Depth of origin of sputtered atoms: Exploring the dependence on relevant target properties to identify the correlation with low-energy ranges

Abstract

The correlation of the depth of origin (DoO) of sputtered atoms with relevant properties of the bombarded target has not yet been clarified in a satisfactory manner. We have carried out SDTrimSP (Monte Carlo) simulations to study the dependence of depth differential sputtering yields on the following parameters: the position of the primary knock-on atom, the surface binding energy, Es, the target density, N, and the target atomic number, Z2. All calculations were performed with normally incident projectiles, either Xe or target-atom-like, with energies between 40 and 10keV. The mean DoO, 〈x〉, was found to depend on Es as 〈x〉∝Esp, with a power p between 0.10 (0.1keV) and 0.26 (10keV Xe). In prior work, lack of knowledge of this weak Es-dependence did not allow the relation between 〈x〉 and N to be uncovered properly. To proceed further, differential yields were calculated for targets ranging from carbon to uranium with deliberately varied N, but keeping Es fixed (4.72eV). The product 〈x〉N turned out to depend very little on N. Hence, it is appropriate to specify the origin of sputtered atoms in terms of the equivalent areal density rather than in units of depth. For medium-mass or heavy target atoms, i.e., for Z2>20, 〈x〉N was largely independent of Z2. For Z2<20, 〈x〉N increased with decreasing Z2. Projected ranges xp calculated for target-atom-like projectiles at energies ⩽40eV exhibited almost the same Z2-dependence as 〈x〉. This finding suggests that 〈x〉 is determined by the range of the recoils mobilized in the collision cascade. The results of this study are compared with previously published data, with sometimes distinctly different definitions of 〈x〉. The idea is recapitulated that the most appropriate way of quantifying the DoO is to quote the fraction of atoms ejected from the top (or outermost) layer of atoms. The fractions determined by SDTrimSP range from about 98% to 75%, for impact energies between 40eV and 10keV and Z2>20.