Experimental investigations of plastically deformed elemental and III-V semiconductors prove that a high number of vacancies and vacancy clusters are formed. The formation of point defects by the motion of jogged dislocations is analysed. Vacancies formed behind the jog are not stable as a simple chain of vacancies. Instead, they are transformed immediately to stable three-dimensional agglomerates. The stability of various vacancy clusters has been investigated by means of density-functional calculations. The positron lifetime in such clusters was calculated and compared to experimental results. The association of open-volume defects with the dislocation can be derived from positron lifetime measurements. The analysis in a positron-trapping model characterizes the dislocation as a combined defect. The undisturbed dislocation line is a precursor trap for the positron capture in a deep trap related to the vacancies bound to the dislocation.