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

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 N₂ 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 N₂. The discrepancy is primarily due to different fates of oxygen vacancies resulting from different calcination atmospheres.