Ion-assisted phase separation in compound films: An alternate route to ordered nanostructures

Abstract

In recent years, observations of highly-ordered, hexagonal arrays of self-organized nanostructures on binary or impurity-laced targets under normal-incidence ion irradiation have excited interest in this phenomenon as a potential route to high-throughput, low-cost manufacture of nanoscale devices or nanostructured coatings. The currently-prominent explanation for these structures is a morphological instability driven by ion erosion discovered by Bradley and Shipman; however, recent parameter estimates via molecular dynamics simulations suggest that this erosive instability may not be active for the representative GaSb system in which hexagonal structures were first observed.Motivated by experimental and numerical evidence suggesting the possible importance of phase separation in ion-irradiated compounds, we here generalize the Bradley-Shipman theory to include the effect of ion-assisted phase separation. The resulting system admits a chemically-driven finite-wavelength instability that can explain the order of observed patterns even when the erosive Bradley-Shipman instability, and in a relevant simplifying limit, provides an intuitive instability criteria that agrees qualitatively with experimental observations on pattern wavelengths. Finally, we identify a characteristic experimental signature that distinguishes the chemical and morphological instabilities, and highlights the need for specific additional experimental data on the GaSb system. * snorris@smu.edu arXiv:1205.6834v2 [cond-mat.mtrl-sci]

Characterization of ion-irradiation-induced nanodot structures on InP surfaces by atom probe tomography

“…Relevant for the present context is the theoretical demonstration [23] that, apart from the formation of specific nanostructures (ripple or dots), compositional gradients may exist within individual of these features: for example, in ripple structures one component will be enriched in the crests while being depleted in the valleys, and vice versa for the other component. Further theoretical approaches [24][25][26][27] confirmed and refined this possibility.…”

Self-organized nanopatterning of silicon surfaces by ion beam sputtering

“…However, the order of the pattern is expected to be short-ranged in such a case. On the other hand, a generalization of the latter models has been proposed in [248], which includes the effect of ionassisted phase separation. Even when the BH instability, preferential redistribution, and chemical diffusion are absent, the resulting model has a narrow-band instability that can explain the order of observed patterns.…”

Nanocone formation on ion-bombarded InP surfaces

“…Therefore, the ion-induced enrichment (depletion) might be site-specific (e.g., different for crests or valleys in ripples) leading, eventually, to a modulation in composition that can be in or out of phase with the (ripple) topography. Further theoretical approaches [20][21][22][23] experimental investigation is quite challenging. In previous studies [24,25], such possible compositional variations in individual nanodots caused by ion bombardment, were examined by means of atom probe tomography [26].…”