Ion Implantation and Synthesis of Materials - Studium

10 Crystallization and Regrowth of Amorphous Si10.1 IntroductionDuring ion implantation, each ion produces a region of disorder around the iontrack. As the implantation dose increases, the disorder increases until all the atomshave been displaced and an amorphous layer is produced over a depth R p . Thebuildup and saturation of disorder are shown in Fig. 10.1 for 40 keV phosphorusions incident on Si. In this example, about 4 × 10 14 phosphorus ions cm −2 arerequired to form an amorphous layer. Except for low doses or implantation withlight ions, we can anticipate that an amorphous layer is formed during theimplantation process. This assumes that no recovery of lattice order occurs aroundthe ion track.Amorphous regions also can be formed by light ion implantation at suitabletarget temperatures and doses. For light ion implantation the defect density in thesingle collision cascade is quite low, and amorphization occurs when the materialaccumulates enough damage that it reaches a critical threshold in the defectdensity (Rimini 1995). In general, for light ion amorphization, cascade overlap isrequired and the layer becomes amorphous when the free energy of the cascadeinduceddefect-rich region equals the free energy of the amorphous phase.In terms of energy deposition, it has been found that amorphous formationrequires an energy deposition of 6 × 10 23 keV cm −3 at low temperature and lowenergy. The deposited energy for a given implant is given by φν/R p , where φ is thedose and ν is the damage energy (see Chap. 7, Sects. 4 and 5). In the case of a Bimplant at low energy with dE/dx| n ~ 0.6 eV nm −1 , a fluence of 1.6 × 10 15 cm −2 isrequired for amorphization. These estimates are valid at sufficiently lowtemperatures where defect mobility is low and therefore defect recombination islimited. For any ion, the threshold dose for amorphization increases withincreasing target temperature up to a critical value, above which amorphizationdoes not occur. The critical temperature depends on the ion mass and energy, i.e.,on the energy density deposited into nuclear collisions. Experimental results areshown in Fig. 10.2.127