Oxidation-Induced Structure Transformation: Thin-Film Synthesis and Interface Investigations of Barium Disilicide toward Potential Photovoltaic Applications.

Barium disilicide (BaSi) has been regarded as a promising absorber material for high-efficiency thin-film solar cells. However, it has confronted issues related to material synthesis and quality control. Here, we fabricate BaSi thin films via an industrially applicable sputtering process and uncovered the mechanism of structure transformation. Polycrystalline BaSi thin films are obtained through the sputtering process followed by a postannealing treatment. The crystalline quality and phase composition of sputtered BaSi are characterized by Raman spectroscopy and X-ray diffraction (XRD). A higher annealing temperature can promote crystallization of BaSi, but also causes an intensive surface oxidation and BaSi/SiO interfacial diffusion. As a consequence, an inhomogeneous and layered structure of BaSi is revealed by Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The thick oxide layer in such an inhomogeneous structure hinders further both optical and electrical characterizations of sputtered BaSi. The structural transformation process of sputtered BaSi films then is studied by the Raman depth-profiling method, and all of the above observations come to an oxidation-induced structure transformation mechanism. It interprets interfacial phenomena including surface oxidation and BaSi/SiO interdiffusion, which lead to the inhomogeneous and layered structure of sputtered BaSi. The mechanism can also be extended to epitaxial and evaporated BaSi films. In addition, a glimpse toward future developments in both material and device levels is presented. Such fundamental knowledge on structural transformations and complex interfacial activities is significant for further quality control and interface engineering on BaSi films toward high-efficiency solar cells.