Ultra-wide bandgap semiconductors, such as gallium oxide (Ga₂O₃) and aluminum nitride (AlN), have gained significant attention due to their exceptional electronic properties, including bandgaps exceeding 4 eV. These materials are promising for high-power and high-temperature applications, but their performance is heavily influenced by defects, which can introduce deep-level traps, alter carrier transport, and degrade breakdown characteristics. Understanding these defects is critical for material optimization and reliability.

Crystal defects in ultra-wide bandgap semiconductors can be broadly categorized into point defects, extended defects, and impurities. Point defects, such as vacancies, interstitials, and antisite defects, are particularly consequential due to their role in creating deep-level traps. In Ga₂O₃, for example, gallium vacancies (V_Ga) and oxygen vacancies (V_O) are common. Oxygen vacancies often act as shallow donors, contributing to unintentional n-type conductivity, while gallium vacancies can form deep acceptors, compensating the donor behavior. In AlN, nitrogen vacancies (V_N) and aluminum vacancies (V_Al) similarly influence carrier concentrations, with V_N acting as a deep donor and V_Al as a deep acceptor.

Deep-level traps are especially problematic in ultra-wide bandgap materials because their large bandgap allows defects to introduce energy levels far from the band edges. These traps can capture charge carriers, reducing carrier mobility and lifetime. For instance, in β-Ga₂O₃, deep traps associated with iron (Fe) or chromium (Cr) impurities can severely limit conductivity by acting as recombination centers. In AlN, carbon (C) and silicon (Si) impurities create deep levels that affect both n-type and p-type doping efficiency. The presence of these traps can lead to Fermi-level pinning, making it difficult to achieve controlled doping.

The impact of defects on breakdown voltage is a major concern for power electronics. Breakdown occurs when the electric field exceeds the material’s critical value, leading to avalanche multiplication or tunneling. Defects can lower the effective breakdown field by creating localized regions of high electric field or by providing pathways for premature breakdown. In Ga₂O₃, dislocations and grain boundaries act as leakage paths, reducing the breakdown voltage despite the material’s theoretically high critical field of 8 MV/cm. Similarly, in AlN, threading dislocations and point defect clusters can degrade the breakdown performance, even though its theoretical breakdown field exceeds 10 MV/cm.

Defect characterization techniques are essential for identifying and mitigating these issues. Deep-level transient spectroscopy (DLTS) is widely used to probe trap energy levels and concentrations. In Ga₂O₃, DLTS has revealed multiple deep traps between 0.5 eV and 2.0 eV below the conduction band, attributed to intrinsic defects and impurities. Photoluminescence (PL) spectroscopy is another valuable tool, with emissions in Ga₂O₃ often linked to oxygen vacancies or gallium interstitials. For AlN, cathodoluminescence (CL) and PL studies have identified defect-related emissions in the ultraviolet and visible ranges, corresponding to nitrogen vacancies or aluminum interstitials.

Growth techniques play a crucial role in defect control. Molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE) are commonly used for Ga₂O₃ and AlN, but each method introduces distinct defects. MBE-grown Ga₂O₃ tends to have lower oxygen vacancy concentrations due to precise stoichiometric control, while HVPE-grown material may exhibit higher dislocation densities. For AlN, metalorganic chemical vapor deposition (MOCVD) is preferred, but carbon incorporation from precursors can introduce deep-level traps. Post-growth treatments, such as annealing in oxygen or nitrogen atmospheres, can passivate vacancies or reduce impurity concentrations.

The role of hydrogen in defect passivation is another critical aspect. Hydrogen can passivate dangling bonds at vacancies or impurities, effectively neutralizing their electrical activity. In Ga₂O₃, hydrogen is known to passivate oxygen vacancies, converting them from shallow donors to electrically inactive complexes. However, hydrogen itself can become a source of instability if it dissociates under high electric fields or elevated temperatures. In AlN, hydrogen passivates nitrogen vacancies but may also interact with carbon or oxygen impurities, complicating the defect landscape.

The interplay between defects and doping is particularly complex in ultra-wide bandgap materials. Intentional doping is necessary for device functionality, but dopants can interact with native defects, forming complexes that alter electrical properties. For example, silicon doping in Ga₂O₃ can be compensated by gallium vacancies, reducing net carrier concentration. In AlN, magnesium (Mg) is used for p-type doping, but Mg acceptors are often passivated by hydrogen or compensated by nitrogen vacancies, leading to low hole concentrations.

Defect engineering strategies are being developed to mitigate these challenges. One approach involves co-doping, where additional impurities are introduced to suppress unwanted defects. In Ga₂O₃, co-doping with iron or magnesium can compensate oxygen vacancies, improving resistivity. Another strategy is strain engineering, where lattice mismatch is exploited to reduce dislocation densities in epitaxial layers. For AlN, substrate patterning or buffer layers can minimize threading dislocations, enhancing material quality.

Thermal stability of defects is another consideration for high-temperature applications. Some defects may anneal out at elevated temperatures, while others become more pronounced. In Ga₂O₃, oxygen vacancies are stable up to high temperatures, but gallium vacancies may migrate or aggregate, altering trap distributions. In AlN, nitrogen vacancies can diffuse at high temperatures, leading to defect clustering or interaction with impurities. Understanding these dynamics is essential for predicting device reliability under operational stress.

The relationship between defects and optical properties is also significant. Defects can introduce absorption or emission features that affect optoelectronic performance. In Ga₂O₃, oxygen vacancies cause UV absorption, while gallium vacancies contribute to visible luminescence. In AlN, defect-related emissions can interfere with deep-UV optoelectronic applications, necessitating stringent defect control. Tailoring growth conditions or post-processing treatments can minimize these effects.

Future research directions include advanced defect modeling and in-situ characterization during growth. First-principles calculations can predict defect formation energies and electronic structures, guiding experimental efforts. In-situ monitoring techniques, such as reflection high-energy electron diffraction (RHEED) or optical spectroscopy, can provide real-time feedback for defect control. Combining these approaches will be key to unlocking the full potential of ultra-wide bandgap semiconductors.

In summary, defects in ultra-wide bandgap semiconductors present unique challenges due to their deep-level nature and impact on critical parameters like breakdown voltage. Addressing these challenges requires a multidisciplinary approach, combining advanced growth techniques, precise characterization, and innovative defect engineering strategies. Progress in this area will be essential for realizing the promise of these materials in next-generation electronic and optoelectronic applications.