Interband transitions in sol-gel-derived ZrO2 films under different calcination conditions

Abstract

Different types of interband transitions as well as the microstructures and O/Zr ratios in sol-gel-derived ZrO2 films calcined at elevated temperatures in air or N-2 were systematically examined to clarify the formation of different band structures in the structural ZrO2 films. Optical absorptions indicate that the structural ZrO2 films all contain direct-band transitions, while the occurrence of indirect-band and band-tail transitions, depends on O/Zr ratios. Band tails mainly result from imperfect structures in the grain boundaries and appear in the ZrO2 films with O/Zr >= 2. When the ZrO2 films are non-stoichiometric (O/Zr < 2), indirect-band transitions occur due to the folded bands caused by lattice oxygen vacancies. Monoclinic ZrO2 has two direct-band transitions with bandgaps of 5.02-5.08 and 5.83-6.01 eV. The valence-band XPS spectra indicate that an additional occupied sub-band is located above its valence band. Indirect bandgaps in the monoclinic ZrO2 films are in the range of 5.00-5.10 eV. Amorphous ZrO2 films exhibit indirect-band transitions with bandgaps of 5.01-5.47 eV followed by direct-band transitions with bandgaps of 5.90-6.12 eV. In addition, tetragonal ZrO2 contains direct bandgaps of 5.32-5.74 eV and indirect bandgaps of 4.72-5.40 eV. Size-dependent bandgaps are observed in the sol-gel-derived ZrO2 films calcined in air, while the exponential dependence of bandgaps on O/Zr ratios is obtained in the ZrO2 films calcined in N-2. The discrepancy is primarily due to different fates of oxygen vacancies resulting from different calcination atmospheres.