STRUCTURE AND NATURE OF VERY SMALL POINT DEFECT CLUSTERS STUDIES BY POSITRON LIFETIME AND TRANSMISSION ELECTRON MICROSCOPE

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It has been concluded that we can obtain reliable and new information about very small point defect clusters using both positron lifetime measurement and transmission electron microscopy (TEM) technique. TEM observation has analyzed the local structure of defect clusters, and positron lifetime measurement has detected small vacancy clusters whose size is smaller than the TEM resolution. The size of three dimensional vacancy clusters produced by displacement cascades in Cu and Si is smaller than divacancies, and the larger vacancy clusters collapse to stacking fault tetrahedra (SPT) and dislocation loops.Weak-beam dark-field (WBDF) method has been applied to identify the very small point defect clusters of vacancy-type and interstitial-type by TEM images. Systematic change of image-size has been examined for dislocation loops whose sizes are between 1 nm and 2 nm by systematically changing a deviation parameter from the Bragg condition. Nature of dislocation loops has been determined by measuring the change in size of the dislocation loops in Cu irradiated with 600 keV self-ions at 20 K and that after 120 K anneal at which temperature all the single interstitial atoms have already moved to dislocation loops and their sinks. Both vacancy and interstitial types are observed, but systematic change of sizes for each type of loops has been hardly observed. Further work using computer image-simulation is needed.Positron lifetime at SFT site has been determined, but is close to that at a vacancy site. Positron lifetime method has been able to evaluate the decrease in dislocation loop density, but has not been able to separate the effects of vacancy and interstitial types. Stereo measurement using TEM has disclosed the difference between the spatial distribution of vacancy-type defect clusters and that of interstitial-type defect clusters for the first time.