Aluminum nitride (AlN) is a wide bandgap semiconductor with exceptional thermal conductivity, high electrical resistivity, and strong radiation tolerance, making it a promising candidate for space and nuclear applications. Its inherent properties allow it to withstand harsh radiation environments where conventional semiconductors like silicon would degrade rapidly. The radiation hardness of AlN stems from its strong covalent bonding, high displacement energies, and low intrinsic defect concentrations. However, like all materials, it is not immune to radiation-induced damage, and understanding its behavior under irradiation is critical for its deployment in extreme environments.

When exposed to high-energy particles such as protons, neutrons, or heavy ions, AlN undergoes atomic displacements that create point defects, including vacancies, interstitials, and antisite defects. The primary knock-on atoms generate cascades of secondary displacements, leading to defect clusters. Nitrogen vacancies (V_N) and aluminum vacancies (V_Al) are the most common point defects observed in irradiated AlN. These vacancies act as recombination centers, reducing carrier lifetimes and degrading electronic performance. Additionally, nitrogen interstitials (N_i) and aluminum interstitials (Al_i) can form, further complicating the defect landscape. The formation energy of these defects is relatively high compared to narrower bandgap materials, which partially explains AlN's resilience.

Carrier lifetime degradation is a critical concern in radiation environments. Studies have shown that high fluences of particle irradiation introduce deep-level traps that capture free carriers, reducing both electron and hole mobilities. In AlN, the dominant deep levels associated with irradiation-induced defects appear at approximately 0.7 eV and 1.2 eV below the conduction band edge. These traps significantly increase non-radiative recombination rates, impacting device performance in optoelectronic and high-power applications. However, due to AlN's wide bandgap (6.2 eV), the material retains functionality at radiation doses that would render silicon or even gallium arsenide inoperable.

Radiation hardening strategies for AlN focus on defect engineering and material optimization. One approach involves doping with elements that act as defect sinks, such as oxygen or silicon. Oxygen, for instance, can passivate nitrogen vacancies, reducing their impact on carrier lifetimes. Another strategy is the use of epitaxial growth techniques that minimize intrinsic defects before irradiation, thereby delaying the onset of severe degradation. Post-irradiation annealing has also been explored, with temperatures above 1000°C shown to partially recover electrical properties by annihilating point defects. However, annealing at such high temperatures may not always be practical for integrated devices.

Comparing AlN with other wide bandgap semiconductors like silicon carbide (SiC) and diamond reveals distinct advantages and trade-offs. SiC, with a bandgap of 3.3 eV, exhibits excellent radiation tolerance but suffers from higher defect mobility at elevated temperatures, leading to faster degradation under neutron irradiation. Diamond, with its ultra-wide bandgap (5.5 eV) and extreme displacement energies, outperforms both AlN and SiC in terms of displacement damage resistance. However, diamond's lack of a mature doping technology limits its practical use in electronic devices. AlN strikes a balance between radiation hardness and manufacturability, particularly in applications requiring high thermal conductivity and stability under prolonged irradiation.

In nuclear reactor environments, where neutron flux is a primary concern, AlN demonstrates superior resistance to swelling and amorphization compared to SiC. While SiC undergoes significant volumetric expansion at high neutron fluences, AlN maintains structural integrity due to its higher defect recombination rate. Diamond, though highly resistant, is prohibitively expensive for large-scale deployment. For space applications, where proton and cosmic ray exposure dominates, AlN's low atomic number reduces the probability of secondary particle generation, minimizing cumulative damage over time.

Despite its advantages, challenges remain in fully exploiting AlN's radiation hardness. The lack of high-quality bulk substrates compared to SiC limits its widespread adoption. Defect interactions under mixed radiation fields (e.g., protons and gamma rays) are not yet fully understood, requiring further study. Additionally, the role of extended defects such as dislocations in radiation tolerance needs deeper investigation, as they may act as preferential sites for defect accumulation.

In summary, aluminum nitride presents a compelling solution for radiation-hardened applications in space and nuclear systems. Its intrinsic properties provide a robust foundation for resisting displacement damage, while ongoing research into defect engineering continues to enhance its performance. While SiC and diamond offer their own strengths, AlN's combination of thermal, electrical, and radiation-resistant properties positions it as a critical material for next-generation high-radiation environments. Future advancements in growth techniques and defect control will further solidify its role in these demanding applications.