Oxidation-InducedStructure Transformation: Thin-Film Synthesis and Interface Investigationsof Barium Disilicide toward Potential Photovoltaic Applications

摘要

Barium disilicide (BaSi2) 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 BaSi2 thin films via an industrially applicable sputtering process and uncovered the mechanism of structure transformation. Polycrystalline BaSi2 thin films are obtained through the sputtering process followed by a postannealing treatment. The crystalline quality and phase composition of sputtered BaSi2 are characterized by Raman spectroscopy and X-ray diffraction (XRD). A higher annealing temperature can promote crystallization of BaSi2, but also causes an intensive surface oxidation and BaSi2/SiO2 interfacial diffusion. As a consequence, an inhomogeneous and layered structure of BaSi2 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 BaSi2. The structural transformation process of sputteredBaSi2 films then is studied by the Raman depth-profilingmethod, and all of the above observations come to an oxidation-inducedstructure transformation mechanism. It interprets interfacial phenomenaincluding surface oxidation and BaSi2/SiO2 interdiffusion,which lead to the inhomogeneous and layered structure of sputteredBaSi2. The mechanism can also be extended to epitaxialand evaporated BaSi2 films. In addition, a glimpse towardfuture developments in both material and device levels is presented.Such fundamental knowledge on structural transformations and complexinterfacial activities is significant for further quality controland interface engineering on BaSi2 films toward high-efficiencysolar cells.