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

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.