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

Abstract

Cobalt oxide (CoOx) 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.