3.3. Raman spectra
To correlate the changes in Raman spectra of all samples, each spectrum was normalized to the total scattering intensity. To analyse the changes due the deposition parameters, the “true” line shape of the spectra must be found. To recover this line shape, the position and the widths of the scattered bands, a least squares best fit with five Lorentzian profiles was performed. In order to compare the evolution of structural modifications in ZrOxNy samples, the spectra of ZrN and ZrO2, deposited in the same chamber, are depicted in Fig. 2a and b, respectively. In zirconium nitride, first order Raman scattering is forbidden as a consequence of selection rules. However due to disorder, induced by the deposition technique, the translation symmetry is lost, and thus all the modes can participate in the scattering. Raman spectra will be an “image” of the density of vibrational states (DVS) [24]. The Raman spectrum of ZrN, as expected, is dominated by an asymmetric band centred at around 500 cm−1, and in the low frequency region by the presence of two bands centred at around 165 and 220 cm−1, reflecting the material's DVS. In the low frequency range, the bands are attributed to the disorder of single phonons and second order processes. For higher frequencies the presence, and the asymmetry, of the bands is due to the superposed contributions of disorder of optical phonons and second order combination of acoustic and optical processes [25].
The spectrum of as-deposited zirconia, ZrO2, is depicted in Fig. 2b, and is formed by several peaks, well defined, that can be identified as the characteristic features of the Raman spectrum of zirconia's monoclinic phase [14,26,27]. Even if the deposited sample is not a perfect crystal and even if some of the features are not very well resolved, it is noted that 16 of the 18 theoreticalcalculated features for the m-ZrO2 phase are present [10].
In Figs. 3 and 4, the Raman spectra of the coatings deposited with increasing reactive gas flow are presented. The analysis of the evolution of the Raman signal, with the variation of gases flow, will be centered on two aspects: the first one will be the changes in the spectral shapes; the second a quantitative analysis based on band or peak position and the relative intensities of some of the detected peaks. As can be seen from these figures, with the variation of gas flow (in fact inducing the variation of the non metallic content) the lines shapes continuously change, and the appearance of new bands in spectra reflects the changes in composition and, consequently, in the structure. The absence of sharp and well defined peaks could be attributed to the local structural disorder of the prepared films. The first note is that the spectral shape of Fig. 3a and b presents some similarities to spectrum of zirconium nitride. It is clearly seen that in the region below 300 cm−1, two bands occur, which possess positions close to the bands observed in the spectrum of as-deposited ZrN. For frequencies higher than 300 cm−1, the presence of a large band extended up to frequencies around 800 cm−1 is noticed, with lower intensity when compared with the band in the as-deposited ZrN spectrum [see Fig. 2a]. With the increase of the oxygen content, the spectral shape continues to change: a shoulder located at around 420 cm−1 starts to appear, as can be seen in Fig. 3d − e. The overlap of low frequency range features is also observed for flow rates around 10 sccm, as it can be seen in spectrum of Fig. 3e, and the transformation to a asymmetrical band centered at ~170 cm−1. A considerable modification of the spectral shape occurs for flow rates higher than 10 sccm, as can be seen in spectra depicted in Fig. 4, where the number of visible bands increases: the appearance of new bands in the region between 200 and 300 cm−1, and also in the region between 400 and 700 cm−1. For flow rates higher then 13 sccm, another modification of the spectral shape is observed. In the present case, the reduced number of visible Raman bands seems closely related to the overlap of bands in the region between 200 and 400 cm−1 and the decrease in intensity of the band located at 640 cm−1. In consequence the spectra are dominated by a large band centered around 460 cm−1. In all the spectra presented in Fig. 4, some of the features appear in positions close to the expected frequencies of modes belonging to monoclinic and tetragonal phases of zirconium oxide. On the other hand, the presence of broad bands, indicative of local disorder, could be due to a non-stoichiometric phase of ZrOxNy-type structure and to a poorly crystallized Zr3N4 structure, as discussed in XRD results. All these changes reflect modifications in local arrangements that lead to the conclusion that the increase of oxygen fraction induces the formation of new structural arrangements. This observation will be later discussed.
In order to clarify the changes induced by the deposition conditions, in Figs. 5 and 6 are depicted the fittings results of the position for some bands, which have been chosen to perform a qualitative study: two bands situated in the low frequency range, centered around 167 and 220 cm−1; and two bands situated at higher frequencies, centered around 475 and 680 cm−1. As can be seen in both figures, it is possible to divide the evolution of the band position in three zones, and correlate the observed behaviour with the observations that have been discussed for XRD-results. The observed shifts in band position could be attributed to film stresses, to non-stoichiometry of the deposited films and to changes in local structural arrangements, resulting from the deposition conditions itself. A shift, to low frequency positions, is observed in the bands of the first region with the increase of flow rate. Incorporation of either O atoms into Zr nitride lattice, or N atoms in Zr oxide, in interstitial sites or by substitution, is expected.
In spite of the fact that oxygen is more reactive than nitrogen, the nitrogen fraction in the working atmosphere is much more important than the oxygen and, obviously, this fact has consequences in the resulting structural evolution. As already discussed, the displayed spectra, for samples deposited with flow rates up to 7 sccm (in region zirconium nitride-like), are very similar to the as-deposited zirconium nitride sample. With the increase of oxygen content in the films, that increases at an almost constant rate as has been seen by RBS results, some incorporation of oxygen in zirconium nitride lattice is expected, or the substitution of nitrogen atoms by oxygen ones, and consequently the appearance of new bands in Raman spectra: the shoulder located at ∼400 cm−1 and a large band located at ∼690 cm−1 (see Fig. 4). These bands are present for flow rates higher than 8 sccm, but they vanish for flow rates equal to 10 sccm. The effect of oxygen content on Raman spectra is clearly seen in zone T and zone II, where the number of observed bands is higher than those observed in spectra of films of zone I. The observed shifts for the bands located at ∼240 and ∼480 cm−1, to higher and low frequency respectively could not be attributed only to compressive stress, but also to lattice dynamics. In fact the changes in local structural arrangements observed in t-ZrO2 samples were correlated to changes in tetragonality [28]. Some of the band positions, approach the characteristic positions of m-ZrO2 ant t-ZrO2 indicating the possible coexistence of these phases with a ZrOxNy-type structure.
4. Conclusions
Thin Zr–O–N films were prepared by dc reactive magnetron sputtering. Structural analysis carried out by both XRD and Raman revealed a definite correlation of the obtained results with the composition analysis, implying the existence of 3 different structural regimes. The first, zone I, corresponded to films prepared with the lowest reactive gas flows, present a zirconium nitride type structure. For the highest flows, a second zone was observed, zone II, and the XRD results show the possibility to have a bcc γ-Zr2ON2-type structure. Between these two zones, there is a transition zone, whose diffraction patterns revealed the possibility of having the formation of very poorly crystallized oxygen-doped orthorhombic Zr3N4-type structure. The Raman line shapes continuously change with the variation of the material composition. The observed changes reflect the modifications in local arrangements induced by the increase of oxygen fraction in the films.
The appearance of new bands in spectra reflect also the changes in the local structure, induced by the particular composition of the samples. The XRD results, associated with Raman data, revealed the possibility of coexistence of different phases.