AlₓGa₁₋ₓO₃ alloys represent a class of ultra-wide bandgap semiconductors with tunable electronic and structural properties, making them promising candidates for high-power electronics and deep-ultraviolet optoelectronic applications. These alloys exhibit bandgaps ranging from 4.8 eV (Ga₂O₃) to 8.8 eV (Al₂O₃), offering a broader range than conventional AlGaN alloys. The ability to adjust the bandgap through alloy composition (x) enables precise control over optical absorption and emission characteristics, critical for UV photodetectors, LEDs, and high-electron-mobility transistors (HEMTs).

The growth of AlₓGa₁₋ₓO₃ thin films has been achieved using pulsed laser deposition (PLD) and molecular beam epitaxy (MBE). PLD offers advantages in maintaining stoichiometry and achieving high-quality crystalline films at relatively lower substrate temperatures, typically between 500–800°C. MBE, on the other hand, provides superior control over layer-by-layer growth, enabling abrupt heterointerfaces necessary for modulation-doped FETs. Both techniques face challenges related to phase separation due to the large miscibility gap in the Al₂O₃-Ga₂O₃ system, particularly at intermediate compositions (0.2 < x < 0.8). Stabilizing single-phase alloys requires careful optimization of growth parameters, including oxygen partial pressure, substrate choice, and growth rate.

Lattice-matching strategies are crucial for minimizing defects in heterostructures. While Ga₂O₃ has a monoclinic β-phase structure, Al₂O₃ crystallizes in the corundum (α-phase) structure, leading to significant lattice mismatch (~4–6%) depending on orientation. To mitigate strain-induced dislocations, researchers have employed buffer layers, such as AlN or graded AlₓGa₁₋ₓO₃ layers, to transition between dissimilar lattices. The critical thickness for defect-free epitaxy is typically below 100 nm for high Al-content films, beyond which relaxation occurs via misfit dislocations. Recent advances in substrate engineering, including the use of (0001) sapphire or (010) β-Ga₂O₃, have improved crystalline quality and enabled higher critical thicknesses.

Modulation-doped FETs based on AlₓGa₁₋ₓO₃/Ga₂O₃ heterostructures leverage the large conduction band offset (>1 eV for x > 0.4) to achieve high two-dimensional electron gas (2DEG) densities (>10¹³ cm⁻²). The high breakdown field (>8 MV/cm) of Al₂O₃-rich barriers enhances device performance in high-voltage applications. However, challenges remain in reducing interface traps and improving carrier mobility, which currently lags behind AlGaN/GaN systems due to stronger alloy scattering and phonon interactions.

In UV optoelectronics, AlₓGa₁₋ₓO₃ alloys enable tunable emission from 260 nm (Ga₂O₃) to <200 nm (high Al-content), covering the solar-blind spectrum (200–280 nm). Unlike AlGaN, which suffers from efficiency droop at high Al compositions, AlₓGa₁₋ₓO₃ maintains stronger band-to-band transitions, though radiative efficiency is limited by native point defects (e.g., oxygen vacancies). Recent progress in doping control, particularly with Si or Sn for n-type and Mg for p-type, has improved carrier injection in LEDs and photodiodes.

Compared to AlGaN alloys, AlₓGa₁₋ₓO₃ offers several advantages, including lower growth temperatures and the absence of polarization-induced electric fields, which complicate device design in III-nitrides. However, AlGaN retains superior thermal conductivity and more mature p-type doping technology. The trade-offs between these material systems depend on the application: AlₓGa₁₋ₓO₃ excels in high-breakdown-voltage devices, while AlGaN dominates in high-frequency RF and high-brightness UV emitters.

Recent progress in alloy composition control has focused on combinatorial growth techniques and in-situ monitoring (e.g., reflection high-energy electron diffraction) to achieve uniform Al incorporation. Advances in device integration include monolithic growth on Ga₂O₃ substrates and hybrid structures combining AlₓGa₁₋ₓO₃ with other oxides (e.g., ZnO) for multifunctional optoelectronics. Challenges for future development include improving p-type conductivity, reducing defect densities at heterointerfaces, and scaling up epitaxial growth for industrial production.

In summary, AlₓGa₁₋ₓO₃ alloys provide a versatile platform for ultra-wide bandgap semiconductor devices, with ongoing research addressing material stability, doping asymmetry, and heterostructure design. Their unique combination of tunable bandgap, high breakdown strength, and compatibility with existing oxide semiconductor technology positions them as a competitive alternative to AlGaN in next-generation high-power and UV optoelectronic systems.