Ion Implantation and the Steady State Concentration

The steady-state surface concentrations are relatively easy to obtain. Consider the implantation of ion species A into the host material B, where NA and VB are the concentrations (per unit volume) of A atoms and B atoms, respectively, at the surface of the sample. Therefore NA/NB represents the surface composition. Let JA and JB be the flux of sputtered atomic species A and B, respectively. Then 

At steady state, there is no change in the total number of A atoms in the material. Therefore 

This is the steady-state surface composition, which is roughly inversely proportional to the total sputtering yield Y, but multiplied by the preferential sputtering factor r. For r = 1, (12.10) reduces to NA/(NB + NA) = MY. 

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II.2a) Reactive primary ion bombardment Yield enhancement is brought about by direct implantation of a reactive species in the sample by using an O2+ (or 0") or a Cs+ primary beam. Liebl [5] has described the build-up in the surface concentration of implanted species during sputter erosion. After sputtering to a depth of approximately Rp + 2ARp, where Rp is the projected range and p the standard deviation, the sputter rate of previously implanted primary ions will exactly balance the implantation rate. In this steady state situation, an equilibrium surface composition is achieved and the secondary ion yields stabilize. Figure 5 shows that upon commencement of bombard-... 

Even in the ab en(pe of preferential sputtering, i.e, SA=Sg, it is not necessarily the case that the concentration ratio of A and B is uniform with depth under steady state ion bombardment. The subsurface layer can have a different concentration ratio from the bulk or the top surface even if the latter two are identical. This can occur because of the subsurface effects of ion bombardment discussed below, which can produce an altered subsurface composition. As a result, there are two altered layers possible the top surface layer where composition at steady state is completely controlled by RgA and the subsurface layer where composition is controlled by segregation, radiation induced diffusion, radiation induced segregation, recoil implantation and cascade mixing. 

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