Interplay between Growth Mechanism, Materials Chemistry, and Band Gap Characteristics in Sputtered Thin Films of Chalcogenide Perovskite BaZrS₃

Abstract

The prototypical chalcogenide perovskite BaZrS3, characterized by its direct band gap, exceptionally strong light-harvesting ability, and good carrier transport properties, provides fundamental prerequisites for a promising photovoltaic material. This inspired the synthesis of BaZrS3 in the form of thin films, using sputtering and rapid thermal processing, aimed at device fabrication for future optoelectronic applications. Using a combination of short- and long-range structural information from X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD), we have elucidated how, starting from a random network of Ba, Zr, and S atoms, thermal treatment induces crystallization and growth of BaZrS3 and explained its impact on the observed photoluminescence (PL) properties. We also provide a description of the electronic structure and substantiate the surface material chemistry using a combination of depth-dependent photoelectron spectroscopy (PES) using hard X-ray (HAXPES) and traditional Al K α radiation. From the knowledge of the optical band gap of BaZrS3 thin films, synthesized at an optimal temperature of 900 °C, and our estimation of the valence band edge position with respect to the Fermi level, one may conclude that these semiconductor films are intrinsic in nature with a slight n-type character. A detailed understanding of the growth mechanism and electronic structure of BaZrS3 thin films helps pave the way toward their utilization in photovoltaic applications.

Electronic structure characterization by photoelectron spectroscopy of BaZrS3 perovskite powder and thin film

“…In both cases the data were fitted with three peaks at 530.80 eV, 532.11 eV and 533.41 eV, respectively assigned to Zr bound to O, BaSO4 and chemisorbed O, with the proportion of the different O species being different between the powder and the thin film. Similar results were obtained in the paper by Mukherjee et al 18 , where the varying ratios of the chemical species obtained at different annealing temperatures could be attributed to the varied surface oxidation effects.…”

“…The thin film was prepared on a silicon substrate via sputtering from BaS and Zr target in an atmosphere of H2S and Ar, with a consequent annealing process at 900 °C, as described in our previous report 7 . The sample was defined as the best of a temperature annealing series in terms photoluminescence peak, XRD analysis, XAS and XPS characterization 7,18 .…”

“…The XAS data were collected to achieve good statistics up to ∼ 1000 eV beyond the Zr edge position, which translates to wavenumbers as high as ∼ 16 Å −1 . The corresponding Zr K-XAS measurements on the sputtered thin films of BaZrS3 were performed at the BALDER beamline, as reported previously 18 . All XAS data sets were processed and analyzed using Athena-Artemis software suite, a front end to FEFF and IFEFFIT.…”

“…A similar analysis of the BaZrS3 thin films yield unit cell dimensions of a = 7.0511 Å, b = 9.9569 Å, c = 7.0479 Å, rather similar to the unit cell dimensions of the powder sample, but a smaller average crystallite size of ̴ 22 nm as already reported 7 . For the details about the polycrystalline thin film, we refer to the papers 7,18 . The powder and thin films of BaZrS3, although grown using very different synthesis strategies, achieves the same orthorhombic structure with rather comparable unit cell dimensions, suggesting the orthorhombic structure to be energetically favored over other higher symmetry perovskite structures.…”

“…Table 1 compiles the morphological and geometric structure information for the powder in terms of crystallite sizes, unit cell dimensions, Zr-S-Zr interatomic distances, Zr-S-Zr bond angles and Zr-S disorder (σ 2 ), and compares it to the reported parameters for thin film 18 Overall, we find the parameters in Table 1 to be rather comparable, except for the higher crystallinity of the powder, as evidenced by the bigger crystallite size from the XRD peak and lower σ 2 for Zr-S bond for the powder as compared to the film. Nonetheless, this comparison confirms the good quality of the thin film.…”