Nitride semiconductors, particularly gallium nitride (GaN), aluminum nitride (AlN), and their ternary alloy aluminum gallium nitride (AlGaN), have emerged as critical materials for ultraviolet (UV) optoelectronic devices. Their wide and tunable bandgaps, high thermal stability, and robust mechanical properties make them ideal for applications such as UV light-emitting diodes (LEDs) and photodetectors. These devices are essential for sterilization, water purification, medical diagnostics, and secure communications. However, achieving high-efficiency UV emission presents significant challenges, requiring precise bandgap engineering, advanced quantum well designs, and mitigation of material defects.

The bandgap of AlGaN can be tuned from 3.4 eV (GaN) to 6.2 eV (AlN) by adjusting the aluminum composition, enabling coverage of the UV spectrum from near-UV (320–400 nm) to deep-UV (200–280 nm). This tunability is crucial for targeting specific applications, as different UV wavelengths have distinct absorption and interaction properties with materials and biological organisms. For example, deep-UV LEDs are highly effective for germicidal applications due to their ability to disrupt DNA and RNA in microorganisms. However, increasing the aluminum content to achieve shorter wavelengths introduces challenges such as reduced carrier mobility, increased defect densities, and difficulties in doping efficiency.

Bandgap engineering in AlGaN involves optimizing the aluminum composition in both the active and cladding layers to balance carrier confinement and light extraction. The active region typically consists of multiple quantum wells (MQWs) designed to enhance radiative recombination. For deep-UV devices, AlGaN with high aluminum content is used in the quantum wells, while slightly lower aluminum content is employed in the barrier layers to improve carrier injection. The large polarization fields in these heterostructures, caused by the wurtzite crystal structure, can lead to quantum-confined Stark effects (QCSE), which reduce the overlap of electron and hole wavefunctions and thus decrease internal quantum efficiency. To mitigate this, researchers have explored nonpolar and semipolar crystal orientations, as well as polarization-matched heterostructures, to minimize the QCSE.

Quantum well design is another critical factor in optimizing UV optoelectronic performance. The thickness and composition of the wells and barriers must be carefully controlled to maximize radiative recombination while minimizing carrier leakage. Thin quantum wells are often used to reduce the spatial separation of electrons and holes caused by polarization fields. Additionally, the use of superlattices or graded-composition layers can improve carrier transport and reduce defect-related non-radiative recombination. For deep-UV emitters, the incorporation of AlN or high-Al-content AlGaN as barrier layers helps confine carriers within the active region, but this also increases the lattice mismatch with underlying layers, leading to higher threading dislocation densities.

Material quality remains a significant challenge in nitride-based UV optoelectronic devices. Threading dislocations, originating from lattice mismatch between epitaxial layers and substrates, act as non-radiative recombination centers, severely limiting device efficiency. Sapphire and silicon carbide are commonly used substrates, but their lattice and thermal expansion mismatches with AlGaN result in high dislocation densities. The development of bulk AlN substrates has shown promise in reducing these defects, but cost and scalability remain barriers. Alternative approaches include epitaxial lateral overgrowth and the use of patterned substrates to reduce dislocation densities in the active region.

Doping efficiency is another hurdle, particularly for p-type AlGaN with high aluminum content. Magnesium, the standard p-type dopant in GaN, exhibits increasingly deeper acceptor levels as the aluminum content rises, leading to lower hole concentrations and higher resistivity. This results in inefficient hole injection into the quantum wells, reducing overall device performance. Strategies such as polarization-induced doping, modulation doping, and the use of superlattices have been explored to improve p-type conductivity. Similarly, n-type doping with silicon becomes less effective at high aluminum concentrations due to compensation effects and donor ionization energy increases.

Light extraction efficiency is particularly problematic for deep-UV LEDs due to the high absorption coefficients of AlGaN at shorter wavelengths. The majority of emitted light is trapped within the device due to total internal reflection at the semiconductor-air interface. Various techniques have been employed to enhance light extraction, including surface roughening, the use of transparent p-contacts, and the integration of photonic crystals or nanostructured surfaces. Flip-chip designs and the incorporation of reflective layers have also been explored to redirect light toward the escape cone.

Despite these challenges, significant progress has been made in the performance of UV optoelectronic devices. External quantum efficiencies (EQEs) for commercial UV-C LEDs have reached the single-digit percentage range, with laboratory devices demonstrating higher values. Further improvements are expected through advances in material growth techniques, defect reduction, and innovative device architectures. The development of tunnel junctions and polarization-engineered structures may also provide pathways to more efficient carrier injection and reduced resistive losses.

Photodetectors based on AlGaN offer high responsivity and solar-blind operation, making them suitable for flame detection, missile warning systems, and environmental monitoring. The solar-blind region (240–280 nm) is particularly valuable because solar radiation at these wavelengths is absorbed by the Earth’s atmosphere, reducing background noise. AlGaN photodetectors with varying aluminum compositions can be tailored for specific cutoff wavelengths, enabling selective detection. However, similar challenges in material quality and doping efficiency affect detector performance, particularly in achieving low dark currents and high signal-to-noise ratios.

In summary, nitride semiconductors are indispensable for UV optoelectronic applications, but their full potential is yet to be realized. Continued research into bandgap engineering, quantum well optimization, defect reduction, and light extraction strategies will be essential for overcoming current limitations. As material growth techniques advance and novel device designs emerge, the efficiency and reliability of UV LEDs and photodetectors will improve, enabling broader adoption across industrial, medical, and environmental applications.