# Radiation Hardness of Oxide Thin Films Prepared by Magnetron Sputtering Deposition
## 3 Experiment Results and Discussion
### Comparing Thermally Grown a-SiO\({}_{2}\) with Magnetron-Sputtering-Deposited a-SiO\({}_{2}\)
additional ion irradiation with copper beams (18 MeV Cu and 2.5 MeV Cu) was made, and the obtained spectra were compared with the spectrum from the sample that was annealed at 600 \({}^{\circ}\)C for 1 h in vacuum. These spectra show some changes, as shown in Figure 3c, but for their proper evaluation, a detailed analysis was required. This analysis was carried out in the same manner for all spectra we had at our disposal (i.e., spectra obtained from the old, thermally grown a-SiO\({}_{2}\) and new, magnetron-sputtering deposition prepared a-SiO\({}_{2}\)). The procedure was as follows: the most intense peak is composed of two peaks, one at 1044 cm\({}^{-1}\) and another one at 1078 cm\({}^{-1}\), which arise from the Si-O-Si asymmetric stretching TO3 mode, as explained in the first paragraph of this section. Observed changes are well-known and redshift of the maxima [15] is attributed to a decrease of 1078 cm\({}^{-1}\) peak intensity and an increase of 1044 cm\({}^{-1}\) peak intensity [17; 20; 21]. This occurs because the O-Si-O bond angle decreases from 144\({}^{\circ}\) to 129\({}^{\circ}\) due to the compaction of silica. The positions of these two well-known peaks are therefore fixed in the analysis, and a fit to the Gauss function was used to evaluate areas of these two overlapped peaks. In case of thermally grown a-SiO\({}_{2}\), irradiation by 23 MeV I beam increases the area of 1044 cm\({}^{-1}\) peak and decreases the area of 1078 cm\({}^{-1}\) peak, as shown in Figure 3a. These changes evolve rapidly with applied ion fluence, and the kinetics of this process have already been well documented [21]. In contrast to iodine ion irradiations, 1.25 MeV gamma-ray irradiation was found to be much less effective, and doses much larger than 5 MGy were needed to introduce noticeable changes to IR spectra [21]. The magnetron-sputtering-deposited a-SiO\({}_{2}\) showed surprisingly little changes in IR spectra when exposed to 23 MeV I irradiation in the same fluence range, as shown in Figure 3b. The total area of the 1044 cm\({}^{-1}\) and 1078 cm\({}^{-1}\) peaks remains the same, within 5% of the average value, and only slight redshift can be visually observed. This striking difference between spectra shown in Figure 3a,c is associated with different structures of the two a-SiO\({}_{2}\) thin films studied here, but slightly different stoichiometric values hinted by ToF-ERDA could play a role too. Another dedicated experiment would be needed to clarify if this is indeed the case. It is important to note that the ratio of 1078 cm\({}^{-1}\) and 1044 cm\({}^{-1}\) peak areas is \(\sim\)1-2 for magnetron-sputtering-deposited a-SiO\({}_{2}\), for all spectra shown in Figure 3c. In the case of thermally grown a-SiO\({}_{2}\), this ratio is \(\sim\)50 for unirradiated and \(\sim\)3 for the sample irradiated by the highest fluence of 23 MeV I. This finding suggests that as-prepared magnetron-sputtering-deposited a-SiO\({}_{2}\) thin film is already a highly disordered material and further gamma-ray or iodine ion irradiation does not change its initial disordered state much. The proposed explanation, that magnetron-sputtered a-SiO\({}_{2}\) is a highly disordered material, then has to address the visible changes shown in Figure 3e. Upon closer inspection, the similarity of spectra shown in Figure 3c,e is only ostensible. The ratio of 1078 cm\({}^{-1}\) and 1044 cm\({}^{-1}\) peak areas is \(\sim\)0.5 for both copper-irradiated samples and the annealed sample. This low value is much less than the previously reported high-fluence limit of \(\sim\)2 [21] and \(\sim\)1 [17], which suggests that unusual properties of magnetron-sputtering-deposited a-SiO\({}_{2}\) cannot be assigned to a highly disordered state alone. However, we note that in both cases (copper ion irradiation and annealing), a similar state of a thin film was reached, since FTIR spectra show similar features.