Mechanisms of pattern formation in grazing-incidence ion bombardment of Pt(111)

Ripple patterns forming on Pt(111) due to $5\mathrm{keV}$ ${\mathrm{Ar}}^{+}$ grazing-incidence ion bombardment were investigated by scanning tunneling microscopy in a broad temperature range from $100\text{to}720\mathrm{K}$ and for ion fluences up to $3\times {10}^{20}\text{ions}∕{\mathrm{m}}^2$. A detailed morphological analysis together with molecular dynamics simulations of single ion impacts allow us to develop atomic scale models for the formation of these patterns. The large difference in step edge versus terrace damage is shown to be crucial for ripple formation under grazing incidence. The importance of distinct diffusion processes—step adatom generation at kinks and adatom lattice gas formation—for temperature dependent transitions in the surface morphology is highlighted. Surprisingly, ion bombardment effects like thermal spike induced adatom production and planar subsurface channeling are important for pattern ordering.

Figure 1

Schematic side view of experimental geometry during ion exposure. Ions crossing the dashed line in front of an illuminated step at a distance $\xi$ within the interval $[-x_c,0]$ give rise to the large step edge sputtering yield $Y^{\text{step}}\approx 8.4$, whereas all other ions cause only negligible sputtering with $Y^{\mathrm{terr}}\approx 0.08$.

Figure 2

Top view of the damage pattern of a $5\mathrm{keV}$ ${\mathrm{Ar}}^{+}$ ion incident on the step on Pt(111) from the left with $\xi =-3$ (see the text). The arrow indicates the direction of the incoming ion.

Figure 3

Morphology of Pt(111) after an ion fluence of 20 MLE at (a) $250\mathrm{K}$, (b) $350\mathrm{K}$, (c) $450\mathrm{K}$, (d) $550\mathrm{K}$, (e) $625\mathrm{K}$, and (f) $720\mathrm{K}$. The direction of the ion beam projection onto the surface is indicated by a white arrow in (a). The image size is $2450\mathrm{\AA }\times 2450\mathrm{\AA }$ for (a)–(e) and $4900\mathrm{\AA }\times 4900\mathrm{\AA }$ in (f). The insets show parts of the images in doubled magnification. The gray scale-height relation differs from image to image.

Figure 4

(a) Temperature dependence of roughness $\sigma$ for the morphologies of Fig. 3 formed by exposure to ion fluences of 20 MLE. The roughness data point for $100\mathrm{K}$ after 70 MLE added in brackets (compare Fig. 5). (b) Wavelength analysis for the same patterns. Error bars below the symbol size, except for the data point at $720\mathrm{K}$, which is just an estimate with a large uncertainty toward larger values. Inset: normalized standard deviations $\Delta \lambda ∕\lambda$ of the histograms of distances $\lambda$ between valleys in the direction normal to the ion beam versus temperature (see the text).

Figure 5

Morphology of Pt(111) after an ion fluence of 70 MLE at $100\mathrm{K}$. The direction of the ion beam projection onto the surface is indicated by the white arrow. The image size is $1250\mathrm{\AA }\times 1250\mathrm{\AA }$.

Figure 6

The sequence of topographs after ion bombardment at $450\mathrm{K}$ with decreasing ion fluence. (a) 5 MLE, (b) 1.0 MLE, (c) 0.5 MLE. The image size is always $2450\mathrm{\AA }\times 2450\mathrm{\AA }$. The white arrow indicates the direction of the ion beam.

Figure 7

Cartoons illustrating the different scenarios for the interaction of growing, elongated vacancy islands. (a) Vacancy groove separation normal to the beam $d>d_S$: no interaction. (b) $d\approx d_S$: repulsion (see the text). (c) $d<0.5d_S$: coalescence.

Figure 8

Morphology of Pt(111) after an ion fluence of 0.5 MLE at (a) $250\mathrm{K}$, (b) $350\mathrm{K}$, (c) $450\mathrm{K}$, (d) $550\mathrm{K}$, (e) $625\mathrm{K}$, and (f) $720\mathrm{K}$. The direction of the ion beam projection onto the surface is indicated by a white arrow in (a). The black arrow in (e) indicates a chain of vacancy islands with positional order. The image size is $1650\mathrm{\AA }\times 1650\mathrm{\AA }$ for all topographs.

Figure 9

A comparison of the adatom yields for ions impinging with 80° and with 83° toward the target normal. Data show the dependence of the adatom yield calculated by molecular dynamics as a function of the distance from the step location. $\xi =x∕\Delta x$ measures the distance of the ion impact point to the step edge in units of the spacing of dense packed atomic rows on the (111) surface.

Figure 10

Morphologies of Pt(111) after the removal of identical amounts of material. (a) $\vartheta =79°$, $550\mathrm{K}$, 14 MLE $5\mathrm{keV}$ ${\mathrm{Ar}}^{+}$. (b) $\vartheta =83°$, $550\mathrm{K}$, 20 MLE $5\mathrm{keV}$ ${\mathrm{Ar}}^{+}$ at $550\mathrm{K}$. The direction of the ion beam is indicated in each topograph by a white arrow. The image sizes are $2450\mathrm{\AA }\times 2450\mathrm{\AA }$.

Figure 11

Cartoons of surface cross sections in the direction normal to the ion beam with three vacancy grooves separated by two remaining top layer terraces. The center vacancy groove has a larger width than the neighboring ones. (a) Initial situation (full lines), centers of top layer terraces (dotted lines), and changes due to subsequent ion exposure (dashed lines). (b) The final situation (full lines) and centers of the remaining top, terraces (dashed lines). The lateral shift of these centers upon exposure is indicated by arrows.