Understanding the growth mechanism of BaZrS$_3$ chalcogenide perovskite thin films from sulfurized oxide precursors

TL;DR

This paper investigates the synthesis mechanism of BaZrS3 thin films from oxide precursors, revealing a two-step formation process involving amorphization and sulfur diffusion, with implications for optimizing material properties for energy applications.

Contribution

It provides a detailed formation mechanism of BaZrS3 from oxide precursors, highlighting the role of temperature and sulfur diffusion in the synthesis process.

Findings

Sulfur diffusion is the rate-limiting step in BaZrS3 formation.

Higher temperatures increase the conversion fraction from oxide to sulfide.

Stoichiometric BaZrS3 forms even under Zr-rich conditions, with ZrO2 as a secondary phase.

Abstract

Barium zirconium sulfide (BaZrS3) is an earth-abundant and environmentally friendly chalcogenide perovskite with promising properties for various energy conversion applications. Recently, sulfurization of oxide precursors has been suggested as a viable solution for effective synthesis, especially from the perspective of circumventing the difficulty of handling alkali earth metals. In this work, we explore in detail the synthesis of BaZrS3 from Ba-Zr-O oxide precursor films sulfurized at temperatures ranging from 700 {\deg}C to 1000 {\deg}C. We propose a formation mechanism of BaZrS3 based on a two-step reaction involving an intermediate amorphization step of the BaZrO3 crystalline phase. We show how the diffusion of sulfur (S) species in the film is the rate-limiting step of this reaction. The processing temperature plays a key role in determining the total fraction of conversion from oxide to sulfide phase at a constant flow rate of the sulfur-containing H2S gas used as a reactant. Finally, we observe the formation of stoichiometric BaZrS3 (1:1:3), even under Zr-rich precursor conditions, with the formation of ZrO2 as a secondary phase. This marks BaZrS3 quite unique among the other types of chalcogenides, such as chalcopyrites and kesterites, which can instead accommodate quite a large range of non-stoichiometric compositions. This work opens up a pathway for further optimization of the BaZrS3 synthesis process, straightening the route towards future applications of this material.