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Register a new userPhase separation in ion-irradiated compound semiconductors: an alternate route to ordered nano-structures
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Scott A. Norris
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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.
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During the ion bombardment of targets containing multiple component species, highly-ordered arrays of nanostructures are sometimes observed. Models incorporating coupled partial differential equations, describing both morphological and chemical evolution, seem to offer the most promise of explaining these observations. However, these models contain many unknown parameters, which must satisfy specific conditions in order to explain observed behavior. The lack of knowledge of these parameters is therefore an important barrier to the comparison of theory with experiment. Here, by adapting the recent theory of crater functions to the case of binary materials, we develop a generic framework in which many of the parameters of such models can be estimated using the results of molecular dynamics simulations. As a demonstration, we apply our framework to the recent theory of Bradley and Shipman, for the case of Ar-irradiated GaSb, in which ordered patterns were first observed. In contrast to the requirements therein that sputtered atoms form the dominant component of the collision cascade, and that preferential redistribution play an important stabilizing role, we find instead that the redistributed atoms dominate the collision cascade, and that preferential redistribution appears negligible. Hence, the actual estimated parameters for this system do not seem to satisfy the requirements imposed by current theory, motivating the consideration of other potential pattern-forming mechanisms.
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