Topographical changes induced by high dose carbon-bombardment of graphite
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D. F. Pedraza Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6091
S. P. Withrow Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6048 (Received 12 February 1993; accepted 28 May 1993)
Highly oriented pyrolytic graphite has been implanted at room temperature with 165 keV C+-ions at doses from 6 X 1017 to 3 X 1019 ions/m 2 . Implantation-induced topographical changes of differing size scales were studied by optical, scanning electron, scanning tunneling, and atomic force microscopies. Defects with atomic resolution are seen for the lower dose implants. The formation of a vacancy line is revealed for the first time. At the higher doses a dendrite-like system of deep surface cracks is observed. This cracking develops as a result of the large basal plane contraction produced by irradiation which generates high shearing stresses between the implanted, damaged surface layer and the underlying material. Two independent systems of ridges have been characterized. One appears to follow a crystallographic direction while the other appears as a dense, intricate, generally curvilinear network with short ramifications. Additional experiments in which both the ion energy and dose rate have been varied indicate that ridge evolution progresses with increased energy and fluence, but is independent of dose rate. It is suggested that the ridge networks may form as a result of C transport by diffusion from the heavily damaged near-surface region or of a tectonic-plate-like motion or both. The geometric features of the ridge networks are related to the subsurface radiation damage as well.
I. INTRODUCTION In the past 20 years numerous studies on the effects of light ion bombardment of graphitic materials have been conducted.1 The effects of ion implantation have attracted interest not only regarding structural and microstructural changes, which may occur both in the bulk and at the surface, but also upon topographical alterations at the microscopic level that are promoted by ion bombardment. Pyrolytic graphites have been reported either to wrinkle or to crack and flake under He-,2"4 D-,5~7 or Ar-ion bombardment.8 Twinning was reported to occur under He- or D-bombardment9 as well as under C-bombardment.10 Implantations at 100 keV with a variety of ions including C were found to produce considerable surface damage at high doses, for all the irradiating species.11 More recently, scanning tunneling microscopy has been employed to examine radiation-induced topography changes at the atomic scale in highly oriented pyrolytic graphite (HOPG), with the aim of gaining direct knowledge of damage production. After irradiating with 20 keV C-ions to fluences < 1 X 1017 ions/m 2 , Coratger et al.12 observed the evolution of small hillocks
( 350 °C. However, anisotropic changes taking place under neutron irradiation at any temperature always involve an expansion along the c-axis direction and a contraction parallel to the basal plane.38 Evidence that similar anisotropic changes o
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