arXiv subjects

Ida Sadeghi

Publications and source records attributed to Ida Sadeghi.

6 recordsLinked to original sources

arXiv2026

Anion Ordering and Phase Stability Govern Optical Band Gaps in BaZr(S,Se)3

Chalcogenide perovskites have emerged as promising lead free materials for photovoltaic and thermoelectric applications. Among them, BaZrS3 has attracted particular attention due to its thermal and chemical stability, favorable optoelectronic properties, and low thermal conductivity. Here, we combine molecular dynamics and Monte Carlo simulations based on machine learned interatomic potentials with scanning transmission electron microscopy to investigate mixing thermodynamics and phase stability in the BaZr(S,Se)3 system. We identify an unusual ordered structure that persists at room temperature, most prominently at 33% S, where S and Se atoms form alternating layers within the crystal. Free energy calculations yield the temperature composition phase diagram, including a nonperovskite delta phase in the Se rich limit and a perovskite phase in the S rich limit, separated by a broad two phase region. Analysis of the dielectric function and the absorption coefficient demonstrates that composition, crystal structure, and anion ordering jointly control the optical band gap. Selenium alloying enables tuning between approximately 1.6 and 1.9eV, while anion ordering within a given composition reduces the gap by about 0.12eV. Lastly, variations between structural polymorphs give rise to band gap differences of up to 0.4eV.

cond-mat.mtrl-sci↗

arXiv2025

Electronic mobility, doping, and defects in epitaxial $\mathrm{BaZrS_3}$ chalcogenide perovskite thin films

We present the electronic transport properties of $\mathrm{BaZrS_3}$ (BZS) thin films grown epitaxially by gas-source molecular beam epitaxy (MBE). We observe n-type behavior in all samples, with carrier concentration ranging from $4 \times 10^{18}$ to $4 \times 10^{20} \mathrm{cm^{-3}}$ at room temperature (RT). We observe a champion RT Hall mobility of 11.1 $\mathrm{cm^2V^{-1}s^{-1}}$, which is competitive with established thin-film photovoltaic (PV) absorbers. Temperature-dependent Hall mobility data show that phonon scattering dominates at room temperature, in agreement with computational predictions. X-ray diffraction data illustrate a correlation between mobility and stacking fault concentration, illustrating how microstructure can affect transport. Despite the well-established environmental stability of chalcogenide perovskites, we observe significant changes to electronic properties as a function of storage time in ambient conditions. With the help of secondary-ion mass-spectrometry (SIMS) measurements, we propose and support a defect mechanism that explains this behavior: as-grown films have a high concentration of sulfur vacancies that are shallow donors ($\mathrm{V_S^\bullet}$ or $\mathrm{V_S^{\bullet \bullet}}$), which are converted into neutral oxygen defects ($\mathrm{O_S^\times}$) upon air exposure. We discuss the relevance of this defect mechanism within the larger context of chalcogenide perovskite research, and we identify means to stabilize the electronic properties.

cond-mat.mtrl-sci↗

arXiv2024

Atomic Structure of Self-Buffered BaZr(S,Se)$_3$ Epitaxial Thin Film Interfaces

Understanding and controlling the growth of chalcogenide perovskite thin films through interface design is important for tailoring film properties. Here, the film and interface structure of BaZr(S,Se)$_3$ thin films grown on LaAlO$_3$ by molecular beam epitaxy and post-growth anion exchange is resolved using aberration-corrected scanning transmission electron microscopy. Epitaxial films are achieved from self-assembly of an interface ``buffer'' layer, which accommodates the large film/substrate lattice mismatch of nearly 40\% for the alloy film studied here. The self-assembled buffer layer, occurring for both the as-grown sulfide and post-selenization alloy films, is shown to have rock-salt-like atomic stacking akin to a Ruddlesden-Popper phase. Above this buffer, the film quickly transitions to the perovskite structure. Overall, these results provide insights into oxide-chalcogenide heteroepitaxial film growth, illustrating a process that yields relaxed, crystalline, epitaxial chalcogenide perovskite films that support ongoing studies of optoelectronic and device properties.

cond-mat.mtrl-sci↗

arXiv2024

A Processing Route to Chalcogenide Perovskites Alloys with Tunable Band Gap via Anion Exchange

We demonstrate synthesis of BaZr(S,Se)3 chalcogenide perovskite alloys by selenization of BaZrS3 thin 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 100x 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.

cond-mat.mtrl-sci↗

arXiv2022

A new semiconducting perovskite alloy system made possible by gas-source molecular beam epitaxy

Optoelectronic technologies are based on families of semiconductor alloys. It is rare that a new semiconductor alloy family is developed to the point where epitaxial growth is possible; since the 1950s, this has happened approximately once per decade. Here we demonstrate epitaxial thin film growth of semiconducting chalcogenide perovskite alloys in the Ba-Zr-S-Se system by gas-source molecular beam epitaxy (MBE). We stabilize the full range y = 0 ... 3 of compositions BaZrS$_{(3-y)}$Se$_y$ in the perovskite structure, up to and including BaZrSe$_3$, by growing on BaZrS$_3$ epitaxial templates. The resulting films are environmentally stable and the direct band gap ($E_g$) varies strongly with Se content, as predicted by theory, covering the range $E_g$ = 1.9 ... 1.4 eV for $y$ = 0 ... 3. This creates possibilities for visible and near-infrared (VIS-NIR) optoelectronics, solid state lighting, and solar cells using chalcogenide perovskites.

cond-mat.mtrl-sci↗

arXiv2021

Making BaZrS3 chalcogenide perovskite thin films by molecular beam epitaxy

We demonstrate the making of BaZrS3 thin films by molecular beam epitaxy (MBE). BaZrS3 forms in the orthorhombic distorted-perovskite structure with corner-sharing ZrS6 octahedra. The single-step MBE process results in films smooth on the atomic scale, with near-perfect BaZrS3 stoichiometry and an atomically-sharp interface with the LaAlO3 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 high-quality epitaxial thin films, as has been long-established for other systems including Si-Ge, III-Vs, and II-Vs. The methods demonstrated here also represent a revival of gas-source chalcogenide MBE.

cond-mat.mtrl-sci↗