Oxide semiconductors have emerged as promising candidates for ultraviolet (UV) photodetection due to their intrinsic wide bandgap, excellent thermal and chemical stability, and tunable optoelectronic properties. Materials such as zinc oxide (ZnO), gallium oxide (Ga₂O₃), and indium gallium zinc oxide (IGZO) exhibit bandgaps ranging from approximately 3.0 eV to 5.0 eV, making them inherently sensitive to UV radiation while remaining blind to visible light. This spectral selectivity is critical for applications in flame detection, environmental monitoring, and secure communications, where minimizing false signals from ambient light is essential.
The photoresponse mechanism in oxide semiconductors begins with the absorption of UV photons with energy exceeding the material’s bandgap. Upon absorption, electron-hole pairs are generated, contributing to photoconductivity. In pristine ZnO, for example, the bandgap of ~3.3 eV corresponds to a cutoff wavelength of ~375 nm, placing it firmly in the UV-A spectrum. Ga₂O₃, with a bandgap of ~4.8 eV, extends this cutoff to ~260 nm, enabling detection in the solar-blind UV-C region. The photogenerated carriers are then separated and collected under an applied electric field, with the resulting photocurrent serving as the measurable signal.
Key figures of merit for UV photodetectors include responsivity, detectivity, and response speed. Responsivity (R) quantifies the photocurrent generated per unit of incident optical power, typically expressed in A/W. High-quality ZnO-based photodetectors have demonstrated responsivities exceeding 10³ A/W under strong UV illumination, attributed to photoconductive gain mechanisms involving trapped holes and prolonged electron lifetimes. Detectivity (D*) accounts for both responsivity and noise, providing a measure of the detector’s ability to discern weak signals. A typical high-performance Ga₂O₃ detector may achieve D* values above 10¹² Jones, indicating excellent sensitivity. Response speed, characterized by rise and fall times, is crucial for dynamic applications. Oxide semiconductors generally exhibit response times ranging from microseconds to milliseconds, influenced by defect-mediated recombination processes.
Device architectures play a pivotal role in determining performance metrics. Metal-semiconductor-metal (MSM) configurations are widely employed due to their simplicity and compatibility with planar fabrication. In an MSM detector, interdigitated metal electrodes are deposited on the semiconductor surface, creating a lateral photoconductive path. The absence of doping gradients simplifies processing, but the reliance on surface contacts can introduce Schottky barriers that influence dark current and responsivity. For instance, Au/ZnO/Au MSM detectors often show low dark currents but may suffer from slower response due to surface state trapping.
P-n junction photodetectors offer an alternative with potentially superior performance. By forming a junction between p-type and n-type oxide layers, built-in electric fields facilitate efficient carrier separation. However, achieving stable p-type doping in oxides like ZnO remains challenging due to self-compensation effects. Recent advances in heterojunction designs, such as n-ZnO/p-NiO or n-Ga₂O₃/p-Si, circumvent this limitation by leveraging established p-type materials. These heterostructures exhibit improved rectification and lower noise, enhancing detectivity. For example, a ZnO/NiO junction detector has demonstrated a responsivity of 0.5 A/W at 360 nm with a fast response time of 20 ns.
The performance of oxide semiconductor photodetectors is strongly influenced by material quality and defect engineering. Native point defects, such as oxygen vacancies or zinc interstitials in ZnO, can act as recombination centers or unintentional donors, altering carrier concentrations and recombination kinetics. Post-growth treatments, including annealing in oxygen or nitrogen atmospheres, are routinely employed to passivate defects and optimize optoelectronic properties. Doping with elements like Al or Mg can further tailor conductivity and band alignment. In Ga₂O₃, Sn doping has been shown to enhance responsivity by increasing carrier mobility while maintaining low dark current.
Environmental stability is another critical advantage of oxide semiconductors. Unlike organic or perovskite-based detectors, oxides such as ZnO and Ga₂O₃ are resistant to moisture and oxidation, ensuring long-term operational reliability. This robustness is particularly valuable for outdoor or harsh-environment applications. However, surface states can still influence performance over time, necessitating appropriate encapsulation or surface passivation techniques.
Emerging trends in oxide semiconductor photodetectors include the integration of nanostructures and hybrid systems. One-dimensional ZnO nanowires or two-dimensional Ga₂O₃ nanosheets provide high surface-to-volume ratios, enhancing light absorption and carrier collection efficiency. Additionally, combining oxides with plasmonic nanoparticles or quantum dots can extend spectral sensitivity or improve responsivity through localized field enhancement or energy transfer mechanisms.
In summary, oxide semiconductors offer a compelling platform for UV photodetection, balancing wide bandgap properties with environmental resilience. Advances in material synthesis, defect control, and device engineering continue to push the boundaries of responsivity, detectivity, and speed. While challenges such as p-type doping persist, innovative heterostructures and nanostructured designs provide viable pathways for next-generation detectors. As research progresses, these materials are poised to play an increasingly vital role in both commercial and specialized UV sensing applications.