A defect model for ion-induced crystallization and amorphization
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I. INTRODUCTION The solid-state thermal epitaxial crystallization of amorphous silicon has been studied in detail1"3 because of its importance for the recovery of ion implantation damage. This process proceeds at measurable rates above 400 °C or so and has an Arrhenius temperature dependence as shown in Fig. 1. Expitaxial crystallization can also be induced by ion irradiation at temperatures as low as 150 °C.4~8 In this case the recovery process is in competition with the damage created by the ions. Recently, detailed experimental measurements have been made9"16 of crystallization and amorphization rates in the regime where these two processes occur at almost equal rates. At low temperatures and high ion fluxes, amorphization proceeds. At higher temperatures or lower fluxes, crystallization occurs. This suggests that the ions are creating damage that promotes amorphization, and at the same time, creating defects that provide atomic mobility at the amorphous-crystalline interface to promote crystallization. The experimental arrangement is shown schematically in Fig. 2. The sample is initially bombarded with silicon ions to create an amorphous layer about 1000 A thick at the surface. The sample is then irradiated with high-energy ions. The range of these ions is greater than the thickness of the amorphous layer and they create damage deeper in the crystal at their end of range. On passing through the amorphous crystalline interface, these ions cause motion of the interface in a direction that depends on both temperature and flux. The interface remains planar. Calculations of the atomic displacements created by various ions as a function of depth are shown in Fig. 3. In the model presented below, it is assumed that each ion converts a small volume of crystal at the inter1218
J. Mater. Res. 3(6), Nov/Dec 1988
http://journals.cambridge.org
face to the amorphous state, virtually instantaneously. The ion also leaves behind some defects that move around in the interface region, converting the amorphous phase back to the thermodynamically stable crystalline phase. These defects do not move very far from the ion path during their lifetime, and their population decays significantly before the next ion arrives in the same area. The interface motion depends on the balance between the amorphization created by the ion and the
TEMPERATURE °C 1400
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l I
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RECIPROCAL TEMPERATURE 1000/T(K)
FIG. 1. Temperature dependence of the rate of crystallization of amorphous silicon (after Ref. 3).
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K. A. Jackson: Defect model for ion-induced crystallization and amorphization
(a) INITIAL STATE
created by an ion are uniformly distributed across the cross section of a cylinder of diameter l0 about the ion path, as illustrated in Fig. 4. Actually, the defect distribution will be Gaussian about the path, but our simpler assumption should give the proper statistics. The defect density cr
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