Influence of the Al 2 O 3 partial-monolayer number on the crystallization mechanism of TiO 2 in ALD TiO 2 /Al 2 O 3 nanolaminates and its impact on the material properties

TiO 2 /Al 2 O 3 nanolaminates are being investigated to obtain unique materials with chemical, physical, optical, electrical and mechanical properties for a broad range of applications that include electronic and energy storage devices. Here, we discuss the properties of TiO 2 /Al 2 O 3 nanolaminate structures constructed on silicon (1 0 0) and glass substrates using atomic layer deposition (ALD) by alternatively depositing a TiO 2 sublayer and Al 2 O 3 partial-monolayer using TTIP–H 2 O and TMA–H 2 O precursors, respectively. The Al 2 O 3 is formed by a single TMA–H 2 O cycle, so it is a partial-monolayer because of steric hindrance of the precursors, while the TiO 2 sublayer is formed by several TTIP–H 2 O cycles. Overall, each nanolaminate incorporates a certain number of Al 2 O 3 partial-monolayers with this number varying from 10–90 in the TiO 2 /Al 2 O 3 nanolaminate grown during 2700 total reaction cycles of TiO 2 at a temperature of 250 °C. The fundamental properties of the TiO 2 /Al 2 O 3 nanolaminates, namely film thickness, chemical composition, microstructure and morphology were examined in order to better understand the influence of the number of Al 2 O 3 partial-monolayers on the crystallization mechanism of TiO 2 . In addition, some optical, electrical and mechanical properties were determined and correlated with fundamental characteristics. The results show clearly the effect of Al 2 O 3 partial-monolayers as an internal barrier, which promotes structural inhomogeneity in the film and influences the fundamental properties of the nanolaminate. These properties are correlated with gas phase analysis that evidenced the poisoning effect of trimethylaluminum (TMA) pulse during the TiO 2 layer growth, perturbing the growth per cycle and consequently the overall film thickness. It was shown that the changes in the fundamental properties of TiO 2 /Al 2 O 3 nanolaminates had little influence on optical properties such as band gap and transmittance. However, in contrast, electrical properties as resistivity and mechanical properties as hardness and elastic modulus were shown to be very dependent. From these analyses, several applications could be suggested for different kinds of nanolaminates obtained in this work.