Ion Implantation and Synthesis of Materials - Studium

12.7 Concentrations in High-Dose Ion Implantation 173the Si content in the PtSi sample because of Si preferential sputtering. Thecomposition in the implanted sample is determined by a competition betweenimplantation and sputtering.Consider the implantation of atomic species A into the alloy AB. Having an ABalloy to begin with sets a different boundary condition at the steady state. Insteadof J A = J i as given in (12.8), the new condition isJA − JB = Ji(12.23)at steady state. The total sputtering yield is still (J A + J B ) = Y J i , (12.7). Therelation expressed in (12.6) still holds. Thus, combining (12.23) and (12.7) yieldsJ = ( Y + 1) J /2(12.24)ABiJ = ( Y −1) J /2(12.25)iSubstituting J A and J B into (12.6) [J B /J A = r(N B /N A )] yieldsN / N = r( Y + 1)/( Y −1)(12.26)ABThis gives the steady-state surface composition.When Y 1 , (12.26) reduces to N A /N B = r, which is the same result as inertgasion sputtering of alloy AB. On a physical basis, this is because, with highsputtering yield Y, very few of the implanted atoms are retained in the material.For the example of implantation into PtSi, the steady-state surface compositionis given by (12.26), i.e., N Si /N Pt = r(Y + 1)/(Y − 1), with r = 1/2. The surfacecomposition is therefore dependent upon the total sputtering yield, Y. Thisdependence is plotted in Fig. 12.10. For Y > 3, the implanted PtSi sample becomesdepleted of Si, because Y is sufficiently large that not enough implanted Si atomsstay in the sample to overcome the preferential sputtering of Si. For Y = 3, the SiimplantedPtSi sample remains PtSi. For Y < 3, more implanted Si atoms stay in,and the sample becomes Si enriched.12.7 Concentrations in High-Dose Ion ImplantationSince the majority of sputtered atoms have relatively low energies and emergefrom the first few atomic layers near the surface, the probability of sputtering isvery sensitive to surface conditions. A thin layer of surface contaminants or oxide