High Temperature Implantation in Graphite
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HIGH TEMPERATURE IMPLANTATION IN GRAPHITE G. BRAUNSTEIN,* B.S. ELMAN,* M.S. DRESSELHAUS, +* G. DRESSELHAUS# Departme t of Physics; +Department of Electrical Engineering and Computer Science; Francis Bitter National Magnet Laboratory; Massachusetts Institute of Technology, Cambridge, MA 02139, USA T. VENKATESAN Bell Laboratories,
Murray Hill, NJ 07974,
USA
ABSTRACT In previous studies it was found that when highly oriented pyrolytic graphite (11OPG) is implanted at room temperature, the damage caused by the implantation could be completely annealed by heating the sample to 0 temperatures higher than - 2500 C. However at these high temperatures, the implanted species was found to diffuse out of the sample, as evidenced by the disappearance of the impurity peak in the Rutherford backscattering (PBS) spectrum. If, on the other hand, the 1OPGcrystal was held 0 at a high temperature (2 600 C) during the implantation, partial annealing could be observed. The present work further shows that it is possible to anneal the radiation damage and simultaneously to retain the implants in the graphite lattice by means of high temperature implantation 0 0 (Ti 2 450 C) followed by annealing at 2300 C.
INTRODUCTION Graphite is an interesting prototype material for ion implantation studies because of the high anisotropy in its structure and physical properties [1]. For example, the layered nature of the graphite lattice gives rise to a regrowth behavior completely different from that observed in widely studied semiconductors [2,3]. In addition, interesting applications of ion implanted graphite can be envisaged in terms of modification to the electrical, thermal and magnetic properties of the host material [4]. There are two main ways to change the physical properties of graphite: a) by substitutional doping or b) by intercalation. In both cases the number of species which can be introduced into the graphite lattice by thermal or chemical means is very limited, while in principle any ion can be introduced into the graphite host by means of ion implantation. Initial studies on radiation damage in ion implanted highly oriented pyrolytic graphite (HOPO) [2,3] have shown that the recrystallization (graphitization) of the disordered lattice is a two step process, as also occurs in the graphitization of pyrocarbons [5,6]. The first step involves two-dimensional or in-plane ordering, which occurs for annealing 0 0 temperatures in the approximate range 1500 C < Ta < 2300 C [2]. The second step of graphitization occurs at higher temperatures where three dimensional or c-axis ordering takes place [2]. Unfortunately when the implantation is done at room temperature, the amount of implanted species retained by the graphite lattice decreases with increasing annealing 0 temperature, until, at a temperature of approximately 2300 C (slightly dependent on implantation dose), all the impurities diffuse out of the substrate [7]. If on the other hand, the HOPG crystal is held at about 0 600 C during the implantation, significant preservation of crysta
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