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The angular dependence of subsurface channeling is investigated for the case of 5 keV Xenon impacts. The angle of incidence has been varied from 88◦ to 78.5◦ with respect to the surface normal for a fixed ion fluence of 1.4 · 10−3 _{MLE. Fig. 4.17 shows a}

part of the experimental results for 83◦ [Fig. 4.17(a)], 80◦ [Fig. 4.17(b),(c)] and 78.5◦ [Fig. 4.17(d)] and one MD simulation snapshot performed at 78.5◦ on the flat terrace. In all images subsurface channeling is clearly visible. Two important observations can be made.

First, decreasing the angle of incidence ϑ increases the surface damage. Whereas at 83◦the damage almost exclusively results from subsurface channeling at step edges, 80◦ and 78.5◦ _{show an increased number of adatoms and vacancies resulting from terrace}

impacts. The energy of motion in the direction normal to the surface becomes larger if the angle of incidence is decreased (E⊥= 74eV for 83◦, E⊥= 151eV for 80◦, E⊥=

199eV for 78.5◦_{). This results in an enhanced probability for large scattering events on}

the terrace. This result is also in accordance with previous research where the angular dependent morphological evolution has been investigated for grazing incidence ions [33]. The second observation is linked to Fig.4.17 (c),(d) at 80◦ and 78.5◦. Here surface trenches are visible on the terraces which cannot be linked to ascending step edges. For 80◦ the number is still very low but at 78.5◦ most trenches start and end on the flat terrace. Two scenarios can be imagined in order to understand this observation.

The first possibility is based on a two-event scenario. A large scattering event on the terrace creates a stable adatom-vacancy cluster. Then a subsequent impact into the vacancy or at the adatom cluster leads to a significant change of the ion trajectory which results in a penetration into the subsurface. It is however hard to imagine how a large scattering event at a surface defect results in a stable channeling trajectory in the subsurface.

Figure 4.17: STM topographs of the damage produced by 5keV Xenon single ion im- pacts at different angles of incidence. (a) 83◦, (b) 80◦.(c) Surface trench on the flat terrace at 80◦ and many surface trenches on the flat terrace at 78.5◦(d). Image sizes: 650˚_{A × 650˚}A. (e) Molecular dynamics snapshot after an ion impact on the flat terrace. An angle of incidence of 78.5◦ and 5 keV Xenon ions have been used [118].

scattering event. Due to the relatively large angle of incidence of the ions with respect to the surface strings, the crystal becomes partially transparent and the ion enters into the subsurface. This scenario is very similar to deeper layer channeling events already discussed in the preceding sections and is well known from surface channeling experiments [74;79] (see also section2.4.1). The ions are able to enter most easily in

4. SURFACE DAMAGE BY SINGLE ION IMPACTS 78 80 82 84 86 88 200 300 400 500 600 m a xi m u m ch a n n e l i n g l e n g t h ( Å ) angle of incidence (°)

Figure 4.18: Five longest channeling events as a function of the angle of incidence ϑ for 5 keV Xenon impacts.

the subsurface if they hit the crystal parallel to a close packed direction between two atomic rows. Here the repulsive potential of the surface atoms reaches its minimum. Danailov et al. [74] deduced the maximum transverse energy, in order to achieve total reflection on Pt(111) along the [110] direction, from computer calculations [see section

2.4.1]. This transverse energy defines the critical angle for total reflection [formula

2.16]. For the case of Xenon a value of 79◦ is deduced. This is in nice agreement with the experimental finding which shows surface trenches on the flat terrace at an angle of incidence of 80◦.

In order to corroborate this finding we analyzed the damage on the flat terrace by molecular dynamics simulations. At an angle of incidence of 80◦ _{no surface trenches}

are observed on the flat terrace. Specular reflection occurs and no damage is induced at the surface. By reducing the angle of incidence to 78.5◦ the situation changes and trenches are produced by the impinging ions on the flat terrace. A snapshot of such a simulation is shown in Fig.4.17(e) where the ion entered the subsurface without a large scattering event and induces a surface trench of 310 ˚A before leaving the simulation cell. In total, 24% of all ions hitting the flat terrace at an angle of 78.5◦ _{enter into the}

subsurface. They perform at least one oscillation between the strings of atoms before dechanneling occurs.

terrace with 5 keV Xenon ions, has been estimated with STM to lie between 83◦-80◦. From MD simulations an angle between 80◦ to 78.5◦ has been deduced. However one has to keep in mind that no experiments have been performed between 80◦_-83◦ _{and no}

simulations between 80◦-78.5◦. Since the experimentally measured number of surface trenches at 80◦is low and with the input from literature (79◦from ref. [74]) the onset of surface vacancy trenches on the flat terrace is most probably between 80◦_{± 1}◦. The MD simulations performed at 0 K probably underestimate the angle of incidence (T=0 K). Moreover one has to keep in mind that the calibration of the angle of incidence in experiment has an error of ±0.5◦_.

The distribution of the five longest channeling events has been investigated as a function of the angle of incidence and is shown in Fig.4.18. The graph has to be treated with proper skepticism since the evaluation suffers from severe simplifications, i.e. the five longest channeling events are the result of similar ion trajectories, similar energy loss etc. In the case of the angular dependent measurements this is very doubtful. Still, the graph shows a trend towards longer channeling distances at less grazing angles. At 78.5◦ most channeling events enter on the terrace in contrast to larger angles of incidence where this is not the case. It is very likely that the ions at 78.5◦ _{are not}

only confined in one layer during channeling but switching between different layers frequently occurs. Deeper layer channeling might lead to an enhanced probability to be captured longer in the channel since a switching event upwards will not lead to dechanneling. An event of deeper layer channeling at 78.5◦ (the longest trajectory found in the experiment) is shown in Fig.4.19 (a). The surface trench is visible over a length of 760 ˚A and crosses a descending step edge. The channeling distance is roughly 50% longer as the longest channeling event shown for 88◦ [Fig. 4.19]. The number and the position of the adatom clusters are shown in Fig. 4.19(b),(d) (red dots). The blue dots mark the clusters which have a significantly lower apparent height. They are probably due to occasionally adsorbed Xenon atoms as it will be discussed in section

4.8. The distribution of the adatom clusters in Fig.4.19(b) differs from the 88◦ _adatom

distribution shown in Fig.4.19(d). At 78.5◦ the number of adatom clusters per length scale on the upper terrace is smaller than at the lower terrace. The lower terrace adatom density is comparable to 88◦_{. This points towards deeper layer channeling at}

4. SURFACE DAMAGE BY SINGLE ION IMPACTS

Figure 4.19: (a) STM topography of 78.5◦ 5 keV Xenon impacts. The ion trajectory crosses a descending step edge and the channeling distance equals to 760 ˚A. (b) Cor- responding sketch of the adatom cluster distribution (red dots). The blue small dots indicate the position of Xenon adsorbates decorating the surface trench (see section

4.8). (c) Channeling event at 88◦ ion incidence with the corresponding sketch (d). Image size: 1010˚_{A × 1010˚}A.