Radiation-induced Chemical Disorder in Covalent Materials
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Radiation-induced Chemical Disorder in Covalent Materials Manabu Ishimaru1, Yanwen Zhang2,3, and William J. Weber3,2 1 The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 3 Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA ABSTRACT Chemical disorder in ion-irradiated SiC and GaN has been examined by means of transmission electron microscopy. Radial distribution functions obtained by a quantitative analysis of electron diffraction intensities revealed that homonuclear bonds, which do not exist in the crystalline state, are formed in both ion-irradiated specimens. The origin of the homonuclear bonds is quite different between SiC and GaN. The constitute elements mix on the atomic-scale in amorphous SiC, while phase separation induced by irradiation is attributed to the formation of self-bonded Ga atomic pairs in amorphous/nanocrystalline GaN. INTRODUCTION Radiation effects in solids are one of the important issues in research fields ranging from microelectronics devices to nuclear fusion reactors. In particular, structural changes during irradiation are extensively studied in order to control the physical properties of materials and to predict the fate of materials under radiation environments. It is known that energetic particles produce extensive damage, and may eventually lead to amorphization or nanocrystallization. To clarify the amorphization and nanocrystallization mechanisms, it is important to obtain information about atomistic structures of ion-irradiated materials. For the amorphous networks formed by different atomic species, not only topological disorder but also chemical disorder should be considered. This information can be obtained from the radial distribution function in which these structures are characterized by the probability of finding another atom at a distance between r and r+dr from a specific atom [1]. Radial distribution functions can be obtained by x-ray, neutron, and electron diffraction techniques. Among the diffraction techniques, electron diffraction has the following merits: (1) strong interaction between the material and electrons has the advantage of detecting light atoms; (2) thanks to short wavelength of high-energy electrons, an intensity profile up to high scattering angles is easily obtained; (3) local structures on the nano-scale can be obtained by a combination of other techniques, such as high-resolution electron microscopy and nano-beam spectroscopy analysis. In the present study, we characterized radiation-induced chemical disorder in covalent materials using transmission electron microscopy (TEM).
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EXPERIMENTAL PROCEDURES High-energy ion irradiation experiments were performed using the 3.0 MV tandem accelerator facilities in the Environmental Molecular Sciences Laboratory at the Pacific Northwest National Laboratory. 4H-SiC single crystals were irradiated at room tempera
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