Crystallization Instead of Amorphization in Collision Cascades in Gallium Oxide

TL;DR

This study investigates how gallium oxide can undergo crystallization instead of amorphization during collision cascades, revealing the kinetic factors and thresholds that favor polymorph transitions over amorphous states.

Contribution

It identifies the disorder threshold for the eta-to-\gamma polymorph transition in ext{Ga}_2 ext{O}_3 and explains the kinetic mechanisms favoring crystallization in collision cascades.

Findings

Disorder threshold for irreversible eta-to-\gamma transition identified.

Kinetic favorability of ext{Ga} atoms transitioning to ext{γ}-like configuration.

Energy considerations explain why ext{γ}-polymorph is favored below amorphous state.

Abstract

Disordering of solids typically leads to amorphization, but polymorph transitions, facilitated by favorable atomic rearrangements, may temporarily help to maintain long-range periodicity in the solid state. In far-from-equilibrium situations, such as atomic collision cascades, these rearrangements may not necessarily follow a thermodynamically gainful path, but may be kinetically limited. In this Letter, we focused on such crystallization instead of amorphization in collision cascades in gallium oxide (\ce{Ga2O3}). We determined the disorder threshold for irreversible $\beta$-to-$\gamma$ polymorph transition and explained why it results in elevating energy to that of the $\gamma$-polymorph, which exhibits the highest polymorph energy in the system below the amorphous state. Specifically, we demonstrate that upon reaching the disorder transition threshold, the \ce{Ga}-sublattice kinetically favors transitioning to the $\gamma$-like configuration, requiring significantly less migration for \ce{Ga} atoms to reach the lattice sites during post-cascade processes. As such, our data provide a consistent explanation of this remarkable phenomenon and can serve as a toolbox for predictive multi-polymorph fabrication.