Magnetoreflection in Ion-Implanted Graphite
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MAGNETOREFLECTION IN ION-IMPLANTED GRAPHITE*
E MCNEIL,$ B.S. ELMAN,$ M.S DRESSELHAUS,#" G. DRESSELHAUS.+ Department of Electrical Engineering and Computer Science; 'Department of Physics; +Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139 T. VENKATESAN Bell Laboratories,
Murray Hill, NJ 07974
ABSTRACT 0
The use of a hot stage (T - 600 C) for ion implantation into graphite permits the introduction of foreign species into the host material while eliminating most of the lattice damage associated with ion implantation at room temperature. This permits the use of the magnetoreflection technique for examination of changes in the electronic band structure induced by implantation Samples of 1 graphite implanted with q P and 11B at various energies and fluences are examined, and the in-plane and c-axis disorder are characterized using Raman spectroscopy and Rutherford Backscattering Spectrometer (RBS) techniques. Implantation-induced changes in the electronic band structure are interpreted in terms of the Slonczewski-WeissMcClure band model. Small changes are found relative to the band parameters that describe pristine graphite.
INTRODUCTION Ion implantation makes possible the physical introduction of foreign species which, for various reasons, cannot be chemically introduced into the host material. This mechanism induces a number of changes in the host material, including displacement of atoms of the host material (radiation damage), changes in the carrier concentration of the host, and modification of the electronic structure of the host material [1]. The relationship between the implantation-induced damage and changes in the electronic structure of the semimetal graphite is the subject of this paper. The electronic structure is affected both by lattice damage due to the incoming ions and the secondary carbon ions, and by the presence of the foreign ions once the ions come to rest. Implantation-induced damage was characterized by two complementary techniques: Raman spectroscopy, which is sensitive mainly to the degree of in-plane disorder within the optical skin depth (6 - 800 A); and Rutherford Backscattering Spectrometry (RBS), which is sensitive to the c-axis disorder to a depth of a few microns. Changes in the electronic structure were measured using the magnetoreflection technique, in which the relative changes in the infrared reflectivity from the sample surface are measured as a function of applied magnetic field. This technique is especially suited to the examination of ion-implanted materials since the infrared skin depth (6) is comparable to the penetration depth of the implanted ions (R ), so that the region of maximum sensitivity corresponds to the region of physical interest. Necessary conditions for the observation of magnetooptical effects include wcr > I and 1iw > kT. The condition w cv > 1 (where w c is the * * angular cyclotron frequency wc = eHlm cc, mc is the cyclotron effective mass and T is the relaxation time), implies that an electron or hole
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