Strain-Free Perovskite Hetero-Chalco-Epitaxy with Giant Lattice Constant Mismatch Enabled by Self-Assembled Surface Passivation Using Gas-Source MBE

Chemical intuition, first-principles calculations, and experiments suggest that sulfides and selenides in the perovskite and related crystal structures - chalcogenide perovskites - are promising semiconductors for optoelectronics and energy conversion applications [1]. Chalcogenide perovskites feature the large dielectric response familiar in oxide perovskites, but also have band gap in the VIS-IR and strong light absorption with direct band gap. Chalcogenide perovskites feature excellent excited-state charge transport properties (<i>e.g.</i>, long diffusion lengths), while also being thermally-stable and comprised of abundant and non-toxic elements. Nearly all experimental results on chalcogenide perovskites have been obtained on powders and microscopic crystals; thin film synthesis is in its infancy. Realizing the promise of a new family of semiconductor alloys requires developing methods for epitaxial thin film synthesis on-par with established semiconductor materials (Si-Ge, III-Vs, <i>etc</i>.) and complex oxides.<br/>We report the first epitaxial synthesis of chalcogenide perovskite thin films by molecular beam epitaxy (MBE): BaZrS₃ films on (001)-oriented LaAlO₃ substrates. The films are stoichiometric and oxygen-free, as confirmed by X-ray fluorescence and electron energy loss spectroscopy. The perovskite phase is confirmed by X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and Raman spectroscopy. The films are atomically-smooth over large areas, and STEM data show an atomically-abrupt substrate/film interface with no chemical interdiffusion.<br/>The sulfide perovskite film has a pseudo-cubic lattice constant more than 30% larger than the oxide perovskite substrate. Strain considerations predict rotated-cube-on-cube growth, wherein the cube diagonals of the substrate align with the cube edges of the film, reducing the lattice constant mismatch. Remarkably, we find that growth is instead dominated by the formation of a fully-strain-relaxed film with cube edges aligned with the substrate, enabled by a self-assembled buffer layer. Direct, rotated-cube-on-cube epitaxy is observed only at substrate step edges, and constitutes a minority of the film. The propensity for buffered epitaxy can be controlled by the H₂S gas flow during growth. We will report detailed studies of the role of H₂S in passivating the substrate, leading to the formation of an atomically sharp buffer layer with van der Waals characteristics, supporting the growth of fully-relaxed, epitaxial films of a sulfide perovskite on an oxide perovskite (hetero-chalco-epitaxy) despite the giant lattice constant mismatch. We will report on the generality of this phenomenon for different oxide substrates, and on the use of substrates with higher step-edge density to enhance the propensity for direct epitaxy. We will present detailed STEM and high-resolution XRD studies that investigate the self-assembled buffer layer, and provide clues to its formation. If time allows, we will also present on the use of BaZrS₃ films to stabilize BaZr(S,Se)₃ alloys in the perovskite structure, using H₂S and H₂Se sources, providing a path towards tuning the direct band gap in the range 1.3 – 1.9 eV.<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. Our methods also represent a revival of gas-source chalcogenide MBE, with potential for impact on research on other sulfur- and selenium-containing compounds.<br/>[1] Jaramillo, R. & Ravichandran, J. In praise and in search of highly-polarizable semiconductors: Technological promise and discovery strategies. <i>APL Materials</i> <b>7,</b> 100902 (2019).<br/>[2] Sadeghi, I., Ye, K., Xu, M., Li, Y., LeBeau, J. M. & Jaramillo, R. Making BaZrS₃ Chalcogenide Perovskite Thin Films by Molecular Beam Epitaxy. <i>Advanced Functional Materials</i> <b>n/a,</b> 2105563 (2021); DOI 10.1002/adfm.202105563.