Mechanical properties of epilayers of metastable $\alpha$- and $\varepsilon$-Ga$_{2}$O$_{3}$ phases studied by nanoindentation

The resistance to deformation and crack formation in epitaxial layers of metastable $\alpha$- and $\varepsilon(\kappa)$-Ga$_{2}$O$_{3}$ polymorphs grown on sapphire substrates has been studied by nanoindentation techniques. The epilayers of $\alpha$-Ga$_{2}$O$_{3}$ (0001) and $\varepsilon(\kappa)$-Ga$_{2}$O$_{3}$ (001) polymorphs are characterized by hardness $H$ (18.7 and 17.5 GPa) and Young’s modulus $E$ (283.4 and 256.1 GPa), respectively. It is established that the critical stress intensity factor (characterizing the cracking resistance) for $\varepsilon(\kappa)$-Ga$_{2}$O$_{3}$ is K$_{1c}$ $\sim$ 0.67 MPa $\cdot$ m$^{1/2}$ and that for $\alpha$-Ga$_{2}$O$_{3}$ is K$_{1c}$ $\sim$ 0.70 MPa $\cdot$ m$^{1/2}$.