Reaction Mechanisms of Non-hydrolytic Atomic Layer Deposition of Al₂O₃ with a Series of Alcohol Oxidants

Abstract

Atomic layer deposition (ALD) of Al 2 O 3 using trimethylaluminum (TMA) and H 2 O is the most widely and deeply studied ALD process owing to the superior properties of the deposited Al 2 O 3 thin films and usability of TMA and H 2 O. However, H 2 O can cause undesirable substrate oxidation during ALD. While previous studies have shown that alcohol oxidants can be used to deposit Al 2 O 3 thin films with less substrate oxidation, the reaction mechanism of ALD Al 2 O 3 with alcohol oxidants has not been elucidated yet. In this study, the reaction mechanism of ALD of Al 2 O 3 thin films using various alcohol oxidants was systematically investigated by computational and experimental methods. Various possible reaction pathways are considered for the oxidation of Al−CH 3 with methanol (MeOH), ethanol (EtOH), and propanol (n-PrOH). It is found that the feasible reaction mechanism for removal of the surface-adsorbed alkoxy group is autocatalytic liberation of alkene through β-hydrogen transfer. ALD processes were developed using the alcohol oxidants. Our process using EtOH showed a growth rate of 0.96 Å/cycle and a moderate level of carbon impurities (2.6%). In addition, we investigated the properties of ALD-deposited Al 2 O 3 thin films with alcohol oxidants, which indicated superior electrical properties and decreased formation of interfacial oxide on the Si substrate.

Rational Molecular Design for Non-aqueous Atomic Layer Deposition of Zinc Oxide

“…Reaction 3 that produces ether via α-C coupling shows E a values greater than those for reaction 2, of 47.86 kcal/mol for EtOH and 40.15 kcal/mol for 3B2O. Therefore, reaction 2, which releases alkene and alcohol as gaseous byproducts, can be considered as a major reaction path for saturated alcohols with a β-H atom, similar to previously reported Al 2 O 3 ALD …”

“…Then, a surface alkoxy (Zn-OR*) is formed, which needs to be removed from the substrate to deposit carbon-free oxide films. Alcohols are known to be dehydrated by various heterogeneous catalysts to produce alkene, aldehyde or ether, whose activation energy is strongly dependent on the molecular structure of the alcohol species. − In our previous work on alcohol-based ALD of Al 2 O 3 using ethanol or propanol, it was found that there were several possible reaction pathways for conversion of surface alkoxy to hydroxyl (OH*) . Some of the reactions proposed in the study are as follows (Figure a).…”

“…Also, for these reactants, the reaction 2 serves as the rate-determining step (RDS) since E a for this step is greater than that of reaction 0 (initial adsorption of alcohol on Zn–Et*). It is noted that alcohols without a β-C (methanol and its derivates) are not considered in this study since they can only undergo reaction 1 with large E a so that their reactivity toward oxide ALD would be low …”

“…However, due to high reactivity of the oxygen-containing reactants, it has been difficult to apply ZnO ALD onto chemically sensitive substrate materials. For example, ZnO can be used as a charge transport layer in photovoltaic cells; however, for hybrid organic–inorganic perovskites, usage of water during ALD may irreversibly damage the substrate and degrade the device performance. , The reactants may also oxidize the surface of the substrate to form an interfacial oxide, which can cause leakage current and degrade device performance. , In general, the ALD growth behavior and the physicochemical properties of the resulting oxide films are highly dependent to the type of the oxygen source. ,− The oxygen vacancies on the ZnO thin film in the ALD process can be controlled by changing oxygen reactants, which can directly affect the electrical properties of the thin films . Therefore, it is desirable to develop novel alternative reactants with controllable reactivity in order to expand ZnO ALD to various wider applications.…”

“…Previously, ALD of oxide thin films using metal alkoxides as an alternative oxygen source were reported. , However, zinc alkoxides are often non-volatile and thus inappropriate for vacuum deposition . On the other hand, for ALD of other oxides such as Al 2 O 3 or ZrO 2 , it is known that alcohols can be used as a chemically milder oxygen source. − For example, the presence of a beta-hydrogen (β-H) in the molecular structure of the alcohol was found to be crucial for reactivity as an ALD reactant . Recently, ZnO ALD using DEZ with organic 1,5-pentanediol as an alternative oxygen source has been reported, in contrast to shorter diols such as 1,2-ethanediol which yields thin films of organic–inorganic hybrid “zincone”, indicating strong dependence of the deposition chemistry on the exact molecular structure of the alcohol reactant.…”