A critical analysis on the origin, stability, relative sputtering yield and related phenomena of textured surfaces under ion bombardment

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Abstract
A critical analysis on the origin, stability, relative sputtering yield and related phenomena of surface texturing under ion bombardment, is presented in an attempt to clarify some “apparent” discrepancies in the experimental results reported by different groups. It is shown that clean surfaces under ion bombardment develop cones and/or pyramids (faceted cones) mainly from surface discontinuities reshaped by the irradiation. The discontinuities result from either sub-surface defects which are preferentially eroded with respect to neighbour areas or by pre-existing and/or bombardment-induced asperities with convex-up curvature. It is shown additionally, that the impurity-induced mechanism for pyramid development, on surfaces seeded with atom other than those of the substrate, may involve the formation of asperities as a necessary first step. Therefore. the impurity-induced mechanism may not be a different alternative. On the other hand, it is shown that seed materials need not possess lower sputtering yield than the substrate in order to generate pyramids, as previously suggested. It is shown that the reduced or enhanced sputtering yield of textured surfaces with respect to smooth surfaces of the same material, as observed by different groups, might reflect the behaviour of the textured surfaces under different irradiation conditions. Therefore the discrepancy may not be real. In addition, it is demonstrated that certain textures developed on “clean” surfaces are generally unstable under ion bombardment, i.e., they disappear after high enough irradiation dose. The relatively stable textures developed on “clean” surfaces with particular crystallographic orientation, which may represent an exception to the generally observed instability, are analyzed also. The behaviour of textured surfaces under ion bombardment is analyzed with particular emphasis on phenomena related to their application for inner walls of fusion devices. Experiments which can clarify the “apparent discrepancies” and help obtain a better assessment of the fusion-related properties of textured surfaces, are proposed.