Integrating ALD with Anion Exchange Chemistry to Tune p-type CuO_xS_y Semiconductors with Atomic Precision

To enable p-type metal oxide semiconductors that are competitive with their n-type counterparts, numerous material systems and synthesis strategies have been explored. Cuprous oxide (Cu₂O) has been identified as a promising p-type oxide material for functional devices due to its high mobility (&gt; 100 cm²/Vs^-1)¹ and moderate direct bandgap (2.1-2.6 eV).^1,2 However, Cu₂O often suffers from interfacial defects in solar cells, high contact resistance in thin film transistors, and poor stability in photoelectrochemical cells. Therefore, new synthesis strategies for high-quality p-type Cu₂O requires atomically-precise control of film composition, structure, and morphology. Among the various deposition methods, atomic layer deposition (ALD) facilitates conformal deposition with sub-nm precision in thickness and composition, allowing for atomically-precise engineering of surface and interfacial properties.<br/><br/>We have previously introduced a new process for plasma-enhanced ALD (PE-ALD) of CuO_x films with tunable phase, oxidation state, and morphology.³ This is facilitated by alternating exposures of oxygen and hydrogen plasma, which allows for sequential oxidation and reduction half-reactions, respectively. The resulting p-type Cu₂O films were incorporated into bottom-gate TFTs, and on/off current ratios of ∼10⁵ were achieved for the films with the largest Cu(I) fraction and largest grain size.<br/><br/>One way to tune the structural, optical and electronic properties of semiconductors is through alloying.⁴ Here, we introduce a solution anion exchange step after the ALD process, which facilitates substitutional incorporation of sulfur into the oxygen sublattice. This novel approach to fabricate oxysulfide films allows for tunable stoichiometry and phase, while circumventing the need for flammable and toxic H₂S gas in the ALD process. By varying the solution molarity and time, the sulfur incorporation in the CuO_xfilms were tracked using both grazing-incidence XRD and XPS depth profiling. To describe this ion exchange process, a diffusion model was developed, which enables the prediction and control of the sulfur profile within the film. In addition to compositional control, the film morphology was also observed to evolve as a function of solution concentration, time, and temperature, which was quantified using AFM.<br/><br/>To demonstrate the power of this integrated PE-ALD + sulfur anion exchange process, conformal coating of 3-D nanowire architectures was demonstrated, generating core-shell structures with tunable shell thickness, composition, and structure. Furthermore, area-selective anion exchange was demonstrated, illustrating the potential of this simple and scalable process to pattern surface and interfacial chemistry in layered devices. Finally, electrical measurements after sulfur anion exchange chemistry indicate a reduction in the sheet resistance of the starting Cu₂O phase by five orders-of-magnitude, signifying the potential of this integrated approach to tune interfacial resistance in electronic devices.<br/><br/>Works Cited:<br/><br/>1. Z. Wang, P.K. Nayak, J.A. Caraveo-Frescas, and H.N. Alshareef, Advanced Materials <b>28</b>, 3831 (2016).<br/>2. Y. Wang, P. Miska, D. Pilloud, D. Horwat, F. Mücklich, and J.F. Pierson, Journal of Applied Physics <b>115</b>, 073505 (2014).<br/>3. Lenef, Julia D., Jaesung Jo, Orlando Trejo, David J. Mandia, Rebecca L. Peterson, and Neil P. Dasgupta. The Journal of Physical Chemistry C <b>125,</b> 9383 (2021).<br/>4. Stevanović, Vladan, Andriy Zakutayev, and Stephan Lany. Physical Review Applied <b>2</b>, 044005 (2014).