Composition. Knowing the exact composition of the ZrN material is important to investigate the influence of Zr/N ratios and impurities such as carbon and oxygen on the catalytic NRR activity. The influence of oxygen heteroatoms and the formation of oxynitrides as the active NRR phase has already been suggested for VN and CrN. The composition of the ZrN films on Si and GC were investigated by complementary analysis methods. With RBS in combination with NRA, the bulk composition of the films was determined, whereas XPS allows differentiation between Zr-N and Zr-O species on the surface as well as bulk information through depth profiling with Ar + ionsputtering. From RBS/NRA analysis the specific amounts of Zr, N, C and O of the films grown on Si(100) and GC were determined. These results are summarized in Table . At 750 °C on both Si and GC slightly N rich films were deposited with Zr/N ratios of 0.87 and 0.84, respectively, with the lowest amount of C and O contamination. At lower deposition temperatures of 650 °C and 550 °C, the amount of C and O contamination increases significantly. The enlarged amount of C hints an incomplete precursor reaction at lower deposition temperatures and is a known phenomenon for MOCVD processes. Interestingly, at 850 °C the C incorporation in the film is significantly enhanced which was speculated from the findings in XRD analysis (Figure a) -c)). It is most likely due to uncontrolled precursor decomposition at higher temperatures, where there is no sufficient time for the byproducts to leave the surface. A significant oxygen incorporation during the deposition process can be excluded, because as otherwise at elevated process temperatures, crystalline ZrO2 would appear in the XRD pattern as shown by previous studies. More likely is a post deposition oxygen incorporation during sample handling under ambient conditions that is well known for ZrN and similar TMNs which are prone to oxidation. The RBS plots in Figure ) and d) show an increasing broadness and asymmetry of the Zr peak at higher deposition temperatures indicating a more continuous film with a higher surface roughness accompanied by a less defined interface between the film and the substrate. To investigate the nature of the different Zr species on the surface and as a function of film thickness, XPS of the surface and after Ar + -sputtering was conducted for optimized process parameters of 750 °C for Si(100) and GC substrates. Depth profiling by Ar + -sputtering (2 keV, 60 s per cycle) was performed to obtain information of the bulk composition. In total 19 cycles were measured resulting in a depth of about 76 nm based on a sputtering rate of 4 nm min -1 determined by profilometry. Figure shows the Zr 3d and N 1s high resolution spectra of the ZrN film on GC substrate. The Zr 3d spectra consist of spin-orbit doublet peaks (Zr 3d5/2 and Zr 3d3/2) which were fitted with three components corresponding to different Zr species. The surface of the ZrN film is significantly oxidized apparent from Zr 3d5/2 signal at a binding energy (BE) of 182.3 eV (component z3 in Figure )). Minor components z2 (181.2 eV) and z1 (180.0 eV) can be allocated to the oxynitride and nitride species, respectively, indicating a contribution from the underlying ZrN phase below the oxidized surface. Thus, the XPS sampling depth of 10 nm 45 is greater than the thickness of the surface oxide layer which was described in literature to be around 3.7 nm. The oxynitride can be present as a particular phase, but was also described as interfacial layer between the oxide and nitride layers for different TMNs. The Zr 3d spectra of ZrN film deposited on Si substrates (Figure The component n4 (403.5 eV) was either assigned as molecular N2 or oxidized nitrogen species. The high resolution N 1s spectra of ZrN-Si sample (Figure )) shows similar components, but lower contributions of n1 and n2 in comparison to n3 supporting the hypothesis of a thicker oxide layer as discussed for the Zr 3d spectra. Due to prone surface oxidation of TMNs when exposed to ambient atmosphere, XPS analysis of nitrides is often done on sputter-cleaned surface. Moreover, XPS depth profiling by sputtering with Ar + -ions (also applied here) enables analysis of the film's bulk composition. However, sputtering also introduces uncertainties to the analysis due to various effects like preferential sputtering of one element in a multielement samples that could result in variation in the atomic concentration or cascade mixing where smearing of one element results in the appearance of this element in deeper levels than in the pristine sample. This complexity has to be considered in the evaluation of XPS depth profiling. Here, Ar +sputtering (2 keV, 60 s, 19 cycles) was performed to analyze the bulk composition of the ZrN films which resulted in a total depth of 76 nm. High resolution spectra after 16 cycles (64 nm depth) are compared to the surface spectra. Zr 3d spectra of ZrN-GC sample (Figure )) shows increased nitride (z1, 179.7 eV) and oxynitride (z2, 181.0 eV) components, but still a large contribution of the oxide phase (z3, 182.7 eV) is present. Thus, surface oxides are smeared from the surface to the bulk due to sputtering or oxides are formed from containing oxygen in the pristine film as determined by RBS/NRA (Table ). Because no oxide phase is visible in XRD measurements, observed oxide components are of amorphous nature from oxidation under ambient conditions. For the ZrN-Si(100) sample a similar behavior is visible with even higher contribution of the oxide phase in comparison to the nitride component (Figure )). Again, high resolution N 1s spectra correspond to these observations with increasing components of nitride (n2) and oxynitride (n1) (Figure cross section SEM images of the 750 °C samples, oriented columnar growth is apparent on both substrates and thicknesses of 330 nm and 370 nm on Si and GC were estimated, respectively. To further investigate the films in terms of crystallite shape, element distribution and crystal structure, BF-TEM, STEM-EDS and SAED analysis were conducted on ZrN films grown at 750 °C on Si and GC. Cross sectional BF-TEM images (Figure )) reveals the columnar growth of the thin films on both substrates. Interestingly, the initial film growth on Si leads to a film of higher density compared to the rest of the sample, as it is significantly darker compared to the rest of the film. The STEM-EDS of the film on Si (Figure )) reveals varying N concentration throughout the film. At the substrate/film interface, Zr (red) and N (green) signals are evenly distributed, whereas close to the nitride surface, the amount of N is significantly increased. Simultaneously, the density of the film is decreasing in the N rich regime. Noteworthy, STEM-EDS on GC reveals a more evenly distribution of Zr and N throughout the whole film (Figure )). This can be explained by the difference in film thicknesses, as the film on Si is four times thicker compared to the one on GC. The decreasing Zr/N ratio, as well as the initial film growth with higher density, can be attributed to a substrate-dependent growth particularly in the initial film deposition stage. After the initial growth, the ZrN formation is preferred on the existing crystallites which is facilitating the columnar growth. Furthermore, the rather porous structure of the films allows a post deposition oxidation beyond the surface and could explain the significant oxygen content in the bulk of the films. SAED of the films on both substrates was carried out revealing distinct diffraction spots along the circles corresponding to cubic ZrN further confirming the growth of crystalline ZrN.