Highly conductive and stable Co₉S₈ thin films by atomic layer deposition: from process development and film...

Co₉S₈ is an interesting sulfide material with metallic conductivity that has shown promise for various energy applications. Herein, we report a new atomic layer deposition process producing crystalline, pure, and highly conductive Co₉S₈ thin films using CoCl₂(TMEDA) (TMEDA = N,N,N',N'-tetramethylethylenediamine) and H₂S as precursors at 180-300 °C. The lowest resistivity of 80 μΩ cm, best uniformity, and highest growth rate are achieved at 275 °C. Area-selective deposition is enabled by inherent substrate-dependency of film nucleation. We show that a continuous and conductive Co₉S₈ film can be prepared on oxide-covered silicon without any growth on Si-H. Besides silicon, Co₉S₈ films can be grown on a variety of substrates. The first example of an epitaxial Co₉S₈ film is shown using a GaN substrate. The Co₉S₈ films are stable up to 750 °C in N₂, 400 °C in forming gas, and 225 °C in O₂ atmosphere. The reported ALD process offers a scalable and cost-effective route to high-quality Co₉S₈ films, which are of interest for applications ranging from electrocatalysis and rechargeable batteries to metal barrier and liner layers in microelectronics and beyond.