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1. 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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2. Epitaxial growth of the first two members of the Ba n +1In n O2.5 n +1 Ruddlesden–Popper homologous series

   https://doi.org/10.1116/6.0002205  

   Hensling, Felix_V_E; Smeaton, Michelle_A; Show, Veronica; Azizie, Kathy; Barone, Matthew_R; Kourkoutis, Lena_F; Schlom, Darrell_G (November 2022, Journal of Vacuum Science & Technology A)

   We demonstrate the epitaxial growth of the first two members, and the n=∞ member of the homologous Ruddlesden–Popper series of Ban+1InnO2.5n+1 of which the n=1 member was previously unknown. The films were grown by suboxide molecular-beam epitaxy where the indium is provided by a molecular beam of indium-suboxide [In2O (g)]. To facilitate ex situ characterization of the highly hygroscopic barium indate films, a capping layer of amorphous SiO2 was deposited prior to air exposure. The structural quality of the films was assessed by x-ray diffraction, reflective high-energy electron diffraction, and scanning transmission electron microscopy. 

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3. Epitaxial growth of α-(Al x Ga1− x )2O3 by suboxide molecular-beam epitaxy at 1 µm/h

   https://doi.org/10.1063/5.0170095  

   Steele, Jacob; Azizie, Kathy; Pieczulewski, Naomi; Kim, Yunjo; Mou, Shin; Asel, Thaddeus_J; Neal, Adam_T; Jena, Debdeep; Xing, Huili_G; Muller, David_A; et al (April 2024, APL Materials)

   We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow α-(AlxGa1−x)2O3 films on (110) sapphire substrates over the 0 < x < 0.95 range of aluminum content. In S-MBE, 99.98% of the gallium-containing molecular beam arrives at the substrate in a preoxidized form as gallium suboxide (Ga2O). This bypasses the rate-limiting step of conventional MBE for the growth of gallium oxide (Ga2O3) from a gallium molecular beam and allows us to grow fully epitaxial α-(AlxGa1−x)2O3 films at growth rates exceeding 1 µm/h and relatively low substrate temperature (Tsub = 605 ± 15 °C). The ability to grow α-(AlxGa1−x)2O3 over the nominally full composition range is confirmed by Vegard’s law applied to the x-ray diffraction data and by optical bandgap measurements with ultraviolet–visible spectroscopy. We show that S-MBE allows straightforward composition control and bandgap selection for α-(AlxGa1−x)2O3 films as the aluminum incorporation x in the film is linear with the relative flux ratio of aluminum to Ga2O. The films are characterized by atomic-force microscopy, x-ray diffraction, and scanning transmission electron microscopy (STEM). These α-(AlxGa1−x)2O3 films grown by S-MBE at record growth rates exhibit a rocking curve full width at half maximum of ≊ 12 arc secs, rms roughness <1 nm, and are fully commensurate for x ≥ 0.5 for 20–50 nm thick films. STEM imaging of the x = 0.78 sample reveals high structural quality and uniform composition. Despite the high structural quality of the films, our attempts at doping with silicon result in highly insulating films. 

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4. γ -phase inclusions as common structural defects in alloyed β -(Al x Ga1− x )2O3 and doped β -Ga2O3 films

   https://doi.org/10.1063/5.0038861  

   Chang, Celesta_S; Tanen, Nicholas; Protasenko, Vladimir; Asel, Thaddeus_J; Mou, Shin; Xing, Huili_Grace; Jena, Debdeep; Muller, David_A (May 2021, APL Materials)

   β-Ga2O3 is a promising ultra-wide bandgap semiconductor whose properties can be further enhanced by alloying with Al. Here, using atomic-resolution scanning transmission electron microscopy, we find the thermodynamically unstable γ-phase is a ubiquitous structural defect in both β-(AlxGa1−x)2O3 films and doped β-Ga2O3 films grown by molecular beam epitaxy. For undoped β-(AlxGa1−x)2O3 films, we observe γ-phase inclusions between nucleating islands of the β-phase at lower growth temperatures (∼500–600 °C). In doped β-Ga2O3, a thin layer of the γ-phase is observed on the surfaces of films grown with a wide range of n-type dopants and dopant concentrations. The thickness of the γ-phase layer was most strongly correlated with the growth temperature, peaking at about 600 °C. Ga interstitials are observed in the β-phase, especially near the interface with the γ-phase. By imaging the same region of the surface of a Sn-doped β-(AlxGa1−x)2O3 after ex situ heating up to 400 °C, a γ-phase region is observed to grow above the initial surface, accompanied by a decrease in Ga interstitials in the β-phase. This suggests that the diffusion of Ga interstitials toward the surface is likely the mechanism for growth of the surface γ-phase and more generally that the more-open γ-phase may offer diffusion pathways to be a kinetically favored and early forming phase in the growth of Ga2O3. However, more modeling and simulation of the γ-phase and the interstitials are needed to understand the energetics and kinetics, the impact on electronic properties, and how to control them. 

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5. Enhanced T C in SrRuO3/DyScO3(110) thin films with high residual resistivity ratio

   https://doi.org/10.1063/5.0156344  

   Schreiber, Nathaniel_J; Miao, Ludi; Nair, Hari_P; Ruf, Jacob_P; Bhatt, Lopa; Birkholzer, Yorick_A; Kotsonis, George_N; Kourkoutis, Lena_F; Shen, Kyle_M; Schlom, Darrell_G (November 2023, APL Materials)

   Epitaxial untwinned SrRuO3 thin films were grown on (110)-oriented DyScO3 substrates by molecular-beam epitaxy. We report an exceptional sample with a residual resistivity ratio (RRR), ρ [300 K]/ρ [4 K] of 205 and a ferromagnetic Curie temperature, TC, of 168.3 K. We compare the properties of this sample to other SrRuO3 films grown on DyScO3(110) with RRRs ranging from 8.8 to 205, and also compare it to the best reported bulk single crystal of SrRuO3. We determine that SrRuO3 thin films grown on DyScO3(110) have an enhanced TC as long as the RRR of the thin film is above a minimum electrical quality threshold. This RRR threshold is about 20 for SrRuO3. Films with lower RRR exhibit TCs that are significantly depressed from the intrinsic strain-enhanced value. 

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