|Tipo di tesi||Tesi di dottorato di ricerca|
|Titolo||Film di TiO2 drogati con Nb e preparati mediante DC-magnetron sputtering ad alta velocità di deposizione|
|Titolo in inglese||Nb-doped TiO2 films prepared by high deposition rate DC-magnetron sputtering|
|Settore scientifico disciplinare||FIS/01 - FISICA SPERIMENTALE|
|Corso di studi||PHYSICS AND NANO SCIENCES|
|Data inizio appello||2017-03-02|
|Disponibilità||Accessibile via web (tutti i file della tesi sono accessibili)|
L’ossido di titanio (TiO2) in forma di anatasio drogato con Nb, risulta essere un valido candidato per sostituire l’ossido di indio drogato con stagno (ITO) e l’ossido di stagno drogato con fluoro (FTO), che nella forma di elettrodo trasparente vengono usati in diverse applicazioni tecnologiche come: monitor a schermo piatto, celle solari in silicio sottile amorfo o nelle celle a colorante (cioè celle di Grätzel).
Nb-doped TiO2 in anatase form (TNO) is a viable candidate for the substitution of indium-tin-oxide (ITO), and fluorine-tin-oxide (FTO) as a transparent electrode in several technological applications like flat panel displays, thin film amorphous silicon in solar cells or dye-sensitized solar cells (i.e. Grätzel cells). In this thesis work, transparent and conducting Nb-doped anatase TiO2 films (TNO) have been produced at high deposition rate (tens of nm per minute) by reactive DC-magnetron sputtering from metal targets (Ti and Nb). High deposition rate was obtained by an active control of the oxygen gas flow in order to stabilize the poisoning of the targets surfaces. Low resistivity (10-3 Ωcm range) Nb-doped TiO2 thin films have been obtained after thermal treatment (450–600 °C) in reductive atmosphere (high vacuum). The deposition and post deposition parameters were changed to produce films with different proprieties as it concerns doping, structure, phases stability, thickness and electrical resistivity. The exposure of the films to ambient air condition, has been found to play a crucial role in determining the electrical resistivity of the material. In particular, air exposure in the tens of hours range had a dramatic effect on the film resistance. The resistivity of the film increased from 1 to 4 order of magnitude with a power-law behaviour and it was influenced by structure and the thickness of the film. In order to understand the increasing of the electrical resistivity, TNOs were studied from chemical and structural point of view. X-ray Photoelectron Spectroscopy (XPS) analysis showed a re-oxidation of the surface of films after ambient exposure. However the cathodoluminescence (CL) measurements revealed that the stoichiometry of the films in the bulk was not changed upon air exposure. XPS and CL data demonstrated that re-oxidation takes places in the first surface layers of the films and therefore the resistivity of the films do not substantially increase upon re-oxidation. After re-oxidation, scanning electron microscope (SEM) imaging revealed the presence of fractures, which extend through the whole films thickness. The number and size of the fractures are related to the films structure and exposure time of TNO in environmental condition.