Chalcogenide semiconductors, characterized by their composition of chalcogen elements (S, Se, Te) combined with metals or metalloids, exhibit unique optoelectronic properties influenced by intrinsic and extrinsic defects. These defects play a critical role in determining carrier concentrations, recombination dynamics, and overall device performance. Understanding their nature, formation mechanisms, and compensation strategies is essential for optimizing material properties for applications such as phase-change memory, photodetectors, and thin-film photovoltaics.

Intrinsic defects in chalcogenides arise from stoichiometric deviations or lattice imperfections during synthesis. Common intrinsic defects include vacancies, interstitials, and anti-site defects. For example, in selenium-based chalcogenides like GeSe or Sb2Se3, selenium vacancies (V_Se) are prevalent due to the high vapor pressure of selenium during growth. These vacancies act as shallow acceptors, introducing hole carriers into the system. Conversely, metal interstitials (e.g., Ge_i or Sb_i) can form donor states, compensating for the acceptor-like vacancies. The balance between these defects determines the Fermi level position and conductivity type.

Extrinsic defects result from intentional or unintentional doping with foreign elements. For instance, introducing Ag into GeSe can lead to substitutional Ag_Ge or interstitial Ag_i defects, both of which modify the electronic structure. Halogens like Cl or Br are often used as n-type dopants in chalcogenides, substituting for chalcogen sites (Cl_Se) and donating electrons. However, extrinsic dopants may also form complexes with intrinsic defects, such as (V_Se-Cl_Se), which can passivate or deepen trap states depending on the binding energy.

Defect compensation mechanisms are crucial for achieving desired electronic properties. In p-type chalcogenides, excess metal or chalcogen vacancies can be counteracted by introducing donor dopants or annealing under chalcogen-rich atmospheres. For example, annealing Sb2Se3 in Se vapor reduces V_Se concentration, lowering hole density. Conversely, n-type materials may require acceptor doping or metal-rich growth conditions to suppress donor-like defects. The interplay between defects and compensation is often modeled using defect formation energies (E_f) under different chemical potentials. For instance, E_f for V_Se in GeSe is lower under Se-poor conditions, explaining its prevalence in non-stoichiometric samples.

Density functional theory (DFT) modeling provides insights into defect thermodynamics and electronic structure. Generalized gradient approximation (GGA) or hybrid functionals (e.g., HSE06) are commonly employed to calculate defect formation energies, charge transition levels, and binding energies. For example, DFT studies on As2Se3 reveal that V_Se introduces mid-gap states, acting as recombination centers, while Se interstitials (Se_i) form shallow levels. Supercell sizes must be carefully chosen to minimize defect-defect interactions, with 200-atom supercells being typical for accurate results.

Charge neutrality conditions further constrain defect populations. The Brouwer approximation simplifies mass-action equations for defect equilibria, linking defect concentrations to Fermi level and temperature. For instance, in CuInSe2, the [V_Cu] / [In_Cu] ratio follows a power-law dependence on Cu chemical potential, as predicted by DFT-calculated formation energies. Finite-temperature effects, including vibrational entropy, can be incorporated via ab initio molecular dynamics or quasi-harmonic approximations.

Experimental validation of DFT predictions involves techniques like positron annihilation spectroscopy for vacancy detection or admittance spectroscopy for trap characterization. In GeTe, DFT-predicted Te vacancies (E_f ≈ 0.2 eV below conduction band) align with experimental activation energies from Hall measurements. Discrepancies often arise from approximations in exchange-correlation functionals or neglect of defect clusters. Advanced methods like GW corrections or machine-learning potentials are being explored to improve accuracy.

Defect engineering strategies leverage these insights. In phase-change memory materials (e.g., GeSbTe), controlled vacancy ordering enhances switching speed. For photovoltaics, passivating V_Se in Sb2Se3 via Cl doping improves minority-carrier lifetime. Table 1 summarizes key defects and their impacts in selected chalcogenides.

Material | Dominant Defect | Type | Effect
GeSe | V_Se | Acceptor | p-type conductivity
Sb2Se3 | Sb_Se | Donor | Self-compensation
CuInSe2 | V_Cu | Acceptor | Shallow holes
GeTe | Te_i | Donor | n-type switching

Future directions include high-throughput DFT screening of dopant-defect combinations and in-situ characterization during growth. Chalcogenides remain a rich platform for defect studies, with implications for next-generation electronic and energy devices.