Cratering-energy regimes: From linear collision cascades to heat spikes to macroscopic impacts

Abstract

Using classical molecular-dynamics simulations we examine the formation of craters during 0.4-100-keV Xe bombardment of Au. Our simulation results, and comparison with experiments and simulations of other groups, are used to examine to what extent analytical models can be used to predict the size and properties of craters. We do not obtain a fully predictive analytical model ͑with no fitting parameters͒ for the cratering probability, because of the difficulty in predicting the probability of cascades splitting into subcascades, and the relation of the heat spike lifetime and energy density. We do, however, demonstrate that the dependence of the crater size on the incident ion energy can be well understood qualitatively in terms of the lifetime of the heat spike and the cohesive energy of the material. We also show that a simple energy density criterion cannot be used to predict cratering in a wide ion energy range because of the important role of the heat spike lifetime in high-energy cascades. The cohesive energy dependence differs from that obtained for macroscopic cratering ͑observed, e.g., in astrophysics͒ because of the crucial role of melting in the development of heat spikes.

On the stabilization of ion sputtered surfaces

“…There is growing evidence that although the BradleyHarper predictions successfully explain some features of experiments ͑e.g., exponential growth of ripples at early stage dynamics, temperature dependence of the wavelength of the ripples, 7 and 90°rotation of ripples in many experiments as is increased͒, there are also some glaring inconsistencies. This is clearly demonstrated in, e.g., the recent work of Ziberi et al, 20 who found a "window of stability" for Si surfaces at room temperature bombarded by ϳ1 -2 keV noble gas ions at an intermediate range of angles 1 34,35 and atomistic simulations 36,37 that have measured the shape change of a smooth solid surface in the vicinity of an impingement by a single energetic monatomic ion or cluster ion. These studies show significant deviations from the predictions of the ellipsoidal Gaussian form.…”

“…These studies demonstrate that after ion impact, a crater forms around the impact point of the penetrating ion, surrounded by rims elevated from the original surface. [34][35][36][37] This behavior, where ⌬h Ͼ 0 in the rim, is completely different from the erosive response functions described above. We investigate whether such response functions can cause the stability of a flat surface.…”

“…In this case, Feix et al still found linear instability of a flat surface. Moreover, in many cases, [34][35][36] including low-energy ͑0.5 keV͒ bombardment of an amorphous silicon surface, 37 the response of the surface is the formation of craters with rims. This type of response, involving the accumulation of matter at some locations, is in clear contradiction to the purely erosive response predicted by Sigmund's model using a Gaussian ellipsoid collision cascade.…”

“…This type of response, involving the accumulation of matter at some locations, is in clear contradiction to the purely erosive response predicted by Sigmund's model using a Gaussian ellipsoid collision cascade. The occurrence of craters with rims has been attributed to thermal spikes 36 or to ion-stimulated surface mass transport. 37 These observations raise the interesting question of how robust are the predictions of BH to the precise shape of the local response to an ion impact.…”

Crater formation by single ions, cluster ions and ion “showers”

“…3 clearly shows the that explosive mechanism can be expected at fairly high fluxes (/ = 1.3 Â 10 28 cm À2 s À1 ), at least at the explored impact energy. Although a single Au ion can form a crater on Au surface, this can happen only at the impact energy of about 5 keV and higher [16]. The 500 eV Au single ions form a linear atomic cascade, which due to the high flux overlap and form a crater according to the thermal spike mechanism.…”