Molecular Layer Deposition of Titanicone Films using TiCl₄ and Ethylene Glycol or Glycerol: Growth and Properties

Abstract

Molecular layer deposition (MLD) techniques were used to grow titanium-containing hybrid organic–inorganic films known as “titanicones” using titanium tetrachloride (TiCl4) and either ethylene glycol (EG) or glycerol (GL). The surface chemistry for titanicone MLD was self-limiting versus TiCl4 and either EG or GL exposures. Quartz crystal microbalance (QCM) measurements observed a film growth rate of ∼83 ng/cm2/cycle using TiCl4 and EG from 90 to 115 °C. The growth rate then decreased significantly at 135 °C. X-ray reflectivity (XRR) studies yielded a growth rate of ∼4.5 Å/cycle with a constant density of ∼1.8 g/cm3 from 90 to 115 °C. The growth rate measured using XRR also decreased to 1.5 Å/cycle at 135 °C. Titanicone films were grown using TiCl4 and GL at higher temperatures between 130 and 210 °C. GL should increase the bridging between the polymer chains in the titanicone film and change film properties and improve film stability. The film growth rates decreased with temperature from 49 ng/cm2/cycle at 130 °C to 34 ng/cm2/cycle at 210 °C. XRR studies were consistent with a temperature-dependent film growth and measured growth rates of 2.8 Å/cycle at 130 °C and 2.1 Å/cycle at 210 °C. Nanoindentation experiments revealed that the elastic modulus and hardness of the titanicone films grown using GL were much higher than titanicone films grown using EG. Annealing the titanicone films to 600 °C in air removed the carbon constituents and yielded TiO2 films with a density of ∼3.3 g/cm3 that is slightly higher than the density of TiO2 ALD films grown at 115 °C. The titanicone films absorbed light in the ultraviolet, and the absorption threshold was consistent with an optical bandgap of ∼3.6 eV. Prolonged ultraviolet exposures on the titanicone films produced TiO2 films with a low density of 2.7 g/cm3.