Dislocations structure investigation in neutron irradiated silicon detectors using AFM and microhardness measurements

The structure, microhardness and deformation character for silicon detectors were investigated following a neutron irradiation, using optical and atomic force (AFM) microscopes. The results of these investigations have given an important contribution to the understanding of silicon damage process by neutron irradiation. It was shown that in the interval of neutron fluences 9.9 × 10¹⁰ ≤ Φ ≤ 3.12 × 10¹⁵ n/cm², the shape of damage is accumulative (from small punctual to large defects). Abrupt changes of microstructure together with the electrical and mechanical properties [Bosetti M, Croitoru N, Furetta C, Pensotti S, Rancoita M, Rattagi M, Redaelli M, Seidman A. Nucl Instr Methods B 1995;95:21; Croitoru N, Gambirasio A, Rancoita PG, Seidman A. Nucl Instr Methods B 1996;111:297; Croitoru N, Rancoita G, Rattaggi M, Rossi M, Seidman A. Nucl Instr Methods B 1996;114:120; Fretwurst N, Claussen N, Croitoru N, Papendick B, Pein U, Schatz H, Schultz T, Wunstorf R. Nucl Instr Methods A 1993;326:357; Croitoru N, Dahan R, Rancoita PG, Rattaggi M, Rossi G, Seidman A. Nucl Instr Methods B 1997;124:542], were found for Φ ≥ 10¹⁴ n/cm². Different kinds of defects (dislocations and interstitials) and their complexes appeared under neutron irradiation. For all fluences the regions ('White' - 'W') with a microhardness smaller than in nonirradiated silicon were observed. Microhardness is larger in the regions where the concentration of dislocation loops is high. The 'W' regions have a small number of the dislocations loops, and single punctual defects were seen there using atomic force microscope. The dislocation loops are placed in specific ('Black' - 'B') regions, which increase in size with the increase of neutron fluence due to a process of vacancies and interstitials accumulation.