Diamond has emerged as a promising material for deep ultraviolet (UV) optoelectronics, particularly in the 200–250 nm wavelength range, due to its unique combination of intrinsic properties. Its wide bandgap of 5.47 eV enables transparency to UV light, while its high thermal conductivity and exceptional carrier mobility make it suitable for high-power and high-frequency applications. The material’s excitonic effects further enhance its potential for efficient UV light emission and detection. However, significant challenges remain in realizing practical diamond-based UV devices, particularly in doping, contact engineering, and device design.

One of the most critical advantages of diamond for deep UV applications is its intrinsic UV transparency. Unlike conventional semiconductors such as silicon or gallium nitride (GaN), diamond does not exhibit significant absorption in the 200–250 nm range, allowing for high transmission of UV light. This property is essential for UV photodetectors and optical windows, where minimizing absorption losses is crucial. Additionally, diamond’s high breakdown electric field and radiation hardness make it suitable for harsh environments, including space and high-energy physics applications.

Excitonic effects in diamond play a significant role in its UV optoelectronic performance. Due to its large exciton binding energy of approximately 80 meV, excitons in diamond remain stable at room temperature, unlike in many other wide-bandgap materials. These excitons contribute to efficient light emission and absorption processes, particularly in the UV range. For instance, free exciton recombination in diamond produces sharp emission peaks near 235 nm, making it an attractive candidate for UV light-emitting diodes (LEDs). However, harnessing these excitonic effects for practical devices requires precise control over material quality and defect engineering.

Despite these advantages, achieving efficient diamond-based UV LEDs remains a major challenge. One of the primary obstacles is the difficulty in achieving reliable p-type and n-type doping. Boron is the most commonly used p-type dopant in diamond, but its relatively deep acceptor level (0.37 eV) limits hole concentration at room temperature. N-type doping is even more challenging, with phosphorus and nitrogen being the primary candidates. Phosphorus-doped diamond exhibits a donor level at 0.6 eV, which results in low free electron concentrations. Recent advances in delta doping and superlattice structures have shown promise in improving carrier injection, but further optimization is needed to achieve low-resistance ohmic contacts.

Contact engineering is another critical issue for diamond UV optoelectronic devices. The high Schottky barrier between metals and diamond often leads to poor carrier injection and high contact resistance. For p-type diamond, ohmic contacts have been demonstrated using heavily boron-doped layers combined with high-work-function metals such as gold or platinum. However, n-type contacts remain problematic due to the lack of shallow donors. Innovative approaches such as carbide-forming metallization and surface termination modifications are being explored to mitigate these challenges.

Experimental prototypes of diamond-based UV photodetectors have demonstrated promising performance. Metal-semiconductor-metal (MSM) photodetectors fabricated on single-crystal diamond substrates exhibit high responsivity in the 200–250 nm range, with dark currents as low as picoamperes. The spectral response of these devices is sharply peaked in the deep UV, with negligible sensitivity to visible light, making them ideal for solar-blind detection. However, achieving high external quantum efficiency (EQE) remains difficult due to carrier recombination at defects and insufficient carrier collection efficiency. Recent work on nanostructured diamond and avalanche photodiodes has shown potential for improving EQE, but further research is needed to optimize device architectures.

For UV LEDs, the internal quantum efficiency (IQE) of diamond remains limited by non-radiative recombination at defects and impurities. Dislocations and point defects such as nitrogen-vacancy centers act as recombination centers, reducing light emission efficiency. High-quality homoepitaxial diamond growth techniques, including microwave plasma chemical vapor deposition (MPCVD), have improved material quality, but defect densities still need to be reduced further. Additionally, the extraction of UV light from diamond LEDs is hindered by its high refractive index (2.4), which leads to significant total internal reflection. Surface texturing and photonic crystal structures have been explored to enhance light extraction, but these approaches add complexity to device fabrication.

The quantum efficiency limits of diamond-based UV devices are still being explored. Theoretical calculations suggest that the maximum IQE for diamond UV LEDs could approach 90% under ideal conditions, but current experimental devices achieve only a fraction of this value. Similarly, the EQE of diamond photodetectors is typically below 50%, primarily due to incomplete absorption and carrier loss mechanisms. Advances in doping, defect control, and device design are necessary to bridge this gap between theory and practice.

Recent progress in diamond growth and processing has enabled more sophisticated device demonstrations. For example, p-i-n diodes fabricated on diamond substrates have shown electroluminescence at 235 nm, corresponding to free exciton emission. While these devices represent a significant step forward, their wall-plug efficiency remains low, often below 1%. Improvements in current injection efficiency and reduction in series resistance are critical for enhancing device performance. Additionally, the integration of diamond with other wide-bandgap materials, such as AlN or BN, could enable novel heterostructure devices with improved carrier confinement and optical properties.

In summary, diamond holds immense potential for deep UV optoelectronics due to its intrinsic UV transparency, robust excitonic effects, and exceptional material properties. However, the realization of efficient diamond-based UV LEDs and photodetectors requires overcoming substantial challenges in doping, contact engineering, and defect control. While experimental prototypes have demonstrated feasibility, achieving high quantum efficiency and practical device performance will demand continued advancements in material synthesis and device physics. The unique advantages of diamond in the deep UV range make it a compelling candidate for next-generation optoelectronic applications, provided these technical hurdles can be addressed.