Compositionally Modulated Ripples Induced by Sputtering of Alloy Surfaces

Sputtering of an amorphous or crystalline material by an ion beam often results in the formation of periodic nanoscale ripple patterns on the surface. In this Letter, we show that, in the case of alloy surfaces, the differences in the sputter yields and surface diffusivities of the alloy components will also lead to spontaneous modulations in composition that can be in or out of phase with the ripple topography. The degree of this kinetic alloy decomposition can be altered by varying the flux of the ion beam. In the high-temperature and low-flux regime, the degree of decomposition scales linearly with the ion flux, but it scales inversely with the ion flux in the low-temperature, high-flux regime.

Figure 1

Schematic of a sinusoidal surface perturbation with a wave vector along the $x$ direction induced by an ion beam incident in the $xz$ plane at an angle $\theta$. The height modulations will also lead to gradients in composition, as indicated by the color variations along the surface.

Figure 2

(Top) Wave number of the fastest growing mode obtained from the numerical solution of Eq. (6) for material parameters in Table . (Bottom) Comparison of the numerical and analytical results [from Eq. (8)] for $k_{\max }/k_A$. The brown curves correspond to $k_B/k_A$.

Figure 3

Phase angle between the composition and height modulations, $\varphi$ obtained from the numerical solution of Eq. (6) for material parameters in Table . The peaks (valleys) are enriched with the $A$ species when $\varphi =0$ ($\varphi =\pi$).

Figure 4

(Top) Modulation in composition for ripples with an amplitude $h_0=20\mathrm{nm}$, obtained from the numerical solution of Eq. (6) for material parameters in Table . (Bottom) Numerical and analytical results [from Eq. (7)] for $|{\zeta }_0|$ plotted for two different values of $D_A/D_B$.