From phase transformation to amorphization: damage accumulation in Yb-implanted $β-Ga_2O_3$
This study provides a comprehensive analysis of the radiation response and structural evolution of differently oriented$β-Ga_2O_3$ single crystals subjected to Yb ion implantation over a wide fluence range from $5 \times 10^{12}$ to $1 \times 10^{16}$~cm$^{-2}$ ($0.04$--$74$~dpa). A multi-technique approach (RBS/c, PAS, HRTEM, and HRXRD) was employed to investigate the mechanisms of damage accumulation. The results reveal a multi-stage process of defect evolution. At a critical threshold of around $0.4$~dpa, the accumulation of lattice strain triggers a phase transformation from monoclinic $β$-Ga$_{2}$O$_{3}$ to a defective spinel structure of $γ$-Ga$_{2}$O$_{3}$. Notably, the formation of this new phase is accompanied by strain relaxation. With further irradiation, defects develop within the crystal structure of $γ$-Ga$_{2}$O$_{3}$. The associated atomic reorganization at this stage is reflected by a distinct dip in the damage accumulation curve and the appearance of stacking faults in the subsurface region of the implanted layer. In contrast to previous reports suggesting high radiation stability of this phase, the present study clearly demonstrates that continuous defect accumulation results in a significant increase in both displaced atoms and vacancy-type defects, with a strong depth dependence in their type and density. Ultimately, at an irradiation level of approximately $7$~dpa, the surface layer amorphizes. With further irradiation, the amorphous layer expands, gradually replacing the transient $γ$-Ga$_{2}$O$_{3}$ phase. These findings reveal that the radiation tolerance of gallium oxide is highly sensitive to ion-specific interactions and strain-induced instabilities, thereby challenging the previously assumed robustness of this material under high-fluence ion irradiation.