First-principles insights into the incorporation of arsenic in BaSi2 thin films grown by molecular beam epitaxy

Abstract

Barium disilicide (BaSi₂) is one of the promising non-toxic and abundant materials for thin-film solar cells. However, defective n-type BaSi₂ layers hinder BaSi₂ homojunction solar cells from achieving higher conversion efficiencies. In recent years, in situ doping of arsenic (As) has been investigated to form n-type BaSi₂ layers. However, a-axis-oriented As-doped BaSi₂ epitaxial layers grown by molecular beam epitaxy (MBE) have a low electron concentration. The donor activation of As-doped BaSi₂ layers grown by MBE was then evaluated. The highest donor activation ratio in this study was found to be approximately 14%. To clarify the reason for this, we used first-principles calculations to evaluate the adsorption of As₂ used in the MBE growth on BaSi₂ (1 0 0) surface and the As-related defects in bulk BaSi₂. The calculations reveal that adsorbed As₂ tend not to dissociate into singular As adatoms. Furthermore, the 2 As_Si defect complex, which has the lowest formation energy compared to other As-related defects, was found to deactivate the donor based on its electronic structure. This study provides valuable insight into donor deactivation in As-doped BaSi₂ layers and serves as a starting point for future investigations on how to control it.