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

结果表明，利用正电子寿命测量和透射电子显微镜（TEM）技术可以获得非常小的点缺陷簇的可靠的新信息。TEM观察分析了缺陷团簇的局部结构，正电子寿命测量检测到尺寸小于TEM分辨率的小空位团簇。Cu和Si中由位移级联产生的三维空位团簇尺寸小于空位团簇，较大的空位团簇坍塌为层错四面体（SPT）和位错环。采用弱光束暗场（WBDF）方法，对TEM图像中非常小的空位型和间隙型点缺陷簇进行了识别。通过系统地改变Bragg条件的偏差参数，研究了尺寸在1 nm至2 nm之间的位错环的图像尺寸的系统变化。通过测量位错环的大小变化来确定位错环的性质，在600 keV自离子照射下，在20 K和120 K退火后，所有的单间隙原子已经移动到位错环和它们的汇。空位型和间隙型都可以观察到，但几乎没有观察到每一种环的大小有系统的变化。需要使用计算机图像模拟进行进一步的工作。已经确定了SFT位置的正电子寿命，但与空位位置的正电子寿命接近。正电子寿命法能够评价位错环密度的降低，但不能区分空位型和间隙型的影响。利用TEM立体测量首次揭示了空缺型缺陷簇和间隙型缺陷簇的空间分布差异。

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.