The nucleation of partial dislocations from a surface is studied. A model is considered for three elementary cases related to the state of the surface (steps or plane). A complex condition is analysed, involving two Shockley partials gliding on the (111) plane of a fee crystal with a (100) surface. The stress-stacking-fault energy plane (sigma-gamma) can then be divided into three parts where complete dissociated dislocation or a partial will or will not nucleate. Quantitative results are provided for silicon, and the behaviour is predicted, consistent with previous observations. [References: 25]
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