The making of BaZrS 3 thin films by molecular beam epitaxy (MBE) is demonstrated. BaZrS 3 forms in the orthorhombic distorted-perovskite structure with corner-sharing ZrS 6 octahedra. The single-step MBE process results in films smooth on the atomic scale, with near-perfect BaZrS 3 stoichiometry and an atomically sharp interface with the LaAlO 3 substrate. 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 highquality 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.

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InGaAs and InAlAs epilayers were grown on InP(111)B substrates by molecular beam epitaxy. Rather than focusing on a specific off-cut angle, the growths were done on rounded wafer edges, which expose a broad spectrum of vicinal surfaces with varying off-cut angle and off-cut azimuth. The epilayers were grown at several different growth conditions by varying the growth temperature, growth rate, and arsenic (As) overpressure. The epitaxial layers were characterized at the center and the edge of the wafers using Nomarski differential interference contrast microscopy and atomic force microscopy. It was shown that a minimum misorientation angle of ∼0.4° should be used in order to avoid pyramidal hillocks. At higher misorientations, 1.7°–3°, step bunching can lead to surface roughening.

Perovskite sulfides, with tunable and direct bandgaps in the visible regime and excellent physical properties comparable to existing oxide perovskites, show promise as future optoelectronic and solar applications [1,2,3]. With the recent success of epitaxial thin film growth of BaZrS 3 (BZS) via molecular beam epitaxy [4], new opportunities are available to engineer material properties through interface and defect engineering. As such, characterizing the film structure, defects, and the filmsubstrate interface at the atomic scale is essential for establishing growth behavior and improving film quality and functionality.