Boron nitride (BN) is a material with exceptional optical properties, making it a strong candidate for applications in deep-ultraviolet (UV) optoelectronics and photonic circuits. Its wide bandgap, high UV transparency, and thermal stability distinguish it from other semiconductor materials, particularly in environments requiring high-performance optical components. This article explores BN’s optical characteristics, focusing on its bandgap, transparency, and applications in photodetectors, light-emitting diodes (LEDs), and photonic circuits.

BN exists in multiple polymorphs, with hexagonal boron nitride (hBN) being the most studied for optoelectronic applications. hBN exhibits an indirect bandgap of approximately 5.9 eV, though some studies suggest variations depending on sample quality and measurement techniques. This wide bandgap enables hBN to operate efficiently in the deep-UV spectrum, a region where conventional semiconductors like silicon or gallium nitride (GaN) face limitations due to their narrower bandgaps. The material’s large exciton binding energy, estimated to be around 130 meV, further enhances its optical performance by promoting stable excitonic emissions even at room temperature.

One of the most notable optical properties of BN is its high transparency in the UV range. hBN demonstrates over 80% transmittance for wavelengths between 200 nm and 300 nm, making it suitable for deep-UV optical components. This transparency, combined with its low absorption coefficient in the UV spectrum, allows hBN to function as an effective window material for UV photodetectors and lenses. Unlike many wide-bandgap oxides, BN does not suffer from significant defect-related absorption, ensuring minimal optical losses in high-energy photon detection systems.

Deep-UV photodetectors based on BN have shown promising performance metrics. Devices fabricated from high-quality hBN exhibit responsivities in the range of 0.1 to 1 A/W under 200-250 nm illumination, with response times as fast as a few milliseconds. The material’s intrinsic resistance to radiation damage and chemical stability further enhance its suitability for harsh-environment applications, such as space-based UV sensing or flame detection systems. BN photodetectors also demonstrate low dark currents, a critical parameter for high-sensitivity detection, due to the material’s low intrinsic carrier concentration at room temperature.

In the realm of light-emitting devices, BN has been explored for deep-UV LEDs. While the indirect bandgap of hBN poses challenges for efficient light emission, advances in defect engineering and doping have enabled electroluminescence in the 215-250 nm range. By introducing controlled impurities or creating quantum-confined structures, researchers have achieved external quantum efficiencies (EQEs) of up to 1% in prototype devices. Though this remains lower than mature III-nitride UV LEDs, BN-based emitters benefit from superior thermal conductivity and reduced efficiency droop at high injection currents, suggesting potential for high-power UV light sources.

BN also plays a critical role in photonic circuits, particularly as a low-loss dielectric for UV waveguides. Its refractive index in the deep-UV region ranges between 1.7 and 2.1, with an extinction coefficient below 0.01, enabling efficient light confinement and propagation. Waveguides fabricated from hBN exhibit propagation losses below 5 dB/cm at 250 nm, outperforming many conventional UV-transparent materials. Additionally, BN’s ability to form atomically smooth heterostructures with other 2D materials allows for the design of hybrid photonic devices with tailored optical responses.

The material’s nonlinear optical properties further expand its utility in photonics. hBN exhibits strong second-harmonic generation (SHG) due to its non-centrosymmetric crystal structure, making it valuable for frequency conversion in UV laser systems. Studies have reported effective nonlinear coefficients comparable to those of beta-barium borate (BBO), a widely used nonlinear crystal, but with superior thermal and mechanical stability. This property opens possibilities for compact UV light sources and integrated photonic signal processing.

Challenges remain in fully exploiting BN’s optical potential. The difficulty in achieving p-type doping with high carrier concentrations limits the development of bipolar devices like LEDs and laser diodes. However, recent progress in ion implantation and defect engineering has shown promising pathways to overcome this limitation. Another area of ongoing research involves improving the crystalline quality of large-area BN films to reduce scattering losses in optical components.

In summary, boron nitride’s unique combination of wide bandgap, deep-UV transparency, and thermal stability positions it as a key material for next-generation optoelectronic systems. Its applications in photodetectors, LEDs, and photonic circuits demonstrate capabilities beyond conventional semiconductors, particularly in the demanding deep-UV spectral range. Continued advancements in material synthesis and device engineering will likely expand BN’s role in emerging optical technologies, offering solutions for high-performance UV optoelectronics and integrated photonics.