Radiation damage accumulation in $\alpha$-Ga$_2$O$_3$ under P and PF$_4$ ion bombardment

We study radiation damage accumulation in alpha polymorph of gallium oxide ($\alpha$-Ga$_2$O$_3$) epitaxial layers under irradiation with 40 keV monatomic P and 140 keV molecular PF$_4$ ions. The distribution of stable structural damage is bimodal in both cases. The growth rate of the surface disordered layer under PF$_4$ ion irradiation is significantly higher than that under monatomic P ion bombardment. At the same time, monatomic ion irradiation is more efficient in the bulk defect peak formation. Thus, the density of displacement cascades strongly affects the formation of stable damage in $\alpha$-gallium oxide. The doses required to create the same level of disorder in the metastable $\alpha$-polymorph are higher than that in the thermodynamically stable $\alpha$-. Mechanisms of damage formation in these polymorphs are different.