A critical assessment of dislocation multiplication laws in germanium

The issue concerning possible dislocation multiplication laws before the upper yield point of covalent materials is approached in 〈123〉 oriented Ge single crystals using the technique of stress relaxation and transient creep tests. Unlike in metals, relaxation tests exhibit a linear decrease of stress with time and inverse creep is observed. It is shown that these features are the signature of multiplication processes during the transient tests. Attempts at interpreting such particular behaviours are presented using three dislocation multiplication laws proposed in the literature for metals, for covalent crystals in general (based on etch-pit experiments) and for Si (based on computer simulations). The numerical reconstruction of the transient test curves shows that the last law is less satisfactory. This is explained by different stress exponents of the dislocation velocity, close to 1 in Si and to 2 in Ge. A realistic multiplication law should consider the heterogeneity of deformation at the onset of plasticity of these crystals.