Ion Implantation and Synthesis of Materials - Studium

114 9 Doping, Diffusion and Defects in Ion-Implanted Si9.2 DefectsIon implantation and subsequent thermal processing will form defects. Defectsmay be categorized as (1) point defects, (2) line defects, (3) planar defects, and (4)volume defects. Table 9.1 lists examples of these four types of defects, andFig. 9.7 shows schematically some of the point defects in a two-dimensionalsimple cubic lattice.9.2.1 Point DefectsA point defect is a deviation in the periodicity of the lattice arising from a singlepoint. Other defects, such as dislocations and stacking faults, extend over manylattice sites. Point defects may form due to nonequilibrium conditions such asthose that occur during crystal growth or during thermal or mechanical processingof the material. Point defects may be categorized as (1) native defects, such as avacancy, and (2) impurity-related defects due to the introduction of an impurityatom into the lattice. For semiconductors, point defects not only cause structuraldisturbances but also often introduce electronic states in the band gap. If anattractive potential exists between a native defect and an impurity atom, they mayinteract and form a defect complex, such as a vacancy-impurity pair.9.2.2 Native Defects and Shallow DopantsFor Si, there are three types of native defects: the vacancy, the interstitial, andthe interstitialcy. The vacancy, V, is an empty lattice site. Depending on theconfiguration of the unsatisfied bonds due to the missing atom, a vacancy in Sican be either neutral, negatively or positively charged. A vacancy is also referredto as a Schottky defect. A Si atom residing in the interstices of the Si lattice isdefined as a self-interstitial. A Frenkel pair is a vacancy-interstitial pair formedwhen an atom is displaced from a lattice site to an interstitial site. An interstitialcyTable 9.1. Defects in SemiconductorsDefect typePointLinePlanarVolumeExamplesvacancies, interstitials, impurity atoms, antisite defectsdislocationsstacking faults, twins, and grain boundariesprecipitates and voids