Atomic-scale simulations of cascade overlap and damage evolution in silicon carbide
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In a previous computer-simulation experiment, the accumulation of damage in silicon carbide (SiC) from the overlap of 10 keV Si displacement cascades at 200 K was investigated, and the damage states produced following each cascade were archived for further analysis. In the current study, interstitial clustering, system energy, and volume changes are investigated as the damage states evolve due to cascade overlap. An amorphous state is achieved at a damage energy density of 27.5 eV/atom (0.28 displacements per atom). At low-dose levels, most defects are produced as isolated Frenkel pairs, with a small number of defect clusters involving only four to six atoms; however, after the overlap of five cascades (0.0125 displacements per atom), the size and number of interstitial clusters increases with increasing dose. The average energy per atom increases linearly with increasing short-range (or chemical) disorder. The volume change exhibits two regimes of linear dependence on system energy and increases more rapidly with dose than either the energy or the disorder, which indicates a significant contribution to swelling of isolated interstitials and antisite defects. The saturation volume change for the cascade-amorphized state in these simulations is 8.2%, which is in reasonable agreement with the experimental value of 10.8% in neutron-irradiated SiC.
I. INTRODUCTION
Both neutron irradiation and ion implantation in silicon carbide (SiC) create defects and a minor fraction of small defect clusters at low doses1–3 whereas at high doses, it is well established experimentally that irradiationinduced amorphization occurs below a critical temperature that ranges from about 300 to 500 K, depending on irradiation conditions.4,5 These irradiation-damage processes have received much attention because of the potential technological applications for SiC in electronic and optoelectronic devices,6 the first wall and blanket of nuclear fusion reactors,7 and coated fuel particles for gas-cooled fission reactors.8–10 Experimental studies of irradiation effects in SiC using electrons, neutrons, and ions have been carried out for several decades. It is now well established that irradiation below a critical temperature results in the creation of interstitials, vacancies, antisite defects, and defect clusters that interact to produce long-range structural and topological disorder. The
a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http:// www.mrs.org/publications/jmr/policy.html.
accumulation and interaction of these nonequilibrium defects lead to amorphization of SiC after a critical local concentration is exceeded. Numerous experimental studies have investigated the crystalline-to-amorphous transition (c–a) in SiC irradiated with electrons,11–13 neutrons,14 and ions4–6,15–18 as a function of temperature. The critical damage energies for
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