These impurities represent an imperfection or defect in the homogeneous crystal. A few imperfections behave in a well-determined manner, while most impurities have properties that are very complex. This complexity is a result of the strong potential of the defect and the amount it samples all of the electron states in the crystal. Impurities are not the only type of defects. Dislocations and small deviations from stoichiometry also represent imperfections in the crystal lattice.
All defects represent possible sinks or traps for electrons and holes.
If the defect binds a carrier strongly it is called a deep level defect. Many analytical techniques have been developed to observe deep levels. DLTS is especially useful as it gives the ionization energy of defects detected and the electrical concentration. Optical techniques, such as photoluminescence and absorption spectroscopies, are also useful. Our group has looked at several different of impurities in many compound semiconductors.