# Radiation Hardness of Oxide Thin Films Prepared by Magnetron Sputtering Deposition
## 3 Experiment Results and Discussion
### Radiation Hardness of Magnetron-Sputtering-Deposited a-Al\({}_{2}\)O\({}_{3}\) and a-MgO  
In Figure 4a, we show the FTIR spectra of a-Al\({}_{2}\)O\({}_{3}\) irradiated with high-energy heavy ions to the highest fluences, and in Figure 4b, the FTIR spectra of a-Al\({}_{2}\)O\({}_{3}\) irradiated with gamma-rays and a-Al\({}_{2}\)O\({}_{3}\) annealed at 600 \({}^{\circ}\)C for 1 h in vacuum. In all cases, including a spectrum from a non-irradiated sample, a single, very broad peak (FWHM \(\sim\)290 cm\({}^{-1}\)) is found centred at \(\sim\)700 cm\({}^{-1}\). This indicates that the material is completely disordered, with a wide distribution of Al-O bond lengths and coordination geometries. In that case, different Al-O stretching and bending modes, usually found around 430-500, 550-630 cm\({}^{-1}\), 650-720 cm\({}^{-1}\), and 750-850 cm\({}^{-1}\) overlap, and therefore, it is very difficult to resolve their individual contributions [37; 38; 39; 40]. An unusually large width of the peak (FWHM \(\sim\)290 cm\({}^{-1}\)) also points to many different bond strengths, which could be due to the low density of a-Al\({}_{2}\)O\({}_{3}\) prepared by magnetron sputtering deposition. Such a broad feature that appears in the FTIR spectrum is therefore indicative of highly disordered amorphous alumina. A feature around 667 cm\({}^{-1}\) in all presented FTIR spectra is an artefact caused by atmospheric CO\({}_{2}\) absorption. Only in the case of thermal annealing, we observe narrowing of the peak (FWHM \(\sim\)255 cm\({}^{-1}\)), indicating some recovery of the material structure. In contrast to a-SiO\({}_{2}\), irradiation with an 18 MeV Cu beam does not yield a similar effect, indicatingthat electronic excitations are not sufficiently intense to induce material recovery in a similar manner. This is in line with previous observations that crystalline Al\({}_{2}\)O\({}_{3}\) is more radiation resistant than SiO\({}_{2}\)[12, 21]. However, crystalline Al\({}_{2}\)O\({}_{3}\) is very prone to ion track recrystallisation, and this could be a cause for its high radiation hardness. For this reason, dedicated experiments were carried out [41], and it was found that the threshold for ion track formation, when recrystallisation is suppressed, is still much higher than the threshold for ion track formation in quartz SiO\({}_{2}\)[42]. Finally, in Figure 5a, we show the FTIR spectra of a-MgO irradiated with high-energy heavy ions to the highest fluences, and in Figure 5b, the FTIR spectra of a-MgO irradiated with gamma-rays and a-MgO annealed at 600 \({}^{\circ}\)C for 1 h in vacuum. A broad peak centred around \(\sim\)415 cm\({}^{-1}\) can be observed [43, 44]. Due to technical limitations posed by the used spectrometer, the spectra exhibit slightly higher noise at the end of the spectral range, but reliable fitting was still possible. The FTIR spectra from a-MgO samples irradiated by high-energy heavy ions, even up to the highest fluences, do not show much difference compared to the spectrum obtained from the as-prepared sample. We concluded that, similar to the case of high-energy heavy-ion-irradiated a-Al\({}_{2}\)O\({}_{3}\), all of the used ion beams are ineffective in changing the structure of a-MgO because electronic stopping is below the threshold for ion track formation in this material. Again, this is in line with previous observations that both Al\({}_{2}\)O\({}_{3}\) and MgO are very radiation-resistant materials [12]. The spectra shown in Figure 5b show significant differences. The spectrum obtained from the sample that was annealed at 600 \({}^{\circ}\)C for 1 h in vacuum shows a moderate redshift of the peak for 20 cm\({}^{-1}\). This finding is assigned to relaxation of the as-prepared material (that is usually under compressive and/or tensile stress when deposition is carried out at room temperature) and consequently the observation of transverse optical Mg-O mode in a less strained environment.