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Solution processed low-dimensional oxide semiconducting structures and devices

Periodic Reporting for period 1 - SUPERSOL (Solution processed low-dimensional oxide semiconducting structures and devices)

Reporting period: 2015-09-07 to 2017-09-06

Everyone who can view this summary on a personal or laptop computer with a flat panel display uses Thin Film Transistor (TFT) technology. Exploitation of TFT technology allows the industry to fabricate low-cost applications widely used in everyday life by the vast majority of people on earth such as flat panel displays, sensors and simple electronic circuits. The most common application of a thin film transistor is as the switch of a pixel of a flat panel display permits or not the light to reach our eye and is produced by depositing a semiconducting layer on a dielectric layer over a transparent substrate. Progress in TFTs has been based primarily on the development of new materials and on implementation of innovative device architectures together with new material combinations. Decreasing the cost of fabrication processes, the ability to fabricate transistors in large scale and the enhancement of the transistor performance are the most important aspects of the technology. Solution based fabrication techniques which have been developed the past few years allow us to achieve fabrication of large area electronics on flexible transparent substrates due to low temperature processing, with low fabrication cost using low cost equipment. More importantly, they are ideal for producing low-dimensional films. Two of the simplest and cost-effective deposition techniques used in this project are the spin casting and spray coating. In spin casting, the solution containing the semiconductor or the dielectric material is casted on a substrate and then is spread by spin for a few seconds, while in spray coating the solution is sprayed on the substrate. The final step of the fabrication process is to anneal the samples at temperatures that are compatible with plastic substrates, below 200°C, in order to convert the solution on the substrate into a solid semiconductor film. Fully soluble metal oxide materials, offers the ability to fabricate TFTs with a high mobility beyond the established silicon technology used until today and thus potentially replace the current technology. However, we are still far away from that point where a single layer of metal oxide semiconductor can reveal a mobility high enough so as to exceed the mobility of silicon transistors. This is mainly due to poor quality of the semiconducting film and the maximum mobility limitations by the intrinsic properties of the semiconductor employed restricting the performance of the transistor. On the other hand, the fabrication techniques that have been used for high quality inorganic films are expensive, complicated and demand high processing temperature and therefore makes them inappropriate for fabricating electronics on flexible substrates. SUPERSOL addresses the challenge of fabricating solution-processed metal oxide TFTs on flexible substrates exhibiting a high mobility which exceeds the mobility of silicon technology prevailing until today. Spin casting and/or spray coating fabrication techniques have been used to deposit metal oxide semiconductors like ZnO, In2O3, Ga2O3, InxZnyOz with carrier density changing at will, dielectric films like ZrO2 and Al2O3 and solutions containing mixed semiconductors like ZnO and In2O3 nanoparticles. In order to overcome the intrinsic semiconductor mobility limits, the project has combined the advantage of charge carrier confinement appears at the interfaces of multiple inorganic semiconducting layer structures with the advantages of simplicity and low-cost solution fabrication processes to enhance the mobility of the transistors. SUPERSOL investigated the underlying conduction mechanisms of the multiple layer transistor structures in order to uncover the aspects leading to high mobilities and took advantage to develop high mobility thin film transistors exceeding the mobility limits until today. Finally, the project has delivered optimized multiple layer transistors and other electronic devices like memristors and resonant tunnelling diodes. In conclusion, SUPERSOL has developed and demonstrated novel high mobility solution processed metal oxide thin film transistors on flexible substrates that will benefit the relevant technology by breaking the transistor mobility limits and lower the production costs for large area electronics industry. Potentially, it will lead to high-end products, more easy to use, to a huge variety of every day applications that will have a large impact to society. A brief summary of a few key results of the SUPERSOL project are illustrated in Figure 1.
In the framework of the project we have: I) Developed new metal oxide materials in order to explore materials that form high quality interfaces that could be used for applications in thin film transistors, optoelectronic and quantum devices. The improvement of the quality of the interfaces was critical since the charge confinement occurs at or close to the interfaces. Additionally, metal oxides used as dielectrics were necessary for developing quantum devices at low deposition temperatures on plastic substrates. II) Optimized the deposition solution processes of metal oxides and consequently the quality of the semiconducting films. A large number of deposition parameters have been investigated so as to change the number of free carriers in the semiconducting films, their thickness, lowering the deposition temperature and improve the quality of interfaces. III) Exploited the optimum quality of semiconducting films discovered and developed innovative device architecture by changing the sequence of adjacent layers in order to achieve the maximum transistor mobilities. We have fabricated single, double layer and multilayer transistors using the best semiconducting films and materials to achieve the best device engineering for charge confinement. The mobility of multilayer devices has reached at high values exceeding the intrinsic properties of the materials employed. IV) Identified the conduction mechanisms of single, double layer and multilayer structures using various characterization techniques. Experimental results has been theoretically simulated, combined with the results of various experimental techniques and the unconventional conduction mechanisms prevailing to the transistor performance have been obtained. V) Fabricated quantum devices like memristors and resonant tunnelling diodes on plastic substrates. We have demonstrated the fabrication and operation of quantum devices using solution processed metal oxide dielectric layers in combination with optimized semiconducting metal oxide materials.
Although the interest of the industry on solution processed metal oxide TFTs is huge, several limitations in the performance of TFTs did not allow the technology applications to expand further. SUPERSOL has addressed the challenge and overcome the mobility values achieved for the state-of-the-art silicon TFTs by incorporating multilayer channel structures. The innovative combination of new materials from solution at low temperatures (<200°C) along with the use of new recipes for known semiconductors will have a great impact on the relevant industry technology. Quantum devices that had been fabricated during the project proves that the current technology can use only solution processes on flexible substrates to achieve better, low-cost and high performance devices.