Study of Vacancy clusters and their electronic structure using positrons

正电子湮灭技术已成为研究金属和半导体中空位型缺陷的有力工具。正电子被灵敏地捕获在缺陷处，并与周围的电子一起湮灭，通过发射的两个伽马光子传递有关缺陷位置周围局部电子环境的重要信息。在该领域，已经采用了正电子寿命和多普勒展宽方法。然而，更复杂的正电子湮没技术“二维湮没角相关（2D-ACAR）”可以为我们提供有关缺陷的更重要的信息。本研究针对Si、SiC及其相关材料开展了2D-ACAR实验。此外，我们还对2D-ACAR谱进行了第一性原理计算。对Si中单负双空位的2D-ACAR分布的有趣特征进行了实验和理论研究。对于具有对齐双空位的样本，成功检测到分布的各向异性，并通过基于二元（正电子-电子）密度泛函理论的第一原理计算很好地重现了分布的各向异性。本计算表明，各向异性反映了双空位周围电子的特征分布，表明 2D-ACAR 是提供空位型缺陷微观信息的有效工具。对于硅、碳化硅、金刚石和石墨的完美晶体以及其中的空位也获得了非常有用的信息。

The positron annihilation technique has emerged as a powerful tool to study vacancy-type defects in metals and semiconductors. Positrons are sensitively trapped at defects and are annihilated there with the surrounding electrons, conveying significant information on the local electronic environment around the defect site with the emitted two gamma photons. In this field, positron lifetime and Doppler broadening methods have been emplyed. However, a more sophisticated positron annihilation technique "two dimensinal angular correlation of annihilation (2D-ACAR)"can provide us with more significant information about the defects. In this study, 2D-ACAR experiments have been carried out on Si, SiC and their related materials. Furthermore we have made first-principles calculations for 2D-ACAR spectrum.Interesting features of 2D-ACAR distribution for singly negative divacancies in Si were studied experimentally and theoretically. Anisotropy of the distribution is successfully detected for a specimen with aligned divacancies and well repreduced by first-principles calculations based on the two-component (positron-electron) density functional theory. The present calculation demonstrates that the anisotropy reflects the characteristic distribution of electrons around the divacancies, indicating that the 2D-ACAR is an effective tool to provide microscopic information on vacancy-type defects. Very useful information has been also obtained for perfect crystals of Si, SiC,diamond and graphite, and vacancies in them.