 In volume - free charge just after their creation - self-trapped charge again capable of movement by processes such hopping from trapping site to other trapping site [Mott-77]. (ii) In volume or in interface boundary - charge deeply trapped on defects, or more generally modification of the state of defect charge, in volume or in interface. (iii) At the interface with silicon strictly speaking - interface states. Interface states are, in principle, sites that are able to exchange charge with the silicon in indefinitely reversible manner, in thermodynamic equilibrium with the electronic population of the surface of the silicon. This must be done in a finite time (generally in the order of a microsecond for so-called "rapid states" that are usually the most common). These sites are therefore located practically in the "interfacial layer" and, in the sense of extended wave functions of this interface, they belong both to the silicon and to the oxide. It is important to note that interface states are not charged by themselves, but "porters" (sites) of charge (Bardeen states [Scho-39]). It is therefore natural to think that lattice defects, notably those caused at the interface by secondary effects of irradiation, are the cause of such electrically-active sites (e.g. Pb centers) (c.f. also mechanisms of Revesz-Griscom, reminded hereafter). But one can also note that some states cannot be defects in the crystalline sense of the term, but quantum levels caused by the passage of a system to an other (states of Tamm [Tamm- 32]). Some authors have imagined that charges trapped close to the interface could create allowed levels in the forbidden band-gap of the interfacial silicon by simple electrostatic influence. III-13 The standard classification is by convention that resulting from the "Deal Commission" [Deal- 80], approved by the Electrochemical Society and the IEEE (fig. 10). Bulk Si02 SILICON Amorphous Structure Volume Defects DIOXIDE Oxygen Vacancies Strained Bonds 30 to 50 Angstroems SILICON monolayer Si203, Si0, Si