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Comparative Growth and Functional Integration of CeO2 Films via Plasma-Enhanced and Thermal ALD Using a Tailored
Yewon Seo1, Debananda Mohapatra1,2, Sola Moon1
1Graduate School of Semiconductor Materials and Devices Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulju-gun, Ulsan 44919, Republic of Korea.
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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 heteroleptic amidinate framework liquid precursor, bis-(n-propylcyclopentadienyl)-(N,N'- diisopropylpropionamidinato)-Ce-(III)-[(n-PrCp)2(iPr2-pamd)-Ce-(III)], via thermal ALD (Th-ALD) and plasma-enhanced ALD (PE-ALD) processes with O2 and O2 plasma coreactants, respectively. Both methods were optimized at 200 °C, achieving growth per cycle values of 1.7 Å/cycle (Th-ALD) and 1.2 Å/cycle (PE-ALD), which revealed striking contrasts in film properties. PE-ALD produced highly crystalline cubic 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. Notably, the PE-ALD-CeO2 device achieved a high dynamic range (a resistance ratio of 7.6 × 102), underscoring the importance of oxygen-vacancy-mediated analog synaptic weight updates via plasma-activated ALD for reliable artificial synapse functionality. 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.
