Optical properties of Ga 2 O 3 thin films grown by atomic layer deposition using GaI 3 and O 3 as precursors

Properties of Ga 2 O 3 thin films grown by atomic layer deposition from GaI 3 and O 3 on Si(100) and amorphous SiO 2 substrates were investigated. The Ga 2 O 3 films deposited on the bare Si and SiO 2 substrates at 450–550 °C contained the κ-Ga 2 O 3 phase while it was not possible to exclude the presence of the ε-Ga 2 O 3 phase in these films, either. On the substrates coated with α-Cr 2 O 3 , α-Ga 2 O 3 was obtained at 275–550 °C. The formation of crystalline phases caused a marked increase in the density, roughness, and in the growth rate as well. The refractive indices, determined for the κ/ε-Ga 2 O 3 and α-Ga 2 O 3 films with thicknesses over 70 nm, were 1.96 ± 0.03 and 2.01 ± 0.02, respectively. Corresponding values of amorphous films, deposited on Si at 150–425 °C, on SiO 2 at 310 °C, and on α-Cr 2 O 3 /Si at 200–234 °C, were 1.86 ± 0.03 at 633 nm. The optical bandgap energies of amorphous Ga 2 O 3 , κ/ε-Ga 2 O 3 , and α-Ga 2 O 3 were found to be 4.96, 5.22–5.28, and 5.28 eV in the approximation of direct optical transitions and 4.27, 4.43, and 5.09 eV, respectively, in the approximation of indirect optical transitions. The films of κ/ε-Ga 2 O 3 and α-Ga 2 O 3 served as effective antireflection coatings on the surface of silicon reducing the reflection down to 0.20% and 0.12%, respectively.