Beyond this fluence, ripples disappear and small mounds as well as faceted structures evolve (which grow further with increasing fluence) which is evident from Figures 4b,c,d,e,f. Figure 4 AFM images of silicon exposed to 500 eV argon ions at 72.5° incidence angle. At fluences of (a) 1 × 1017, (b) 2 × 1017, (c) 5 × 1017, (d) 10 × 1017, (e) 15 × 1017, and (f) 20 × 1017 ions cm-2,

respectively. The corresponding height scales for (a to f) are the following: 4, 3.6, 73.9, 85.9, 165.2, and 154.1 nm. For clarity, (a, b) have a scan size of 1 × 1 μm2, whereas (c to f) have a scan size of 2 × 2 μm2. Insets show Ivacaftor concentration the 2D autocorrelation functions for corresponding images. The insets of all the images shown in Figures 3 and 4 represent corresponding 2D autocorrelation functions. In Figure 3, ripple anisotropy is clearly observed at the fluence of 1 × 1017 ions cm-2, whereas the same in Figure 4 is evident up to the fluence of 2 × 1017 ions cm-2. The average values (calculated from the AFM images shown in Figures 3 and 4) of ripple wavelength, feature height,

and base width of mounds/facets are listed in Table 1 for different fluence values. An increasing trend in height and base CX-4945 width of mounds/facets is observed for both angles of incidence with increasing Ar ion fluence albeit the effect is more prominent at 72.5°. Table 1 Calculated values of ripple wavelength ( λ ), feature height ( h ), and base width Rucaparib manufacturer of mounds/facets Angle of incidence

Fluence (ions cm-2) λ (nm) Average feature height (nm) Average base width (nm) 70° 1 × 1017 34 2 – 2 × 1017 57 5 – 5 × 1017 – 16 131 10 × 1017 – 22 152 15 × 1017 – 30 199 20 × 1017 – 56 357 72.5° 1 × 1017 26 1 – 2 × 1017 27 2 – 5 × 1017 – 28 237 10 × 1017 – 50 363 15 × 1017 – 78 486   20 × 1017 – 90 525 To explain the transition from a rippled surface to faceted structures, we invoke the shadowing condition stated in Equation 2. Let us first consider the case of 70° and the fluence of 1 × 1017 ions cm-2 where the calculated value of 2πh 0/λ turns out to be 0.369, whereas tan(π/2 – θ) is 0.364. Thus, 2πh 0/λ is slightly above the limiting condition which indicates the shadowing effect to start playing a role at this fluence itself. In the case of 2 × 1017 ions cm-2, the shadowing effect becomes more prominent since 2πh 0/λ turns out to be 0.551. As a result, crests of the ripples should undergo more erosion compared to troughs, and hence, there is a likelihood of mounds/facets to evolve. This explains the observation of mounds at this fluence. Similar behaviour is observed in the case of 72.5°. For instance, in the case of 1 × 1017 ions cm-2, 2πh 0/λ equals to 0.242, while tan(π/2 – θ) turns out to be 0.315. Thus, the condition for no shadowing, i.e. tan(π/2 – θ) ≥ 2πh 0/λ gets satisfied here, and ripples are expected to be seen. The observation of sinusoidal ripples in Figure 4a supports this theoretical prediction.