Study of Vacancy clusters and their electronic structure using positrons

正电子湮没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，并在那里与周围的电子一起湮灭，用发射的两个伽马光子传递关于缺陷部位周围的局部电子环境的重要信息。在这一领域中，正电子寿命和多普勒展宽方法已得到应用。然而，一种更先进的正电子湮没技术“二维角相关湮没（2D-ACAR）”可以为我们提供更有意义的缺陷信息。本文对Si、SiC及其相关材料进行了二维ACAR实验研究。此外，我们还对2D-ACAR谱进行了第一性原理计算，从实验和理论上研究了Si中单负双空位的2D-ACAR分布的一些有趣特征。的分布的各向异性被成功地检测到与对齐的双空位和repreduced的第一性原理计算的基础上的两个组件（正电子-电子）密度泛函理论的试样。计算结果表明，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Si、SiC、金刚石和石墨的完整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.