Optical absorption at its onset in sputter deposited hafnia–titania nanolaminates

Abstract

The onset of the fundamental optical absorption edge in sputter deposited HfO2–TiO2 nanolaminate films grown on unheated substrates was investigated. Three bilayer architectures were examined, representing overall film chemistry from 0.51 to 0.72 atom fraction Hf. The goal was to determine the absorption coefficient, α(E), versus incident photon energy, E, and to model this dependence in terms of the absorption behavior of specific functional units within the nanolaminate. Persistence and amalgamation models were applied, representing the extremes of segregated cation and mixed cation structures, respectively, and both were found to be unsatisfactory. Consideration of physiochemical data for the nanolaminates led to the development of a modified persistence model for absorption. α(E) was decomposed into contributions from (I) broad (9 nm-thick) interfacial regions that were chiefly o-HfTiO4, and (II) regions remote from interfaces that contained material based on a highly defective m-HfO2 lattice. The absorption edge at its onset in all nanolaminates was determined by short-range atomic order characteristic of o-HfTiO4. An indirect band gap of EG=3.25±0.02 eV was determined for this compound.