Enhanced Deep-Ultraviolet Responsivity in Aluminum-Gallium Oxide Photodetectors via Structure Deformation by High-Oxygen-Pressure Pulsed Laser Deposition

Al Coordination and Ga Interstitial Stability in a β-(Al0.2Ga0.8)2O3 Thin Film

Alloying Al₂O₃ with Ga₂O₃ to form β-(Al_xGa_1-x)₂O₃ opens the door to a large number of new possibilities for the fabrication of devices with tunable properties in many high-performance applications such as optoelectronics, sensing systems, and high-power electronics. Often, the properties of these devices are impacted by defects induced during the growth process. In this work, we uncover the crystal structure of a β-(Al_0.2Ga_0.8)₂O₃/β-Ga₂O₃ interface grown by molecular beam epitaxy. In particular, we determine Al coordination and the stability of Al and Ga interstitials and their effect on the electronic structure of the material by means of scanning transmission electron microscopy combined with density functional theory. Al atoms can substitutionally occupy both octahedral and tetrahedral sites. The atomic structure of the β-(Al_0.2Ga_0.8)₂O₃/β-Ga₂O₃ interface additionally shows Al and Ga interstitials located between neighboring tetrahedrally coordinated cation sites, whose stability will depend on the number of surrounding Al atoms. The presence of Al atoms near interstitials leads to structural distortions in the lattice and creates interstitial-divacancy complexes that will eventually form deep-level states below the conduction band (E_c) at E_c -1.25 eV, E_c -1.68 eV, E_c -1.78 eV, E_c -1.83 eV, and E_c -1.86 eV for a Ga interstitial surrounded by zero, one, two, three, and four Al atoms, respectively. These findings bring new insight toward the fabrication of tunable β-(Al_xGa_1-x)₂O₃ heterostructure-based devices with controlled electronic properties.

Effect of extended defects on photoluminescence of gallium oxide and aluminum gallium oxide epitaxial films

In this work, a systematic photoluminescence (PL) study on three series of gallium oxide/aluminum gallium oxide films and bulk single crystals is performed including comparing doping, epitaxial substrates, and aluminum concentration. It is observed that blue/green emission intensity strongly correlates with extended structural defects rather than the point defects frequently assumed. Bulk crystals or Si-doped films homoepitaxially grown on (010) β-Ga₂O₃ yield an intense dominant UV emission, while samples with extended structural defects, such as gallium oxide films grown on either (-201) β-Ga₂O₃ or sapphire, as well as thick aluminum gallium oxide films grown on either (010) β-Ga₂O₃ or sapphire, all show a very broad PL spectrum with intense dominant blue/green emission. PL differences between samples and the possible causes of these differences are analyzed. This work expands previous reports that have so far attributed blue and green emissions to point defects and shows that in the case of thin films, extended defects might have a prominent role in emission properties.

Impact of Al doping on a hydrothermally synthesized β-Ga2O3 nanostructure for photocatalysis applications

Aluminum (Al)-doped beta-phase gallium oxide (β-Ga₂O₃) nanostructures with different Al concentrations (0 to 3.2 at%) are synthesized using a hydrothermal method. The single phase of the β-Ga₂O₃ is maintained without intermediate phases up to Al 3.2 at% doping. As the Al concentration in the β-Ga₂O₃ nanostructures increases, the optical bandgap of the β-Ga₂O₃ increases from 4.69 (Al 0%) to 4.8 (Al 3.2%). The physical, chemical, and optical properties of the Al-doped β-Ga₂O₃ nanostructures are correlated with photocatalytic activity via the degradation of a methylene blue solution under ultraviolet light (254 nm) irradiation. The photocatalytic activity is enhanced by doping a small amount of substitutional Al atoms (0.6 at%) that presumably create shallow level traps in the band gap. These shallow traps retard the recombination process by separating photogenerated electron–hole pairs. On the other hand, once the Al concentration in the Ga₂O₃ exceeds 0.6 at%, the crystallographic disorder, oxygen vacancy, and grain boundary-related defects increase as the Al concentration increases. These defect-related energy levels are broadly distributed within the bandgap, which act as carrier recombination centers and thereby degrade the photocatalytic activity. The results of this work provide new opportunities for the synthesis of highly effective β-Ga₂O₃-based photocatalysts that can generate hydrogen gas and remove harmful volatile organic compounds.

The photocatalytic activity is correlated with different parameters affecting the photocatalytic reactions; redox potential (RP), surface area (SA), crystal defect (CD), oxygen defect (OD), and grain-boundary induced defect (GD).

Characterization of defect levels in β-Ga2O3 single crystals doped with tantalum

Optical properties and defect characterization of Ta-doped β-Ga₂O₃ single crystals grown by the optical floating zone method.