Structural, mechanical, and optical properties of atomic layer deposited (Cr_1−xAl_x)₂O₃ films on sapphire substrates

Structural, mechanical, and optical properties of atomic layer deposited (ALD) chromium oxide (Cr₂O₃), aluminum oxide, and ternary chromium–aluminum oxide (Cr_1−xAl_x)₂O₃ thin films on sapphire substrates were investigated. Variation of Al content in the ternary films was achieved by changing the Cr(thd)₃ + O₃ subcycle number (z) in the ALD supercycle formula z (Cr(thd)₃ + O₃) + (TMA + O₃). X-ray absorption spectroscopy confirmed the presence of Cr³⁺ in octahedral coordination across all compositions of films on c-plane sapphire. The films were epitaxially oriented on sapphire, and the (0001) [112̄0] film ‖ (0001) [112;̄0] c-plane Al₂O₃ epitaxial relationship of the grown film with the substrate was confirmed by high-resolution φ-scan and in-plane XRD measurement. Mechanical characterization exhibited a peak hardness of ∼28 GPa for the films deposited by ALD supercycle with z = 38 and an estimated relative ratio of cations Al/(Cr + Al) = 0.50, attributed to the formation of a composite structure. The optical properties displayed dependence on the chemical composition of the films, where both the indirect transition band gap (2.9 eV) and the direct transition band gap (3.7 eV) in pure Cr₂O₃ increased to 3.75 eV and 4.25 eV, respectively, for the film deposited by the ALD supercycle of 38 (Cr(thd)₃ + O₃) + (TMA + O₃).