied Surface Science Division | Tuesday Sessions |
| Session AS+TF-TuA |
| Session: | Problem Solving Using Surface Analysis in the Industrial Laboratory |
| Presenter: | Grzegorz Greczynski, Linköping University, Sweden |
| Authors: | G. Greczynski, Linköping University, Sweden L. Hultman, Linköping University, Sweden |
| Correspondent: | Click to Email |
X-ray photoelectron spectroscopy (XPS) is an irreplaceable analytical tool in materials research providing information about surface chemistry and composition. The unambiguous bonding assignment relies, however, on the correct measurement of binding energy (BE) values, which is often a nontrivial task due to the lack of an internal BE reference. C 1s signal from ubiquitous carbon contamination on samples forming during air exposure, so called adventitious carbon (AdC) layers, is the most common BE reference in XPS studies. Our literature review shows that in 58% of the top-cited papers dealing with XPS studies of magnetron sputtered films published between 2010 and 2016 in peer-reviewed journals, C 1s of AdC was used as a BE reference, while, alarmingly, the remaining papers lack information about any referencing method used. Within the first group, C 1s peak was set quite arbitrary at the BE varying from 284.0 to 285.2 eV. This serious inconsistency contradicts the very notion of a BE reference, which per definition should be connected with one single-energy value.
In this work, we examine the reliability of using AdC for XPS BE referencing by measuring the position of C 1s peak for a series of transition metal (TM) nitride thin film layers that exhibit a well-defined Fermi edge cut-off serving as an internal BE reference. Measurements are performed as a function of the AdC layer thickness, which scales with the air exposure time. We show that the BE of C 1s peak EFB varies by as much as 1.44 eV, depending on the underlying (TM)N.[i] This is a factor of ten more than the typical resolvable difference between two chemical states of the same element, which makes BE referencing against the C 1s peak unreliable and thus inadvisable for the purpose. Surprisingly and reassuringly, we find that C 1s shifts correlate to changes in sample work function ΦSA, such that the sum EFB+ΦSA is constant at 289.50±0.15 eV, irrespective of materials system and air exposure time, indicating vacuum level alignment. This discovery allows for significantly better accuracy of chemical state determination through a complementary measurement of ΦSA and referencing to C 1s set at 289.50-ΦSA, which as we demonstrate, yields consistent results for the whole series of TM nitrides, irrespective of air exposure time. Our findings are not specific to nitrides and likely apply to all systems where charge transfer at the AdC/substrate interface is negligible.[i] G. Greczynski and L. Hultman, ChemPhysChem 18 (2017) 1507