Phase separation in ion-irradiated compound semiconductors: an alternate route to ordered nano-structures

摘要

In recent years, observations of highly-ordered, hexagonal arrays ofself-organized nanostructures on binary or impurity-laced targets undernormal-incidence ion irradiation have excited interest in this phenomenon as apotential route to high-throughput, low-cost manufacture of nanoscale devicesor nanostructured coatings. The currently-prominent explanation for thesestructures is a morphological instability driven by ion erosion discovered byBradley and Shipman; however, recent parameter estimates via molecular dynamicssimulations suggest that this erosive instability may not be active for therepresentative GaSb system in which hexagonal structures were first observed. Motivated by experimental and numerical evidence suggesting the possibleimportance of phase separation in ion-irradiated compounds, we here generalizethe Bradley-Shipman theory to include the effect of ion-assisted phaseseparation. The resulting system admits a chemically-driven finite-wavelengthinstability that can explain the order of observed patterns even when theerosive Bradley-Shipman instability, and in a relevant simplifying limit,provides an intuitive instability criteria that agrees qualitatively withexperimental observations on pattern wavelengths. Finally, we identify acharacteristic experimental signature that distinguishes the chemical andmorphological instabilities, and highlights the need for specific additionalexperimental data on the GaSb system.