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1. Epitaxial growth of the first two members of the Ba n+1 In n O 2.5n+1 Ruddlesden–Popper homologous series

   https://doi.org/10.1116/6.0002205  

   Hensling, Felix V. ; Smeaton, Michelle A. ; Show, Veronica ; Azizie, Kathy ; Barone, Matthew R. ; Kourkoutis, Lena F. ; Schlom, Darrell G. ( December 2022 , Journal of Vacuum Science & Technology A)

   We demonstrate the epitaxial growth of the first two members, and the [Formula: see text] member of the homologous Ruddlesden–Popper series of [Formula: see text] of which the [Formula: see text] 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 [[Formula: see text]O (g)]. To facilitate ex situ characterization of the highly hygroscopic barium indate films, a capping layer of amorphous [Formula: see text] 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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2. Initial nucleation of metastable γ-Ga 2 O 3 during sub-millisecond thermal anneals of amorphous Ga 2 O 3

   https://doi.org/10.1063/5.0087093  

   Gann, Katie R. ; Chang, Celesta S. ; Chang, Ming-Chiang ; Sutherland, Duncan R. ; Connolly, Aine B. ; Muller, David A. ; van Dover, Robert B. ; Thompson, Michael O. ( August 2022 , Applied Physics Letters)

   Beta-phase gallium oxide ([Formula: see text]-Ga 2 O 3 ) is a promising semiconductor for high frequency, high temperature, and high voltage applications. In addition to the [Formula: see text]-phase, numerous other polymorphs exist and understanding the competition between phases is critical to control practical devices. The phase formation sequence of Ga 2 O 3 , starting from amorphous thin films, was determined using lateral-gradient laser spike annealing at peak temperatures of 500–1400 °C on 400 μs to 10 ms timescales, with transformations characterized by optical microscopy, x-ray diffraction, and transmission electron microscopy (TEM). The resulting phase processing map showed the [Formula: see text]-phase, a defect-spinel structure, first nucleating under all annealing times for temperatures from 650 to 800 °C. The cross-sectional TEM at the onset of the [Formula: see text]-phase formation showed nucleation near the film center with no evidence of heterogeneous nucleation at the interfaces. For temperatures above 850 °C, the thermodynamically stable [Formula: see text]-phase was observed. For anneals of 1–4 ms and temperatures below 1200 °C, small randomly oriented grains were observed. Large grains were observed for anneals below 1 ms and above 1200 °C, with anneals above 4 ms and 1200 °C resulting in textured films. The formation of the [Formula: see text]-phase prior to [Formula: see text]-phase, coupled with the observed grain structure, suggests that the [Formula: see text]-phase is kinetically preferred during thermal annealing of amorphous films, with [Formula: see text]-phase subsequently forming by nucleation at higher temperatures. The low surface energy of the [Formula: see text]-phase implied by these results suggests an explanation for the widely observed [Formula: see text]-phase inclusions in [Formula: see text]-phase Ga 2 O 3 films grown by a variety of synthesis methods. 

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3. Epitaxial growth of β -Ga 2 O 3 by hot-wall MOCVD

   https://doi.org/10.1063/5.0087571  

   Gogova, Daniela ; Ghezellou, Misagh ; Tran, Dat Q. ; Richter, Steffen ; Papamichail, Alexis ; Hassan, Jawad ul ; Persson, Axel R. ; Persson, Per O. ; Kordina, Olof ; Monemar, Bo ; et al ( May 2022 , AIP Advances)

   The hot-wall metalorganic chemical vapor deposition (MOCVD) concept, previously shown to enable superior material quality and high performance devices based on wide bandgap semiconductors, such as Ga(Al)N and SiC, has been applied to the epitaxial growth of β-Ga 2 O 3 . Epitaxial β-Ga 2 O 3 layers at high growth rates (above 1 μm/h), at low reagent flows, and at reduced growth temperatures (740 °C) are demonstrated. A high crystalline quality epitaxial material on a c-plane sapphire substrate is attained as corroborated by a combination of x-ray diffraction, high-resolution scanning transmission electron microscopy, and spectroscopic ellipsometry measurements. The hot-wall MOCVD process is transferred to homoepitaxy, and single-crystalline homoepitaxial β-Ga 2 O 3 layers are demonstrated with a [Formula: see text]01 rocking curve width of 118 arc sec, which is comparable to those of the edge-defined film-fed grown ([Formula: see text]01) β-Ga 2 O 3 substrates, indicative of similar dislocation densities for epilayers and substrates. Hence, hot-wall MOCVD is proposed as a prospective growth method to be further explored for the fabrication of β-Ga 2 O 3 . 

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4. Directly measuring the structural transition pathways of strain-engineered VO 2 thin films

   https://doi.org/10.1039/d0nr04776g  

   Evlyukhin, Egor ; Howard, Sebastian A. ; Paik, Hanjong ; Paez, Galo J. ; Gosztola, David J. ; Singh, Christopher N. ; Schlom, Darrell G. ; Lee, Wei-Cheng ; Piper, Louis F. ( September 2020 , Nanoscale)

   null (Ed.)

   Epitaxial films of vanadium dioxide (VO 2 ) on rutile TiO 2 substrates provide a means of strain-engineering the transition pathways and stabilizing of the intermediate phases between monoclinic (insulating) M1 and rutile (metal) R end phases. In this work, we investigate structural behavior of epitaxial VO 2 thin films deposited on isostructural MgF 2 (001) and (110) substrates via temperature-dependent Raman microscopy analysis. The choice of MgF 2 substrate clearly reveals how elongation of V–V dimers accompanied by the shortening of V–O bonds triggers the intermediate M2 phase in the temperature range between 70–80 °C upon the heating–cooling cycles. Consistent with earlier claims of strain-induced electron correlation enhancement destabilizing the M2 phase our temperature-dependent Raman study supports a small temperature window for this phase. The similarity of the hysteretic behavior of structural and electronic transitions suggests that the structural transitions play key roles in the switching properties of epitaxial VO 2 thin films. 

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5. Anomalous Hall effect and perpendicular magnetic anisotropy in ultrathin ferrimagnetic NiCo 2 O 4 films

   https://doi.org/10.1063/5.0097869  

   Chen, Xuegang ; Wu, Qiuchen ; Zhang, Le ; Hao, Yifei ; Han, Myung-Geun ; Zhu, Yimei ; Hong, Xia ( June 2022 , Applied Physics Letters)

   The inverse spinel ferrimagnetic NiCo₂O₄possesses high magnetic Curie temperature T_C, high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo₂O₄thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo₂O₄films on MgAl₂O₄substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films down to 1.5 unit cell (1.2 nm), while T_Cfor 3 unit cell and thicker films remains above 300 K. The sign change in the anomalous Hall conductivity [Formula: see text] and its scaling relation with the longitudinal conductivity ([Formula: see text]) can be attributed to the competing effects between impurity scattering and band intrinsic Berry curvature, with the latter vanishing upon the thickness driven metal–insulator transition. Our study reveals the critical role of film thickness in tuning the relative strength of charge correlation, Berry phase effect, spin–orbit interaction, and impurity scattering, providing important material information for designing scalable epitaxial magnetic tunnel junctions and sensing devices using NiCo₂O₄.

    

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