High-temperature post-annealing effect on the surface morphology and photoresponse and electrical properties of B-doped BaSi2 films grown by molecular beam epitaxy under various Ba-to-Si deposition rate ratios

Semiconducting barium disilicide (BaSi2) is one of the emerging materials for light absorber layers in solar cell applications. We investigated the effect of post-annealing on various properties of lightly B-doped BaSi2 films by molecular beam epitaxy grown under various Ba-to-Si deposition rate ratios (RBa/RSi) in the range 0.4-4.0. Postannealing was performed at 900 degrees C for 2 min in an Ar atmosphere. As-grown samples formed under Si-rich conditions (RBa/RSi = 0.4 and 1.2) showed p-type conductivity, while those formed under Ba-rich conditions (RBa/RSi = 2.2, 2.6, 3.0 and 4.0) showed n-type conductivity. The conductivity type and the carrier concentration remained almost unchanged after the post-annealing. In contrast, there was a significant difference in surface morphology and photoresponsivity. We compared the photocurrent density (Jph) obtained from integrating the product of each sample's external quantum efficiency spectrum and AM 1.5 spectrum. For samples grown under Ba-rich conditions (RBa/RSi >= 2.2), cracks were generated on the surface, and the Jph decreased. The Jph particularly decreased by approximately two orders of magnitude for the sample formed at RBa/RSi = 2.6. On the other hand, the cracks were hardly observed by optical microscopy when grown under Si-rich conditions (RBa/ RSi = 0.4 and 1.2), and the Jph increased. Transmission electron microscopy observation revealed the presence of crystalline Si islands around the BaSi2/Si interface caused by the aggregation of excess Si atoms in the sample formed at RBa/RSi = 1.2.