Phase-controlled growth of cobalt oxide thin films by atomic layer deposition

Cobalt oxide (CoO_x) thin films were deposited on thermally grown SiO₂ substrates by atomic layer deposition (ALD) using bis(1,4-di-iso-propyl-1,4-diazabutadiene)cobalt (C₁₆H₃₂N₄Co) and oxygen (O₂) as reactants at deposition temperatures ranging from 125 to 300 °C. X-ray diffraction (XRD) and Raman spectroscopic analysis indicated that a mixed-phase oxide consisting of CoO and Co₃O₄ was deposited at temperatures ranging from 125 to 250 °C. However, single-phase Co₃O₄ was deposited above the deposition temperature of 275 °C. Further, analyses by Rutherford backscattering spectrometry, transmission electron microscopy, and selected area electron diffraction along with XRD and Raman spectroscopy revealed that the single-phase cobalt oxide film was stoichiometric crystalline (spinel structure) with negligible N and C impurities. The optical band gap of the single-phase Co₃O₄ film was 1.98 eV and increased with decreasing deposition temperature. It was also shown that the mixed-phase cobalt oxide thin films could be converted into single-phase spinel Co₃O₄ by annealing at 350 °C in O₂ ambient. It was further observed that the phase of the ALD-grown cobalt oxide thin film could be controlled by controlling the precursor or reactant pulsing condition. The study revealed that pure Co₃O₄ phase could be grown at a relatively low temperature (250 °C) by using water vapor as a reactant. Therefore, this work systemically demonstrated several pathways to grow single-phase Co₃O₄ by ALD using a novel metalorganic cobalt precursor.