Comparative Study on the CeO2 Thin Film Prepared by Plasma-Enhanced and Thermal Atomic Layer Deposition with a New Liquid Ce Precursor for the Switching Material of the Artificial Synaptic Devices

Abstract

The development of high-performance switching cerium dioxide (CeO2) thin films is critical for advancing neuromorphic computing technologies, where atomic layer deposition (ALD) offers unparalleled control over the conformality, stoichiometry, and microstructure of oxide thin films. Here, we report the CeO2 deposition using a newly developed homoleptic amidinate framework liquid precursor, bis(n-propylcyclopentadienyl)cerium(III)N,N′-diisopropylacetamidinate [Ce(nPrCp)2(N- iPr-amd)], via thermal ALD (Th-ALD) and plasma-enhanced ALD (PE-ALD) processes with O2 and O2 plasma co-reactants, respectively. Both methods were optimized at 200 °C, achieving growth per cycle values of 1.7 Å/cycle (Th-ALD) and 1.3 Å/cycle (PE-ALD), which revealed striking contrasts in film properties. PE-ALD produced highly crystalline CeO2 with larger grains (~7 nm), higher density (~7.1 g/cm3), and greater surface roughness (~1.1 nm), while Th-ALD yielded nanocrystalline, smoother, and less dense films. X-ray photoelectron spectroscopy confirmed near-stoichiometric Ce1O2.1 composition without detectable impurities for PE-ALD, whereas Th-ALD films were oxygen- deficient (Ce1O0.8) and carbon-contaminated. Optical analysis revealed a refractive index of 2.5 and a well-defined bandgap (3.2 eV) for PE-ALD films, compared to 1.8 for Th-ALD. Despite reduced step coverage (~56%) on high-aspect-ratio features compared to Th-ALD (excellent conformality, ~100%), only PE-ALD-CeO2 enabled analog resistive switching in Pt/ CeO2/Pt devices, demonstrating synaptic behavior essential for neuromorphic computing. These findings reveal how precursor chemistry and plasma processes govern film quality and synaptic behavior, offering a scalable route to oxide-based artificial synaptic devices.