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1. Band Alignment of Al ₂ O ₃ on α-(Al _x Ga _1-x ) ₂ O ₃

   https://doi.org/10.1149/2162-8777/ac546f  

   Xia, Xinyi; Al-Mamun, Nahid Sultan; Fares, Chaker; Haque, Aman; Ren, Fan; Hassa, Anna; von Wenckstern, Holger; Grundmann, Marius; Pearton, S. J. (February 2022, ECS Journal of Solid State Science and Technology)

   X Ray Photoelectron Spectroscopy was used to measure valence band offsets for Al 2 O 3 deposited by Atomic Layer Deposition on α -(Al x Ga 1-x ) 2 O 3 alloys over a wide range of Al contents, x, from 0.26–0.74, corresponding to a bandgap range from 5.8–7 eV. These alloys were grown by Pulsed Laser Deposition. The band alignments were type I (nested) at x <0.5, with valence band offsets 0.13 eV for x = 0.26 and x = 0.46. At higher Al contents, the band alignment was a staggered alignment, with valence band offsets of − 0.07 eV for x = 0.58 and −0.17 for x = 0.74, ie. negative valence band offsets in both cases. The conduction band offsets are also small at these high Al contents, being only 0.07 eV at x = 0.74. The wide bandgap of the α -(Al x Ga 1-x ) 2 O 3 alloys makes it difficult to find dielectrics with nested band alignments over the entire composition range. 

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2. MOCVD growth of β-phase (Al _x Ga _1−x ) ₂ O ₃ on ( 2 ¯01) β-Ga ₂ O ₃ substrates

   https://doi.org/10.1063/5.0025478  

   Bhuiyan, A F; Feng, Zixuan; Johnson, Jared M.; Huang, Hsien-Lien; Hwang, Jinwoo; Zhao, Hongping (October 2020, Applied Physics Letters)

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3. Thermal stability of epitaxial α -Ga ₂ O ₃ and (Al,Ga) ₂ O ₃ layers on m-plane sapphire

   https://doi.org/10.1063/5.0064278  

   McCandless, J. P.; Chang, C. S.; Nomoto, K.; Casamento, J.; Protasenko, V.; Vogt, P.; Rowe, D.; Gann, K.; Ho, S. T.; Li, W.; et al (August 2021, Applied Physics Letters)
4. Al Composition Dependence of Band Offsets for SiO ₂ on α-(Al _x Ga _1−x ) ₂ O ₃

   https://doi.org/10.1149/2162-8777/ac39a8  

   Xia, Xinyi; Fares, Chaker; Ren, Fan; Hassa, Anna; von Wenckstern, Holger; Grundmann, Marius; Pearton, S. J. (November 2021, ECS Journal of Solid State Science and Technology)

   Valence band offsets for SiO 2 deposited by Atomic Layer Deposition on α -(Al x Ga 1-x ) 2 O 3 alloys with x = 0.26–0.74 were measured by X-ray Photoelectron Spectroscopy. The samples were grown with a continuous composition spread to enable investigations of the band alignment as a function of the alloy composition. From measurement of the core levels in the alloys, the bandgaps were determined to range from 5.8 eV (x = 0.26) to 7 eV (x = 0.74). These are consistent with previous measurements by transmission spectroscopy. The valence band offsets of SiO 2 with these alloys of different composition were, respectively, were −1.2 eV for x = 0.26, −0.2 eV for x = 0.42, 0.2 eV for x = 0.58 and 0.4 eV for x = 0.74. All of these band offsets are too low for most device applications. Given the bandgap of the SiO 2 was 8.7 eV, this led to conduction band offsets of 4.1 eV (x = 0.26) to 1.3 eV (x = 0.74). The band alignments were of the desired nested configuration for x > 0.5, but at lower Al contents the conduction band offsets were negative, with a staggered band alignment. This shows the challenge of finding appropriate dielectrics for this ultra-wide bandgap semiconductor system. 

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5. Growth of α-Ga ₂ O ₃ on α-Al ₂ O ₃ by conventional molecular-beam epitaxy and metal–oxide-catalyzed epitaxy

   https://doi.org/10.35848/1347-4065/acbe04  

   McCandless, J. P.; Rowe, D.; Pieczulewski, N.; Protasenko, V.; Alonso-Orts, M.; Williams, M. S.; Eickhoff, M.; Xing, H. G.; Muller, D. A.; Jena, D.; et al (March 2023, Japanese Journal of Applied Physics)

   Abstract We report the growth of α -Ga 2 O 3 on m -plane α -Al 2 O 3 by conventional plasma-assisted molecular-beam epitaxy and In-mediated metal–oxide-catalyzed epitaxy (MOCATAXY). We report a growth rate diagram for α -Ga 2 O 3 ( 10 1 ¯ 0 ), and observe (i) a growth rate increase, (ii) an expanded growth window, and (iii) reduced out-of-lane mosaic spread when MOCATAXY is employed for the growth of α -Ga 2 O 3 . Through the use of In-mediated catalysis, growth rates over 0.2 μ m h −1 and rocking curves with full width at half maxima of Δ ω ≈ 0.45° are achieved. Faceting is observed along the α -Ga 2 O 3 film surface and explored through scanning transmission electron microscopy. 

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