Surface Processing and Characterization of Stoichiometry-Varied BaZrS₃ Thin Films

The chalcogenide perovskite BaZrS3 is a semiconductor that exhibits a high absorption coefficient and is composed of earth-abundant elements, making it a promising candidate for sustainable optoelectronic devices. To integrate BaZrS3 thin films into devices, one needs to obtain clean surfaces and characterize them thoroughly. Herein, we report a sputtering-annealing method to produce clean surfaces of prefabricated BaZrS3 thin films with varying metal ratios (Ba-rich, stoichiometric, and Zr-rich). This method combines Ar sputtering with high-temperature annealing (600 and 750 °C) in ultra-high vacuum. Depth-profiling via photoelectron spectroscopy with soft (950 eV) and hard X-rays (6.6 keV) confirms that this processing route substantially mitigates undesired surface oxidation of the films, revealing predominantly core level peaks characteristic of the BaZrS3 perovskite. As a drawback, the sputtering process also produces Zr0, which persists in the Zr-rich sample even after the annealing treatment. In contrast, the Ba-rich and stoichiometric BaZrS3 samples converge to similar surface compositions free of Zr0, and the low roughness of the Ba-rich thin film indicates its preference for device integration. While the thermal treatment modifies the surface chemistry, the bulk characteristics, e.g., nominal metal-ratio, orthorhombic structure, and crystallite sizes, remain unaffected. However, high-temperature annealing affects band realignment with respect to the Fermi level, resulting in n-type doping characteristics. By correlating the experimentally measured valence band to the density functional theory calculated molecular orbital picture, we assign the valence band features to specific elemental orbitals and their interactions. The proposed cleaning procedure has the potential to advance the application of BaZrS3 in layered devices, such as photovoltaic cells and photodetectors.