Thin Films of TiO2 by MOCVD : Precursor Development, Film Growth and Microstructure, Optical and Electrical Properties

Abstract

TiO2 is a well-known and very attractive material owing to its chemical stability, biocompatibility, and remarkable electrical and optical properties. Indeed, it exhibits a high dielectric constant and resistivity, a high refractive index, and a good optical transparency over a wide spectral range, as well as a relatively high hardness. As a result, TiO2 thin films have several applications like electric insulators, antireflective layers for optical devices and protective coatings. TiO2 films have also been considered as future candidates to the high-k gate dielectric application in microelectronic transistor devices. Furthermore, TiO2-anatase is a well-known photocatalyst which exhibit a band gap (Eg=3.2 eV). Owing to these properties TiO2 is a major candidate in new devices, including gas sensors and supported photocatalysts used for the decontamination and purification of environmental pollutants.

The work focuses on growing and characterizing the properties of thin films of TiO2 via METAL- ORGANIC chemical vapor deposition (MOCVD) with its potential exploits to Devices as high-k dielectric application. An effort was also made in the direction of doping the TiO2 with Cobalt as TiO2 thin films have also been observed to exhibit dilute magnetism when doped with certain transition elements.

A novel liquid precursor for TiO2 was synthesized in this work and characterized for its suitability for MOCVD. A comparison was made with a solid powder precursor material, of which the liquid precursor (Ti-complex developed in this work) is suspected to be a derivative. FT-IR spectroscopy and CHN analysis were performed and a probable structure was suggested, the verification of which needs further investigation. The compatibility of the liquid Ti-complex was adjudged with an already reported Cobalt precursor for the purpose of growing Cobalt doped Titanium Oxide.

The developed precursor was successfully employed to MOCVD of TiO2 thin films. The TiO2 thin films grown by MOCVD in this work were characterized for surface morphology using SEM and AFM. The phase identification was done by grazing angle X-ray diffraction method. The cross- section TEM was also performed for interface analysis which revealed the grain size in nanometer range. The cross-section transmission electron microscopy (TEM)also showed the presence of interfacial SiO2 layer. The films were optically characterized by UV-spectroscopy to understand the effect of reduction in grain size on the optical band-gap energy. FT-IR spectroscopy was also performed on the grown films as a part of the investigation.

Electrical measurements were also carried out in the MOS configuration to extract the dielectric constant and understand the current leakage mechanism involved for the TiO2 thin films grown in this work. The growth process was modified to reduce the thickness of the interfacial layer which indeed resulted in increase in the effective dielectric constant of the grown TiO2 thin film. The current voltage relationships were analyzed to understand the effect of growth process modification on the leakage of current through the films under bias.