Microstructural control of epitaxial alfa-quartz films.

Abstract

Epitaxial films of piezoelectric α-quartz could enable the fabrication of sensors with unprecedented sensitivity for prospective applications in electronics, biology and medicine. However, a prerequisite is harnessing the crystallization of epitaxial α-quartz, tailoring suitable film microstructures for nanostructuration. In my PhD work, we bring new insights into the crystallization of epitaxial α-quartz films on Silicon(100) from the thermal devitrification of nanoporous silica and the control the film microstructures: We show that by increasing the quantity of devitrifying agent (Sr) it is possible to switch from an α-quartz microstructure consisting of porous flat film to one dominated by larger and fully dense α-quartz crystals. The mechanism of Sr-assisted devitrification was also investigated simultaneously. Then, we found that film thickness, annealing temperature, relative humidity and the nature of surfactant also play an important role in the control of the microstructure and homogeneity of the films. By increasing its, thickness it is possible to switch from a partly crystalline and porous film to fully a crystalline and dominated by dense crystals one. Annealing temperature can impact the crystallization process very deeply for it can change the dynamics and reactivity of Sr within the silica film. High relative humidity cooperates with a suitable surfactant to create perforations on the films via a water-induced phase separation phenomenon. This perforation can also influence the film crystallinity by altering the distribution of Sr inside film. All these studies on the one hand give us a better understanding of the mechanism of Sr-assisted devitrification and on the other hand can show us a versatile microstructural control of the epitaxial α-quartz film. Besides, via a multilayer deposition method, we have extended the maximum thickness of the α-quartz films from a few hundreds of nm into the µm range. Moreover, in my thesis, we report unprecedented large-scale fabrications of ordered arrays of piezoelectric epitaxial α-quartz nanostructures on silicon(100) substrates by the combination of three cost-effective lithographic techniques: (i) laser transfer lithography, (ii) soft nanoimprint lithography on Sr-doped SiO2 sol-gel thin films and (iii) self-assembled SrCO3 nanoparticles reactive nanomasks. Epitaxial α-quartz nanopillars with different diameters (down to 50 nm) and heights (up to 2000 nm) were obtained for the first time. This part of my PhD work demonstrates the control over the shape, micro- and nano-patterning of α-quartz thin films while preserving its crystallinity, texture and piezoelectricity, which opens the opportunity to fabricate new high frequency resonators and high sensitivity sensors relevant in different fields of application.

Date of Award	4 Oct 2019
Original language	English
Supervisor	Carretero Genevrier Adrian (Director), Marti Gich Garcia (Director) & Lluis Casas Duocastella (Tutor)