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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. MnO(001) thin films on MgO(001) grown by reactive MBE using supersonic molecular beams

   https://doi.org/10.1063/5.0198832  

   Pedersen, Andrew J; Liu, Junchen; Li, Fanxing; Lamb, H Henry (April 2024, The Journal of Chemical Physics)

   MnO(001) thin films were grown on commercial MgO(001) substrates at 520 °C by reactive molecular beam epitaxy (MBE) using Mn vapor and O2-seeded supersonic molecular beams (SMBs) both with and without radio frequency (RF) plasma excitation. For comparison, MnO(001) films were grown by reactive MBE using O2 from a leak valve. X-ray photoelectron spectroscopy confirmed the Mn2+ oxidation state and 10%–15% excess oxygen near the growth surface. Reflection high-energy electron diffraction and x-ray diffraction evidenced that the films were rock salt cubic MnO with very strong (001) orientation. High-angle annular dark field scanning transmission electron microscopy with energy-dispersive x-ray spectroscopy demonstrated abrupt MnO/MgO interfaces and indicated [(001)MnO||(001)MgO] epitaxial growth. Ex situ atomic force microscopy of films deposited without RF excitation revealed smooth growth surfaces. An SMB-grown MnO(001) film was converted to Mn3O4 with strong (110) orientation by post-growth exposure to an RF-discharge (RFD) SMB source providing O atoms; the surface of the resultant film contained elongated pits aligned with the MgO110 directions. In contrast, using the RFD-SMB source for growth resulted in MnO(001) films with elongated growth pits and square pyramidal hillocks aligned along the MgO110 and 100 directions, respectively. 

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3. Molecular beam epitaxy growth of superconducting tantalum germanide

   https://doi.org/10.1063/5.0189597  

   Strohbeen, Patrick J; Banerjee, Tathagata; Brook, Aurelia M; Levy, Ido; Sarney, Wendy L; van_Dijk, Jechiel; Orth, Hayden; Mikalsen, Melissa; Fatemi, Valla; Shabani, Javad (February 2024, Applied Physics Letters)

   Developing alternative material platforms for use in superconductor–semiconductor hybrid structures is desirable due to limitations caused by intrinsic microwave losses present in commonly used III/V material systems. With the recent reports on tantalum superconducting qubits that show improvements over the Nb and Al counterparts, exploring Ta the superconductor in hybrid material systems is promising. Here, we study the growth of Ta on semiconducting Ge (001) substrates grown via molecular beam epitaxy. We show that at a growth temperature of 400 °C, the Ta diffuses into the Ge matrix in a self-limiting nature resulting in smooth and abrupt surfaces and interfaces with roughness on the order of 3–7 Å as measured by atomic force microscopy and x-ray reflectivity. The films are found to be a mixture of Ta5Ge3 and TaGe2 binary alloys and form a native oxide that seems to form a sharp interface with the underlying film. These films are superconducting with a TC∼1.8−2 K and HC⊥∼1.88 T, HC∥∼5.1 T. These results show this tantalum germanide film to be promising for future superconducting quantum information platforms. 

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4. Epitaxial growth and characterization of Bi1− x Sb x thin films on (0001) sapphire substrates

   https://doi.org/10.1063/5.0190217  

   Huang, Yu-Sheng; Islam, Saurav; Ou, Yongxi; Ghosh, Supriya; Richardella, Anthony; Mkhoyan, K. Andre; Samarth, Nitin (February 2024, APL Materials)

   We report the molecular beam epitaxy of Bi1−xSbx thin films (0 ≤ x ≤ 1) on sapphire (0001) substrates using a thin (Bi,Sb)2Te3 buffer layer. The characterization of the films using reflection high energy diffraction, x-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy reveals the epitaxial growth of films of reasonable structural quality. This is further confirmed via x-ray diffraction pole figures that determine the epitaxial registry between the thin film and the substrate. We further investigate the microscopic structure of thin films via Raman spectroscopy, demonstrating how the vibrational modes vary as the composition changes and discussing the implications for the crystal structure. We also characterize the samples using electrical transport measurements. 

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5. Rotationally Aligned Hexagonal Boron Nitride on Sapphire by High-Temperature Molecular Beam Epitaxy

   Ryan Page1, Yongjin Cho2 (July 2019, Physical review and Physical review letters index)

   Hexagonal boron nitride (hBN) has been grown on sapphire substrates by ultrahigh-temperature molecular beam epitaxy (MBE). A wide range of substrate temperatures and boron fluxes have been explored, revealing that high crystalline quality hBN layers are grown at high substrate temperatures, >1600℃ , and low boron fluxes, ∼1 × 10%& Torr beam equivalent pressure. In situ reflection high-energy electron diffraction revealed the growth of hBN layers with 60° rotational symmetry and the [112+ 0] axis of hBN parallel to the [11+ 00] axis of the sapphire substrate. Unlike the rough, polycrystalline films previously reported, atomic force microscopy and transmission electron microscopy characterization of these films demonstrate smooth, layered, few-nanometer hBN films on a nitridated sapphire substrate. This demonstration of high-quality hBN growth by MBE is a step toward its integration into existing epitaxial growth platforms, applications, and technologies. 

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