Anisotropic Core-Shell Swift Heavy Ion Tracks in beta-Ga2O3

Abstract

Swift heavy ion (SHI) irradiation generates nanoscale ion tracks through intense electronic excitation, yet the microscopic mechanisms governing their morphology and phase stability in low symmetry oxides remain poorly understood. Here, a multiscale atomistic simulation framework is employed to investigate SHI-induced track formation and recovery in monoclinic beta-Ga2O3 over a wide range of electronic energy losses (Se) and crystallographic orientations. A sequence of distinct structural responses is identified with increasing Se: complete lattice recovery at low Se, recrystallization into a metastable gamma-Ga2O3 phase at intermediate Se, and the formation of core-shell ion tracks at high Se, consisting of an amorphous core surrounded by a recrystallized gamma-phase shell. Despite the essentially isotropic initial energy deposition, the final ion-track morphology exhibits pronounced crystallographic anisotropy, governed by orientation-dependent recovery dynamics. The superior recrystallization along [010] direction is attributed to its exceptionally high elastic stiffness. Notably, SHI irradiation perpendicular to the (100) plane induces a more severe structural response at low Se (less than 10 keV/nm), however, at higher Se, it yields a smaller residual ion track compared with the other orientations. The simulated ion-track sizes show excellent quantitative agreement with available experimental measurements over a broad range of Se values. These findings establish a unified atomic-scale picture of core-shell track formation and anisotropic recovery in beta-Ga2O3.

Figures (4)

• Figure 1: Spatiotemporal lattice-energy evolution and ion-track morphology under low and high $S_\mathrm{e}$.a,b Spatiotemporal profiles of lattice energy obtained from TTM simulations at $S_\mathrm{e}$ of 10 keV/nm and 44 keV/nm, respectively. c-h Cross-sectional morphologies from MD simulations with $S_\mathrm{e}=10$ keV/nm and 44 keV/nm, colored by atomic displacement magnitude. All images correspond to the central cross section of the simulated region, representing an effective sample thickness of 5 nm. The view direction is normal to the (100) plane.
• Figure 2: Structural analysis of $\beta$-Ga2O3 under SHI irradiation perpendicular to (100) direction at different electronic energy losses.a O FCC order parameter evolution at different $S_\mathrm{e}$ values. b Atomic configurations at 30 ps and 320 ps ($S_\mathrm{e} = 10$ keV/nm). Red (blue) atoms are Ga (O) atoms. c Evolution of Ga–Ga RDF of the track core (region 1, $r \leq 2.5$ nm). d Atomic configurations at 30 ps and 320 ps ($S_\mathrm{e} = 44$ keV/nm). e The overall RDF of the track core (Region 2, $r \leq 4.5$ nm). f Bond-angle distribution (BAD) of Region 2 at 320 ps. g Coordination-number distributions of Region 2.
• Figure 3: Final state of oxygen lattice order mapping under different SHI irradiation perpendicular to (100) plane.
• Figure 4: Ion-track morphology characterized by local structural entropy in $\beta$-Ga2O3. a--d Spatial maps of the local structural entropy for ion tracks formed under SHI irradiation perpendicular to the (100), (010), (001), and (-201) crystallographic planes at $S_\mathrm{e}$ of 44 keV/nm. e--h One-dimensional entropy profiles extracted along the lateral principal crystallographic axes, averaged within a 1.5 nm wide slab, as a function of the radial distance from the track center. The red line denotes the mean entropy at each distance. All analyses are performed within a 5 nm thick region centered on the simulation cell.