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Figure 11. RBS channeling spectra of Ba-irradiated quartz [71]. The Ba ion fluence was 1 x 1015/cm2 and the sample temperature varied from RT up to 900 "C. One notes that the SiO2 matrix gets amorphized during implantation at room temperature and stays damaged near the surface up to and including 900 " C, while the thickness of the amorphous layer decreases for rising sample temperature. The Ba depth profile is essential immobile at all temperatures.

quartz were generally not successful up to about 1270 K and led to partial epitaxy at most [75-78]. This is particularly true concerning thermal annealing of the irradiated samples in vacuo, in which case the results for different ion species, energies, and fluences are somewhat conflicting, depending on the type and concentration of impurities in the matrix before ion implantation. For instance, when irradiating natural Brazilian quartz with Si and O ions at a fluence of 1 x 1016 ions/cm2 and annealing the samples in air, Devaud et al. [77] observed SPEG of the matrix to be completed at 1120 K and concluded that impurity OH groups were responsible for recrystallization. As early as 1980, Arnold and Peercy [75] in their implantation studies of noble gas ions in Li2O • 2SiO2 and of Li ions in quartz had found evidence of the special role of alkali ions for epitaxy in silicates. These authors obtained crystalline Li2O • 2SiO2 when annealing the irradiated sample at 775 K, and recrystallization of Li-doped SiO2 at 9701070 K, respectively. After the implantation of 3-MeV Ar ions in quartz, Wang et al. found recrystallization to occur below half the melting temperature [76]. More recently, Harbsmeier et al. [78] carried out similar experiments on a-quartz after 50-keV carbon and nitrogen ion implantations at a fluence of 1016/cm2 and annealing in air up to 1670 K. In these experiments, SPEG was found to set in at 820 K (after N-doping) and at 1470 K (after C-doping), visible as a reduced thickness of the amorphous layer and a movement of the amorphous/crystalline (a/c) interface towards the surface. However, no full recrystallization was achieved below 1670 K, the highest annealing temperature used in these experiments.

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