Quantifying the Order of Spontaneous Ripple Patterns on Ion-Irradiated Si(111)
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Quantifying the order of spontaneous ripple patterns on ion-irradiated Si(111) H. Bola George1, Ari-David Brown2*, Matthew R. McGrath3, Jonah Erlebacher2 & Michael J. Aziz1 1 Division of Engineering & Applied Sciences, Harvard University, Cambridge, MA 02138 2 Department of Materials Science & Engineering, Johns Hopkins University, Baltimore, MD 21218 3 Physics & Astronomy Department, Vanderbilt University, Nashville, TN 37235 * Current address: NASA Goddard Space Flight Center, Greenbelt, MD 20771 ABSTRACT Uniform keV ion irradiation causes a morphological instability known to result in the spontaneous formation of topographic ripple and dot patterns. The degree of order of these patterns, which has important implications for non-lithographic patterning applications, varies markedly with fabrication conditions. We investigate the influence of systematic variations of fabrication conditions, including current density, ion fluence and ion energy, on the degree of order of argon ion bombarded Si(111) surfaces. For quantifying order in sputter rippled topographic images, we develop an algorithm that evaluates the density of topological defects, such as ripple bifurcations and terminations, and suitably normalizes the result in order to present a scalar figure of merit: the normalized defect density. We discuss fabrication conditions that lead to extremely well ordered dot and ripple patterns upon irradiation. INTRODUCTION Sputter patterning employs unfocused low-energy ion beams to sputter etch surfaces and is envisioned to play an important role in nanoscale device fabrication – it offers an alternate, highthroughput means of forming periodic structures possessing nanometer length scales. Following ion irradiation, self-organization of one-dimensional ripple arrays have been observed on metal 14 , semiconductor 5-7, and insulator 8,9surfaces. In addition, two-dimensional dot arrays have been shown to form under a variety of ion patterning conditions and substrates. Frost and co-workers 10,11 observed the formation of dots possessing hexagonal and rectangular symmetry during ion bombardment of simultaneously rotating InP substrates. Hexagonal dot arrays have also been observed on Si surfaces following ion sputtering at normal incidence with 12-16 and without 17 sample rotation; oblique incidence ion sputtering under sample rotation also results in highly ordered GaSb dot arrays 18. Recently, we reported 19-21 the formation of sub-micron sized dots possessing quasi-rectangular symmetry resulting from oblique incidence ion bombardment of heated Si(111) surfaces. A drawback of the sputter patterning technique pertains to the regularity and order of structure. While short-range order (SRO) of these dots can be quite high, the dots are often separated by defects resulting in dots arrays possessing poor long-range order (LRO) envisaged for some applications. Generally 10,12,18,22, it has been observed that enhanced dot order is proportional to the ion fluence (sputtering time). The effect of substrate tempera
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