1.02.4.1 Formation

Electronic defects are formed when single or small groups of atoms in a crystal have their electronic structure changed (e. g., electrons removed, added, or excited). In particular, they are formed when an electron is excited from its ground state configuration into a higher energy state. Most often this involves a valence electron, although electrons from inner orbits can also be excited if sufficient energy is available. In either case, the state left by this transition, which is no longer occupied by an electron, is usually termed a hole. These defects can be generated thermally, opti­cally, by radiation or through ion beam damage. The excited electron component may be localized on a single atomic site and if the electron is transferred to another center, it is represented as a change in the ionization state of the ion or atom to which it is localized. This is sometimes described as a small polaron or trapped electron. Such electronic defects might migrate through the lattice via an activated hopping process. An example of a small polaron electron is a Ce3+ ion in CeO2_x.16 Alternatively, the excited electron may be delocalized so that it moves freely through the crystal. In this case, the electron occupies a conduction band state, which is formed by the superposition of atomic wave functions from many atoms. This is the case with most semi­conductor materials. Similarly, the hole may also be localized to one atomic center and be represented as a change in the ionization state of the ion or atom. Holes may also move via an activated hopping pro­cess. An example is a Co3+ ion in Co1_xO. Similarly, the hole may also be delocalized. Intermediate situa­tions may occur with the hole or electron being localized to a small number of atoms or ions (known as a large polaron) or a specific type of hole state associated with a particular chemical bond.

The relationship between doping and its influence on electronic defects is of great technological impor­tance in the field of semiconducting materials. For example, doping silicon with defect concentrations in the order of parts per million is sufficient for most microelectronic applications. Incorporation of a phosphorous atom in silicon results in a shallow state below the conduction band that will easily donate an electron to the conduction band. The remaining four valence electrons of the phosphorous dopant will form sp3 hybrid bonds with the four neighboring tetrahedral silicon atoms. Recently, it has been suggested that the state from which the electron is removed is associated with the dopant species and the four silicon atoms surrounding it; in other words, it is associated with a cluster.17