Spontaneous Pattern Formation on Ion Bombarded Si(001)

Abstract

Pattern formation on surfaces undergoing low-energy ion bombardment is a common phenomenon. Here, a recently developed in situ spectroscopic light scattering technique was used to monitor periodic ripple evolution on Si(OO1) during Ar+ sputtering. Analysis of the rippling kinetics indicates that under high flux sputtering at low temperatures the concentration of mobile species on the surface is saturated, and, surprisingly, is both temperature and ion flux independent. This is due to an effect of ion collision cascades on the concentration of mobile species. This new understanding of surface dynamics during sputtering allowed us to measure straightforwardly the activation energy for atomic migration on the surface to be L2 f 0.1 eV. The technique is generalizable to any material, including high temperature and insulating materials for which surface migration energies are notoriously difficult to measure. PRL, 1999 Low energy (500 -LOO0 eV) ion bombardment is a common technique used in many thin film applications such as forming shallow junctions, sputter etching and deposition, ion beam assisted growth, reactive ion etching, and plasma assisted chemical vapor deposition. Under certain conditions, ion sputtering is known to produce patterns on surfaces. Features such as ripples, bumps or cones are common [ 1,2,3]. Typical length scales of these features are of order 10-1000 n m In some cases, these features are nuisances, such as in sample thinning for transmission microscopy or depth profiling by secondary ion mass spectroscopy. However these nano-scale patterns also hold promise in applications as varied as optical devices, templates for liquid crystal orientation, and strain-free patterned substrates for heteroepitaxial growth of quantum dots or wires.Rippling has been observed in amorphous materials (Si02 In this study, a recently developed in situ light scattering spectroscopic technique [7] was used to monitor ripple evolution on Si(OO1) during Arc sputtering. The technique allowed the first systematic study of the temporal and spatial evolution of both ripple wavelength and ripple amplitude as functions of both temperature and ion beam flux.Theoretical models describe rippling as arising from competition between ion beam roughening/etching and surface diffusion or viscous flow mediated relaxation 181. Our results here help illuminate the range of validity of these models. Additionally, we describe the method by which the activation energy for surface migration can be found.A salient feature of these experiments was our use of a very high ion flux compared to other rippling experiments. At high fluxes, the annihilation process for mobile species becomes dominated by collision cascades. Sputter-induced rippling can, in principle, occur on the surface of any material. As such, it holds great promise as a method for measuring the activation energy for surface migration on high temperature or insulating materials, which are generally notoriously difficult to measure.Rippling experiments were perf...

Ion-beam sculpting at nanometre length scales

“…Lacking more detailed information, it is reasonable to assume a single mobile species which, for simplicity, we will call an "adatom". It has been shown that the assumption of adatom annihilation by direct ion impingement and by diffusion to surface defects or other sinks is consistent with observations of sputterinduced ripple amplification on Si(001) 23,24 .…”

Microtextured Silicon Surfaces for Detectors, Sensors & Photovoltaics

“…Periodic surface ripples, conical microstructures, whisker-like structures, and pyramidal microstructures have all been observed following high-energy ion-bombardment during sputtering experiments [40,41,42,43,44,45,46,47]. In work that is closely related to our research, conical structures formed after irradiation with nanosecond laser pulses have been observed by several research groups [7,8,10,12,13,14,15,17,48,49,50].…”

Nanopore sculpting with noble gas ions

“…Low energy noble gas ion beams have been applied as an effective tool to modify surface structures and properties on various materials at the nanoscale [7][8][9][12][13][14][15]. Here we present ion beam sculpting results for a variety of noble gas ions and ion beam fluxes to drastically change some of the potentially relevant parameters of the process.…”

“…Other methods of nanopore formation are also being actively explored [6]. The physics responsible for the ion beam sculpting process appears to be consistent with an adatom diffusion model [1,7,8]. However, another approach based on the notion of stimulated viscous flow has been advanced to account for the potentially related phenomena of ion beam induced ripple formation [9][10][11], and it must be admitted that the explanation for the ion sculpting phenomena may yet to be fully elucidated.…”