Tungsten oxide (WO3-x) is a versatile semiconducting material known for its flexible structure and wide-ranging properties, which can be finely tuned through structural manipulation. This thesis explores the multifaceted properties and potential applications of optical-quality WO3-x films, particularly in functional near-infrared transparent-conductive oxide (TCO) and plasmonic devices. High-quality WO3-x films, with thickness ranging from 30 nm to 200 nm, were fabricated by non-reactive RF-sputtering, followed by post-thermal annealing treatments in dry air, N2, and vacuum, spanning temperature from 300 ℃ to 650 ℃, to tailor their structures, stoichiometries and properties. A comprehensive investigation was conducted, encompassing morphological and structural assessments, optical and electrical examinations, and dielectric analyses. Long-range structural characteristics were studied by XRD and Raman spectroscopy, confirming the transition from amorphous to crystalline phases, with evidence of WO3 and WO2 formation, respectively, in air- and vacuum-annealed films. XAS provided insights into the local atomic and electronic structures, revealing distortions in WO6 octahedral units and shifts of orbitals within the W 5d band as a result of annealing. And EDXS analysis provided the relative compositional ratio between O and W, supporting the oxidation or reduction effects of WO3-x films by annealing in different conditions. Optical transparency and electrical conduction were assessed, with notable balance observed in films of 90 nm thick, annealed in air at 300 ℃. The quantitative evaluation on the figure of merit for transparent-conductive performance indicated the superiority of WO3-x films than commercial TCOs, particularly in NIR range. Ellipsometry and FTIR were employed for the extraction of complex dielectric functions over a wide spectral range from ultraviolet to mid-infrared. In particular, innovative and dedicated fitting models were established in Ellipsometry study, accounting for the grading effect along the depth of WO3-x films as a result of non-homogeneous annealing. The successful extraction of complex dielectric functions paves the way for the design and production of plasmonic structures. Electron beam irradiation is prospected as a promising method for controllably writing plasmonic patterns on WO3 with high spatial resolution and flexibility in shape.
L'ossido di tungsteno (WO3-x) è un versatile materiale semiconduttore, noto per il suo polimorfismo e per il suo ampio spettro di proprietà, che possono essere finemente accordate modificando la struttura del materiale. Questa tesi esplora le proprietà e le potenziali applicazioni dei film di WO3-x di qualità ottica, con particolare riguardo all’ impiego come ossido trasparente conduttivo (TCO), funzionale a dispositivi nel vicino infrarosso, e per dispositivi plasmonici. Film di alta qualità di WO3-x, con spessore compreso tra 30 nm e 200 nm, sono stati fabbricati mediante tecnica di deposizione per RF-sputtering non reattivo, seguita da trattamenti termici in aria secca, N2 e vuoto, a temperature comprese tra 300 ℃ e 650 ℃, per variarne la struttura, stechiometria e proprietà. È stata condotta un'indagine esaustiva, comprensiva di valutazioni morfologiche e strutturali, esami ottici ed elettrici, e analisi dielettriche. Le caratteristiche strutturali a lungo raggio sono state studiate mediante XRD e spettroscopia Raman, confermando la transizione dalle fasi amorfe a cristalline, con evidenza della formazione di WO3 e WO2, rispettivamente, nei film trattati in aria e in vuoto. XAS ha fornito approfondimenti sulle strutture atomiche ed elettroniche locali, rivelando distorsioni nelle unità ottaedriche WO6 e spostamenti degli orbitali all'interno della banda W 5d a seguito del trattamento termico. L'analisi EDXS ha fornito il rapporto compositivo relativo tra O e W, supportando gli effetti di ossidazione o riduzione dei film di WO3-x mediante trattamento termico. La trasparenza ottica e la conduzione elettrica sono state valutate, mostrando un interessante valore di compromesso nei film spessi 90 nm, scaldati in aria a 300 ℃. La valutazione quantitativa della figura di merito per le prestazioni trasparenti-conduttive ha indicato la superiorità dei film di WO3-x rispetto ai TCO commerciali, in particolare nell’intorno spettrale NIR. L'Ellissometria spettroscopica e la FTIR sono stati impiegate per ricavare la funzione dielettrica complessa su un'ampia gamma spettrale dall'ultravioletto al medio-infrarosso. In particolare, sono stati sviluppati modelli innovativi e dedicati per interpolare i dati Ellissometrici, tenendo conto di un possibile effetto di gradiente in profondità delle proprietà fisico-chimiche dei film di WO3-x a seguito del riscaldamento. Avere ottenuto i valori della funzione dielettrica complessa apre la strada alla progettazione di strutture plasmoniche in WO3-x. L’irraggiamento a fascio elettronico è considerato un metodo promettente per scrivere in modo controllabile strutture plasmoniche alla superficie di film di WO3-x con alta risoluzione spaziale e flessibilità di forma.
Sub-stoichiometric tungsten oxide thin films : towards infrared transparent-conductive oxides and plasmonics
CHEN, HAO
2023/2024
Abstract
Tungsten oxide (WO3-x) is a versatile semiconducting material known for its flexible structure and wide-ranging properties, which can be finely tuned through structural manipulation. This thesis explores the multifaceted properties and potential applications of optical-quality WO3-x films, particularly in functional near-infrared transparent-conductive oxide (TCO) and plasmonic devices. High-quality WO3-x films, with thickness ranging from 30 nm to 200 nm, were fabricated by non-reactive RF-sputtering, followed by post-thermal annealing treatments in dry air, N2, and vacuum, spanning temperature from 300 ℃ to 650 ℃, to tailor their structures, stoichiometries and properties. A comprehensive investigation was conducted, encompassing morphological and structural assessments, optical and electrical examinations, and dielectric analyses. Long-range structural characteristics were studied by XRD and Raman spectroscopy, confirming the transition from amorphous to crystalline phases, with evidence of WO3 and WO2 formation, respectively, in air- and vacuum-annealed films. XAS provided insights into the local atomic and electronic structures, revealing distortions in WO6 octahedral units and shifts of orbitals within the W 5d band as a result of annealing. And EDXS analysis provided the relative compositional ratio between O and W, supporting the oxidation or reduction effects of WO3-x films by annealing in different conditions. Optical transparency and electrical conduction were assessed, with notable balance observed in films of 90 nm thick, annealed in air at 300 ℃. The quantitative evaluation on the figure of merit for transparent-conductive performance indicated the superiority of WO3-x films than commercial TCOs, particularly in NIR range. Ellipsometry and FTIR were employed for the extraction of complex dielectric functions over a wide spectral range from ultraviolet to mid-infrared. In particular, innovative and dedicated fitting models were established in Ellipsometry study, accounting for the grading effect along the depth of WO3-x films as a result of non-homogeneous annealing. The successful extraction of complex dielectric functions paves the way for the design and production of plasmonic structures. Electron beam irradiation is prospected as a promising method for controllably writing plasmonic patterns on WO3 with high spatial resolution and flexibility in shape.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/220794