Ion Implantation and Synthesis of Materials - Studium

116 9 Doping, Diffusion and Defects in Ion-Implanted Sicarrier (holes in an n-type semiconductor, for example) captured at an impuritycenter. The minority carrier stays at the center for a period of time and is thenejected thermally into the band from which it came; this impurity center is knownas a trap. Before ejection, if a majority carrier is captured, recombination of thecarrier pair takes place and the center is therefore a recombination center.9.2.4 Line DefectsLine defects in a crystalline material are known as dislocations. Dislocations areformed due to nonequilibrium conditions such as ion implantation and thermalprocessing. Under equilibrium conditions, there is no requirement for the presenceof dislocations or any other defect (except native point defects) in the crystal. Anedge dislocation may be viewed also as having an extra half-plane inserted intothe crystal (see Fig. 9.9).Au levels in SiE C0. 54 eVE A0.35 eVE DE VFig. 9.8. Au in Si has an acceptor state, E A (~ E C – 0.54 eV) and a donor state,E D (~ E V + 0.35 eV). For strongly p-type Si, the Fermi level, E F , is below E D ; the donorstates are ionized (the electron in the state is given away) and become positively charged.The acceptor state at E A is not ionized (the state has not received an electron because E F isfar below E A ). For strongly n-type Si, E F is far above E A ; the acceptor states are ionized (thestate has received an electron) and become negatively charged. The Au donor states are notionized in strongly n-type Si