Projects / Programmes
Oxide-based components for transparent electronics
| Code |
Science |
Field |
Subfield |
| 2.09.01 |
Engineering sciences and technologies |
Electronic components and technologies |
Materials for electronic components |
| Code |
Science |
Field |
| P250 |
Natural sciences and mathematics |
Condensed matter: structure, thermal and mechanical properties, crystallography, phase equilibria |
| Code |
Science |
Field |
| 2.02 |
Engineering and Technology |
Electrical engineering, Electronic engineering, Information engineering |
transparent conducting oxides, high-K dielectrics, thin films, patterning, transparent electronics
Researchers (19)
Organisations (2)
Abstract
Transparent electronics for everyday applications should contain invisible electronic circuitry deposited on transparent and in some cases also flexible substrates. Transparent Conductive Oxides (TCOs) are the materials, which possess two usually contradicting properties, namely a reasonable electrical conductivity and a high optical transmittance. Transparent electronic components on glass substrates, such as transparent capacitors or transparent Thin Film Transistors (TFTs) have been predominantly realized by physical vapor deposition routes, such as sputtering and further patterning. The values of charge carrier mobility in amorphous films, prepared by physical vapor deposition routes, are comparable to the values obtained in crystalline films, enabling the processing of oxide semiconductors at low temperatures, even below 200/250 oC.
The goal of the project is to realize transparent electronic components, such as transparent capacitors and TFTs from solution routes without any need for lithography steps, that is, with direct patterning and to correlate the chemical composition and the conditions of the processing, the chemical and phase composition, and microstructure, and the electrical and optical properties.
The studied n-type materials will mainly include In-free compositions, based on doped ZnO or other mixed oxides, such as ZnO-SnO2. The established In-based materials, such as ZnO-In2O3 or ZnO-In2O3-Ga2O3 will be used as reference. The high-K dielectrics will be based on Ta2O5-SiO2-Al2O3.
The reagents and the conditions of the synthesis of solution precursors will be designed with the aim of obtaining a high level of chemical homogeneity. When considering the selection of ligands and/or solvents, we should keep in mind, that for spin coating and ink-jet printing quite opposite properties of the liquids are needed. The solutions for spin-coating should have a low viscosity (a few mPa.s) and a low wetting angle on a selected substrate, while the solutions for ink-jet printing, or inks should have the viscosity between 10 - 12 mPa.s, surface tension between 28 – 32 mN/m and a high enough wetting angle on a selected substrate to keep the required pattern.
In both spin-coating and ink-jet printing, the sequence of the depositions, drying and/or heating steps, and the atmosphere will be studied as they influence the film consolidation, and consequently the functional properties. Our target processing temperatures for the films on glass substrates should not exceed 450 oC. Spin-coating will be mainly used to prepare the high-K dielectric and TCO films; this will allow us to determine their properties: phase composition, microstructure, optical transmittance of the films on glass substrates and specific electrical properties and optical transmittance.
The last part of the project is to realize transparent electronic components, namely a capacitor and thin film transistor. In brief, the former consists in cross-section view of a bottom conductive layer, a dielectric and a top conductive layer on a transparent substrate and the latter of layers of a conductor (gate), dielectric, channel (semiconductor) and source and drain (conductors) pads on the glass substrate. The majority of the layers will be ink-jet printed and the processing conditions for such multilayered structures will be established keeping in mind that the critical values of optical and electrical properties for all constituent materials should be reached. In addition to the materials characterization of individual oxide layers, the focus will be on the analysis of the interfaces. The measurements optical transmittance and selected electrical properties (i.e., capacitance, voltage-current characteristics) will serve as the feedback for optimization of the processing of selected transparent components.
Significance for science
Transparent electronics encompasses a new field of research and technology based on oxide materials with a large enough band-gap and a high enough mobility of charge carriers - transparent conducting oxides (TCOs) which enable transparent electronic components, such as thin film transistors (TFTs), which cannot be realized by silicon technology. Also dielectrics employed in such components should exhibit high dielectric permittivity and high optical transmittance. Substrates, typically glass, require use of low processing temperatures, not exceeding 500 oC. This new research field has encountered extremely fast evolution in the last years. This project contributed to knowledge on solution processing of components for transparent electronic devices, mainly on amorphous high-K dielectric Ta2O5-based thin films and 2D structures. The formulation of the coating solution and the processing were designed to enable the removal of functional groups at low temperatures, to densify the film and to obtain suitable electrical properties, comparable to those obtained by physical vapor deposition. The investigated Ta2O5-based thin films from solution processed at temperatures not exceeding 400 °C, exhibited promising properties for both transparent passive and active electronic devices. The precursor solutions originally designed for CSD of thin films were further adapted for piezoelectric inkjet printing. The printing procedure and the heat treatment not exceeding 350 °C were optimized in order to pattern uniform structures with high electrical performance (dielectric permittivity, leakage current). The capacitors printed on glass showed an electrical performance comparable with the one of the spin coated thin films, and thus promising for further applications. The development of transparent TFTs requires high-quality amorphous high-K dielectrics processed on glass substrates at low temperatures. The influence of heating temperature upon structural, microstructural, optical and electrical properties of the Ta2O5-based thin films was studied, and gave a proof of concept for their use in TFTs, which exhibited the on/off ratio exceeding 10^8.
Significance for the country
The research in the field of transparent electronics has been mainly application driven. Transparent electronics for everyday applications consisting of both passive and active components based on transparent conductive oxides and high-K dielectrics are usually processed by physical vapor deposition (PVD) and patterned by lithography. In contrast to PVD, chemical solution deposition (CSD) of thin films is cost efficient and it enables quick modifications of chemical composition and thus properties. The project was focused on research of thin-film components of transparent electronics, processed from solutions. The films were prepared by CSD and spin-coating. Inkjet printing of inks with suitable rheological properties allowed direct patterning of 2D structures. The latter proved to be an efficient technology for patterning almost without residues and has got a strong potential for industrial use.
Most important scientific results
Annual report
2011,
2012,
2013,
final report,
complete report on dLib.si
Most important socioeconomically and culturally relevant results
Annual report
2011,
2012,
2013,
final report,
complete report on dLib.si
