The effect of interface layers and doping on the multiferroic properties of bismuth titanate oxide thin films on silicon

Abstract

For the development of magnetoelectric memory devices, the development of artificial magnetoelectric materials is necessary. Previous work in the field had focused on the multiferroic properties of this material in bulk form, however, for a practical device, a thin film investigation is needed. This thesis details the author's investigation into the multiferroic properties of bismuth titanate oxide, Bi4Th012, (BTO) thin films doped with lanthanum, niobium, iron and cobalt, to form the novel material Bi3.2slao.7sLTb.sNbo.2sFeo.mCoo.m012 (BTFC), and how different interface layers influence their structural and electronic properties when they are deposited on silicon substrates. The aim of this research is to achieve a multiferroic BTO thin film with strong ferroelectric and ferromagnetic properties by utilising a novel combination of dopants. Chapter 1is the introduction, providing insight into the layout and structure of the thesis making it easier for the reader to follow. Chapter 2 is the background literature review, which discusses the real life industrial demands of this work, and is followed by a discussion of the physics and material science principles behind this investigation. Chapter 3 is the experimental literature review, in which the material synthesis, device fabrication and characterisation techniques utilised in this work are discussed. The experimental chapters begins with Chapter 4, in which the effect of a zinc oxide (ZnO) interface layer on crystalline properties of BTFC on silicon is investigated. Chapter 5 follows this investigation up by investigating ferroelectric propertied of BTFC thin films on silicon with ZnO interface layers. Finally chapter 6 looks at BTFC thin films on silicon substrates with platinum interface layers, and both simultaneous ferroelectric and ferromagnetic properties were present, confirming multiferroic behaviour. Initial investigations into the deposition onto silicon were unsuccessful, but were overcome by utilising ZnO interface layers. The ZnO interface layer eliminated some critical difficulties; however high resolution transmission electron microscopy (HRTEM) analysis showed zinc atoms from inside the interface layer diffusing into the BTFC thin film. The investigation into the ferroelectric properties of thin films using the triangular voltage waveform method confirmed ferroelectric domain switching, but were inhibited by the need for a vacuum annealing environment to prevent the oxidation of the silicon substrate. This resulted in the generation of oxygen vacancies within the BTFC thin film, which limited driving voltage during the measurement, presenting ferroelectric saturation. The final investigation was focused on BTFC deposited on platinised-silicon substrates. Platinum is proved to be the superior interface layer, due to its chemical and thermal stability. The investigation found high quality crystalline BTFC, with a high dielectric constant and leakage current, which can be attributed to the doping effect. The ferroelectric measurements demonstrated a fully saturated ferroelectric loop, and a remnant polarisation and coercivity of 2Pr = 11.03!lC/cm 2, and 2Ec = 196.5kV/em on the optimised thin films. Ferromagnetic measurements of the sample were challenging, due to the small total magnetisation of the thin film resulting from its low volume and mass. Using a Superconducting Quantum User Interface Device {SQUID) vibrating sample microscopy {VSM), Ferromagnetisum was presented in the bulk and thin film form, however, the remnant magnetisation of the thin film could not be determined, due to its noise level value. This discovery proves the existence of simultaneous ferroelectric and ferromagnetic phases in BTFC thin film, confirming muitiferroic behaviour of the deposited thin films had been achieved with the chosen dopants.