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Abstract The synthesis of BaZr(S,Se)₃chalcogenide perovskite alloys is demonstrated by selenization of BaZrS₃thin films. The anion‐exchange process produces films with tunable composition and band gap without changing the orthorhombic perovskite crystal structure or the film microstructure. The direct band gap is tunable between 1.5 and 1.9 eV. The alloy films made in this way feature one‐hundred‐times stronger photoconductive response and a lower density of extended defects, compared to alloy films made by direct growth. The perovskite structure is stable in high‐selenium‐content thin films with and without epitaxy. The manufacturing‐compatible process of selenization in H₂Se gas may spur the development of chalcogenide perovskite solar cell technology. 

Abstract The making of BaZrS₃thin films by molecular beam epitaxy (MBE) is demonstrated. BaZrS₃forms in the orthorhombic distorted‐perovskite structure with corner‐sharing ZrS₆octahedra. The single‐step MBE process results in films smooth on the atomic scale, with near‐perfect BaZrS₃stoichiometry and an atomically sharp interface with the LaAlO₃substrate. The films grow epitaxially via two competing growth modes: buffered epitaxy, with a self‐assembled interface layer that relieves the epitaxial strain, and direct epitaxy, with rotated‐cube‐on‐cube growth that accommodates the large lattice constant mismatch between the oxide and the sulfide perovskites. 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 systems including Si‐Ge, III‐Vs, and II‐VIs. The methods demonstrated here also represent a revival of gas‐source chalcogenide MBE.