Ion Implantation and Synthesis of Materials - Studium

138 10 Crystallization and Regrowth of Amorphous Sitemperatures as low as 250°C at a rate of 0.007 nm s −1 . with an irradiation of600 keV Kr at a dose rate of 1 × 10 12 Kr cm −2 s −1 . At this temperature the purethermal regrowth rate is negligible (~10 −11 nm s −1 ).As shown in Fig. 10.10, the ion beam-induced regrowth of amorphous Sidepends on the energy density deposited into nuclear collisions, as demonstratedby the correlation between the growth rate and the number of vacancies generatedby the 600 keV Kr beam irradiation. (Vacancies were calculated by the SRIMcode.) This trend suggests that the ion-induced growth rate is coupled with theproduction of point defects, or in a more general way, with the energy lost throughelastic collisions at the α–c interface by the impinging ions.The role of the α–c interface in Ion Beam Induced Epitaxial Crystallization(IBIEC) is demonstrated in Fig. 10.11, which shows that the regrowth rate isorientation dependent. The data shows that the rate is much lower (almost by afactor of 4) for 〈111〉 substrates relative to 〈100〉 substrates. These results suggestthat the same interfacial defects that are responsible for thermal regrowth also areimportant in IBIEC, with the role of the ion beam being that of changing theaverage defect concentration.The dependence of the growth rate on temperature is illustrated in Fig. 10.12,for both IBIEC and thermal annealing. The growth rate is reported both in Å s −1 .2.5Growth Rate [nm/(10 13 at/cm 2 )]2.0151.510TRIM1.050.50.00 25 50 75 100 125Depth (nm)Vacancies (#/ion/nm )Fig. 10.10. Growth rate versus depth for an α−Si layer recrystallized at 350°C by 600 Krions (after Priolo et al. 1989)