Development of a thin film growth system to produce nanostructures through laser interference

Abstract

This thesis seeks to initiate an innovative process paradigm for the production of dense arrays of identical nanostructures of precise size, shape, and composition by overcoming all the limitations of conventional nanostructuring. The combination of light-based material structuring, with the advantages of a state-of-the-art thin film growth technique, provides a single step, cost-effective and up-to-date capability for next-generation ordered arrays of nanostructures. New methods to achieve such structures are a vital requirement for the exploitation of devices at the nanometer regime. In our approach, we have developed a system that combines interferometric light patterning with Aerosol Assisted Chemical Vapor Deposition (AACVD). To merge these two techniques, a multidisciplinary setup is required. The system is divided into independent modules that have been designed to complete the hardware, which are; pulsed laser, aerosol generator, beam delivery optics, and reactor. Every single subsystem is defined so it can be easily changed to meet the different needs of each of the processes that have been developed. Therefore, different subsystems must be assembled and validated for the three different processes targeted in this thesis. Firstly, for the AACVD of thin films, the reactor chamber and the gas system are integrated. To validate the deposition technique, Zinc Oxide (ZnO) thin films have been grown. The effects of deposition parameters, such as aerosol flow or substrate temperature are studied, showing a wurtzite crystallization in all cases (from 350 ºC to 400 ºC), of which the one with a higher preferential orientation along the c-axis are the grains grown at 375 ºC. From the growth kinetics study, it is extracted that the activation energy of the aerosol assisted chemical reaction is 1.06 eV. Furthermore, ZnO thin films have been optically and electrically characterized. Secondly, nanosecond lasers have been used to assist the chemical reaction of the AACVD. The laser-matter interaction has been studied through two thermal models, one to study the single pulse thermal effect at the nanosecond scale and the second one to study the thermal accumulation produced by the train of pulses. The thermal accumulation results are corroborated by experimental measurements. When including the AACVD technique in the setup, the laser produces local reaction processes that provide energetically favorable sites for the nucleation or structure of the material. Initial experimental results of the performance of this innovative technique are described in which the temperature stability has distinguished itself as the principal technology limiter. Subsequently, precision laser interference optics and state-of-the-art pulsed lasers have been integrated within materials reactors to produce concentrated light patterns with a pitch of fractions of the laser wavelength. Two lasers with different wavelengths (355 nm and 1064 nm) have been used together with two interference optics approaches. With the 355 nm laser, the light pattern induces local photothermal modifications on the grown surface creating nanostructures. The nanostructures show a concordance between theoretical and experimental periods, 792 nm and 800 nm respectively. The dependence of the height of the nanostructures on the number of pulses follows the Marangoni theory developed for this kind of processes. Gold nanostructured thin films have also been achieved using the LI+AACVD technique, completing one-step processing, which supposes an improvement of the previous similar nanocorrugation techniques. Finally, gas sensor devices to detect NO2 have been developed as an application for the nanostructured ZnO thin films. The nanostructured ZnO-based sensors offer several key advantages compared to the only annealed sensors, such as more responsivity, room temperature gas detection, reduction of the baseline resistance, and improvement of NO2 measurements in wet conditions. Therefore, nanostructured ZnO-based sensors are a step forward for the next generations of NO2 gas detectors.