Optical characterization of nanowires for optoelectronic devices and energy harvesting

Resumo

This thesis presents a detailed investigation of the optical properties of Bi2Te3, GaN, InN, InP and GaN/AlN nanowires grown by different techniques for their application on future optoelectronic and thermoelectric devices. The thesis also includes a prototype of hybrid solar cell. The outline of the thesis is organized as follows: Chapter 1 presents a bird's eye view of the content of the thesis. Chapter 2 reviews basic notions of epitaxial growth, fundamentals in the synthesis of the nanowires and experimental techniques used for characterization and their underlying theory. Although several characterization techniques are employed in this work, Raman spectroscopy is applied as the major research method for this thesis. Concepts of Raman scattering theory as well as the mainly used Raman setups are presented. Chapter 3 reports the growth and optical characterization of long single GaN nanowires. Surface modes characteristics of these nanostructures are identified by Raman spectroscopy. On the other hand, it explains the appearance of extra peaks on Bi2Te3 tiny nanowires, which are erroneously assigned in the literature as infrared (IR) activated forbidden Raman modes. Stochiometric and non-stochiometric Bi2Te3 nanowires ensembles are characterized. Chapter 4 presents Resonance Raman Spectroscopy (RRS) of ultrathin GaN and GaN/AlN nanowires in order to obtain information of the electronic structure and polarity of the material. In the experimental characterization of the ensembles, anomalous results appears; e.g. supplementary peaks attributed to forbidden silent mode and huge Raman intensities. In order to shed some light into the physical origin of these observations DFT calculations of the lattice dynamics in GaN were discussed. A thorough analysis of the different physical mechanisms allowing the forbidden mode to appear is carried out for understanding the physics behind the experimental results. Chapter 5 shows the optical characterization of individual InN and InP nanowires. In this chapter these nanowires are localized in a marked substrate and investigated by means of Raman and SERS. In order to increase the Raman signal from SERS measurements, gold nano-branched particles onto the nanowires are deposited. The signal enhancement in the Raman spectra plus the possibility of accessing to forbbidden modes by SERS technique are discussed. In Chapter 6, a novel prototype of hybrid solar cell using ZnO nanowires is performed. This Chapter shows the growth of ZnO nanowires by Chemical Vapour Deposition, their optical characterization and the fabrication of the photovoltaic cell.Finally, the major results of this thesis are summarized in Chapter 7.