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Kinetics of Irradiation-Damage Processes in (6H) SiC W. J. Weber,(a,b) W. Jiang,(a,c) and S. Thevuthasan Supported by Office of Basic Energy Sciences. (a) EMSL User. (b) Environmental and Health Sciences Division. (c) Postdoctoral Research Fellow. Single crystals of 6H-SiC from Cree Research, Inc. were irradiated with 550 keV Si+, 360 keV Ar+, 550 keV C+, and 390 keV He+ ion beams over a range of ion fluences at temperatures between 160 and 190 K. Additional irradiations with 550 keV C+ were carried out over a range of ion fluences (1 to 50 ions/nm2) and temperatures (180 to 870 K). Profiles of atomic disorder on the Si sublattice were measured in situ by 2.0 MeV He+ Rutherford backscattering spectrometry in a <0001> axial channeling geometry (RBS/C). The equivalent dose in displacements per atom (dpa) has been estimated based on TRIM-97 calculations using minimum threshold displacement energies of 35 eV for the Si sublattice and 20 eV for the C sublattice. The relative disorder on the Si sublattice is shown in Figure 9.10 as a function of dose for different ions. The relative disorder, which is due to both the accumulation of point defects in the structure (e.g., interstitials) and the formation of amorphous material, shows a sigmoidal dependence on dose for all ions. The Ar+ and Si+ results are in good agreement; however, there is a shift in the curves to higher doses with decreasing ion mass (damage energy density) for the C+ and He+ results. This suggests a higher fraction of defects lost to simultaneous recombination processes at the low damage energies. Although irradiation with He+ in the PNNL study was not carried out to doses high enough to achieve full amorphization, irradiation with He+ to higher doses has been shown to produce complete amorphization (Grimaldi et al. 1997). Figure 9.10. Relative disorder on the Si sublattice, at the damage peak, as a function of dose in a-SiC irradiated with different ions. The effect of temperature on the accumulation of relative disorder on the Si sublattice is shown in Figure 9.11. Irradiation at 180 and 300 K results in a fully-disordered (or amorphous) state, and the amorphization dose at 300 K is about a factor of two higher than at 180 K. At irradiation temperatures of 470 K and higher, full amorphization is no longer achieved, which is consistent with the critical temperatures determined previously (Weber et al. 1997, 1998). The relative disorder at 180 and 300 K exhibits a sigmoidal dependence on dose. At 470 K, where full amorphization is not achieved, the relative disorder is much less sigmoidal, and at 670 and 870 K, the dependence of the relative disorder on dose follows a simple defect accumulation model (Weber 1981) represented by an exponential rise to a maximum. Figure 9.11. Relative disorder on the Si sublattice as a function of dose in a -SiC irradiated with 550 keV C+ ions at different temperatures. Isochronal and isothermal annealing were used to study the damage recovery in situ using RBS/C. The isochronal anneals were performed for 20 minutes in a sequence of annealing steps, from the irradiation temperature up to 1070 K, for samples irradiated with Si+, C+, and He+ ions over a range of fluences. Isothermal anneals over a sequence of time intervals at several temperatures were performed on specimens irradiated at 180 K with 550 keV C+ to 8 ions/nm2 (constant initial damage state). RBS/C measurements after each isochronal or isothermal annealing step were made at temperatures well below the annealing temperature to minimize the defect recombination during data acquisition. The isochronal recovery of the relative Si disorder on the Si sublattice is shown in Figure 9.12 as a function of annealing temperature for three different Si+ ion fluences. For low ion fluences (0.1 and 0.5 ions/nm2), complete recovery of the irradiation-induced disorder (freely migrating defects) occurs at 300 K. At an ion fluence of 1.0 ions/nm2, there is still significant recovery at room temperature, and the results suggest that there may be another recovery stage between 470 and 670 K, which is followed by gradual recovery until full recovery is obtained at 1070 K. The amount of recovery at 300 K is comparable for irradiation fluences of 0.5 and 1.0 ions/nm2, which may suggest that the number of freely migrating defects saturates at a fluence of 0.5 ions/nm2 (0.025 dpa at the damage peak) under these Si+ irradiation conditions. Above this fluence more stable defect clusters form or amorphization occurs. Figure 9.12. Isochronal (20 min.) recovery of disorder on the Si sublattice for a -SiC irradiated with 550 keV Si+ ions. The isochronal recovery of relative disorder on the Si sublattice for He+ irradiations to similar damage levels indicates some residual defects remain after annealing to 300 K. This suggests the formation of helium-defect complexes that inhibit full recovery of the He+-induced damage. Isochronal and isothermal annealing results are shown in Figures 9.13a and b, respectively, for samples irradiated under identical conditions (550 keV C+ at 180 K) to a fluence of 8.0 C+/nm2 (0.17 dpa at the damage peak) to provide a constant initial damage state. Isothermal annealing at 180 K for over two hours after irradiation shows no evidence for recovery. The isochronal and isothermal recovery data can be used to separate different kinetic processes and isolate a single-activated process with constant activation energy. Arrhenius plots obtained from the combination of isochronal and isothermal annealing data (Figure 9.13) are shown in Figure 9.14. At low temperatures (below room temperature), the estimated activation energy is of the order 0.25 ± 0.1 eV. At higher temperatures (between about 570 and 870 K), the estimated activation energy is about 1.5 ± 0.3 eV, which is in agreement with the activation energy (1.6 eV) previously determined for recovery of neutron damage in SiC in this same temperature range (Primak 1956). The data (Figure 9.13) suggest another recovery stage may be present between 350 and 570 K, but the current data could not be used to estimate an activation energy for this process. These results suggest that there are at least two distinct activation energies and associated recovery processes on the Si sublattice. Further studies are planned to determine the activation energies more accurately and to uniquely determine the nature of the recovery processes associated with the activation energies. However, it should be noted that the activation energy for thermal recovery at low temperatures (0.3 eV) is similar in magnitude to the activation energy (0.3 eV) determined for simultaneous recovery processes during amorphization (Weber et al. 1998). Figure 9.13. Recovery of the relative Si disorder on the Si sublattice: (a) isochronal (20 min.) and (b) isothermal annealing. Figure 9.14. Arrhenius plot of logarithmic annealing time in isothermal steps versus 1/kT in isochronal annealing. References Grimaldi, M. G., L. Calcagno, P. Musumeci, N. Frangis, and J. Van Landuyt, J. Appl. Phys. 81, 1 (1997). Primak, W., L. H. Fuchs, and P. P. Day, Phys. Rev. 103, 1184 (1956). Weber, W. J., N. Yu, L. M. Wang, and N. J. Hess, Mater. Sci. and Engr. A 253, 62 (1998). Weber, W. J., N. Yu, L. M. Wang, and N. J. Hess, J. Nucl. Mater. 244, 258 (1997). Weber, W. J., J. Nucl. Mater. 98, 206 (1981). William R. 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