Al 2 O 3 films were deposited on β-Ga 2 O 3 (010) and β-Ga 2 O 3 substrates by atomic layer deposition at 250 C, and their interface state densities (D it) at shallow energies were evaluated using a high–low capacitance–voltage (C–V) method. Al 2 O 3/β-Ga 2 O 3 (010) showed lower D it values (5.9× 10 10 to 9.3× 10 11 cm− 2 eV− 1) than Al 2 O 3/β-Ga 2 O 3(2.0× 10 11 to 2.0× 10 12 cm− 2 eV− 1) in an energy range of− 0.8 to− 0.1 eV. Cross-sectional transmission electron microscopy analysis indicated the formation of a uniform amorphous Al 2 O 3 layer on the β-Ga 2 O 3 substrate. In contrast, a crystalline Al 2 O 3 interlayer with a thickness of 3.2±0.7 nm with an amorphous Al 2 O 3 top layer was formed on the β-Ga 2 O 3 (010) substrate, which effectively decreased D it. Moreover, thicker interlayers showing lower D it values at deep state levels were formed at deposition temperatures higher than 100 C, which were evaluated by shifts in the C–V curves.