Ion Implantation and Synthesis of Materials - Studium

9.2 Defects 115VacancySubstitutionalSelfinterstitialFig. 9.7. Various points defects in a simple cubic latticeconsists of two atoms in nonsubstitutional positions configured about a single latticesite. Because of the similarity between an interstitial and an interstitialcy, a distincsubstitutionaldefect. Such a defect, when surrounded only by Si atoms on theirtion is generally not made; both are abbreviated as “I” in the literature. When an impuritysuch as a shallow dopant (e.g., As in Si) occupies a lattice site, it is known as anormal sites, is identified as “A”. When a vacancy V forms next to A, it is known asan impurity-vacancy pair, usually referred to as an AV defect. If one of the atoms inthe interstitialcy defect is a dopant atom, A, the defect is referred to as an AI defect.If the impurity atom occupies an interstitial site, it is designated as A i ; if the impurityatom occupies a substitutional site, it is designated as A s .9.2.3 Deep Level CentersShallow dopant impurities have small ionization energies (such as As and P in Siwith ionization energies of ~0.04 eV). There are chemical impurities and chargedpoint defects that form deep energy states in semiconductors. The energy levels ofthese centers in the band gap are usually far away from the band edges; thus theyare called deep level centers. Typical deep level impurities are oxygen andmetallic elements in Si. A deep level impurity may have several energy levels,with each energy level being either an acceptor state or a donor state. For example,Au in Si has an acceptor state at 0.54 eV between the conduction band edge, E C ,and a donor state 0.35 eV from the valence band edge, E V (Fig. 9.8). Other thanchemical impurities, charged point defects, such as a negatively charged vacancy,V − in Si, also form deep level states.A deep center may act either as a trap or as a recombination center, dependingon the impurity, temperature, and other doping conditions. Consider a minority