MnO x deposits on porous gas diffusion layer (GDL) substrates for application as catalysts for the oxygen reduction reaction (ORR) in metal−air batteries have been prepared using atomic layer deposition (ALD). The saturation behavior of the bis(ethylcyclopentiadienyl) manganese ((EtCp) 2 Mn) and H 2 O ALD system has been investigated. The observed saturation behavior is in disagreement with previous reports in literature. (EtCp) 2 Mn + H 2 O depositions exhibited non-ALD behavior, as demonstrated by a high growth per cycle (GPC) at substrate temperatures (T sub ) = 40−50 °C and nonsaturating reactions at T sub ≥ 60 °C. The introduction of a forming gas (FG) (95% N 2 + 5% H 2 ) plasma between the (EtCp) 2 Mn and H 2 O doses promoted precursor saturation with a constant GPC (1.15 Å/cy) in the T sub range of 100−200 °C. The effect of saturation behavior on porosity coverage was investigated by coating porous carbon electrodes with ALD MnO x . Energy dispersive X-ray (EDX) spectroscopy, electrochemical surface area measurements, and oxygen reduction activity all indicate that the saturating behavior of the (EtCp) 2 Mn + FG + H 2 O deposition resulted in superior coverage compared with the (EtCp) 2 Mn + H 2 O depositions.
Atomic layer deposition (ALD) surface reactions are comprised of several elementary surface interactions (such as physisorption, desorption, and chemisorption) occurring at the substrate. Since ALD processes are often far from thermodynamic equilibrium, the surface saturation behavior is controlled by the kinetics of these involved interactions. In this article, we present a first-order kinetic model for ALD reaction, to simulate the cumulative effect of precursor exposure (tA), post-precursor purge (tP1), reactant exposure (tB), post-reactant purge (tP2), and substrate temperature (Tsub) on the resulting growth per cycle (GPC) in an ABAB… pulsed ALD process. Furthermore, to simulate the effect of inadequate reactor purges (tP1, and/or tP2) and undesired non-ALD side reactions, reaction pathways to account excess GPC are also taken into consideration. From our model calculations, we simulate GPC vs Tsub trends observed in ALD growth experiments and demonstrate that the process temperature window (ΔTALD) for a constant GPC depends upon the deposition cycle parameters tA, tP1, tB, and tP2. The modeled GPC vs Tsub trends are discussed and compared with SiNx, ZrN, and ZnO PEALD growth experiments.
Although atomic layer deposition (ALD) of ZnO using diethyl zinc (DEZ) precursor has been extensively reported, variation in growth-per-cycle (GPC) values and the range of substrate temperature (Tsub) for ALD growth between related studies remain unexplained. For identical processes, GPC for the characteristic self-limiting ALD growth is expected to be comparable. Hence, a significant variation in GPC among published ZnO ALD studies strongly suggests a concealed non-ALD growth component. To investigate this, the authors report plasma-enhanced ALD growth of ZnO using DEZ precursor and O2 inductively coupled plasma. The effect of Tsub on ZnO GPC was studied with deposition cycles (1) 0.02 s–15 s–6 s–15 s, (2) 0.10 s–15 s–15 s–15 s, and (3) 0.20 s–15 s–30 s–15 s, where the cycle parameters t1–t2–t3–t4 denote duration of DEZ pulse, post-DEZ purge, plasma exposure, and postplasma purge, respectively. The non-ALD growth characteristics observed at Tsub ≥ 60 °C are discussed and attributed to DEZ precursor decomposition. The authors demonstrate ZnO growth at Tsub = 50 °C to be self-limiting with respect to both t1 and t3 giving GPC of 0.101 ± 0.001 nm/cycle. The effect of precursor decomposition related (non-ALD) growth at Tsub ≥ 60 °C is illustrated from comparison of optical dielectric function, electrical resistivity, and surface roughness of ZnO films deposited at Tsub = 50, 125, and 200 °C.
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