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

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 O-2 or H2O vapor ambient. When deposited under O-2 gas flow, ((2) over bar 01)-oriented beta-Ga2O3 grew on sapphire C-planes at 300 degrees C for both ascrystallization and solid-phase crystallization. In contrast, (110)-oriented beta-Ga2O3 grew on sapphire Aplanes above 600 degrees C. Randomly-oriented polycrystalline beta-Ga2O3 was formed on Si(100) substrates. These experimental results indicate that while a high temperature is needed to form beta-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 alpha-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 gamma-Ga2O3. Preferentially (311)-oriented gamma-Ga2O3 was obtained on sapphire C-planes. While pseudomorphic growth of alpha-Ga2O3 occurred on sapphire A-planes above 800 degrees C, small gamma-Ga2O3 crystallites coexisted with them. Conversion between the phases was not observed even at 800 degrees C, reflecting a high activation barrier for rearrangement of atoms that constitute crystalline lattices. (C) 2015 Elsevier Ltd. All rights reserved.