One and Two-Dimensional Pattern Formation on Ion Sputtered Silicon
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One and Two-Dimensional Pattern Formation on Ion Sputtered Silicon Ari-David Brown1,2, H. Bola George3, Michael J. Aziz3, and Jonah Erlebacher2 1
Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, U.S.A. 2 Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218, U.S.A. 3 Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, U.S.A. ABSTRACT The evolution of surface morphology during ion beam erosion of Si(111) at glancing ion incidence (60o from normal, 500 eV Ar+, 0.75 mA/cm2 collimated beam current) was studied over a temperature range of 500-730o Celsius. Keeping ion flux, incident angle, and energy fixed, it was found that one-dimensional sputter ripples with wavevector oriented perpendicular to the projected ion beam direction form during sputtering at the lower end of the temperature range. For temperatures above approximately 690o Celsius, growth modes both parallel and perpendicular to the projected ion beam direction contribute to the surface morphological evolution. This effect leads to the formation of bumps (“dots”) with nearly rectangular symmetry. INTRODUCTION Sputter rippling is a term commonly used to describe the self-assembly of periodic nanostructures on surfaces etched by an ion beam. The surface instability leading to rippling is caused by a competition between ion-induced roughening on one hand, and smoothing due to surface relaxation mechanisms such as surface diffusion on the other. Ripples have been produced on the surfaces of a variety of different materials including metals [1, 2], semiconductors [3], and amorphous solids [4, 5]. In addition, many different ion species and ion energies have been used [3, 6] to form sputter ripples. One- and two-dimensional features with well-defined symmetry have been produced by sputter rippling. Arrays consisting of small bumps or “nano-dots” with hexagonal symmetry have been fabricated on GaSb [7] and Si [8] surfaces using normal incidence sputtering; similar structures were formed on InP [9] during simultaneous ion bombardment and sample rotation. Typically, only one-dimensional features form during glancing incidence sputtering, and typically two-dimensional features tend to exhibit only hexagonal ordering. Here we report the evidence of two-dimensional pattern formation with nearly rectangular symmetry on Si(111) using glancing incidence sputtering. In fact, we find that different sputter ripple morphologies on Si(111) may be generated simply by varying the substrate temperature. A theory explaining the morphological evolution of ion sputtered surfaces was formulated by Bradley and Harper (BH) [10]. BH use a curvature-dependent sputter yield derived from Sigmund's model for ion sputtering [11] and couple this ion-induced roughening mechanism to surface smoothing by surface diffusion. The cooperative
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behavior between roughening and smoothing mechanisms leads to functional relationships between ripple wavelength, ion flux and subst
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