Thermal Growth of He-cavities in Si Studied by Cascade Implantation
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Thermal growth of He-cavities in Si studied by cascade implantation E. Ntsoenzok*, R. El Bouayadi**, G. Regula**, B. Pichaud**, S. Ashok*** * CNRS/CERI, 3A rue de la Ferollerie 45071 Orléans, France ** TECSEN, case 151, Faculte des Sciences ST-Jerome, 13397 MARSEILLE, France *** Department of Engineering Science and Mechanics, the Pennsylvania State University, 212 Earth and Engineering Science Building, University Park, PA 16802, USA ABSTRACT Float Zone (FZ) silicon samples have been cascade-implanted with helium ions at energies decreasing from 1.9 MeV to 0.8 MeV in steps of 0.1 MeV, with flux maintained between 5 x 1012 and 1 x 1013 He cm-2s-1. The dose was 5×1016 He cm-2 for all the energies except 0.8 MeV where a lower dose of 3×1016 He cm-2 was used. After thermal annealing, the sample was studied by cross section transmission electron microscopy (XTEM) using a Field Emission Gun Microscope (Jeol 2010F). Our results clearly demonstrate that these cavities mainly grow by the Ostwald ripening mechanism. This means a growth by exchange of He and vacancies from smaller to bigger cavities. Further this study provides essential data for resolving the controversy on the growth mechanism governing He-cavities. INTRODUCTION The key elements to the thermal growth of He-implantation induced cavities in Si are vacancies, interstitials, impurities and implanted atoms [1,2]. Formation and growth of nanocavities are very complex and further investigations are needed to obtain efficient models describing the actual phenomena . For cavity formation, a very good description was provided by Corni et al [1]. These authors stipulate the presence of three distinct layers beneath the Si surface: A surface layer with vacancy-like defects, an intermediate layer with He and vacancies, and a deeper third layer containing mobile He in interstitial sites. According to them, the most stable complexes formed inside the intermediate layer, and with annealing helium diffuses from the third layer towards helium vacancy complexes and then fills them. The thermal growth of these nanoscale entities is however subject to many questions and controversies, the most important being the mechanism leading to their thermal growth. Many authors favor growth by migration coalescence [3] while others attribute it to the Ostwald ripening mechanism [4]. In this study our goal is to use cascade implantation under differing implantation parameters to determine the most probable mechanism involved. EXPERIMENTAL PROCEDURE FZ Si samples have been cascade-implanted with helium ions at energies starting at 1.9 MeV and progressively reducing to 0.8 MeV in steps of 0.1 MeV, with the flux maintained between 5 ×1012 and 1 x 1013 He cm-2s-1. The dose was kept constant and equal to 5 × 1016 He cm-2 for all the energies except 0.8 MeV (3 × 1016 He cm-2). During the whole implantation procedure, the sample temperature did not exceed 100 °C. These multipleimplantation were performed by the means of the 3.5 MeV Van de Graaff.
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After thermal annealing at temp
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