TiO2-SiO2 mixed oxide deposited by low pressure PECVD: Insights on optical and nanoscale electrical properties

Abstract

TixSi1-xO2 thin films appear as promising materials to replace SiO2 for DRAM devices. For the effective down scaling, these mixed oxides should combine the best properties from SiO2 (low leakage current) and TiO2 (high dielectric constant). In this study, TixSi1-xO2 thin films were deposited by low pressure PECVD. Varying the Titanium Tetra-Isopropoxide (TTIP) and Hexamethyldisiloxane (HMDSO) precursors flow rate allows us to obtain nanostructured films with tunable properties. Their surface chemical composition was assessed by X-ray Photoelectron Spectroscopy (XPS). Atomic Force Microscopy (AFM) revealed that the surface roughness increases with the Ti content. Spectroscopic Ellipsometry (SE) showed that a small amount of Ti is enough to induce a decrease of the energy gap and an increase of the refractive index, while preserving a low surface roughness. Since these mixed oxide films exhibit nanodomains with three different environments with randomly distributed Si-O-Ti bonds, SiO2-like and TiO2-like bonds, electrical properties are investigated at nanoscale in terms of dielectric permittivity, charge injection, trapping and transport via electrical modes derived from AFM as Electrostatic Force Microscopy (EFM), Kelvin Force Probe Microscopy (KPFM) and Conductive AFM (C-AFM). Despite a chemical composition evolving at the nanoscale, this work highlights that the electrical behavior can be considered as homogeneous (at the AFM resolution scale). A small amount of Ti induces a decrease in the amount of trapped electrons due to charge transport increase and a strong decrease of trapped holes. Moreover, after injection, the rate of charge decay increases with the Ti content. These features were interpreted on the basis of the energy diagrams deduced from the Valence Band measurement highlighting the n-type semiconducting character of the Ti-rich films. As a conclusion, TixSi1-xO2 films with x = 0.33 allows the best compromise in terms of dielectric permittivity improvement, charge injection and transport behavior.