First Principles Study of Reactions in Hybrid Film Growth: The Role of the Organic Precursor

Abstract

Hybrid materials are a special class of materials with tunable features. This is related to their diphasic structures that leads to multifunctional materials with a wide range of applications. Molecular layer deposition (MLD) enables the fabrication of ultrathin and conformal hybrid films using sequential, selflimiting reactions, similar to ALD. Despite growing interest in these materials, there is little understanding of the atomistic mechanism of MLD film growth.In this contribution, we explore the molecular mechanisms of the growth of different hybrid organic−inorganic thin films by using density functional theory (DFT). The prototype for MLD organic−inorganic hybrids are composed of thin films of aluminium alkoxides, known as ”alucones”. We investigate the MLD reaction products between the post-TMA Monomethyl-Al2O3 (Al-CH3-Al2O3) and Dimethyl-Al2O3 (Al(CH3)2-Al2O3) surface and the organic precursors ethylene glycol (EG) and glycerol (GL). The DFT calculations show that while the organic precursors can bind to the TMA fragments via formation of Al-O bonds and loss of CH4, it is most favorable for the organic precursors to lie flat and create so-called double reactions through two terminal hydroxyl groups with the surface fragments, where the terminal groups bind to Al. For EG this potentially removes all the active sites and growth will be less favorable. For GL the third hydroxyl group is available and growth can proceed. We also showed that the TMA in the next pulse reacts favourably with this OH from GL.We also explored the growth mechanism of magnesium containing hybrid films known as “magnesicone” from the reaction of EG and GL at MgCp-terminated MgO(100) as a model system. Interaction energies presented some clear differences in reaction mechanism of films grown with EG and GL. While the ligand elimination process is favorable for both precursors, EG prefers to orient in a flat configuration and interacts at the MgO(100) surface and GL species prefers to lie in an upright position. As a consequence the change in thickness for GL-based process is larger than for EG-based process, resulting in a thicker GL-based magnesicone compared to the EG-based magnesicone.Finally we use DFT to explore the growth mechanism of titanium-containing hybrid organic−inorganic films known as “titanicones”. We investigate the reaction mechanism of titanicones grown using titanium tetrachloride (TiCl4) as metal source and either ethylene glycol (EG) or glycerol (GL) as organic reactants. This investigation contributes to the understanding of growth process of EG and GL based hybrid films at the molecular level and is valuable in supporting experimental data on hybrid film growth.