Stabilizing BaZr(S,Se)₃ Chalcogenide Perovskite Alloys by Molecular Beam Epitaxy

<b>Symposium EQ04—Emerging Chalcogenide Electronic Materials—Theory to Applications</b><br/> <br/>Chemical intuition, first-principles calculations, and recent experimental results suggest that chalcogenide perovskites are an outstanding class of semiconductors. Chalcogenide perovskites feature the large dielectric response familiar in oxide perovskites, but also have band gap in the VIS-IR and strong light absorption [1]. Preliminary results suggest that chalcogenide perovskites feature excellent excited-state charge transport properties familiar in halide perovskites, while also being thermally-stable and comprised of abundant and non-toxic elements. Nearly all experimental results on chalcogenide perovskites to-date were obtained on powder samples and microscopic single crystals, and thin film synthesis is in its infancy. The history of complex oxide science teaches us that advances in fundamental understanding and development for applications will hinge on the availability of high-quality, controllable thin film synthesis, and that the best film quality and control is achieved by molecular beam epitaxy (MBE).<br/> <br/>We recently reported the first epitaxial synthesis of chalcogenide perovskite thin films by MBE: BaZrS₃ films on (001)-oriented LaAlO₃ substrates [2]. The films are atomically-smooth, and scanning transmission electron microscopy (STEM) data show an atomically-abrupt substrate/film interface. The sulfide perovskite film has a pseudo-cubic lattice constant more than 30% larger than the oxide perovskite substrate. This strain is fully accommodated by a remarkable, self-assembled interface buffer layer that enables epitaxial growth of strain-free films, and that the propensity for buffered epitaxy can be controlled by the H₂S gas flow during growth.<br/><br/>We further demonstrate control of the band gap by alloying BaZrS₃ with Se. We have made the first epitaxial BaZrS_(3-<i>y</i>)Se<i>_y</i> films with varying Se composition, up to and including a pure selenide perovskite BaZrSe₃. BaZrSe₃ is theoretically predicted to be stable in a non-perovskite, needle-like phase with very low band gap. We instead stabilized perovskite BaZrSe₃ film on a BaZrS₃ buffer layer grown on LaAlO₃ substrate using MBE. We support these findings with experiments including high-resolution STEM, high-resolution X-ray diffraction (HRXRD), and photocurrent spectroscopy (PCS). The films are smooth with an Rms roughness ranging from 0.65 to 3 nm. The out-of-plane HRXRD measurement confirmed the presence of the perovskite BaZrS₃ buffer layer and the perovskite BaZrS_(3-<i>y</i>)Se<i>_y</i> in the films. The in-plane XRD measurements and reciprocal space maps showed that the BaZrS_(3-<i>y</i>)Se<i>_y</i> layer grows strained, however, BaZrSe₃ grows relaxed on the BaZrS₃ buffer layer. The PCS measurements on the BaZrS_(3-<i>y</i>)Se<i>_y</i> films showed bandgap tunability, with bandgap decreasing from 1.57eV (y=1) to 1.50eV (y=1.3) to 1.49eV (y=2) with increasing Se content.<br/> <br/>This work sets the stage for developing chalcogenide perovskites as a family of semiconductor alloys with properties that can be tuned with strain and composition in high-quality epitaxial thin films, as has been long-established for other semiconductor materials.<br/><br/>[1] R. Jaramillo, J. Ravichandran, APL Materials 7(10) (2019) 100902.<br/>[2] I. Sadeghi et al., Adv. Func. Mater., (2021) 2105563.