(Color online) AFM images of Ge samples in the various morphological regimes. Samples were irradiated with different ion incidence angles at 250 eV at fluences of 4.5 × 1018 ions/cm2 reckoned in a plane perpendicular to the ion beam. The projected ion beam direction is from the bottom to the top of the page. Scale bar lengths are 200 nm for AFM images, and 100 μm−1 for the FFT inset. Ion incidence angles, resulting surface morphologies, and height scales are (a) 5°, smooth, 1 nm, (b) 60°, parallel-mode ripples, 6 nm, (c) 75°, holes, 4 nm, and (d) 80°, perpendicular-mode ripples, 2 nm. Inset: FFT of image D, showing the emergence of perpendicular-mode ripples.
(Color online) Pattern formation phase diagram. Qualitative patterning behavior is equivalent for both 250 and 500 eV ion energy. Filled circles: smoothening; open circles: parallel-mode ripples; crosses: perpendicular-mode ripples. Superposed circles and crosses indicate a hole pattern, which we interpret as the superposition of parallel- and perpendicular-mode ripples.
Wavelengths of pattern formation in linear regime, as measured with FFT.
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