11. Halide Vapor Phase Epitaxy 2

Halide vapor phase epitaxy of metastable α- and ε-Ga₂O₃ is reviewed. The both polymorphs were grown using GaCl and O₂ as precursors. Phase-pure corundum α-Ga₂O₃ was heteroepitaxially grown on (0001) sapphire at temperatures of approximately 550 °C or lower. The n-type electrical conductivity was controlled by Ge doping using GeCl₄ as the dopant source, and very low resistivity of 8.6 mΩ cm was achieved. Epitaxial lateral overgrowth was shown to be effective in improving the crystal quality, and the dislocation density was reduced from 10¹⁰ cm⁻² to less than 5 × 10⁶ cm⁻² in the laterally grown wing region. Morphology of α-Ga₂O₃ islands was controlled such that inclined facets well develop, and dislocation density above mask openings remarkably decreased due to dislocation bending caused by the inclined facets. Orthorhombic ε-Ga₂O₃ was grown on (0001) GaN and (0001) AlN using virtually the same growth recipe as that used for α-Ga₂O₃. Fundamental material properties of ε-Ga₂O₃, such as the optical bandgap energy, thermal stability, and thermal expansion coefficient, were investigated using the epitaxial film.