Features of the iron charge states in semi-insulating $\beta$-Ga$_2$O$_3$:Fe identified by high-frequency electron paramagnetic resonance

The charge states of non-Kramers Fe$^{2+}$ ions and Fe$^{3+}$ ions in the octahedral and tetrahedral positions of the crystal lattice in a commercial substrate of semi-insulating gallium oxide $\beta$-Ga$_2$O$_3$ before and after proton irradiation with an energy of 15 MeV have been identified by high-frequency EPR.