Reaction and Growth Mechanisms in Al₂O₃ deposited via Atomic Layer Deposition: Elucidating the Hydrogen Source

Abstract

In this work, we have quantitatively elucidated the source of the hydrogen content in the atomic layer deposition of Al2O3 at different temperatures (80–220 °C), by replacing the H2O precursor with heavy water (D2O) to use as a tracer and discern between the H coming from the unreacted metal precursor ligands and that from the unreacted −OD (hydroxyl) groups coming from the (heavy) water. The main source of impurities arises from the unreacted hydroxyl groups (−OD), reaching ∼18 atom % of deuterium at a deposition temperature of 80 °C. Reconsidering carefully our own and literature experimental data, we concluded that the generally accepted mechanism of steric hindering by monodentate Al­(CH3)2 adsorbates (dimethylaluminum) cannot be solely responsible for the retention of hydroxyls during atomic layer deposition (ALD). On this regard, we propose two additional mechanisms that can lead to sterically hinder hydroxyl groups which will then remain unreacted in the film: surface rehydroxylation resulting in the reconfiguration of bidentate or tridentate adsorbates into monodentate adsorbates and hindered subsurface hydroxyl groups during the (heavy) water pulse and the hydroxylation of sterically hindered dissociated methyl chemisorbed species. Based on these three steric hindrance mechanisms, we constructed a growth model that consists of the initial chemisorption configurations of trimethyl-aluminum molecules with the alumina surface and the subsequent reconfiguration of the resulting adsorbates into a monodentate configuration that consequently leads to sterically hindered hydroxyl groups. The fraction of AlOx adsorbates arranged in monodentate and bidentate configurations entails a specific number of O/Al atoms and unreacted hydroxyl groups inside the film. This model was able to explain the deuterium content, the O/Al ratio, and the density obtained from Rutherford back-scattering and heavy ion elastic recoil detection analysis measurements. Furthermore, this model was able to predict more accurately the growth per cycle to what has been reported to be the ALD window of alumina. Our findings will spur further detailed investigations of the reaction and growth modes in ALD films.

Controlled deposition of titanium oxide overcoats by non-hydrolytic sol gel for improved catalyst selectivity and stability

“…as the amount of Ti precursor required to saturate the surface. Similarly to ALD, the first monolayer will not cover the surface with one continuous layer of TiO 2 due to the space left behind by the precursor ligands [51]. In contrast, precursor dosing according to OH density, as performed by Yan et al [30] resulted in undesired excess TiO 2 formation and poor deposition control.…”

Suppressing the Photocatalytic Activity of TiO2 Nanoparticles by Extremely Thin Al2O3 Films Grown by Gas-Phase Deposition at Ambient Conditions

“…As this number is not reasonable for the assumption that all the –OH groups are on the surface of the powder (i.e., it is theoretically and practically impossible to have 95 OH –groups on 1 nm 2 ), the calculated values suggest that most of the –OH groups are located inside the coating layer. This is caused by the incomplete consumption of –OH groups, which leads to hydrogen impurity in the film [66]. The existence of –OH strongly affects the density of Al 2 O 3 [61].…”

“…Nevertheless, the experimental data obtained by Ylivaara et al have demonstrated that the low-temperature deposition of Al 2 O 3 using TMA and H 2 O is inherently associated with a considerable amount of impurities, especially hydrogen and carbon, which increase with decreasing deposition temperature [74]. The hydrogen impurity arises from the unreacted hydroxyl groups, which has recently been verified by Guerra-Nunez et al [66]. The degree of the GPC decrease at low temperatures is not well determined, and strongly depends on experimental conditions.…”

“…The reaction mechanism in ALD of Al 2 O 3 using TMA and H 2 O has been intensively investigated in the past decades, both theoretically and experimentally [21,56,57,58,59,60,61,62,63,64,65,66,67]. Accordingly, the surface chemical reactions that lead to the deposition of Al 2 O 3 in the ideal case can be divided into two half-reactions.…”

Functional Metal Oxides in Perovskite Solar Cells

“…Al 2 O 3 is the most used TFE material that can protect against water and oxygen. [65] The 20 nm intermediate Al 2 O 3 layer reacts with water molecules and slowly produces a more compact Al 2 O 3 layer. Chang et al fabricated a 50 nm ALD Al 2 O 3 -coated PET layer as the encapsulation layer for PSCs, which significantly improved the environmental stability of the device.…”