Temporal evolution of Ge surface topography under keV ion irradiation: Combined effects of curvature-dependent sputter erosion and atomic redistribution

Abstract We report on our comprehensive experimental studies on 100 keV Kr+-ion irradiation induced nanoscale ripple pattern evolution on Ge surface over a large angular window and a wide range of ion fluence. Using the present experimental parameters, theoretical estimations have also been carried out in order to unveil the underlying physical processes causing the observed pattern formation. We observe the formation of periodic ripple patterns, with wave-vectors parallel to the ion-beam projection onto the surface, in the range of oblique incidence angles of 45–70°. On the other hand, for angles of incidence in the range of 80–85°, patterns having wave-vectors perpendicular to the projected ion-beam direction are seen to evolve. In contrast, the surface remains stable (no pattern is formed) for incidence angles between 0–40° and 75°. Corresponding theoretical estimations clearly demonstrate the simultaneous roles of curvature-dependent sputter erosion and ion-induced prompt atomic redistribution behind the morphological evolution, albeit sputter erosion becomes dominant at grazing angles of incidence. Consequently, observed patterning process of Ge surface at the present energies turns out to be analogous to those of Si and SiO2 in both medium (up to tens of keV) and low energy (up to a few keV) regimes. The present study, therefore, implies a generality in pattern evolution on these surfaces (for ion energies of few tens of keV) and thus, is significant towards taking a step ahead in understanding ion-induced pattern evolution on other solid surfaces as well in a universal framework. Further, investigations on temporal evolution of Ge ripples, in terms of surface roughness and slope distribution, reveal a transition from the linear to nonlinear regime, leading to a gradual transformation of sinusoidal surface patterns to faceted structures. Depending upon the ion fluence, the evolution of the faceted structures is attributed to the formation of undercompressive shocks and the ion-beam shadowing phenomenon.