Dislocation patterning in fatigued silicon single crystals

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Dislocation patterning in fatigued silicon single crystals M. Legros1, O. Ferry2, J.-P. Feiereisen2, A. Jacques2 and A. George2 CEMES-CNRS, 29 rue J. Marvig, 31055 Toulouse, France 2 LPM, Ecole des Mines, Parc de Saurupt, 54042 Nancy, France

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ABSTRACT Tension compression fatigue tests and subsequent TEM observations were conducted on single crystalline silicon in a temperature and strain rate domain where lattice friction is still effective: 800-900°C and 1.5 to 6x10-4s-1. Samples oriented for single slip conditions were cyclically loaded under plastic strain amplitude control. For amplitudes ranging from 6x10-4 to 10-2, cyclic stress-strain curves exhibit two different stages of hardening and pass through a maximum before saturation is reached. TEM observations suggest that strain localization takes place near the maximum cyclic stress and beyond. Before mechanical saturation, edge dislocation dipoles sit mainly in thick rectilinear walls. Once the maximum stress is reached, these thick walls "condense" in much thinner walls that seem to carry out the imposed deformation while other regions become inactive. In this case, the dislocation structure anneals out and a loop structure is created from the dipolar walls. INTRODUCTION Persistent slip bands (PSBs) are one of the most striking examples of dislocation patterning. They are observed in fcc metals strained in tension compression fatigue [1-5] and consist of ladder-like structures, in which the rungs are made of dipolar dislocation walls. These PSBs form the softer region of the fatigue dislocation structure and thus concentrate the deformation. Because mobile dislocations are confined between dipolar dislocation walls, the interwall spacing (or channel width) controls the macroscopic cyclic stress that saturates once the PSBs are established in the material [6]. When the plastic strain per cycle (γp) is increased, the macroscopic stress remains constant as only the volume fraction of PSBs increases. Such relation between stress and strain is much complex in the case of BCC metals [1], and wavy slip alloys [7, 8]. From a metallurgical point of view, silicon is an attractive material for fatigue experiments : it combines the slip modes of fcc metals, a rather low stacking fault energy, (comparable to that of Cu) and the large lattice friction which characterizes most of bcc metals [9, 10]. Because of their brittleness, the pioneering experiment of Scoble and Weissmann [11] was, until recently, the only available work concerning fatigue of semiconductors. We have now shown that uniaxial tension/compression testing of single crystalline Si was feasible under strain amplitude control and that PSB-like structures could exist after a large number of cycles [12]. However, it has been pointed out that the ladder structure may not be as common in Si as in fcc metals and that new dislocation patterns may be more representative [13, 14]. The aim of this paper is to analyze some of these structures in relation to the measured mechanical properties between 825°C an

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