The effect of nanocrystallite size in monoclinic HfO2 films on lattice expansion and near-edge optical absorption

Abstract

Nanocrystalline monoclinic HfO2 films were sputter deposited on fused silica substrates, air annealed at 573 to 1273 K to affect crystallite growth, and analyzed by x-ray diffraction and spectrophotometry. Lattice expansion occurs with diminishing crystallite size. O 2p→Hf 5d interband absorption dominates the optical edge at energy E≥6.24 eV, with an optical band gap, Eo=5.48±0.023, which is independent of crystallite size. However, the strength of a localized resonant band, with onset at 5.65 eV and maximum at 5.94 eV, is affected by crystallite size. Its polaronic origin in a perfect HfO2 lattice is discussed.