Studies of ferroelectrics films using micro-Raman spectroscopy and photoluminescence measurements

Abstract

Ferroelectric materials have attracted considerable interests from perspectives of nonvolatile memories, piezoelectric sensors/actuators, infrared detectors and optoelectronic devices etc., due to their unique electrical and optical properties. Three kinds of ferroelectric materials were selected in this thesis: 1) SrxBa1-xNb2O6 (SBN) which possesses tetragonal tungsten bronze (TTB) structure and have been be used as pyroelectric infrared detectors, electro-optics and photorefractive devices; 2) BiFeO3 (BFO), one of the well-known multiferroics, exhibits coexistence of ferroelectric and antiferromagnetic ordering above room temperature and has attracted great attention due to its potentials for novel devices as well as material for fundamental physics investigation; and 3) K0.5Na0.5NbO3 (KNN), one of the most promising lead-free alternatives to Pb(ZrxTi1-x)O3(PZT)-based compositions due to its high piezoelectric coefficient. Rare-earth (from La3+ to Yb3+), alkaline earth (Ca2+), and alkali elements (Na+, K+) modified SEN ceramics were prepared and investigated to assess their potential for pyroelectric applications. Thin films of the same compositions were epitaxially grown on single crystal MgO (001) substrates using pulsed laser deposition (PLD) method. Temperature dependent (30 oC ~ 400 oC) unpolarized micro-Raman spectroscopy and low-temperature (12K) photoluminescence (PL) measurements were carried out to study the stress states, ferroelectric phase transitions, and energy structures of the thin films. The PL emission band (~2.1eV) observed in bulk SBN ceramics shifted to higher frequencies (~2.4eV) and was greatly suppressed in the thin film form. The background noises from the substrate and the laser source made the observation difficult. The stress in thin films was evaluated qualitatively by Raman spectroscopy. Behavior of the Raman spectra around phase transition temperature was found to be dependent on the nature of the ferroelectrics, i.e. 'relaxor' or 'normal' ferroelectrics. Pure and rare-earth (Sm3+, Eu3+, Gd3+, and Dy3+)-doped BFO thin films were expitaxially grown on STO(001) and conductive Nb-doped STO(001) (NSTO) single crystal substrates by PLD method. Two broad PL emission bands at ~3.0eV and ~2.3eV were observed. The former was found to be related with bandgap transition; while the latter was shown to be sensitive to compositions and material form (thin film or ceramic). The origin of the latter was believed to be arisen from oxygen vacancies. Emission lines from the rare-earth ions were shown to be less dependent on the host ferroelectric material. Lead-free piezoelectric (K0.5Na0.5)0.96Li0.04(Nb0.8Ta0.2)O3 thin films were prepared by PLD method on MgO (001) and conductive NSTO (001) single crystal substrates. Around the phase transition temperatures, 300 oC for cubic-to-tetragonal transition and 120 oC for tetragonal-to-orthorhombic transition, the Raman spectra exhibited obvious anomalous behaviors. The PL spectra of the ceramic samples presented one broad peak located at ~2.2eV. In thin film form, the emission band was suppressed. SBN-based composite thin films of SBN+BaxSr1-xTiO3 and SBN+LiTaO3 were prepared in order to study the effects of B-site dopants on the bandgap energies, phase transitions and dielectric properties of the doped SEN thin films. BFO-based composite (SBN+BFO) and multilayered (KNSBN+BFO) thin films were also fabricated by PLD method. They were characterized by ferroelectric and magnetic properties measurements, UV-Vis transmittance measurements, temperature dependent Raman spectroscopy and PL studies. Temperature dependent Raman spectra were shown to be valuable in investigating the interactions between different ferroelectric materials in these artificial structures. All these composite/multilayered films were shown to be of great interest and value to the materials community.