The thickness-dependent band gap and defect features of ultrathin ZrO2 films studied by spectroscopic ellipsometry

Abstract

The band gap and defect features of ultrathin ZrO₂ films with varying thicknesses have been investigated by spectroscopic ellipsometry through the point-by-point data inversion method. The ε₂-sprectra in the 3–6 eV range are extracted based on an optical model consisting of a Si substrate/effective ZrO₂ film/air ambient structure where the effective ZrO₂ film is a combination of interfacial layers and ZrO₂. Evident widening of the band gap with a reducing size is observed when the effective ZrO₂ films are below a critical thickness, somewhere between 8.80 nm and 17.13 nm. This is due to quantum-confinement and amorphous effects. Moreover, the sub-band-gap defects at interfacial layers and in bulk ZrO₂ are identified and present strong thickness dependence as well. The interfacial defects at 3.26, 4.13, 4.43, and 4.77 eV mainly exist below the critical thickness and exhibit a significant suppression with increasing film thickness. The bulk defects at 4.15 eV and 4.46 eV dominate in ZrO₂ films once they are over the critical thickness. The evolution of the band gap and defects is closely related to variance in the electronic structure of amorphous ZrO₂. Our results may be helpful in understanding controversial problems concerning the size effect on ultrathin high-k oxide films and exploring the further miniaturization of electronic devices based on them.