The role of Si vacancies in the segregation of O, C, and N at silicon grain boundaries: An ab initio study

TL;DR

This study uses first-principles calculations to explore how silicon vacancies influence impurity segregation and electronic properties at grain boundaries, revealing that vacancies significantly affect C and O behavior but have complex effects on N.

Contribution

It provides new insights into the role of vacancies in impurity segregation and electronic property modifications at silicon grain boundaries, especially for C, N, and O impurities.

Findings

C and O segregation increases with vacancies

Electronic properties are unaffected by O but altered by C

N shows complex behavior with vacancies and segregation

Abstract

Grain boundaries (GBs) are defects originating in multi-crystalline silicon during crystal growth for device Si solar cell fabrication. The presence of GBs changes the coordination of Si, making it advantageous for charge carriers to recombine, which brings a significant reduction of carrier lifetimes. Therefore, GBs can be highly detrimental for device performances. Furthermore, GBs easily form vacancies with deep defect electronic states and are also preferential segregation sites for various impurity species, such as C, N, and O. We studied from first principles the correlation between structural, energetics, and electronic properties of the $\Sigma$3{111} Si GB with and without vacancies, and the segregation of C, N, and O atoms. C and O atoms strongly increase their ability to segregate when vacancies are present. However, the electronic properties of the $\Sigma$3{111} Si GB are not affected by the presence of O, while they can strongly change in the case of C. For N atoms, it is not possible to find a clear trend in the energetics and electronic properties both with and without vacancies in the GB. In fact, as N is not isovalent with Si, as C and O, it is more flexible in finding new chemical arrangements in the GB structure. This implies a stronger difficulty in controlling the properties of the material in the presence of N impurity atoms compared to C and O impurities.