Physical mechanisms affecting critical angle for nanopatterning in irradiated thin films: II. Collision cascade details

TL;DR

This paper investigates how the detailed collision cascade shape influences the critical angle for nanopatterning in ion-irradiated thin films, addressing discrepancies between experimental results and previous theoretical predictions.

Contribution

It derives geometric expressions linking collision cascade shape to interface relations, refining understanding of energy, ion, and target effects on critical angle.

Findings

Derived closed-form expressions for interface relation based on collision cascade shape

Revealed dependence of critical angle on ion energy, species, and target material

Improved theoretical model aligning better with experimental observations

Abstract

Ion-beam irradiation of an amorphizable material such as Si or Ge may lead to spontaneous pattern formation beyond some critical angle of the beam versus the surface. It is known from experimental results that this critical angle varies according to beam energy, ion species and target material. However, most prevailing theoretical analyses predict a critical angle of $45^{\circ}$ independent of energy, ion and target, disagreeing with experiment. In this second part of a set of papers, we consider the influence of the relationship between the upper and lower interfaces of the amorphous thin film (the ``interface relation"). From our previous work, we are motivated to derive from a geometric argument closed-form expressions describing the interface relation in terms of the collision cascade shape. This feature leads to a refined characterization of the influence of ion-, target- and energy-dependence on critical angle selection.