Substrate dependent growth behaviors of plasma-enhanced atomic layer deposited nickel oxide films for resistive switching application

In this study, NiO thin films were deposited via a plasma-enhanced atomic layer deposition (PEALD) on metal (Pt, Ru, and W) substrates using a bis-methylcyclopentadienyl-nickel ([MeCp]₂Ni) precursor followed by a reaction with plasma-enhanced oxygen gas. The ALD temperature regime of NiO films was defined between 150 and 250 C, while substrate temperature higher than this region induced the thermal cracking of precursors. The saturated PEALD rates of NiO film on Pt, Ru, and W substrates were 0.48, 0.58, and 0.84 Å/cycle, respectively, even though it has been usually regarded that the substrate effect on the saturated ALD rate vanishes after covering the entire surface with the growing films. At the initial stage of film growth, the NiO film showed enhanced nucleation behavior on the W and Ru substrates, whereas it did not show enhanced growth behavior on the Pt substrate. X-ray photoelectron spectroscopy revealed that the surface of a NiO film, which is thick enough for the W substrate not to influence the analysis, contains WO₃ bonding states while the films grown on other metal substrates did not show any oxidation states of the substrate metal species. This could be due to the fact that the diatomic bond strength of W-O is stronger than that of Ni-O, which may induce the layer inversion during the ALD of NiO on the W substrate, and the surface W-O promotes the surface chemical reaction. This can result in the eventual increase of the saturated growth rate even in the ALD mode. The supply of oxygen to the adsorbing Ni-precursor by the reduction of a previously oxidized Ru substrate enhanced the initial growth rate of NiO film but this does not affect the steady-state growth rate on the Ru substrate. The small lattice mismatch between the NiO and Pt, as well as the identical crystal structure of the two materials results in the local epitaxial growth of NiO film on Pt substrate even though the growth temperature was only 250 C. The NiO films on the W substrate showed reliable bipolar resistance switching in a wide temperature range (25-100 C), which provides new opportunities for the next generation nonvolatile memory applications.