Ultraviolet (UV) rays, situated on the electromagnetic spectrum just beyond visible light, are incredibly important in our lives. Comprising UVA, UVB, and UVC rays, they have distinct roles, both beneficial and potentially harmful. UVB rays, for instance, facilitate the synthesis of vitamin D in our skin, vital for our overall health. Moreover, moderate exposure to UVA and UVB rays can promote skin health.

On the flip side, UVC rays, while the most dangerous, are fortunately absorbed by the Earth's atmosphere, serving as a natural shield against these harmful rays. Therefore, it is vital to have ultraviolet photodetectors which are designed to convert UV light into electrical signals, which are pivotal in diverse fields. They play a central role in UV radiation monitoring, contributing to public safety by assessing UV exposure risks, and are instrumental in environmental monitoring, helping scientists understand the impact of UV radiation on ecosystems. Furthermore, UV photodetectors are essential in astronomy for observing celestial phenomena beyond Earth's UV-absorbing atmosphere and are indispensable tools in medical research, where they enable insights into molecular processes through fluorescence microscopy and DNA analysis. In addition, they have crucial roles in water and air purification by ensuring efficient UV-C disinfection and contributing to safer water and air for consumption and breathing.

In the early days, UV detection was primarily done via thermal detectors, charged coupled devices and photomultiplier tubes, however, these devices were inefficient, slow, fragile, bulky and had wavelength independent response. Therefore, new and improved semiconductors like silicon-based UV photodiodes and GaAs-based photodiodes proved to be a better option mainly due to their low cost, lightweight, insensitiveness to magnetic field and faster response. However, Si-based UV photodiodes show some limitations typically due to the building up of the passivation layer which reduces the quantum efficiency over time in the gaping UV range, and device ageing mostly because of overexposure to high energy radiation. Moreover, Si-based photodiodes must have a cool active surface in order to reduce dark current, however, this attracts contaminants which can lower detectivity.

On the other hand, UV photodetectors based on wideband gap semiconductors like Sic, diamond and III-nitrides proved to be a superior alternative, by having room temperature operation and intrinsic visible blindness. Figure 1 compares the key parameters of various materials used in UV photodetectors and proves that WBG semiconductor-type material has better overall characteristics.

Figure 1: Crucial key parameters for different materials used in UV photodetectors.

It is clearly seen that WBG materials have a higher thermal conductivity which enables them to be suitable for high power and high temperature applications. Moreover, WBG photodiodes have high electron velocity and strong chemical bonds with the capability of having negative electron affinity that renders them a perfect candidate for blind visible UV photocathodes. In spite of all these superior characteristics of WBG photodiodes, there are a few drawbacks like low crystal quality which is due to the addition of low-quality substrates in homoepitaxy in crystal growth which can cause structural defects and leakage of current. On the other hand, WBG photodiodes require high energy for the activation of dopants, which in turn requires manufacturers to include huge doping impurities which greatly reduces carrier mobility. Despite these drawbacks, semiconductor materials like Sic, diamond and III-nitrides are being studied and introduced to the market slowly.

Ⅰ. Exploring Diverse Materials in WBG Photodetectors: From SiC to Diamond Semiconductors

The first ever WBG photodetectors to reach the consumer markets were the Sic p-n junction photodiodes for varied applications like combustion monitoring, fire alarms, and discharge detection. These photodiodes were manufactured on p-type 6H-SiC substrates and were heavily doped to 1019 cm-3, in order to have a low reverse current of approximately 10-11 A cm-2 even at a high temperature of 200°C which made it the most viable option. Moreover, due to the unique ability of radiation hardness, SiC was the prime source of interest in particle detection, as the readings from a commercial SiC photodiode remain unchanged even after being exposed up to 1000 kGy gamma dose.

On the other hand, Diamond as a photodetector has been garnering a lot of attention as it has unusually high thermal conductivity, low dielectric constant and high carrier mobilities which makes it suitable for high frequency and high power applications. However, Diamond is less superior than its SiC counterpart as it is expensive and has diverse quality, moreover, it is difficult to artificially grow diamond devices as doping procedures of crystal lattices need reliable P-type doping which is still inefficient at its current stage. Despite these issues, diamond is most suited for X-ray and particle detection applications as more energy is required to create a vacancy-interstitial defect pair which makes it suitable for harsh environmental conditions. Finally, III-nitrides photodetectors made primarily from compounds like AIN, InN and GaN show an upper hand over other WBG semiconductors in spectral selectivity mainly due to their direct bandgap and high breakdown field. Moreover, AIGaN photoconductors have a high response to different irradiance of light as shown in Figure 2.

Figure 2: Responsivity VS. Irradiance of a AIGaN photodetector

However, III-nitrides photodetectors show persistent photoconductivity that makes it heavily dependent on the amount of time the sample has been kept in the dark. On the other hand, III-nitride materials are used extensively as Schottky photodiodes, metal–semiconductor–metal photodiodes, p–i–n photodiodes, phototransistors, avalanche photodiodes and photocathodes.

Ⅱ. Conclusion

Photodetectors are pivotal devices that convert light or electromagnetic radiation into electrical signals, serving as the backbone for various applications. They are indispensable in imaging, optical communication, scientific research, medical diagnostics, environmental monitoring, and aerospace, and defense. Additionally, photodetectors contribute to renewable energy production, enable security systems, and support biometric identification. Their ability to capture and measure light's properties plays a crucial role in advancing technology, science, and our daily lives. Breakthroughs in wide-bandgap (WBG) semiconductor technology have created opportunities to develop affordable selective ultraviolet (UV) photodetectors suitable for operation in challenging environmental conditions. Nonetheless, the technology associated with these materials is still in its infancy.

One of the most persistent challenges is in consistently producing high-quality substrates for homoepitaxial growth, but the most formidable hurdle lies in achieving substantial doping levels and establishing dependable ohmic and Schottky contacts. Photodetectors constructed from materials such as silicon carbide (SiC), diamond, and III-nitrides have already been successfully introduced into the market for applications including fire sensors, engine control, and environmental monitoring. Consistency and reliability are two prime factors in deciding the success of these photodetectors in the market. While wide-bandgap semiconductors are not set to immediately replace silicon detectors, they do present an unparalleled potential to cater to a specialized sensor market focused on high-temperature and challenging environmental settings.