Relationship Between Stress and Surface Roughness in Krypton Implanted MgO
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1. INTRODUCTION It has been shown in several ways [ 1-4]that the surface of a solid subjected to a normal stress, may develop a roughness, if mass transport can occur. The roughness may be fed by surface or bulk diffusion or by plastic deformation. An important number of experimental studies report the observation of surface instabilities in various domains, such as the growth of epitaxial films [2, 5], at the interface between a He crystal and its melt [6-8], during polymerization of diacethylenes [9]. The instability arises through various modes of deformation : surface diffusion, bulk diffusion or plastic deformation. An interesting example
where plastic deformation is involved is that of Villechaise [10] who has observed fluctuations at the surface of a copper crystal plastically deformed by fatigue. The purpose of this paper is to discuss the surface roughness induced by ion implantation ; we will try to show the importance of the radiation induced plastic deformation on the surface instability evolution. The system studied is a (001) MgO surface implanted at a high fluence with 150 keV Kr atoms. MgO is an ionic crystal that shows no detectable deviation from stoichiometry and implantation effects in this material are very-well documented [11-13]. In MgO, implantation defects are created by nuclear collisions induced by the incident ions ; interstitials and vacancies are created in both sublattices (F, F+ centers, V centers), as well as stoichiometric dislocation loops. As a consequence of defect creation, most materials exhibit swelling. It is shown here that in the case of Kr implanted (001) MgO surface, the swelling is not uniform but that the surface is undulated. This paper, attemps to correlate the appearance of such a roughness to a stressdriven rearrangement instability fed by displacements of implantation defects. 2. EXPERIMENTAL (001) surfaces of magnesium oxide single crystals were used. The samples were cleaved a few minutes before implantation. Implantation was performed at room temperature with the mean of a 200 keV Balzers implantor. The fluence rate was about 1012 ions.cm- 2 .s-1 and an angle of 70 was kept between the normal to the sample surface and the beam direction in order to avoid parasiting channeling effects. The samples was subsequently submitted to an ionizing irradiation with the help of a 2 MeV He ions beam generated from a Van de Graaff source 57
Mat. Res. Soc. Symp. Proc. Vol. 356 01995 Materials Research Society
(conditions : room temperature, fluence of 1016 He.cm- 2 ). Investigation of the as-implanted samples before and after reirradiation was made with Rutherford Backscattering Spectroscopy (RBS) to evaluate the implantation damage and the location of the implanted particles and a Vickers microindentation technique to measure the implantation stresses. A Parks Scientific Instruments (PSI) Atomic Force Microscope (AFM) was used to perform a fine study of the surface roughness, immediatly after the implantation and reirradiation. The topographic images were taken in air wi
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