Amido-Amine Co(II) Precursor-Based Atomic/Molecular Layer Deposition Processes for Cobalt-Organic Thin Films and Their Thermal Conversion to CoO Thin Films

Atomic/molecular layer deposition (ALD/MLD) offers a comprehensive process and application portfolio for metal–organic thin films; however, ALD/MLD process development for transition-metal-based materials remains very limited, despite the versatile functional properties of their compounds. In this work, to enrich the chemistry of transition metal precursors in ALD/MLD, an all-nitrogen-coordinated cobalt complex, Co(tmsaedma)₂ (tmsaedma = Bis(N,N-dimethyl(N’-trimethylsilyl)ethane-1-amino-2-amido), was employed as the metal precursor for the first time. The Co-N coordination provides an optimal reactive site for a variety of organic linker groups, as demonstrated here by three organic precursors that share the same rigid benzene backbone but differ in reactive groups: 1,4-benzenediol (hydroquinone; HQ), 1,4-benzenedithiol (BDT), and 1,4-benzenedicarboxylic acid (terephthalic acid; BDC). A comprehensive set of characterization techniques, combined with first principles density functional theory (DFT) calculations, is used to systematically investigate the three new ALD/MLD processes and the stability of the resulting Co(II)-organic thin films: Co-HQ, Co-BDT, and Co-BDC. The reactivity and stability trends of the organics are found as BDC>HQ>BDT and BDC>>BDT>>HQ, respectively. Decomposition mechanisms are provided for Co-HQ and Co-BDT. Furthermore, the preparation of low-density, porous CoO thin films with tunable structural and optical properties, difficult to achieve otherwise, is demonstrated via calcination in N₂ of the Co-BDC thin films.