Periodic Reporting for period 3 - TREND (Transparent and flexible electronics with embedded energy harvesting based on oxide nanowire devices)
Reporting period: 2020-01-01 to 2021-06-30
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.
- 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.
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