Intellectual Merit: Transparent amorphous oxide semiconductors based on amorphous and poly-silicon are an attractive alternative to existing transistor technologies to enable future applications of high-performance, flexible and large-area electronics. Solution-based processes are also needed for ink-jet printable electronics. This BRIGE project will study solution-processed ternary amorphous metal oxide thin films such as zinc tin oxide and zinc gallium oxide using a novel molecular precursor approach. The research seeks to establish the feasibility of using these precursors to fabricate the active layer of thin film transistors and to develop alternate energetic sources to lower the required temperature for high-quality film formation, opening the door to future solution processing of a variety of electronic, optoelectronic and magnetic thin films. The semiconductor physics of these films, such as the nature of defects and donors as well as electronic charge transport, will be explored. Finally, this project seeks to demonstrate the films' potential for future applications in a wide range of demanding circuit applications.
Broader Impacts: The proposed technical work on printable, flexible electronics has the potential to significantly impact technologies for consumer, industrial, and military mobile applications where durability, low weight and low-cost are key design criteria. The project will have broad impact in education and training by involving both undergraduate and graduate researchers in collaborative research alongside the PI. The students will obtain significant experience in fabrication and characterization techniques relevant to micro- and nano-fabrication of electronic materials and devices, and will seek to share their knowledge with the broader community, including school-age students, via outreach programs. Participation in engineering research will be broadened through recruiting, training, mentoring, and outreach to under-represented and disabled students at all levels.
Society desperately needs smarter and less costly sensing systems to help us identify critical events, such as environmental change or personal risk exposure, so that we can focus our limited resources on solving such problems rather than simply detecting them. Printed electronics with radio-frequency performance would enable low-cost and large-area circuitry for many pervasive wireless sensing applications such as ubiquitous indoor and outdoor environmental monitoring, chronic health monitoring, and continuous detection of events such as brain injury for military personnel or professional sports players. In this project we have shown that amorphous zinc tin oxide (ZTO) is a strong candidate for the active layer of such printed electronics. ZTO is a transparent semiconductor, so it can be used in opto-electronic devices and transparent/window-based electronics such as heads’ up displays. Compared to similar materials such as indium gallium zinc oxide, it offers the advantages of being indium-free and non-toxic. This is important because the price and scarcity of indium have increased in recent years due to the increasing demand for transparent indium tin oxide (ITO) electrodes in electronics displays. Here, we use a liquid ink approach to form zinc tin oxide layers, and have demonstrated working transistors using ZTO layers deposited using ink-jet printing or spin-coating. We have explored the ZTO electronic properties by measuring transistor performance as a function of temperature. We showed that our solution-processed ZTO has the same underlying semiconductor physics as similar materials made by expensive and complex vacuum chamber methods, such as sputtering. Advantageously, our solution-processed layers have fewer defect states and more ideal conduction behavior than vacuum-deposited films. In order to build circuits that run at high frequency, it is necessary to make transistors with narrow dimensions, in the same way as silicon transistors have been scaled for decades according to "Moore’s Law." We have investigated methods for implementing transistor channels smaller than one micron (one millionth of a meter). One of these methods is based on a self-assembled solution process. Another requirement for narrow-channel, high-frequency transistors is low contact resistance between the metal electrodes and semiconductor. We have demonstrated record-low contact resistance of molybdenum electrodes to the active ZTO layer. In the future, we hope to combine these different elements to build complex ZTO-based radio frequency (RF) and digital circuitry. In summary, this work shows that solution-based methods for ZTO thin film transistor fabrication provide a feasible technical path toward printed electronics. This work has impact in chemistry, physics, materials science, electrical engineering, and future applications using this material include medical and biological sensors, environmental sensors, energy scavengers/harvesters, and advanced displays. In order to broaden participation, this project has involved seven undergraduate research assistants, including several from groups that are traditionally under-represented in electrical engineering, as well as one graduate student researcher. These students have been trained in cleanroom fabrication and electrical device testing. They also have had opportunities to present their work in oral and poster presentations, online videos, and written reports. This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
