defectlocalized

Defect localization is the spatial confinement of electronic, vibrational, or other excitations around lattice defects within crystalline materials. It encompasses localization of electronic states in the band gap due to impurities or structural disruptions, as well as localized vibrational modes tied to defects.

Electronic localization: In semiconductors and insulators, point defects such as vacancies, interstitials, and substitutional impurities introduce localized energy levels within the band gap. These defect states can trap carriers, act as recombination centers, and alter conductivity and optical absorption. The degree of localization depends on defect type, charge state, and local lattice relaxation.

Vibrational localization: Defects can create local phonon modes whose frequencies lie outside the bulk phonon spectrum. These localized modes affect thermal properties and appear as sharp peaks in Raman or infrared spectra; they can also mediate energy transfer around defects.

Modeling and measurement: First-principles methods (density functional theory) and tight-binding approaches are used to calculate defect levels and wavefunctions. Green's function techniques describe localized states in imperfect crystals. Experimentally, techniques such as deep-level transient spectroscopy (DLTS), electron paramagnetic resonance (EPR), photoluminescence, and scanning probe methods reveal defect-localized states.

Applications: Understanding defect localization is important for semiconductor device performance, radiation-hard materials, and quantum technologies. Certain defect centers, like color centers in wide-bandgap crystals, provide localized electronic states used as qubits and single-photon sources.