SUPERSOL · Solution processed low-dimensional oxide semiconducting structures and devices

Solution processed low-dimensional oxide semiconducting structures and devices

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.

Data: CORDIS, © European Union