Native defects and impurity defects are two fundamental categories of imperfections in semiconductor crystals that significantly influence electronic and optical properties. While native defects arise from deviations in the host lattice, impurity defects result from foreign atoms introduced intentionally or unintentionally. Understanding their distinct roles is essential for controlling semiconductor performance in devices.

Native defects include vacancies, interstitials, and antisites. A vacancy occurs when an atom is missing from its lattice site, creating a point defect. Interstitials are host atoms occupying positions outside regular lattice sites. Antisites form when atoms swap positions in compound semiconductors, such as a Ga atom occupying an As site in GaAs. These defects are intrinsic and exist even in high-purity materials due to thermodynamic equilibrium. Their concentrations depend on growth conditions, temperature, and stoichiometry. For example, in GaAs, arsenic vacancies are more prevalent under arsenic-deficient conditions. Native defects introduce energy levels within the bandgap, acting as traps or recombination centers that affect carrier lifetimes and mobility.

Impurity defects, or dopants, are extrinsic and introduced to modify electrical properties. Shallow impurities like phosphorus in silicon create energy levels near the conduction or valence band edges, ionizing easily to donate or accept charge carriers. Deep-level impurities, such as gold in silicon, introduce states near the mid-gap, acting as non-radiative recombination centers. Unlike native defects, impurity concentrations are controlled during growth or post-processing to achieve desired conductivity. However, unintentional impurities like oxygen or carbon in silicon can degrade performance if not managed.

The impact on conductivity differs between native and impurity defects. Native defects often compensate dopants by counteracting their charge contributions. For instance, a silicon vacancy may act as an acceptor, reducing the effective donor concentration in n-type silicon. In compound semiconductors, antisite defects like EL2 in GaAs can pin the Fermi level, leading to semi-insulating behavior. Impurity defects, when intentionally introduced, dominate conductivity. Shallow donors and acceptors increase free carrier concentrations, while deep-level impurities reduce carrier lifetimes and increase leakage currents in devices.

Compensation effects arise when defects counteract dopant activity. Native defects like vacancies can passivate dopants by forming complexes, such as a phosphorus-vacancy pair in silicon, rendering the dopant electrically inactive. Similarly, impurity-defect interactions can lead to deactivation, as seen with hydrogen passivation of boron in silicon. Compensation is critical in wide-bandgap semiconductors like GaN, where native defects often dominate electronic properties despite heavy doping. Understanding these interactions is necessary for optimizing doping efficiency and device performance.

Identification of defects relies on specialized characterization techniques. Deep-Level Transient Spectroscopy (DLTS) is a powerful tool for detecting both native and impurity defects. DLTS measures capacitance transients caused by carrier emission from traps, providing information on defect energy levels, concentrations, and capture cross-sections. For example, the well-known E-center in silicon, a vacancy-phosphorus complex, is detectable via DLTS around 0.4 eV below the conduction band. Photoluminescence (PL) spectroscopy identifies radiative transitions involving defects, such as the D-X center in AlGaAs. Secondary Ion Mass Spectrometry (SIMS) quantifies impurity concentrations but cannot distinguish electrically active from inactive species. Electron Paramagnetic Resonance (EPR) reveals defect structures by detecting unpaired spins, useful for studying vacancies in silicon.

Native and impurity defects also differ in their thermal stability. Native defects often exhibit higher formation energies and may anneal out at elevated temperatures, whereas impurities remain unless removed by gettering. For example, silicon interstitials migrate and recombine with vacancies at moderate temperatures, while oxygen precipitates require high-temperature treatments. This difference is exploited in defect engineering to optimize material quality.

In optoelectronic devices, defects play contrasting roles. Native defects like gallium vacancies in GaN contribute to yellow luminescence, degrading LED efficiency. Impurity defects like magnesium acceptors in GaN are essential for p-type conductivity but may introduce non-radiative pathways if not properly activated. In solar cells, defects reduce minority carrier lifetimes, lowering conversion efficiency. Mitigating these effects requires careful control of growth conditions and post-growth treatments.

The interplay between native and impurity defects is complex and material-dependent. In silicon, oxygen impurities can interact with vacancies to form beneficial defect complexes that improve mechanical strength but may also generate thermal donors. In III-V semiconductors, stoichiometry deviations during growth introduce native defects that compensate dopants, complicating conductivity control. Wide-bandgap materials like SiC and GaN exhibit high native defect concentrations due to high bonding energies, making defect management critical for device performance.

Defect engineering strategies vary for native and impurity defects. Reducing native defects involves optimizing growth parameters such as temperature, pressure, and stoichiometry. Impurity control requires high-purity source materials and clean processing environments. Post-growth treatments like annealing can passivate or activate defects selectively. For example, hydrogen passivation neutralizes dangling bonds at vacancies or grain boundaries, improving electronic properties.

In summary, native and impurity defects exhibit distinct origins, behaviors, and impacts on semiconductor properties. Native defects are intrinsic and influenced by thermodynamic conditions, while impurity defects are extrinsic and controlled during processing. Both types introduce energy levels that affect conductivity, compensation, and device performance. Advanced characterization techniques like DLTS, PL, and SIMS enable their identification and quantification. Effective defect management is essential for optimizing semiconductor materials for applications ranging from microelectronics to photovoltaics. Understanding these defects at a fundamental level allows for precise material engineering, enabling advancements in semiconductor technology.