Mechanistic study of the atomic layer deposition of scandium oxide films using Sc(MeCp)₂(Me₂pz) and ozone

The atomic layer deposition (ALD) of scandium oxide (Sc₂O₃) thin films is investigated using Sc(MeCp)₂(Me₂pz) (1, MeCp = methylcyclopentadienyl, Me₂pz = 3,5-dimethylpyrazolate) and ozone on hydroxyl-terminated oxidized Si(111) substrates at 225 and 275 °C. In situ Fourier transform infrared spectroscopy reveals that 1 not only reacts with surface hydroxyl groups at 275 °C, as expected but also with the SiO₂ layer, as evidenced by losses in the SiO₂ longitudinal optical and transverse optical phonon modes, resulting in the partial transformation of near-surface SiO₂ to an ScSi_xO_y interface layer. Ozone then combusts the MeCp groups of the O–Sc(MeCp)₂ chemisorbed species, yielding surface carbonates, and oxidizes some of the underlying silicon, evidenced by gains in the SiO₂ phonon modes. The Me₂pz group from the next pulse of 1 reacts with these surface carbonates, leading to Sc–O–Sc bond formation (Sc₂O₃ deposition) and the restoration of an O–Sc(MeCp)₂ surface. The reaction of the SiO₂ substrate with 1 and the oxidation of silicon by ozone are temperature-dependent processes that occur during the initial cycles of film growth and directly impact the changes in the intensities of the SiO₂ phonon modes. For instance, the intensity of the net gains in the phonon modes following ozone exposure is greater at 275 °C than at 225 °C. As the ALD cycle is repeated, the formation of an ScSi_xO_y interface layer and deposition of an Sc₂O₃ film result in the gradual attenuation of the reaction of the SiO₂ substrate with 1 and the oxidation of the underlying silicon by ozone. In addition to the ALD process, characterized by ligand exchange and self-limiting reactions, there are gas-phase reactions between 1 and residual water vapor near the substrate surface that lead to deposition of additional Sc₂O₃ and surface carbonates, the extent of which are also dependent on the temperature of the substrate. After 20 cycles of 1/ozone, the film thicknesses derived from ex situ X-ray photoelectron spectroscopy measurements are 2.18 nm (225 °C) and 3.88 nm (275 °C). This work constitutes the first mechanistic study of an Sc₂O₃ ALD process using ozone as the oxidant and emphasizes the significance of atypical reactions between the substrate and the reactants that influence the growth rate and near-surface stoichiometry during the initial cycles of film deposition.