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1. H trapping at the metastable cation vacancy in α -Ga ₂ O ₃ and α -Al ₂ O ₃

   https://doi.org/10.1063/5.0094707  

   Venzie, Andrew; Portoff, Amanda; Stavola, Michael; Fowler, W. Beall; Kim, Jihyun; Jeon, Dae-Woo; Park, Ji-Hyeon; Pearton, Stephen J. (May 2022, Applied Physics Letters)

   α-Ga₂O₃has the corundum structure analogous to that of α-Al₂O₃. The bandgap energy of α-Ga₂O₃is 5.3 eV and is greater than that of β-Ga₂O₃, making the α-phase attractive for devices that benefit from its wider bandgap. The O–H and O–D centers produced by the implantation of H⁺and D⁺into α-Ga₂O₃have been studied by infrared spectroscopy and complementary theory. An O–H line at 3269 cm⁻¹is assigned to H complexed with a Ga vacancy (V_Ga), similar to the case of H trapped by an Al vacancy (V_Al) in α-Al₂O₃. The isolated V_Gaand V_Aldefects in α-Ga₂O₃and α-Al₂O₃are found by theory to have a “shifted” vacancy-interstitial-vacancy equilibrium configuration, similar to V_Gain β-Ga₂O₃, which also has shifted structures. However, the addition of H causes the complex with H trapped at an unshifted vacancy to have the lowest energy in both α-Ga₂O₃and α-Al₂O₃. 

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2. Atomistic mechanisms of water vapor–induced surface passivation

   https://doi.org/10.1126/sciadv.adh5565  

   Chen, Xiaobo; Shan, Weitao; Wu, Dongxiang; Patel, Shyam Bharatkumar; Cai, Na; Li, Chaoran; Ye, Shuonan; Liu, Zhao; Hwang, Sooyeon; Zakharov, Dmitri N; et al (November 2023, Science Advances)

   The microscopic mechanisms underpinning the spontaneous surface passivation of metals from ubiquitous water have remained largely elusive. Here, using in situ environmental electron microscopy to atomically monitor the reaction dynamics between aluminum surfaces and water vapor, we provide direct experimental evidence that the surface passivation results in a bilayer oxide film consisting of a crystalline-like Al(OH)₃top layer and an inner layer of amorphous Al₂O₃. The Al(OH)₃layer maintains a constant thickness of ~5.0 Å, while the inner Al₂O₃layer grows at the Al₂O₃/Al interface to a limiting thickness. On the basis of experimental data and atomistic modeling, we show the tunability of the dissociation pathways of H₂O molecules with the Al, Al₂O₃, and Al(OH)₃surface terminations. The fundamental insights may have practical significance for the design of materials and reactions for two seemingly disparate but fundamentally related disciplines of surface passivation and catalytic H₂production from water. 

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3. Microscopic and Spectroscopic Investigation of (AlxGa1–X)2O3 Films: Unraveling the Impact of Growth Orientation and Aluminum Content

   https://doi.org/10.1002/admi.202301016  

   Sarker, Jith; Garg, Prachi; Rauf, Abrar; Nirjhar, Ahsiur Rahman; Huang, Hsien‐Lien; Zhu, Menglin; Bhuiyan, A_F_M_Anhar Uddin; Meng, Lingyu; Zhao, Hongping; Hwang, Jinwoo; et al (March 2024, Advanced Materials Interfaces)

   Abstract (Al_xGa_1–x)₂O₃ is an ultrawide‐bandgap semiconductor with a high critical electric field for next‐generation high‐power transistors and deep‐ultraviolet photodetectors. While (010)‐(Al_xGa_1–x)₂O₃ films have been studied, the recent availability of (100), (01)‐Ga₂O₃ substrates have developed interest in (100), (01)‐(Al_xGa_1–x)₂O₃ films. In this work, an investigation of microscopic and spectroscopic characteristics of (100), (01), (010)–(Al_xGa_1–x)₂O₃ films is conducted. A combination of scanning transmission electron microscopy, atom probe tomography (APT), and first‐principle calculations (DFT) is performed. The findings reveal consistent in‐plane chemical homogeneity in lower aluminum content (x = 0.2) films. However, higher aluminum content (x = 0.5), showed inhomogeneity in (100), (010)–(Al_xGa_1–x)₂O₃ films attributed to their spectroscopic properties. The study expanded APT's capabilities to determine Ga─O and Al─O bond lengths by mapping their ion‐pair separations in detector space. The change in ion‐pair separations is consistent with varying orientations, irrespective of aluminum content. DFT also demonstrated a similar trend, concluding that Ga─O and Al─O bonding energy has an inverse relationship with their bond length as crystallographic orientations vary. This systematic study of growth orientation dependence of (Al_xGa_1–x)₂O₃ films’ microscopic and spectroscopic properties will guide the development of new (100) and (01)‐(Al_xGa_1–x)₂O₃ along with existing (010)–(Al_xGa_1–x)₂O₃ films. 

