Point defects in semiconductors are atomic-scale imperfections that significantly influence material properties and device performance. These defects include vacancies, interstitials, and substitutional impurities, each with distinct formation mechanisms and effects on electrical and optical behavior. Understanding and controlling these defects is essential for optimizing semiconductor functionality, particularly in doping and optoelectronic applications.

Vacancies are lattice sites where an atom is missing. In silicon, a silicon vacancy (V_Si) forms when a silicon atom is displaced from its lattice position, leaving an empty site. The energy required to create a vacancy, known as the formation energy, depends on temperature and the surrounding atomic environment. At high temperatures, thermal vibrations increase the likelihood of vacancy formation. Vacancies act as recombination centers for charge carriers, reducing minority carrier lifetime and increasing leakage currents in devices. In compound semiconductors like GaAs, vacancies can be cation (V_Ga) or anion (V_As), each influencing electrical properties differently. For example, V_As in GaAs introduces deep acceptor levels, while V_Ga creates deep donor states.

Interstitials are atoms that occupy positions between regular lattice sites. A silicon interstitial (Si_i) can form when an extra silicon atom is introduced into the crystal, often during high-energy processes like ion implantation or rapid thermal annealing. Interstitials are highly mobile and can diffuse through the lattice, interacting with other defects or impurities. In some cases, interstitials contribute to self-diffusion, where atoms move through the crystal without external impurities. Interstitials can also act as donors or acceptors, altering carrier concentrations. For instance, in germanium, self-interstitials introduce energy levels near the conduction band, increasing n-type conductivity.

Substitutional impurities occur when foreign atoms replace host atoms in the lattice. These impurities are intentionally introduced during doping to modify electrical properties. For example, phosphorus (P) substituting for silicon (Si_P) donates an extra electron, creating n-type conductivity, while boron (B) substituting for silicon (Si_B) accepts an electron, resulting in p-type conductivity. The solubility of substitutional impurities depends on factors like atomic size mismatch and chemical affinity. In III-V semiconductors, silicon can act as either a donor or acceptor depending on whether it substitutes for the group III or group V element. For instance, Si_Ga in GaAs is a donor, while Si_As is an acceptor.

Frenkel defects consist of a vacancy-interstitial pair, where an atom moves from its lattice site to an interstitial position. This defect is common in materials with open crystal structures or under irradiation. The formation energy of Frenkel defects is lower in ionic crystals like NaCl but higher in covalent semiconductors like silicon. Frenkel defects can act as trapping centers, reducing carrier mobility and increasing scattering. Schottky defects, on the other hand, involve a pair of vacancies, such as a missing cation and anion in ionic crystals. In semiconductors, Schottky-like defects can form during high-temperature processing, particularly in compounds like GaN, where nitrogen vacancies (V_N) and gallium vacancies (V_Ga) may coexist.

The impact of point defects on electrical properties is profound. Vacancies and interstitials introduce energy levels within the bandgap, acting as traps or recombination centers. These defects reduce carrier lifetime and increase non-radiative recombination, which is detrimental for optoelectronic devices like LEDs and solar cells. Substitutional impurities, when controlled, are beneficial for doping. Shallow impurities like phosphorus or boron in silicon introduce energy levels close to the conduction or valence band, enabling tunable carrier concentrations. Deep-level defects, such as transition metals (e.g., iron in silicon), create mid-gap states that severely degrade device performance by acting as generation-recombination centers.

Defect engineering is a critical tool for optimizing semiconductor performance. Intentional doping involves introducing specific impurities to achieve desired carrier concentrations. For example, n-type silicon is produced by adding phosphorus or arsenic, while p-type silicon uses boron or gallium. The doping process requires precise control over impurity concentrations and activation energies. Thermal annealing is often used to repair lattice damage and activate dopants. In compound semiconductors, stoichiometry control is essential to minimize native defects. For instance, growing GaAs under arsenic-rich conditions reduces gallium vacancies but may increase arsenic antisites (As_Ga), which are deep donors.

In optoelectronic applications, defect engineering enhances device efficiency. For LEDs, reducing non-radiative recombination centers improves light output. This is achieved by minimizing vacancies and interstitials through optimized growth conditions and post-growth treatments. In solar cells, controlling defect densities maximizes carrier collection and minimizes recombination losses. Passivation techniques, such as hydrogenation, are used to neutralize dangling bonds at defects. For wide-bandgap semiconductors like GaN, managing point defects is crucial for achieving high-power and high-frequency operation. Nitrogen vacancies in GaN, for example, can be suppressed by growing under high nitrogen pressure or using ammonia-rich environments.

Point defects also play a role in advanced materials like perovskites and 2D semiconductors. In hybrid perovskites, iodine vacancies (V_I) and lead interstitials (Pb_i) influence hysteresis and stability in solar cells. Defect tolerance in perovskites arises from the shallow nature of some defects, but deep-level defects still degrade performance. In 2D materials like MoS2, sulfur vacancies (V_S) act as recombination centers but can also be exploited for catalytic applications. Defect engineering in these materials involves chemical treatments or strain modulation to tailor properties.

The study of point defects relies on advanced characterization techniques. Deep-level transient spectroscopy (DLTS) identifies defect energy levels and concentrations. Photoluminescence (PL) spectroscopy reveals defect-related emission peaks, while positron annihilation spectroscopy detects vacancies. Transmission electron microscopy (TEM) provides atomic-scale imaging of defects, though it is more suited for extended defects. Computational methods like density functional theory (DFT) predict defect formation energies and electronic structures, guiding experimental efforts.

In summary, point defects are intrinsic to semiconductors and profoundly impact their electrical and optical properties. Vacancies, interstitials, and substitutional impurities each have unique formation mechanisms and effects. Frenkel and Schottky defects represent specific configurations with distinct behaviors. Defect engineering enables precise control over material properties, essential for doping and optoelectronic applications. By understanding and manipulating these defects, researchers can optimize semiconductor performance for next-generation technologies.