Zinc gallate (ZnGa₂O₄) is a spinel-structured ultra-wide bandgap semiconductor with a bandgap energy of approximately 5 eV, positioning it as a promising candidate for deep-ultraviolet (UV) photodetection and transparent electronics. Its unique combination of optical transparency, high breakdown field, and chemical stability makes it suitable for applications in harsh environments, including space and high-power electronics. This article explores the material's properties, synthesis methods, defect engineering, and performance metrics, while contrasting it with β-Ga₂O₃, another leading ultra-wide bandgap oxide.

The spinel structure of ZnGa₂O₄, with the general formula AB₂O₄, consists of Zn²⁺ ions occupying tetrahedral sites and Ga³⁺ ions in octahedral sites within a cubic close-packed oxygen lattice. This arrangement contributes to its high thermal and chemical stability, as well as its anisotropic electronic properties. The bandgap of ZnGa₂O₄ is direct, enabling efficient optical transitions, which is advantageous for UV optoelectronic applications. The material exhibits high transparency in the visible and UV spectrum, with an absorption edge near 250 nm, corresponding to its 5 eV bandgap.

Synthesis of ZnGa₂O₄ thin films has been achieved using techniques such as pulsed laser deposition (PLD) and radio-frequency magnetron sputtering. PLD offers precise stoichiometric control and high crystalline quality, with growth temperatures typically ranging from 500 to 700 °C under oxygen-rich conditions. Sputtering, on the other hand, is more scalable and compatible with industrial processes, though it often requires post-deposition annealing to improve crystallinity. Epitaxial growth has been demonstrated on substrates like sapphire (Al₂O₃) and magnesium aluminate spinel (MgAl₂O₄), with lattice matching playing a critical role in reducing defect densities.

Defect engineering in ZnGa₂O₄ is crucial for tuning its electronic properties. Off-stoichiometry, particularly Zn/Ga ratio variations, significantly impacts conductivity and carrier concentrations. Excess Ga can lead to n-type conductivity due to the formation of oxygen vacancies and Ga interstitials, which act as shallow donors. Conversely, Zn-rich conditions may compensate for these defects but often result in reduced mobility due to increased scattering centers. Intentional doping with elements like Sn or Si has been explored to enhance n-type conductivity, achieving carrier concentrations in the range of 10¹⁷ to 10¹⁹ cm⁻³. However, p-type doping remains a challenge, with no reliable acceptor dopants identified to date, limiting the development of bipolar devices.

Anisotropic conductivity is another notable feature of ZnGa₂O₄, arising from its spinel crystal structure. Electrical transport measurements reveal that conductivity along the [111] direction is higher than along [100], attributed to the directional arrangement of Ga-O octahedra. This anisotropy must be accounted for in device design, particularly for high-frequency or power applications where uniform current distribution is critical.

In comparison to β-Ga₂O₃, ZnGa₂O₄ exhibits a slightly lower electron mobility, typically in the range of 10–30 cm²/Vs, whereas β-Ga₂O₃ can achieve mobilities exceeding 100 cm²/Vs due to its lower effective mass. However, ZnGa₂O₄ offers a higher breakdown field, estimated at 8–10 MV/cm, compared to β-Ga₂O₃'s 6–8 MV/cm, making it more suitable for high-voltage applications. Additionally, ZnGa₂O₄'s spinel structure provides better chemical stability against humidity and oxidation, a significant advantage for long-term device reliability.

Deep-UV photodetectors based on ZnGa₂O₄ have demonstrated high responsivity in the solar-blind region (200–280 nm), with external quantum efficiencies exceeding 50% in optimized devices. The material's wide bandgap ensures minimal dark current, enhancing signal-to-noise ratios. For transparent electronics, ZnGa₂O₄ thin-film transistors (TFTs) have been fabricated with on/off ratios >10⁶ and subthreshold swings below 0.5 V/decade, though channel mobility remains a limiting factor for high-speed applications.

Substrate compatibility is a persistent challenge for ZnGa₂O₄. While sapphire is commonly used, its lattice mismatch with ZnGa₂O₄ (∼4.5%) induces strain and defects. Alternative substrates like MgAl₂O₄ offer better lattice matching but are costly and less readily available. Heteroepitaxial growth on silicon has also been attempted, though interfacial reactions and thermal expansion mismatches complicate the process.

In summary, ZnGa₂O₄ is a compelling ultra-wide bandgap oxide with distinct advantages for deep-UV photodetection and transparent electronics. Its synthesis and defect engineering are well-studied, though p-type doping and substrate compatibility remain unresolved challenges. When compared to β-Ga₂O₃, it trades some mobility for higher breakdown strength and environmental stability. Future research should focus on optimizing doping strategies, exploring novel substrate solutions, and integrating ZnGa₂O₄ into functional devices to unlock its full potential.