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4. In Situ Dielectric Al ₂ O ₃ /β‐Ga ₂ O ₃ Interfaces Grown Using Metal–Organic Chemical Vapor Deposition

   https://doi.org/10.1002/aelm.202100333  

   Roy, Saurav; Chmielewski, Adrian_E; Bhattacharyya, Arkka; Ranga, Praneeth; Sun, Rujun; Scarpulla, Michael_A; Alem, Nasim; Krishnamoorthy, Sriram (August 2021, Advanced Electronic Materials)

   Abstract High quality dielectric‐semiconductor interfaces are critical for reliable high‐performance transistors. This paper reports the in situ metal–organic chemical vapor deposition of Al₂O₃on β‐Ga₂O₃as a potentially better alternative to the most commonly used atomic layer deposition (ALD). The growth of Al₂O₃is performed in the same reactor as Ga₂O₃using trimethylaluminum and O₂as precursors without breaking the vacuum at a growth temperature of 600 °C. The fast and slow near interface traps at the Al₂O₃/β‐Ga₂O₃interface are identified and quantified using stressed capacitance–voltage (CV) measurements on metal oxide semiconductor capacitor (MOSCAP) structures. The density of shallow and deep level initially filled traps (D_it) are measured using ultraviolet‐assisted CV technique. The average D_itfor the MOSCAP is determined to be 6.4×10¹¹cm⁻²eV⁻¹. The conduction band offset of the Al₂O₃/ Ga₂O₃interface is also determined from CV measurements and found out to be 1.7 eV which is in close agreement with the existing literature reports of ALD Al₂O₃/Ga₂O₃interface. The current–voltage characteristics are also analyzed and the average breakdown field is extracted to be approximately 5.8 MV cm⁻¹. This in situ Al₂O₃dielectric on β‐Ga₂O₃with improved dielectric properties can enable Ga₂O₃‐based high‐performance devices. 

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5. Al Incorporation up to 99% in Metalorganic Chemical Vapor Deposition‐Grown Monoclinic (Al _x Ga _1–x ) ₂ O ₃ Films Using Trimethylgallium

   https://doi.org/10.1002/pssr.202300224  

   Bhuiyan, A. F. M. Anhar Uddin; Meng, Lingyu; Huang, Hsien-Lien; Chae, Christopher; Hwang, Jinwoo; Zhao, Hongping (August 2023, physica status solidi (RRL) – Rapid Research Letters)

   Growths of monoclinic (Al_xGa_1−x)₂O₃thin films up to 99% Al contents are demonstrated via metalorganic chemical vapor deposition (MOCVD) using trimethylgallium (TMGa) as the Ga precursor. The utilization of TMGa, rather than triethylgallium, enables a significant improvement of the growth rates (>2.5 μm h⁻¹) of β‐(Al_xGa_1−x)₂O₃thin films on (010), (100), and (01) β‐Ga₂O₃substrates. By systematically tuning the precursor molar flow rates, growth of coherently strained phase pure β‐(Al_xGa_1−x)₂O₃films is demonstrated by comprehensive material characterizations via high‐resolution X‐ray diffraction (XRD) and atomic‐resolution scanning transmission electron microscopy (STEM) imaging. Monoclinic (Al_xGa_1−x)₂O₃films with Al contents up to 99, 29, and 16% are achieved on (100), (010), and (01) β‐Ga₂O₃substrates, respectively. Beyond 29% of Al incorporation, the (010) (Al_xGa_1−x)₂O₃films exhibit β‐ to γ‐phase segregation. β‐(Al_xGa_1−x)₂O₃films grown on (01) β‐Ga₂O₃show local segregation of Al along (100) plane. Record‐high Al incorporations up to 99% in monoclinic (Al_xGa_1−x)₂O₃grown on (100) Ga₂O₃are confirmed from XRD, STEM, electron nanodiffraction, and X‐ray photoelectron spectroscopy measurements. These results indicate great promises of MOCVD development of β‐(Al_xGa_1−x)₂O₃films and heterostructures with high Al content and growth rates using TMGa for next‐generation high‐power and high‐frequency electronic devices. 

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