Formation of various phases of gallium oxide films depending on substrate planes and deposition gases

• Crystal phases of Ga2O3 films on sapphire and Si(100) have been investigated.
• (2¯01)-oriented β-Ga2O3 is formed on Al2O3(0001) above 300 °C.
• (110)-oriented α-Ga2O3 is formed on Al2O3(112¯0) above 600 °C.
• High temperature is needed to obtain high crystallinity of α-Ga2O3.
• Solid-phase crystallization in the presence of H2O favors formation of γ-Ga2O3.

The crystal phases and orientations of Ga2O3 thin films were investigated by RF magnetron sputtering on sapphire C-planes, sapphire A-planes, and Si(100) substrates either in an O2 or H2O vapor ambient. When deposited under O2 gas flow, (2¯01)-oriented β-Ga2O3 grew on sapphire C-planes at 300 °C for both as-crystallization and solid-phase crystallization. In contrast, (110)-oriented α-Ga2O3 grew on sapphire A-planes above 600 °C. Randomly-oriented polycrystalline β-Ga2O3 was formed on Si(100) substrates. These experimental results indicate that while a high temperature is needed to form β-Ga2O3, the activation barrier of nucleation is greatly reduced by hexagonally packed oxygen atoms on sapphire C-planes. Nucleation and/or crystallization on sapphire A-planes require higher temperatures probably because of sparse surface atoms. Nevertheless, pseudomorphic growth of α-Ga2O3 having the same corundum structure is selected due to the template effect of the substrate at elevated temperatures. Deposition with H2O severely decelerated crystallization and promoted nucleation of γ-Ga2O3. Preferentially (311)-oriented γ-Ga2O3 was obtained on sapphire C-planes. While pseudomorphic growth of α-Ga2O3 occurred on sapphire A-planes above 800 °C, small γ-Ga2O3 crystallites coexisted with them. Conversion between the phases was not observed even at 800 °C, reflecting a high activation barrier for rearrangement of atoms that constitute crystalline lattices.