Aluminum nitride (AlN) is a critical material for deep-ultraviolet (DUV) photonic applications due to its wide bandgap of approximately 6.1 eV, which enables emission and detection in the 200–280 nm wavelength range. This spectral region is essential for applications such as sterilization, water purification, UV curing, and biological sensing. The performance of AlN-based devices is governed by its intrinsic material properties, including band-edge emission characteristics, exciton binding energy, and challenges associated with light extraction efficiency.

Band-edge emission in AlN is dominated by near-band-edge transitions, which are influenced by crystal quality, strain, and defect density. The direct bandgap of AlN allows for efficient radiative recombination, but achieving high internal quantum efficiency requires minimizing non-radiative recombination centers such as dislocations and point defects. The exciton binding energy in AlN is notably high, around 50–80 meV, due to its large bandgap and low dielectric constant. This strong excitonic effect enhances light emission efficiency at room temperature, as excitons remain stable and contribute significantly to radiative processes. However, the high binding energy also necessitates careful engineering of carrier injection and recombination dynamics in device structures.

One of the primary challenges in AlN-based DUV photonics is light extraction efficiency. The refractive index of AlN (~2.2 in the DUV range) creates a significant mismatch with air (n=1), leading to total internal reflection and trapping of emitted light within the material. Additionally, AlN exhibits strong absorption at wavelengths below its bandgap, complicating the design of transparent substrates and contacts. Strategies to mitigate these issues include the use of nanostructured surfaces, photonic crystals, and flip-chip geometries to enhance light outcoupling. Polarization engineering is also critical, as AlN exhibits strong anisotropic optical properties due to its wurtzite crystal structure.

DUV light-emitting diodes (LEDs) based on AlN face several hurdles, including low external quantum efficiency (EQE) and high operating voltages. The EQE is typically limited by poor hole injection due to the high activation energy of p-type dopants (Mg) in AlN, as well as non-radiative recombination at defects. Heterostructure designs incorporating AlGaN layers with graded compositions help improve carrier confinement and injection efficiency. Despite these challenges, AlN-based DUV LEDs have demonstrated emission wavelengths as short as 210 nm, with continuous improvements in output power and lifetime.

DUV lasers using AlN as the active medium or cladding layer are an emerging area of research. The large bandgap and high thermal conductivity of AlN make it suitable for high-power laser operation, but achieving population inversion remains difficult due to the high carrier densities required. Optically pumped AlN lasers have been demonstrated with sub-250 nm emission, while electrically injected devices are still in development. Key challenges include reducing optical losses, improving carrier injection efficiency, and managing thermal effects under high current operation.

For DUV photodetectors, AlN offers advantages such as solar-blind operation (insensitivity to visible light) and high breakdown voltage. Photoconductive and photovoltaic detectors based on AlN exhibit low dark currents and fast response times, making them suitable for flame detection, missile warning systems, and UV astronomy. However, achieving high quantum efficiency requires optimizing the thickness and doping profile of the absorption layer, as well as minimizing surface recombination through passivation techniques.

In summary, AlN is a cornerstone material for DUV photonic applications, but its performance is heavily influenced by material quality and device design. Advances in epitaxial growth and defect reduction have enabled significant progress in DUV LEDs, lasers, and detectors, yet challenges remain in improving efficiency, power output, and reliability. Future developments will likely focus on novel device architectures, improved doping techniques, and integration with other wide-bandgap materials to unlock the full potential of AlN in the DUV spectrum.

The unique properties of AlN position it as a key enabler for next-generation DUV technologies, with ongoing research addressing both fundamental limitations and practical engineering hurdles. As material synthesis and device fabrication techniques continue to mature, AlN-based photonic devices are expected to play an increasingly vital role in industrial, medical, and scientific applications requiring precise and efficient DUV light sources and sensors.