Plastic Deformation of Intrinsic and Extrinsic Silicon Below 500°C

Abstract

In this paper we report an analysis of the plasticity of silicon in a range of low temperatures where it usually responds to a standard uniaxial compression by brittle fracture. Indeed, it has been previously shown that silicon is able to undergo macroscopic permanent strain in compression provided cracks are prevented from propagating, by superimposing an hydrostatic pressure onto the sample1 2. This is but imperfectly attained in microindentation tests where the stress and pressure gradients are not quantitatively known resulting in ambiguous information. Concurrently, a modified version of the Griggs device3, mostly used by geologists, has been used recently with success for spinel4, sapphire5 acid silicon2.It has been shown to offer sufficient accuracy to allow acceptable standard measurements of the yield stress and of the activation parameters of the deformation. Therefore, it was interesting to extend our knowledge of the plasticity of brittle materials to : (i) low temperatures; i.e. temperatures at which the point defects (dopants, impurities, thermal point defects) are no longer mobile. Indeed, aliovalent impurities in ionic crystals as well as electrically active dopants in semiconductors have proved to affect drastically the velocity of dislocations. It is therefore important to carry out experiments at temperatures where the mobility of the point defects is minimized. Furthermore, the core structure of dislocations in silicon is matter of active investigation since, in order to explain several electrical and mechanical properties, it has been repeatedly postulated that it should be associated with (ii) high stresses; i.e. stresses comparable to or larger than γ/b for example. When the stress applied to one partial dislocation is larger than this limit, the partial may overcome the spring force exerted by the stacking fault and would then move independent from the other partial. This has been already observed in silicon2, it was shown that at stresses of the order of 300 MPa, the dislocation microstructure in samples deformed along , is mostly composed of extended stacking faults and twins: features which suggest that the individual rather than the correlated motion of partials has been operating during the deformation.