| AVS 62nd International Symposium & Exhibition | |
| Applied Surface Science | Thursday Sessions |
| Session AS-ThP |
| Session: | Applied Surface Science Poster Session |
| Presenter: | Niklas Hellgren, Messiah College |
| Authors: | N. Hellgren, Messiah College R. Haasch, University of Illinois at Urbana-Champaign S. Schmidt, Linköping University, Sweden L. Hultman, Linköping University, Sweden I. Petrov, University of Illinois at Urbana-Champaign |
| Correspondent: | Click to Email |
X-ray photoelectron spectroscopy (XPS) has for decades been one of the most widely used techniques for analyzing the quantity and bonding states of nitrogen in carbon-nitride compounds, in particular hard carbon-nitride films in wear-protective applications, and more recently for analyzing nitrogen doping states in graphene used for electronic applications. Interpretation of the C1s and N1s spectra, however, can be very challenging due to the many possible bonding configurations of N and C, combined with the inevitable interaction with oxygen and hydrogen on the film surface. A corresponding debate over interpretation has accompanied the field.
In this study we report on XPS studies of magnetron sputtered CNx thin films, with x ranging from ~0.1 to 0.6. Different growth conditions result in films of different structures, from amorphous to graphite-like and fullerene-like. In order to address some of the above-mentioned difficulties, films were analyzed by angular-resolved XPS, first in-situ in the growth and analysis chamber, then after air exposure, and finally after Ar ion etching using ion energies ranging from 500 eV to 4 keV.
The as-deposited films typically exhibit two strong N1s peaks at ~398-398.5eV (usually assigned to pyridine-like nitrogen, C–N=C), and ~400-400.7eV (graphitic nitrogen, –N<), with some nitrile contribution (–C≡N) in between, at ~399eV. Interestingly, the in-situ spectra also show a shoulder in the 402-404 eV range, which is typically attributed to oxidized nitrogen (N-O). However, this peak does not increase upon air exposure, which shows that a different assignment is required for this peak. Instead, air exposure results in the gap between the two main peaks being filled, presumably due to an increase in pyrrole-like nitrogen (>N-H) and/or amino-like nitrogen (>NH2). Meanwhile, the C1s peak broadens on the high-energy side which indicates that the 5-10 at.% oxygen uptake on the film surface is primarily in the form of C-O and possibly H2O, but not N-O.
Argon ion etching readily removes surface oxygen, but also results in a strong preferential sputtering of nitrogen. The N/C ratios rapidly approach equilibria of 0.05-0.2, depending on the initial concentration and the Ar+ beam energy. Furthermore, changes in the N1s peak shape indicate that ion etching causes amorphization of the film surface. Both effects are more pronounced at higher ion energies, and the damage does not appear to be reversible with subsequent low-energy etching. The best methods for evaluating the as-deposited film structure and composition with ex-situ XPS will be discussed.