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

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 Σ3{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 Σ3{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.

Insight into the inclusion of heteroatom impurities in Silicon structures

“…E n X +VGB is the total energies of Si GB containing n X number of impurity atoms in presence of a vacancy and E VGB is the energy of Si GB with a vacancy. [7] 2. Chemical bond…”

“…flat N coordinations). This is linked to the fact that some of these structures are gapless (see SI: Table VIII) [7]. Bond charges get far away from nominal and are compensated by LPs.…”

“…[3][4][5] In silicon-based devices, defects such as impurities, vacancies, interstitial and grain boundaries (GBs) modify the functionality and the performance of the devices. [1,[6][7][8] In this context, polycrystalline silicon (poly-Si) is very attractive for several technological applications [8,9]. However, its efficiency can be strongly modified because poly-Si is characterized by a high density of GBs which are the primary hindrance to its functionality.…”

“…Furthermore, the electrical properties of poly-Si and GBs are additionally altered by the complex interaction with intrinsic point defects (vacancies and/or self-interstitials) and/or impurity atoms, such as carbon, oxygen, nitrogen, and various types of metals. [7,10,11] Moreover, the interaction of these light impurities (C, O, N) with poly-Si under certain circumstances can facilitate the formation of microstructures of SiC, SiO 2 , and Si 3 N 4 [10].…”

“…). This leads to7.98 e around each oxygen atom.When we introduce oxygen as an interstitial, patterns are very regular. Oxygen atoms always appear in between two Si atoms, O-O bonds are never observed.…”