Cobalt oxide is of interest for applications in catalysis, energy storage, and metal/insulator/metal (MIM) tunnel diodes. Atomic layer deposition (ALD), based on purge-separated self-limiting surface reactions between precursors and oxidizing agents, enables precisely controlled deposition of highly conformal thin films. Although there have been a handful of reports of ALD of CoO_x using various metal precursors and oxidizing agents, to date there have been no systematic studies of electrical performance for ALD cobalt oxide and even reports of refractive index are sparse. There is also a lack of agreement in the literature on the fundamental band gap. In this work, the structural, optical, and electrical properties of a low temperature ALD cobalt oxide are investigated as-deposited and after post-deposition annealing.

CoO_x thin films were deposited on p- and n-type silicon substrates via ALD. ALD was performed at 250 °C in a Picosun Sunale R-150 reactor using N₂-purge separated cycles of biscyclopentadienyl-cobalt (Co(Cp)₂) held at 80 ^oC, and a ~10% O₃/O₂ mixture generated with an IN-USA AC2025 ozone generator [1, 2]. CoO_x films were characterized using variable angle spectroscopic ellipsometry (VASE), grazing-incident x-ray diffraction (GIXRD), x-ray reflectivity (XRR), and atomic force microscopy (AFM). Electrical test structures were formed by evaporating either Co or Al metal through a shadow mask to form top electrode areas of ~0.05 mm².

X-ray and optical analysis indicate that as-deposited cobalt oxide on silicon substrates is Co₃O₄. Several GIXRD peaks corresponding to Co₃O₄ phase orientations are found (Fig. 1) and are consistent with previously reported spectra [1, 2]. Additionally, VASE analysis (Fig. 2) yields a refractive index of ~2.8 at a wavelength of 632.8 nm, close to that of bulk Co₃O₄ and an earlier report of ALD Co₃O₄ [3]. AFM (Fig. 3) indicates that the films have an RMS roughness of ~0.4 nm, much smoother than earlier reports [1, 2]. High current density at 1 MV/cm of 1.2 mA/cm² for a Co/Co₃O₄/n-Si device and 16.3 mA/cm² for an Al/Co₃O₄/n-Si device is consistent with the semiconducting (p-type) nature of Co₃O₄. The effects of post deposition annealing in a forming gas (H₂/N₂) environment are expected to reduce the Co₃O₄ to CoO, which is expected to be the higher bandgap phase of cobalt oxide. Electrical characterization will be performed on post-deposition annealed MOS and MIM devices in an attempt to observe the transition to the insulating phase. Additionally, a discrepancy in the reported temperature versus growth behavior of the Co(Cp)₂ / O₃ process [1,2] will be compared to the ALD window measured in this work.

[1] M. Diskus, O. Nilsen, and H. Fjellvåg, Chemical Vapor Deposition, vol. 17, no. 4–6, pp. 135–140, Jun. 2011.

[2] B. Huang, K. Cao, X. Liu, L. Qian, B. Shan, and R. Chen, RSC Adv., vol. 5, no. 88, pp. 71816–71823, 2015.

[3] B. Han, K. Ha Choi, J. Min Park, J. Woo Park, J. Jung, and W.-J. Lee, JVST A, vol. 31, no. 1, p. 01A145, 2013.