The effect of S source on BaZrS₃ crystallization at low temperatures: H₂S vs. elemental S

Chalcogenide perovskites, particularly BaZrS₃, hold great promise for optoelectronic applications. Lowering the synthesis temperature of high-quality BaZrS₃ films has been a long pursuit that has recently yielded significant progress. However, achieving reproducible, device-grade films at moderate temperatures below 600 °C remains a significant challenge. This difficulty is primarily driven by three limiting factors that conventionally hinder BaZrS₃ crystallization: the formation of deep binary thermodynamic sinks, the inherently low atomic mobility of Zr ions, and competing reactions with O. In this work, we use a two-step process of reactive sputtering and post-annealing to demonstrate that the choice of S source in the first step dictates the crystallization pathway in the second. A directional evaporated S₂ beam provides a high chemical potential, which effectively suppresses O incorporation, but risks trapping the system in a rigid intermediate phase. In contrast, diffuse H₂S gas is likely to supply additional kinetic energy to the growing film through energetic ion bombardment. This promotes intimate atomic mixing within the amorphous precursor, bypassing binary phase separation and enabling crystallization in the subsequent post-annealing step even in the presence of O. Ultimately, the diffuse H₂S route decouples perovskite crystallization from the need for S-rich intermediate phases, while the directional beam creates a highly S-rich environment directly within the sputtering process. Both methods offer practical approaches to integrating these materials into optoelectronic devices by crystallizing films at 600 °C and potentially below, each with distinct advantages.