Transparent Conducting Oxides (TCOs) are wide band-gap semiconductors that have, after heavy doping, good electrical conductivity properties combined with high optical transparency in the visible range. New generation photovoltaic devices use these materials mainly as transparent electrodes but the request for cheaper and low environmental impact solutions moved the attention of research to new TCOs like Aluminum-doped Zinc Oxide (AZO). In this work correlation between morphology, structure and functional properties of this material have been studied for nanostructured films deposited using Pulsed Laser Deposition (PLD) that allows to obtain samples with different structure (i.e. doping, defects, nanoscale crystallinity, mesoscale morphology) simply changing oxygen pressure in the chamber during deposition. In order to better investigate the structure-properties relation and in view of an improvement of the functional properties, selected samples have been thermally annealed in high vacuum or argon atmosphere in order to induce crystalline changes in the material structure in the absence of oxygen, since oxygen vacancies play a fundamental role in determining functional properties. Morphology and structure were investigated with Scanning Electron Microscopy, X-Ray Diffraction and Raman spectroscopy while functional properties have been studied using Van der Pauw method (resistivity), Hall effect (mobility and carrier density) and a NIR-Vis-UV absorption spectroscopy (optical). Strong correlation between these properties was observed and some hypothesis to discuss the results are presented.
Gli ossidi trasparenti conduttivi (TCO) sono semiconduttori ad ampio gap che, dopo forte drogaggio, presentano buone proprietà di conduzione elettrica insieme ad un’alta trasparenza ottica nel visibile. I dispositivi fotovoltaici di nuova generazione usano questi materiali principalmente come elettrodi trasparenti ma la richiesta di soluzioni più economiche ed a a basso impatto ambientale muovono l’attenzione della ricerca verso nuovi TCO come l’ossido di zinco drogato con alluminio (AZO). In questo lavoro sono state studiate le correlazioni tra morfologia, struttura e proprietà funzionali di questo materiale per film nanostrutturati depositati tramite Pulsed Laser Deposition (PLD), la quale permette di ottenere campioni con strutture differenti (drogaggio, difetti, cristallinità alla nanoscala, morfologia alla mesoscala) semplicemente cambiando la pressione di ossigeno in camera durante la deposizione. Per meglio investigare le relazioni fra struttura e proprietà ed in vista di un miglioramento delle proprietà funzionali, alcuni campioni selezionati sono stati sottoposti ad un trattamento termico di annealing in alto vuoto o in atmosfera di argon in modo da indurre cambiamenti nella cristallinità in assenza di ossigeno in quanto le vacanze anioniche giocano un ruolo fondamentale nel determinare le proprietà funzionali. Morfologia e struttura sono state investigate usando un microscopio a scansione elettronica (SEM), diffrazione di raggi X (XRD) e spettroscopia Raman mentre le proprietà funzionali sono state studiate utilizzando il metodo di Van der Pauw (per la resistività), l’effetto Hall (per la mobilità e la densità di portatori di carica) e la spettroscopia in assorbimento NIR-vis-UV (per le proprietà ottiche). Sono state trovate forti correlazioni fra queste proprietà e saranno presentate alcune ipotesi per discutere i risultati.
Correlation between morphological, structural and functional properties of nanostructured aluminum doped zinc oxide films
DIANI, MARCO
2012/2013
Abstract
Transparent Conducting Oxides (TCOs) are wide band-gap semiconductors that have, after heavy doping, good electrical conductivity properties combined with high optical transparency in the visible range. New generation photovoltaic devices use these materials mainly as transparent electrodes but the request for cheaper and low environmental impact solutions moved the attention of research to new TCOs like Aluminum-doped Zinc Oxide (AZO). In this work correlation between morphology, structure and functional properties of this material have been studied for nanostructured films deposited using Pulsed Laser Deposition (PLD) that allows to obtain samples with different structure (i.e. doping, defects, nanoscale crystallinity, mesoscale morphology) simply changing oxygen pressure in the chamber during deposition. In order to better investigate the structure-properties relation and in view of an improvement of the functional properties, selected samples have been thermally annealed in high vacuum or argon atmosphere in order to induce crystalline changes in the material structure in the absence of oxygen, since oxygen vacancies play a fundamental role in determining functional properties. Morphology and structure were investigated with Scanning Electron Microscopy, X-Ray Diffraction and Raman spectroscopy while functional properties have been studied using Van der Pauw method (resistivity), Hall effect (mobility and carrier density) and a NIR-Vis-UV absorption spectroscopy (optical). Strong correlation between these properties was observed and some hypothesis to discuss the results are presented.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/81131