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Stacks of alternating conductive and non-conductive oxides studied by ellipsometry
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High-quality video-rate holography is a logic evolution of display technology that requests an extraordinary technological effort. Due to this challenge, this has not been demonstrated yet. For this realization it is necessary to vary the refractive index within a waveguide with high precision, subwavelength spatial resolution and GHz frequency. Within this thesis, the development of a metamaterial for the cladding of such a holographic waveguide was the target. This material is supposed to consist of non-optically absorbing materials where an electrical conductor is laterally separated by an insulating material. In our chosen set of materials, we investigated Gallium Indium Zinc Oxide (IGZO), Indium-Tin-Oxide (ITO) and Indium Tin Zinc Oxide (ITZO) as transparent conductive oxides and Silicon Nitride (Si3N4) and Silicon Oxynitride (SiOxNy) as insulating oxide (IO).
IGZO, a transparent semiconductor oxide (TSO), appeared recently as new material for flexible and transparent thin film transistors. The good electrical conductivity together with high transparency in the visible region (> 80% optical transmittance) at room temperature makes this material a very attractive candidate. ITO is one of the most important transparent conductor oxides (TCOs) that exists. It is applied in different technologies due to its high electrical conductivity and high optical transparency. ITZO has been a promising replacement for ITO as the TCO layer in many opto-electronic applications. The insulator Si3N4 is widely used as a passivation layer, a barrier, or as a masking layer to prevent oxidation or diffusion in underlying materials in patterned areas, and as a final protection layer to finished devices because of its radiation resistance and hardness. Another candidate, SiOxNy, is a material that has been under investigation because it can be considered an intermediate between Si3N4 and SiO2 in terms of physical and chemical properties. Thus, is considered very good for electro-optics and organic light-emitting devices as its properties can be tuned by varying the ratio of the oxide and nitride content.
In this work, we start with describing the production techniques used. All the TSOs were deposited by Direct Current Sputtering, that is a high vacuum physical vapor deposition (PVD) technology. Inductively coupled plasma chemical vapor deposition (ICP-CVD) was the technique used for the deposition of IOs. With the help of ellipsometry it was also possible to determine the optical constants and the thickness of them. Thus, we began by analysing these parameters and, when necessary, altering them in order to approach the main requirements of this project. Finally, several comparative studies on optical properties were carried out to find applicable combinations between the two types of oxide. It was therefore concluded that IGZO is the preferred candidate in combination with SiN or SiON. because of their similar optical constants for certain wavelengths.
In addition, the reliability and sensitivity of ellipsometry were studied to see if this could be used in this application, or whether it was necessary to investigate another method capable of detecting the low optical extinction coefficients of the materials of interest.
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Video-rate holography Refractive index Transparent conductive oxides Insulating oxide Optical properties