Electronic mobility, doping, and defects in epitaxial $\mathrm{BaZrS_3}$ chalcogenide perovskite thin films

TL;DR

This study investigates the electronic transport, doping, and defect mechanisms in epitaxial BaZrS3 thin films, revealing high mobility, defect-related property changes over time, and strategies for stabilization, advancing chalcogenide perovskite applications.

Contribution

It provides the first detailed analysis of transport properties, defect mechanisms, and microstructure effects in epitaxial BaZrS3 thin films, including the impact of environmental exposure.

Findings

Achieved a record RT Hall mobility of 11.1 cm^2V^{-1}s^{-1}

Identified sulfur vacancies as shallow donors that convert to neutral oxygen defects upon air exposure

Correlated mobility with stacking fault concentration and microstructure

Abstract

We present the electronic transport properties of $\mathrm{BaZrS_3}$ (BZS) thin films grown epitaxially by gas-source molecular beam epitaxy (MBE). We observe n-type behavior in all samples, with carrier concentration ranging from $4 \times 10^{18}$ to $4 \times 10^{20} \mathrm{cm^{-3}}$ at room temperature (RT). We observe a champion RT Hall mobility of 11.1 $\mathrm{cm^2V^{-1}s^{-1}}$, which is competitive with established thin-film photovoltaic (PV) absorbers. Temperature-dependent Hall mobility data show that phonon scattering dominates at room temperature, in agreement with computational predictions. X-ray diffraction data illustrate a correlation between mobility and stacking fault concentration, illustrating how microstructure can affect transport. Despite the well-established environmental stability of chalcogenide perovskites, we observe significant changes to electronic properties as a function of storage time in ambient conditions. With the help of secondary-ion mass-spectrometry (SIMS) measurements, we propose and support a defect mechanism that explains this behavior: as-grown films have a high concentration of sulfur vacancies that are shallow donors ($\mathrm{V_S^\bullet}$ or $\mathrm{V_S^{\bullet \bullet}}$), which are converted into neutral oxygen defects ($\mathrm{O_S^\times}$) upon air exposure. We discuss the relevance of this defect mechanism within the larger context of chalcogenide perovskite research, and we identify means to stabilize the electronic properties.