Control of Crystal Structure of Ga 2 O 3 on Sapphire Substrate by Introduction of α‐(Al x Ga 1− x ) 2 O 3 Buffer Layer

An annealed α‐(Al 0.4 Ga 0.6 ) 2 O 3 buffer layer is introduced to achieve either α‐Ga 2 O 3 or ϵ‐Ga 2 O 3 growth on sapphire substrates, depending on the growth temperature, using the mist chemical vapor deposition method. Transmission electron microscopy reveals that the epitaxial relationship between ϵ‐Ga 2 O 3 and the α‐(Al 0.4 Ga 0.6 ) 2 O 3 buffer layer is ϵ‐Ga 2 O 3 [101 ¯ 0] || α‐(Al 0.4 Ga 0.6 ) 2 O 3 [112 ¯ 0], and the two hexagonal lattices are consequently rotated in the ab plane by 30° with respect to each other. The lattice mismatch between the buffer layer and ϵ‐Ga 2 O 3 is 1.2%, while that between the buffer layer and α‐Ga 2 O 3 is 2.2%. When the growth temperature is below 600 °C, ϵ‐Ga 2 O 3 , which had the smaller lattice mismatch, is produced. On the other hand, higher temperature leads to a longer diffusion length, and atoms reach the step edges. Therefore α‐Ga 2 O 3 , which has the same structure as the buffer layer, is grown along the step edges above 600 °C. As a result, ϵ‐Ga 2 O 3 and α‐Ga 2 O 3 grow below and above 600 °C, respectively.