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1. Molecular beam epitaxy of KTaO 3

   https://doi.org/10.1116/6.0002223  

   Schwaigert, Tobias ; Salmani-Rezaie, Salva ; Barone, Matthew R. ; Paik, Hanjong ; Ray, Ethan ; Williams, Michael D. ; Muller, David A. ; Schlom, Darrell G. ; Ahadi, Kaveh ( March 2023 , Journal of Vacuum Science & Technology A)

   Strain-engineering is a powerful means to tune the polar, structural, and electronic instabilities of incipient ferroelectrics. KTaO 3 is near a polar instability and shows anisotropic superconductivity in electron-doped samples. Here, we demonstrate growth of high-quality KTaO 3 thin films by molecular-beam epitaxy. Tantalum was provided by either a suboxide source emanating a TaO 2 flux from Ta 2 O 5 contained in a conventional effusion cell or an electron-beam-heated tantalum source. Excess potassium and a combination of ozone and oxygen (10% O 3 + 90% O 2 ) were simultaneously supplied with the TaO 2 (or tantalum) molecular beams to grow the KTaO 3 films. Laue fringes suggest that the films are smooth with an abrupt film/substrate interface. Cross-sectional scanning transmission electron microscopy does not show any extended defects and confirms that the films have an atomically abrupt interface with the substrate. Atomic force microscopy reveals atomic steps at the surface of the grown films. Reciprocal space mapping demonstrates that the films, when sufficiently thin, are coherently strained to the SrTiO 3 (001) and GdScO 3 (110) substrates. 

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2. Sn-modified BaTiO3 thin film with enhanced polarization

   https://doi.org/10.1116/6.0002208  

   Nunn, William ; Kumar, Abinash ; Zu, Rui ; Nebgen, Bailey ; Yu, Shukai ; Kamath Manjeshwar, Anusha ; Gopalan, Venkatraman ; LeBeau, James M. ; James, Richard D. ; Jalan, Bharat ( January 2023 , Journal of Vacuum Science & Technology A)

   Hybrid molecular beam epitaxy (MBE) growth of Sn-modified BaTiO3 films was realized with varying domain structures and crystal symmetries across the entire composition space. Macroscopic and microscopic structures and the crystal symmetry of these thin films were determined using a combination of optical second harmonic generation (SHG) polarimetry and scanning transmission electron microscopy (STEM). SHG polarimetry revealed a variation in the global crystal symmetry of the films from tetragonal (P4mm) to cubic (Pm3¯m) across the composition range, x = 0 to 1 in BaTi1−xSnxO3 (BTSO). STEM imaging shows that the long-range polar order observed when the Sn content is low (x = 0.09) transformed to a short-range polar order as the Sn content increased (x = 0.48). Consistent with atomic displacement measurements from STEM, the largest polarization was obtained at the lowest Sn content of x = 0.09 in Sn-modified BaTiO3 as determined by SHG. These results agree with recent bulk ceramic reports and further identify this material system as a potential replacement for Pb-containing relaxor-based thin film devices.

    

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3. Microstructure characterization of BaSnO3 thin films on LaAlO3 and PrScO3 substrates from transmission electron microscopy

   https://doi.org/10.1038/s41598-018-28520-9  

   Yun, Hwanhui ; Ganguly, Koustav ; Postiglione, William ; Jalan, Bharat ; Leighton, Chris ; Mkhoyan, K. Andre ; Jeong, Jong Seok ( July 2018 , Scientific Reports)

   Detailed microstructure analysis of epitaxial thin films is a vital step towards understanding essential structure-property relationships. Here, a combination of transmission electron microscopy (TEM) techniques is utilized to determine in detail the microstructure of epitaxial La-doped BaSnO₃films grown on two different perovskite substrates: LaAlO₃and PrScO₃. These BaSnO₃films are of high current interest due to outstanding electron mobility at ambient. The rotational disorder of low-angle grain boundaries, namely the in-plane twist and out-of-plane tilt, is visualized by conventional TEM under a two-beam condition, and the degree of twists in grains of such films is quantified by selected-area electron diffraction. The investigation of the atomic arrangement near the film-substrate interfaces, using high-resolution annular dark-field scanning TEM imaging, reveals that edge dislocations with a Burgers vector along [001] result in the out-of-plane tilt. It is shown that such TEM-based analyses provide detailed information about the microstructure of the films, which, when combined with complimentary high-resolution X-ray diffraction, yields a complete structural characterization of the films. In particular, stark differences in out-of-plane tilt on the two substrates are shown to result from differences in misfit dislocation densities at the interface, explaining a puzzling observation from X-ray diffraction.

    

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4. Making BaZrS 3 Chalcogenide Perovskite Thin Films by Molecular Beam Epitaxy

   https://doi.org/10.1002/adfm.202105563  

   Sadeghi, Ida ; Ye, Kevin ; Xu, Michael ; Li, Yifei ; LeBeau, James M. ; Jaramillo, Rafael ( August 2021 , Advanced Functional Materials)

   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.

    

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5. An optimized sample preparation approach for atomic resolution in situ studies of thin films

   https://doi.org/10.1002/jemt.23130  

   Moatti, Adele ; Sachan, Ritesh ; Prater, John ; Narayan, Jagdish ( October 2018 , Microscopy Research and Technique)

   This work provides the details of a simple and reliable method with less damage to prepare electron transparent samples for in situ studies in scanning/transmission electron microscopy. In this study, we use epitaxial VO₂thin films grown on c‐Al₂O₃by pulsed laser deposition, which have a monoclinic–rutile transition at ~68°C. We employ an approach combining conventional mechanical wedge‐polishing and Focused Ion beam to prepare the electron transparent samples of epitaxial VO₂thin films. The samples are first mechanically wedge‐polished and ion‐milled to be electron transparent. Subsequently, the thin region of VO₂films are separated from the rest of the polished sample using a focused ion beam and transferred to the in situ electron microscopy test stage. As a critical step, carbon nanotubes are used as connectors to the manipulator needle for a soft transfer process. This is done to avoid shattering of the brittle substrate film on the in situ sample support stage during the transfer process. We finally present the atomically resolved structural transition in VO₂films using this technique. This approach significantly increases the success rate of high‐quality sample preparation with less damage for in situ studies of thin films and reduces the cost and instrumental/user errors associated with other techniques.

   The present work highlights a novel, simple, reliable approach with reduced damage to make electron transparent samples for atomic‐scale insights of temperature‐dependent transitions in epitaxial thin film heterostructures using in situ TEM studies.

    

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