Ion Implantation and Synthesis of Materials - Studium

12.4 Sputtering of Alloys and Compounds 165the higher the sputtering yield. Therefore the sputtering yield is proportional to thenuclear stopping power of the incident ion in the near surface region.The main features remain the same for composite materials such as binaryalloys. There are additional complications, however, because there are two kindsof atoms in the material. The two species may not be sputtered at an equal ratebecause of differences in energy sharing (in the collision cascade), ejectionprobabilities, or binding energies. Indeed, preferential sputtering of one speciesover the other has been observed in many alloys and compounds.Since ion implantation is also a bombardment of energetic ions, there is alwayssome sputtering, especially when heavy ions and high doses are used. Sputteringmakes the sample surface recede; therefore it affects the implantation profile andalso removes atoms that have been implanted. This eventually leads to a steadystatecondition in which there is no further increase in the amount of implantedspecies retained in the material.12.4.1 Preferential SputteringIn a description of sputtering from a multicomponent system, the influence ofpreferential sputtering and surface segregation must be included. For ahomogeneous sample with atomic components A and B, and in the absence ofsurface segregation, the surface concentration, N s , is equal to that in the bulk, N b .Therefore, at the start of sputterings s b bA B=A BN / N N / N .(12.11)The partial yield of atomic species A and B is defined byNumber of Ejected Atoms A,BY A,B = (12.12)Incident ParticleThe partial sputtering yield, Y A , of species A is proportional to the surfacessconcentration, NA, and similarly Y B is proportional to NB. The ratio of partialyields is given byYYAB=sAsBNrN(12.13)where the sputtering factor, r, takes into account differences in surface bindingenergies, sputter escape depths, and energy transfers within the cascade. Measuredvalues of r generally are in the range between 0.5 and 2.s sIn the case when r is unity, YA/ YB = NA/NB. In the case when r ≠ 1, thessurface concentrations and yields change from their initial values, N A (0) and