Transition metal perovskite chalcogenides are a new class of versatile semiconductors with high absorption coefficient and luminescence efficiency. Polycrystalline materials synthesized by an iodine-catalyzed solid-state reaction show distinctive optical colors and tunable bandgaps across the visible range in photoluminescence, with one of the materials' external efficiency approaching the level of single-crystal InP and CdSe.

Transition metal perovskite chalcogenides, a class of materials with rich tunability in functionalities, are gaining increased attention as candidate materials for renewable energy applications. Perovskite oxides are considered excellent n-type thermoelectric materials.Compared to oxide counterparts, we expect the chalcogenides to possess more favorable thermoelectric properties such as lower lattice thermal conductivity and smaller band gap, making them promising material candidates for high temperature thermoelectrics. Thus, it is necessary to study the thermal properties of these materials in detail, especially thermal stability, to evaluate their potential. In this work, we report the synthesis and thermal stability study of five compounds, a-SrZrS 3 , b-SrZrS 3 , BaZrS 3 , Ba 2 ZrS 4 , and Ba 3 Zr 2 S 7 . These materials cover several structural types including distorted perovskite, needle-like, and Ruddlesden-Popper phases.Differential scanning calorimeter and thermo-gravimetric analysis measurements were performed up to 1200°C in air. Structural and chemical characterizations such as X-ray diffraction, Raman spectroscopy, and energy dispersive analytical X-ray spectroscopy were performed on all the samples before and after the heat treatment to understand the oxidation process. Our studies show that perovskite chalcogenides possess excellent thermal stability in air at least up to 600°C.

Abstract

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.

The rates of excited-state decay through recombination processes determine the usefulness of a semiconductor for ambipolar devices. We find that recombination rates in chalcogenide perovskites are promising for continued progress towards solar cells.

Chemical disorder in semiconductors is important to characterize reliably because it affects materials performance, for instance by introducing potential fluctuations and recombination sites. It also represents a means to control material properties, to far-exceed the limits of equilibrium thermodynamics. We present a study of highly-disordered Cu-Zn-Sn-S (d-CZTS) films along the Cu2SnS3-Cu2ZnSnS4-ZnS binary line, deposited by physical vapor deposition. Deposition at low temperature kinetically stabilizes compositions that are well outside of the narrow, equilibrium solid solution of kesterite (Cu2ZnSnS4). Here we study d-CZTS and its thermal treatment using complementary characterization techniques: X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). We find that cations in d-CZTS are highly-disordered while the sulfur anions remain in a well-defined, cubic close-packed lattice. On the atomic scale, composition fluctuations are accommodated preferentially by stacking faults. Kinetically-stabilized cation disorder can produce nonequilibrium semiconductor alloys with a wide range of band gap, electronic conductivity, and thermal conductivity. d-CZTS therefore represents a processing route to optimizing materials for optoelectronic device elements such as light absorbers, window layers, and thermal barriers.

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van der Waals (vdW) layered chalcogenides have strongly direction-dependent (i.e., anisotropic) properties that make them interesting for photonic and optoelectronic applications. Orthorhombic tin selenide (α-SnSe) is a triaxial vdW material with strong optical anisotropy within layer planes, which has motivated studies of optical phase and domain switching. As with every vdW material, controlling the orientation of crystal domains during growth is key to reliably making wafer-scale, high-quality thin films, free from twin boundaries. Here, we demonstrate a fast optical method to quantify domain orientation in SnSe thin films made by molecular beam epitaxy (MBE). The in-plane optical anisotropy results in white-light being reflected into distinct colors for certain optical polarization angles and the color depends on domain orientation. We use our method to confirm a high density of twin boundaries in SnSe epitaxial films on MgO substrates, with square symmetry that results in degeneracy between SnSe 90° domain orientations. We then demonstrate that growing on a-plane sapphire, with rectangular lattice-matched symmetry that breaks the SnSe domain degeneracy, results in single-crystalline films with one preferred orientation. Our SnSe bottom-up film synthesis by MBE enables future applications of this vdW material that is particularly difficult to process by top-down methods. Our optical metrology is fast and can apply to all triaxial vdW materials.