Influence of interface layer on optical properties of sub-20 nm-thick TiO₂ films

The sub-20 nm ultrathin titanium dioxide (TiO₂) films with tunable thickness were deposited on Si substrates by atomic layer deposition (ALD). The structural and optical properties were acquired by transmission electron microscopy, atomic force microscopy and spectroscopic ellipsometry. Afterwards, a constructive and effective method of analyzing interfaces by applying two different optical models consisting of air/TiO₂/Ti_xSi_yO₂/Si and air/effective TiO₂ layer/Si, respectively, was proposed to investigate the influence of interface layer (IL) on the analysis of optical constants and the determination of band gap of TiO₂ ultrathin films. It was found that two factors including optical constants and changing components of the nonstoichiometric IL could contribute to the extent of the influence. Furthermore, the investigated TiO₂ ultrathin films of 600 ALD cycles were selected and then annealed at the temperature range of 400-900 °C by rapid thermal annealing. Thicker IL and phase transition cause the variation of optical properties of TiO₂ films after annealing and a shorter electron relaxation time reveals the strengthened electron-electron and electron-phonon interactions in the TiO₂ ultrathin films at high temperature. The as-obtained results in this paper will play a role in other studies of high dielectric constants materials grown on Si substrates and in the applications of next generation metal-oxide-semiconductor devices.