Electric Light Source Theory - Luminous Centers of Electronic Energy Levels
The energy states discussed above apply to ideal crystals. However, in real crystals, various impurity atoms, defects, and crystal surfaces and interfaces inevitably disrupt the periodicity of the lattice. Wherever the lattice periodicity is disrupted, bound states with energy values within the bandgap may form. In other words, in contrast to shared states, electrons (or holes) are confined near these regions, potentially with energies within the bandgap. We call these bound electron (or hole) states localized energy levels.
These localized energy levels play a crucial role in the luminescence process. Because impurities or defects can bind electrons or holes, we often say that impurities or defects form centers within the luminescent material. Localized energy levels formed by defects (including structural and impurity defects) can become luminescence centers.