Defects in semiconductors significantly impact their electronic and optical properties, often leading to non-radiative recombination, reduced carrier lifetimes, and degraded device performance. Defect passivation strategies aim to neutralize these detrimental effects without compromising material quality. Key approaches include hydrogenation, surface coatings, and chemical treatments, each tailored to specific materials like silicon (Si), gallium nitride (GaN), and perovskites. The trade-offs between passivation effectiveness and carrier mobility must be carefully balanced to optimize device performance.

Hydrogenation is a widely used technique for passivating defects in semiconductors. In silicon, hydrogen effectively neutralizes dangling bonds at grain boundaries and interfaces. Atomic hydrogen diffuses into the lattice, bonding with silicon dangling bonds to form Si-H complexes, which reduce trap states in the bandgap. The process typically involves exposure to hydrogen plasma or annealing in a hydrogen-rich atmosphere at temperatures between 200 and 400 degrees Celsius. However, excessive hydrogenation can lead to the formation of hydrogen-induced defects, such as platelets or voids, which degrade electronic properties. In GaN, hydrogen passivates shallow donors and deep-level defects, often introduced during growth or processing. Hydrogen binds to nitrogen vacancies (V_N) and gallium vacancies (V_Ga), reducing their activity as recombination centers. Yet, hydrogen can also passivate intentional dopants, such as magnesium in p-type GaN, necessitating precise control of hydrogenation conditions. For perovskites, hydrogenation is less common but has shown promise in passivating halide vacancies and undercoordinated lead atoms, improving photovoltaic efficiency. The challenge lies in avoiding hydrogen-induced degradation of the perovskite lattice, which is more sensitive than Si or GaN.

Surface coatings provide another effective passivation strategy by physically isolating the semiconductor from environmental factors or chemically modifying surface states. Silicon benefits from dielectric coatings like silicon nitride (SiN_x) or aluminum oxide (Al2O3), which reduce surface recombination velocities. SiN_x deposited via plasma-enhanced chemical vapor deposition (PECVD) introduces hydrogen during growth, enabling simultaneous bulk and surface passivation. Al2O3, grown by atomic layer deposition (ALD), offers excellent field-effect passivation due to its high fixed charge density. GaN surfaces are often passivated with silicon nitride or silicon dioxide (SiO2) to mitigate surface states caused by nitrogen vacancies. However, these coatings must be optimized to avoid strain-induced defects at the interface. Perovskites require more delicate coatings due to their ionic nature and susceptibility to moisture. Thin layers of polymers or metal oxides, such as titanium dioxide (TiO2) or zirconium oxide (ZrO2), are employed to protect against environmental degradation while passivating surface defects. The trade-off here involves maintaining charge transport across the coating-semiconductor interface without introducing additional resistive losses.

Chemical treatments involve the application of reactive agents to modify surface or bulk defect states. For silicon, wet chemical treatments with solutions like hydrofluoric acid (HF) remove native oxides and terminate the surface with hydrogen, reducing interface trap densities. Subsequent immersion in iodine or bromine solutions can further passivate dangling bonds by forming stronger covalent bonds than hydrogen alone. GaN surfaces are treated with sulfur-containing solutions, such as ammonium sulfide ((NH4)2S), which bind to gallium dangling bonds and reduce surface recombination. However, these treatments are often unstable in air, requiring additional protective coatings. Perovskites benefit from chemical passivation using Lewis bases, such as thiophene or pyridine derivatives, which coordinate with undercoordinated lead atoms and halide vacancies. These treatments can significantly enhance photoluminescence quantum yield and device stability. A critical consideration is the potential for these chemicals to introduce impurities or disrupt the perovskite crystal structure if not applied judiciously.

The interplay between defect passivation and carrier mobility is a key consideration in optimizing semiconductor performance. Passivation often involves the introduction of additional species or interfaces, which can scatter charge carriers and reduce mobility. In silicon, hydrogenation improves minority carrier lifetime but may slightly reduce mobility due to increased phonon scattering. The trade-off is generally favorable for solar cells, where recombination suppression outweighs mobility losses. In GaN, passivation of surface states with dielectric coatings can improve two-dimensional electron gas (2DEG) mobility in high-electron-mobility transistors (HEMTs) by reducing Coulomb scattering. However, excessive passivation can introduce strain or interface defects that counteract these benefits. For perovskites, chemical passivation typically enhances mobility by reducing trap-assisted scattering, but over-treatment can lead to insulating layers that hinder charge extraction. The optimal passivation strategy must therefore balance defect neutralization with minimal disruption to the charge transport pathways.

Material-specific challenges further complicate passivation efforts. Silicon’s well-established processing techniques allow for precise control over hydrogenation and coating parameters, but advanced device architectures demand ever-lower defect densities. GaN’s high defect density, particularly in heteroepitaxial layers, requires aggressive passivation, yet the material’s sensitivity to hydrogen and high-temperature processing limits the available options. Perovskites present the greatest challenge due to their soft lattice and dynamic defect behavior, where passivation must be both effective and non-destructive. Each material system demands a tailored approach, considering the nature of its dominant defects and the operational requirements of the intended application.

In summary, defect passivation is a critical step in enhancing semiconductor performance across Si, GaN, and perovskites. Hydrogenation, surface coatings, and chemical treatments each offer unique advantages and limitations, requiring careful optimization to maximize defect suppression while minimizing adverse effects on carrier mobility. The choice of passivation strategy depends on the material’s defect landscape, processing constraints, and device requirements, underscoring the need for continued research into more robust and scalable passivation techniques.