Ion Implantation and Synthesis of Materials - Studium

11.3 The Silicon–Hydrogen System 147−6−7fromEq. (11.1)H diffusion coeffient in silicon [cm 2 s -1 ]−8−9−10−11−12−13−14−15fromEq. (11.2)H 0 H +−161.0 1.5 2.0 2.5 3.0 3.5 4.01000 / T [K]Fig. 11.2. Arrhenius plot of expected H-diffusivity. The predictive diffusivities of neutraland positively charged H-atoms are from Capizzi and Mittiga (1987). Equation (11.1) isfrom Van Wiering and Warmhotz (1956). Equation (11.2) is from Ichimiya and Furuichi(1968)Ion implantation into c-Si breaks chemical bonds and creates point defects in theimplantation zone (Chap. 7). This effect is crucial in the Ion-Cut process: since thediffusivity of H in c-Si is very high, one would expect out-diffusion of the Himplant during the heat treatment of the silicon sample. However, the hydrogengets trapped at the implantation damage by passivating the dangling bonds in theimplantation zone, where the number of lattice defects is very high. As anexample, Fig. 11.3 illustrates possible local Si-H defects, formed at a siliconmonovacancy. A vacancy in the silicon lattice leaves four dangling bonds behind,which can be passivated with H atoms.Detailed information about the evolution of Si–H complexes in H ionimplantedsilicon were gained by the infrared vibrational studies of Weldon et al.(1997) and Chabal et al. (1999). Implanted H atoms form complexes of the formV x H y or I x H y , where V denotes a silicon vacancy and I denotes silicon interstitial.Also observed was the so-called H 2 * complex, a hydrogen molecule formation inwhich one H atom is located at the bond-centered site and the other at the antibondsite, with a silicon lattice atom residing between the H 2 bonds. Upon annealing ofthe H ion-implanted samples, the IR studies uncover a net loss of bound hydrogen