Structural Characterization of BaZrS(3-y)Sey Perovskite Thin Films via Scanning Transmission Electron Microscopy

Chalcogenide perovskites show great potential for becoming solar cell materials due to their tunable, direct bandgaps in the visible range, physics-rich combination of ionic and covalent bonding, and use of nontoxic/abundant elements [1].While high quality thin film growth of these materials was previously difficult, recent advances in chalcogenide molecular beam epitaxy enabled a new route to study their fundamental properties.For example, in this work, by alloying BaZrS 3 and BaZrSe 3 , the bandgap of the resulting material can be tuned from ∼1.8 to ∼1.3 eV with high polarizability for polaron formation and slow defect-assisted recombination rates.Complete control of properties, however, necessitates understanding the formation of other extended defects, such as antiphase boundaries and rotation variants, in the alloy films.In this talk, we expand on previous work [2] by characterizing thin films of the BaZrS (3-y) Se y (BZSSe) perovskite alloy system grown on a BaZrS 3 (BZS) template on a LaAlO 3 (LAO) substrate by molecular beam epitaxy.Specifically, we will discuss the structure of BZSSe films (y = 1 -3) and their defects determined using atomic-resolution scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS).We will show that the crystal structure of both the template and the alloy layers is perovskite, and exhibits two competing epitaxial relationships, as previously seen in pure BZS films.While the film is relaxed overall, there is a high concentration of antiphase boundaries (APBs) in both layers.Those in the template region are predominately oriented perpendicular to the grown plane, while those near the surface in the alloy predominately lie in-plane.Moreover, EDX and EELS indicate a gradient of Se concentration that reaches down to the substrate interface and correlates with the rotation of the of the APBs.Finally, we will also show that structural distortions introduced by the APBs explain features in the X-ray diffraction profiles [3].