Infrared Spectroscopy Study of Functional Metal Oxides

Overview & Synopsis

Functional metal oxides are an integral part of advanced technologies today and they are the basis for innovation in optical and electronic devices. Functional properties such as ferroelectricity, piezoelectricity and superconductivity are obtained through different compositions, structures and physical forms. Metal oxides deposited as nanometer thin films may play a key role in microelectronics as they allow the miniaturization of the devices. The physico-chemical properties of thin layers can differ greatly from those of bulk, due to epitaxial stress, specific surface effects, chemical composition and confinement, hence the importance of probing them as nanometer thin films. Understanding the implications of such differences is essential to the operation of these systems. The present article thesis aims to couple infrared (IR) spectroscopy and Density Functional Theory (DFT) simulations to probe the various phases in thin layers and to determine the differences with their bulk counterparts. Three families of thin films compounds are studied. (1) Nd₀.₈Sr₀.₂NiO₂: the Nickelate family of superconductors, a current challenge to the scientific community due to its unexplained pairing mechanism and its similarities and differences to the Cuprates. In this paper, optical measurements are used to characterize the sample in the normal state using a multilayer stacking model. In addition, an iterative algorithm based on the Mattis-Bardeen formalism is used to extract the gap energy below Tc; (2) PbZr₁₋ₓTiₓO₃: one of the most relevant piezoelectric materials to this date has its tetragonal to cubic phase transition investigated in this published article. A special emphasis is placed on probing phonon signatures for various thicknesses. Using DFT simulations adapted to reproduce the complexity of solid solutions, the polar modes responsible for the loss of ferroelectricity in the paraelectric cubic phase are identified; (3) Hf₀.₅Zr₀.₅O₂, a promising ultra thin lead-free ferroelectric material for which we performed the first optical investigation on films of various thickness and phases. The optical index for different film thickness and phases were extracted and we verified the gradual loss of the ferroelectric phase as thickness increases. The various phonon structures, assigned using DFT calculations, provide possible IR signatures for ferroelectric and non ferroelectric samples.