[Paper Review] Crystallization Instead of Amorphization in Collision Cascades in Gallium Oxide

J. Zhao, Javier García‐Fernández+7 more|arXiv (Cornell University)|Jan 15, 2024

Ga2O3 and related materials6 citations

TL;DR

The paper shows that in collision cascades in Ga2O3, disorder triggers a rapid β-to-γ phase transition rather than amorphization, with a sharp disorder threshold that aligns with experiments and ML-MD simulations.

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 $β$-to-$γ$ polymorph transition and explained why it results in elevating energy to that of the $γ$-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 $γ$-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.

Motivation & Objective

• Understand how collision cascades in β-Ga2O3 lead to a β-to-γ phase transition instead of amorphization.
• Determine the disorder threshold that irreversibly triggers the β-to-γ transition.
• Elucidate atomic migration pathways and the role of Ga sublattice reconstruction in stabilizing the γ phase.
• Provide a predictive framework for multi-polymorph fabrication in Ga2O3.
• Correlate experimental observations with ML-MD simulations to explain defect dynamics during post-cascade evolution.

Proposed method

• Use 58Ni+ ion implantation to create controlled disorder in β-Ga2O3 and characterize with high-resolution STEM and SAED.
• Employ machine-learning molecular dynamics (ML-MD) with a Ga2O3 interatomic potential to model overlapping collision cascades and post-cascade annealing.
• Conduct DFT calculations to map intermediate states and energy landscapes for β to γ migration paths.
• Construct orthogonal β-cell supercells to compare β and γ phases under identical conditions for direct energy comparisons.
• Analyze disordered states via partial radial distribution functions and bond-angle distributions to track phase evolution.
• Cross-validate experimental disorder thresholds (dpa) with simulation crossing points where ΔEp(Beta) and ΔEp(Gamma) intersect.

Experimental results

Research questions

• RQ1 What disorder level (in dpa) triggers the irreversible β-to-γ phase transition in Ga2O3 after collision cascades?
• RQ2 Why does the system preferentially transition to the γ phase rather than amorphize, and what are the atomistic migration pathways involved?
• RQ3 How do post-cascade annealing and defect recombination influence the stability and kinetics of the γ phase?
• RQ4 Can the β-to-γ transition be explained via a minimal set of Ga atom migrations that conserve lattice side lengths while releasing stress?
• RQ5 What is the role of β/γ interfaces in stabilizing the γ phase and enabling rapid reconstructions?

Key findings

• A sharp disorder threshold exists (0.2–0.3 dpa range) that triggers the irreversible β-to-γ transition during post-cascade evolution.
• Beyond the threshold, Ga sublattice rearranges with significantly less migration to reach γ lattice sites, stabilizing the γ phase instead of forming amorphous Ga2O3.
• ML-MD and DFT analyses show β and γ defect energies cross during post-cascade evolution, explaining the preference for γ formation.
• Post-cascade annealing reveals two regimes: partial recovery to β at lower PKAs, and sustained γ/disordered states at higher PKAs, consistent with experiments.
• The reconstructed γ-like state (γ′) involves specific Ga atom displacements (16/32 Ga) and stress release, leading to a final γ phase after external relaxation.
• Interface-mediated pathways enable rapid γ-phase formation within a thin interfacial region, matching experimental observations of sharp β/γ boundaries.