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Study on the structural and dielectric properties of epitaxial Barium Strontium Titanate (BST) for Tunable Microwave applications.

Ismail, Marwa Ahmed Mohamed (2015). Study on the structural and dielectric properties of epitaxial Barium Strontium Titanate (BST) for Tunable Microwave applications. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 218 p.

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The future generation of active telecommunication devices may benefit a lot from components based on ferroelectrics. These tunable microwave devices require thin film ferroelectrics with specific material characteristics such as high dielectric constant that allows reducing the size of the devices, high tunability typically more than 50% for few kV.cm-1, low dielectric loss lower than 0.01 in order to minimize signal attenuation, high Figure-of-Merit (FOM) at microwave range of frequencies (300 MHz to 300 GHz). In order to achieve these characteristics, high quality thin films, better understanding of the material properties and improvement of device designs are needed. The challenge in the recent years is the difficulty to obtain thin films with low dielectric loss and high tunability at the same time. This step is crucial for integrating thin film in tunable microwave devices such as phase shifters, tunable resonators, filters, and tunable antenna. The most promising ferroelectric for this kind of applications is Barium Strontium Titanate, Ba0.6Sr0.4TiO3 (BST) that can have a Curie temperature around room temperature and a dielectric constant that can be varied by more than an order of magnitude under an electric field, but that generally exhibit a relatively high level of losses at high frequency. In this thesis high quality of BST thin films were deposited on MgO substrates by mean of pulsed laser deposition (PLD), in order to study the influence of the film microstructure on the microwave characteristics of the material. We investigated the influence of the deposition parameters such as laser fluence, laser repetition rate, substrate temperature, oxygen background pressure, substrate material type and orientation, and BST target composition on the BST thin films characteristics. Base on this first extensive study, we try to optimize the BST thin film properties by using buffer layers and doping with different elements (Mg, Ni, Mo, Fe, and Ti) in order to obtain low dielectric loss (tan δ) and high tunability at the same time. These extended studies yielding to epitaxial, textured, and polycrystalline BST films revealed that there is a strong correlation between the microstructure properties and the microwave properties of BST thin films, that highlights the important influence of the crystal quality and lattice parameter on the microwave characteristics of the films, such correlation found between the lattice parameter and rocking curve FWHM, the rocking curve FWHM and tunability, the lattice parameter and tunability, and dielectric loss and tunability. These observations helped to better understand the correlations between the dielectric characteristics and the material structural properties. The relation between the tunability and the material lattice parameter was found to obey to the Landau- Devonshire theory whereas the dielectric losses are found to be either intrinsically linked to the tunability or influenced by the presence of a large stress gradient at the substrate interface. The optimum conditions for depositing epitaxial BST thin films on MgO (100) substrates were found to be at 10 Hz for laser repetition rate, 1 J/cm2 for laser fluence, 65 mm for target substrate distance, 10 mTorr oxygen pressure, 800°C deposition temperature and post annealing conditions of 1050°C for 3 hours. In these conditions the rocking curve full-width at half-maximum obtained is of ~0.3, one of the best obtained so far for BST thin films. Interestingly, the BST Figure of Merit (FOM, ratio of tunability on the dielectric losses) of epitaxial and polycrystalline films are found to be comparable, due to high level of losses on highly epitaxial films. For example, the film deposited at 500°C is polycrystalline with a low tunability of 20%, low dielectric loss of 0.025, and a FOM of ~7.5; while the best crystal quality film ( of 0.23°) has the highest tunability of 75% and also the highest dielectric loss of 0.1 but it has a FOM of ~7. The BST-0.6 epitaxial thin film characteristics were further improved using specific design of experiments, intended to reduce the level of stress of epitaxial films. Using, for the first time, ion implantation of Mg on epitaxial thin films and post-implantation annealing, the BST Figure of Merit was improved from ~10 to 17 (tunability of 60% and dielectric loss of 0.035 at 3 GHz and 10 V.μm-1). Our results proves that whereas tunability and losses seem to be intrinsically linked for BST alone, ion implantation gives the opportunity to decrease the level of losses while maintaining a relatively high tunability of the material. This work provides a better understanding of the performance of BST material at microwave frequency and will help to reach the targeted ferroelectric properties for its integration into functional tunable microwave devices.

Type de document: Thèse
Directeur de mémoire/thèse: Chaker, Mohamed
Mots-clés libres: couches minces de BST; Barium Strontium Titanate; titanate de baryum et de strontium; substrats de MgO; micro-ondes accordables
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 06 avr. 2016 20:24
Dernière modification: 06 avr. 2016 20:24
URI: http://espace.inrs.ca/id/eprint/3364

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