Channeling Studies of Thermal Regrowth in Ion Damaged Graphite
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CHANNELING STUDIES OF THERMAL REGROWTH IN ION DAMAGED GRAPHITE
T. VENKATESAN÷ AT&T Bell Laboratories, Murray Hill, New Jersey 07974
B. S. ELMAN, G. BRAUNSTEIN, M. S. DRESSELHAUS and G. DRESSELHAUS Center for Materials Science and Engineering, MIT, Cambridge, MA 02139
ABSTRACT The crystallization of disordered surface layers on highly oriented pyrolytic graphite (HOPG) have been studied by Rutherford backscattering spectrometry (RBS) and channeling techniques. Disordered layers (-1000-3000A thick) are produced on the surface of HOPG by the implantation of various ions. The disordered layers are regrown by thermal annealing of the samples in an inert environment. Isochronal anneals reveal two distinct regrowth processes: one, a rapid process of low activation energy (Ea - 0.15 eV) which is observed primarily in regions where the disorder is sufficient to prevent the channeling of the ions but insufficient to totally destroy the graphitic structure. This low activation energy may indicate annealing of the damage by migration of interstitials where the interstitials are the knock-on carbon atoms produced by the primary ions. A regrowth process with higher activation energy (Ea - 2.0 eV) occurs primarily in regions where the disorder is close to the saturation-disorder produced by ion implantation. Both the regrowth processes are epitaxial in nature and the epitaxial nature of the process may explain the much lower activation energy for 3D AB stacking as measured in ion implanted graphite when compared with results on the bulk graphitization of pyrocarbons.
INTRODUC-ION Crystal growth usually occurs at a solid-liquid or solid-vapor interface at which the perfection of the crystal is controlled by the crystallinity of the interface. The growth of graphite crystals from the vapor has been achieved in the formation of graphite whiskers. The normal synthetic graphites are obtained at a crystal-amorphous solid interface at which small crystallites grow as a result of solid state diffusion. This graphite crystal growth is difficult to monitor and characterize. The present work reports on a series of experiments related to the regrowth of graphite under more controlled conditions making use of ion implantation to introduce disorder into the substrate in a controlled manner. Specifically, the implantation process produces a crystal-disordered solid interface which we monitor by Rutherford backscattering spectrometry (RBS) (with depth resolution of -400A) during a series of isothermal and isochronal anneals during which the crystalline regrowth occurs. The graphite recrystallization studies reported in this paper are to be compared and contrasted with the graphitization studies which have been reported with various precursor carbons.' Carbons are classified into graphitizable and ungraphitizable precursors. The graphitizable carbons generally start with either large or small "crystallite" planes which are to some extent aligned. In the graphitization process these planes grow in lateral extent and decrease the mosaic spread
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