Spontaneous Pattern Formation from Focused and Unfocused Ion Beam Irradiation
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Spontaneous Pattern Formation from Focused and Unfocused Ion Beam Irradiation Alexandre Cuenat and Michael J. Aziz Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138. ABSTRACT We study the formation and self-organization of "ripples" and "dots" spontaneously appearing during uniform irradiation of Si, Ge, and GaSb with energetic ion beams. Features have been produced both with sub-keV unfocused Ar+ ions and with a 30 keV Ga+ Focused Ion Beam. We follow the evolution of features from small amplitude to "nanospikes" with increasing ion dose. It appears that the edge of the sputtered region influences the patterns formed, an effect that may make it possible to guide the self-organization by the imposition of lateral boundary conditions on the sputter instability. INTRODUCTION Self-organization of surface morphological features to produce structures of potential use in nanotechnology is in its infancy. A few processes leading to self-organized patterns, particularly in heteroepitaxial growth, have been identified and studied. However, control of the selforganization process remains an issue. We have been investigating alternative processes leading to self-organized surface structures - in particular, "sputter patterning" produced by unfocused ion irradiation. Spontaneous rippling of surfaces by ion irradiation was first observed on glass [1]. Similar patterns have been obtained on a variety of materials, including semiconductors [25], metals [6] and insulators [7]. These self-organized patterns have been observed for a wide range of ion energy (from 200 eV to 40 keV); the orientation of the ripples depends mainly on the incidence angle of the ion beam relative to the sample normal. The pattern formation mechanism is usually discussed within the linear stability framework of Bradley and Harper [8]. Patterning is due to the dependence of the sputter yield on the curvature of the surface. Because ion energy deposition rate increases as the ion penetrates the solid, concave regions are sputtered more than convex regions, resulting in a growing instability on the surface. This instability is opposed by a smoothening mechanism, e.g. surface diffusion or viscous flow, which has a different dependence on wavelength than the roughening effect. The interplay of these two effects selects a characteristic wavelength for the pattern. This linear theory reproduces many experimental observations [9] but not all [10] [4] [11]. At large amplitudes, nonlinear terms have been proposed [12]. The nonlinear behavior may be especially important in enhancing the uniformity of the size distribution of "quantum dots" recently produced on GaSb surfaces [4] using sub-keV irradiation at normal incidence and room temperature. In this paper, self-organized patterns on Si (001) and Ge (001) using both sub-keV unfocused Ar+ beam and 30 keV Ga+ Focus Ion Beam (FIB) are compared. Boundary effects are shown to have an effect on the self-organization process, opening a way to control the symmetry of the pattern.
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