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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. Structural and electronic properties of Ga ₂ O ₃ -Al ₂ O ₃ alloys

   https://doi.org/10.1063/1.5036991  

   Peelaers, Hartwin; Varley, Joel B.; Speck, James S.; Van de Walle, Chris G. (June 2018, Applied Physics Letters)
3. Phase Selection and Structure of Low-Defect-Density γ-Al ₂ O ₃ Created by Epitaxial Crystallization of Amorphous Al ₂ O ₃

   https://doi.org/10.1021/acsami.0c17606  

   Liu, Rui; Elleuch, Omar; Wan, Zhongyi; Zuo, Peng; Janicki, Tesia D.; Alfieri, Adam D.; Babcock, Susan E.; Savage, Donald E.; Schmidt, J. R.; Evans, Paul G.; et al (December 2020, ACS Applied Materials & Interfaces)

   null (Ed.)

   A multistep phase sequence following the crystallization of amorphous Al2O3 via solid-phase epitaxy (SPE) points to methods to create low-defect-density thin films of the metastable cubic γ-Al2O3 polymorph. An amorphous Al2O3 thin film on a (0001) α-Al2O3 sapphire substrate initially transforms upon heating to form epitaxial γ-Al2O3, followed by a transformation to monoclinic θ-Al2O3, and eventually to α-Al2O3. Epitaxial γ-Al2O3 layers with low mosaic widths in X-ray rocking curves can be formed via SPE by crystallizing the γ-Al2O3 phase from amorphous Al2O3 and avoiding the microstructural inhomogeneity arising from the spatially inhomogeneous transformation to θ-Al2O3. A complementary molecular dynamics (MD) simulation indicates that the amorphous layer and γ-Al2O3 have similar Al coordination geometry, suggesting that γ-Al2O3 forms in part because it involves the minimum rearrangement of the initially amorphous configuration. The lattice parameters of γ-Al2O3 are consistent with a structure in which the majority of the Al vacancies in the spinel structure occupy sites with tetrahedral coordination, consistent with the MD results. The formation of Al vacancies at tetrahedral spinel sites in epitaxial γ-Al2O3 can minimize the epitaxial elastic deformation of γ-Al2O3 during crystallization. 

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4. In situ MOCVD growth and band offsets of Al ₂ O ₃ dielectric on β-Ga ₂ O ₃ and β-(Al _x Ga _1−x ) ₂ O ₃ thin films

   https://doi.org/10.1063/5.0104433  

   Bhuiyan, A F; Meng, Lingyu; Huang, Hsien-Lien; Hwang, Jinwoo; Zhao, Hongping (October 2022, Journal of Applied Physics)

   The in situ metalorganic chemical vapor deposition (MOCVD) growth of Al 2 O 3 dielectrics on β-Ga 2 O 3 and β-(Al x Ga 1−x ) 2 O 3 films is investigated as a function of crystal orientations and Al compositions of β-(Al x Ga 1−x ) 2 O 3 films. The interface and film qualities of Al 2 O 3 dielectrics are evaluated by high-resolution x-ray diffraction and scanning transmission electron microscopy imaging, which indicate the growth of high-quality amorphous Al 2 O 3 dielectrics with abrupt interfaces on (010), (100), and [Formula: see text] oriented β-(Al x Ga 1−x ) 2 O 3 films. The surface stoichiometries of Al 2 O 3 deposited on all orientations of β-(Al x Ga 1−x ) 2 O 3 are found to be well maintained with a bandgap energy of 6.91 eV as evaluated by high-resolution x-ray photoelectron spectroscopy, which is consistent with the atomic layer deposited (ALD) Al 2 O 3 dielectrics. The evolution of band offsets at both in situ MOCVD and ex situ ALD deposited Al 2 O 3 /β-(Al x Ga 1−x ) 2 O 3 is determined as a function of Al composition, indicating the influence of the deposition method, orientation, and Al composition of β-(Al x Ga 1−x ) 2 O 3 films on resulting band alignments. Type II band alignments are determined at the MOCVD grown Al 2 O 3 /β-(Al x Ga 1−x ) 2 O 3 interfaces for the (010) and (100) orientations, whereas type I band alignments with relatively low conduction band offsets are observed along the [Formula: see text] orientation. The results from this study on MOCVD growth and band offsets of amorphous Al 2 O 3 deposited on differently oriented β-Ga 2 O 3 and β-(Al x Ga 1−x ) 2 O 3 films will potentially contribute to the design and fabrication of future high-performance β-Ga 2 O 3 and β-(Al x Ga 1−x ) 2 O 3 based transistors using MOCVD in situ deposited Al 2 O 3 as a gate dielectric. 

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5. Probing Heterogeneous Charge Distributions at the α-Al ₂ O ₃ (0001)/H ₂ O Interface

   https://doi.org/10.1021/jacs.0c01366  

   Piontek, Stefan M.; DelloStritto, Mark; Mandal, Bijoya; Marshall, Tim; Klein, Michael L.; Borguet, Eric (July 2020, Journal of the American Chemical Society)

   null (Ed.)