Process Development and Formation Chemistry of BaZrS₃ Thin Films via a Two-Stage PVD Process

Chalcogenide perovskites like BaZrS3 are promising candidates for tandem solar cells due to their stability, nontoxicity, and ideal bandgap of ∼1.8 eV. However, their thin-film fabrication remains challenging due to the high reactivity of Ba and Zr, leading to oxidation and phase impurities. Here, we report on further development of a two-step physical vapor deposition process that enables BaZrS3 thin films with high crystallinity (FWHM of the XRD peak corresponding to the (121) plane of BaZrS3 decreases by 38%) and exceptionally low oxygen content (3% in bulk), which is difficult to achieve with conventional methods. The process involves cosputtering of Ba–Zr metal precursors followed by thermal treatment in a sulfur-rich atmosphere, with a SnS capping layer protecting the precursor from oxidation during transfer and annealing. A comparative study of cosputtered and layered metal precursors reveals that intermixing of Ba and Zr during deposition is essential for uniform sulfurization and improved crystallinity. Introducing a Zr interlayer effectively prevents reactions between the metal precursor and the substrate, enabling BaZrS3 growth on silicon, quartz, and metallic substrates while minimizing the impurity phase formation. By investigating the reaction mechanism, differential metal reactivity, and detrimental role of ZrO2, we demonstrate that protecting Zr from oxidation is crucial for phase purity and film quality. We demonstrate that slowing down the evaporation of the SnS capping layer using additional SnS vapor during sulfurization effectively blocks oxygen diffusion into the film, resulting in highly crystalline BaZrS3 thin films. These findings provide fundamental insights into BaZrS3 synthesis and offer a scalable approach for integrating chalcogenide perovskites into photovoltaic applications.