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Transparent and flexible electronics with embedded energy harvesting based on oxide nanowire devices

Periodic Reporting for period 2 - TREND (Transparent and flexible electronics with embedded energy harvesting based on oxide nanowire devices)

Reporting period: 2018-07-01 to 2019-12-31

The Internet of Things is shaping the evolution of information society, requiring an increasing number of objects with embedded electronics, sensors and connectivity. This spurs the need for systems where summing to performance and low cost, multifunctionality has to be assured. To date, there is not a technology fulfilling these needs: for instance, thin film microelectronics enables flexibility and large area processing but fails on assuring high levels of electrical performance or energy harvesting; nanoscale Si or III-V materials, while exhibiting outstanding electrical performance, typically fail on multifunctionality and/or low-temperature & large-area processing. In this context, TREND aims to take transparent electronics into as-of-yet unexplored levels of integration, by combining on flexible substrates transparent and high-speed nanocircuits with energy harvesting capabilities, based on multicomponent metal oxide materials, particularly nanowires (NWs). For this end, sustainable and recyclable materials as zinc-tin oxide (ZTO) are being synthesized using low-temperature and low-cost solution processes. The precise control of the density and alignment of these nanostructures is critical for device integration and in TREND different approaches are being considered to tackle this: transfer methods, by using techniques such as spray-coating and tuning them for reproducible deposition below and above percolation thresholds of random NW networks; direct growth of NWs from seed layers patterned by substrate conformable imprint lithography (SCIL) on flexible substrates. Major applications envisaged are nanotransistors and piezo/triboelectric nanogenerators, but other fascinating properties of these materials are also of interest for a multifunctional platform, such as photocatalytic and sensing ones. Oxide thin-film technology is being used as a basis for the transistors and resulting analog/digital circuit blocks, by exploring physical aspects of miniaturizing channel lengths below 1 µm, improving fabrication processes of thin films to accommodate oxide NWs and enable miniaturized vias through multilevel metallization schemes and using circuit design techniques to compensate for transistor performance limitations. At the end a final platform of nanocircuits +nanogenerators is envisaged, making use of NW interconnects and bringing a new dimension to the systems-on-foil concept. The research is being carried out at FCT-NOVA, in a group pioneering oxide electronics.
TREND is thus an ambitious interdisciplinary project motivating advances in materials science, engineering, physics and chemistry, with impact extending from consumer electronics to health monitoring wearable devices. By promoting new ideas for practical ends, it will contribute to place Europe in the leading position of such strategic areas, where sustainability and innovation are key factors.
Until the end of this period major work has been carried out on:

- NW synthesis below 200 °C – ZnSnO3 NWs were synthesized by an hydrothermal method, from ZnCl2 and SnCl4 hydrated precursors; alternatively a conversion from ZnO NWs, also by an hydrothermal process, enabled either ZnSnO3 or Zn2SnO4 NWs, depending on pressure and temperature.

- Metallic NW spray coating as random networks to achieve transparent and conductive electrodes, with sheet resistance below 5 Ohm/sq and average optical transmittance close to 80 % in visible range.

- Implementation of ZnSnO3 NWs on hybrid nanogenerators, be embedding ZnSnO3 into a polymeric network and exploring both piezo and triboelectric effects. Instantaneous power densities above 200 µW.cm-2 could be achieved, showing great potential for energy harvesting in wearables.

- First attempts to create oxide NW transistors, using electrolyte gated ZnSnO3 NWs, resulting in On-Off ratio of 1E4. Also, ZTO was taken to thin film form and by controlling sputtering parameters, particularly composition and H2 incorporation, flexible oxide TFTs with comparable performance to IGZO TFTs (µFE>5 cm2/Vs, Von~0 V, On/Off>1E7) could be obtained, without critical raw materials.

- Exploring circuit design techniques unusual in flexible electronics to tackle limitations of oxide transistor technology compared to conventional Si CMOS, such as switched bootstrapping capacitive loads in logic gates for rail-to-rail operation and level-shifting ability, negative capacitance generators to increase amplifiers bandwidth, fully-transistor based rectifiers operating at NFC or high-frequency range (13.56 MHz) of RFID. Concepts will then be migrated to make use of oxide NW-based transistors.

- Establishment of processes to fabricate oxide electronics with large number of layers (>10) and enable miniaturized vias through multilevel metallization schemes.
Even if many other achievements are expected until the end of TREND, the items listed in the section above already go beyond the state of the art. To the best of our knowledge, our ZTO NW synthesis was the first hydrothermal process resulting in ZnSnO3 NWs without the use of any seed layer, resulting in ideal structures for subsequent transfer processes. Moreover, our ZnO-to-ZTO conversion and flexibility on getting either ZnSnO3 or Zn2SnO4 NWs below 200 °C is also innovative.
On the application side, even if hybrid nanogenerators have been previously reported, seed-layer free ZnSnO3 NWs are being used in TREND for the first time, together with micro-structured polymeric films that improve the efficiency of deformation of the NWs and boost the combined piezo and triboelectric signals.
While oxide NW integration on nanotransistors is yet to be explored in its full potential in TREND after optimization of EBL and SCIL techniques, we could demonstrate in 2018 for the first time a flexible ZTO TFT with similar performance and stability compared to IGZO TFTs with maximum processing temperature of 180 °C. This is a major step towards sustainable flexible electronics.
Finally, on the circuit side, even if the basis of the design of most analog and digital blocks originates from older generations of CMOS nodes, many of the concepts are novel in flexible electronics and enable one to get the best compromise between electrical performance, power consumption and fabrication feasibility.

Major results expected until the end of the project can be summarized as follows:
- Optimized EBL process enabling precise electrode patterning on single ZTO NWs to allow for their detailed electrical characterization and Si molds for SCIL with sub-100 nm features
- SCIL process on flexible substrates with sub-100 nm patterned seeds for oxide NW direct growth
- Oxide transistors with random NW multicomponent oxide semiconductor networks replacing conventional semiconductor thin films
- Oxide nanotransistors with vertically and horizontally aligned oxide NW semiconductors with cut-off frequencies approaching 1 GHz
- Enhanced hybrid nanogenerators using accurate control of density and alignment of ZnSnO3 NWs potentiated by SCIL
- Detailed understanding of NW composition and structure on its piezoelectric properties
- Self-powered analog and digital building blocks based on oxide NWs
ZTO nanostructures – 2nd prize in Science as Art, MRS Fall meeting 2018
Single ZnSnO3 NW by hydrothermal synthesis at 200 °C - electrical characterization
Growth mechanism of seed-layer free ZTO nanostructures