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TRANSPORT PROPERTIES AND ELECTRON MICROSCOPY STUDIES OF ION IMPLANTED GRAPHITE

T.C. CHIEU B.S. ELMAN+, L. SALAMANCA-RIBA+, M. ENDOa AND G. DRESSELHAUS# Department f Electrical Engineering and Computer Science; +Department of Physics; ýFrancis Bitter National Magnet Laboratory; Massachusetts Institute of Technology, Cambridge, MA 02139, USA; #Faculty of Engineering, Shinshu University, Nagano, Japan.

ABSTRACT Graphite fibers with high structural perfection and small diameters (-lIm) provide a sensitive medium to study the effect of ion implantation on the transport properties of graphite and to observe the defect structure associated with the implantation process. Graphite fibers prepared from the thermal decomposition of benzene and subsequent 0 heat treatment to high temperatures (-2900 C) have been shown to achieve the high structural perfection necessary to carry out such experiments. Implantation-induced changes in the fiber resistivity are reported and are found to be larger at low temperature, as expected on the Using the lattice basis of residual resistance arguments. fringe imaging technique of high resolution electron microscopy, the implantation-induced defect structure can be observed directly. The results show local expansion of the interlayer graphite planes, with an interlayer separation ranging up to 3.9 1 in the implanted region, compared with -3.4 1 for the well-ordered layers beyond the ion penetration depth.

INTRODUCTION Graphite fibers with the highest structural perfection yet achieved have been successfully fabricated by thermal decomposition of a mixture of 0 benzene and hydrogen gas [1-3] at a temperature of approximately 1100 C. The fiber axis is oriented along a basal plane direction and the c-axis is 0 radial [4]. When heat treated to temperatures as high as 2900 C, the fibers exhibit graphite crystallites with dimensions larger than 1000 and an unusually high degree of structural order. These fibers also have the highest electrical conductivity, bulk modulus and tensile strength yet Furthermore, for fibers with found in fibrous graphite materials [4-81. diameters less than 0.8 pm, the c-axis lattice image and defect structures can be seen directly using high resolution electron microscopy without any Thus, benzenethinning and sectioning or special sample preparation [9]. 0 derived fibers (BDF), heat treated to 2900 C and above, provide an excellent matrix for the study of structural order associated with the implantation process. The focus of this work is the direct observation by the lattice fringe imaging technique of the defect structures created by the implantation of various ions into these well-ordered graphite fibers. The results show a variation in the defect density with depth and an enlargement in the c-axis graphite spacing from d = 3.36 X to d as large as 3.9 ± 0.1 A. A temperature dependent resistivity curve for a boron implanted benzenederived fiber (THT = 3500°C) is compared to that of the original BDF, showing a degradation in conductivity upon implantation because the