Research
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https://www.ist.hokudai.ac.jp/labo/nano/en/
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Research vision
A research goal in his career is to achieve flexible multi-functional systems interacted with devices and humans that would overcome the next innovative breakthroughs. The keyword is “human-friendly interactive electronics” by combining “material innovations” and “device innovations”.
Nanomaterial printings
High performance electronics on user-defined substrates gets becoming one of the hottest topics for the future electronics. However, crystalline materials, especially inorganic materials, are usually required a crystal substrate for epitaxial growth and/or high temperature process. For the flexible electronics, the substrates are usually amorphous and are not compatible with high temperature processes. These make a high barrier to realize high performance electronics on arbitrary substrates by using inorganic materials. To address the issues, we have proposed some nanomaterial printing techniques where we can uniformly form nanomaterial films on any types of surfaces such as a plastic, paper, and glass as shown in Figures. In our contributions, we aim to demonstrate high performance transistors including integrated circuits, sensors, and actuators as a new class of electronics such as surface interactive electronics.
Flexible electronics
Flexible electronics is a new class of next generation devices that have applications such as wearable electronics. However there are several bottlenecks such as the fact that current standard materials for electronics such as silicon are mechanically rigid. Another problem is even if mechanically flexible materials are used, how integrated circuits can be fabricated on flexible substrates. Currently, there are three major candidates for the fabrication of flexible electronics. Organic materials and epitaxial microfilm transfer are promising approaches for flexible electronics. However there are advantage and disadvantage using these materials/techniques. Our approach is slightly different and unique. Our method is inorganic semiconductor nanomaterial (nanowires and nanotubes) printing or painting on macro-scale flexible substrates. By using these material systems, the performance is significantly improved on even flexible substrates with low cost processing. For example, effective mobilities of Ge/Si core/shell nanowire arrays and semiconductor nanotube networks of ~20 cm2/Vs and ~50 cm2/Vs, respectively, have been demonstrated. In addition to the high performance of the field-effect transistors, the mechanical reliability of devices fabricated using this method is also very high. No performance degradation has been observed while bending the substrates up to 2.5 mm curvature radius. This high reliability is due to two reasons. One reason is nanowire and nanotube diameters are very small, so they are not as affected by strain during bending. Another is transistors are fabricated close to neutral regions of the substrates to decrease strain under bending. Based on our developed technology, we have reported an artificial electronic skin as one example as shown here. This demonstration proves that the proposed approach is truly a promising candidate for future wearable electronics which require low-voltage/power operation (high mobility), macroscale assembly, flexible substrates, and low cost processing. We are now moving forward to demonstrate a system that allows us to use this concept for practical applications. In addition, by developing high sensitive strain sensor, we have also demonstrated an artificial electronic whiskers.
References
5. C. Wang, D. Hwang, Z. Yu, K. Takei, J. Park, T. Chen, B. Ma, A. Javey, “User-interactive electronic-skin for instantaneous pressure visualization”, Nature Materials, Vol. 12, pp. 899-904, 2013.
6. K. Takei, Z. Yu, M. Zheng, H. Ota, T. Takahashi, A. Javey, "Highly sensitive electronic whiskers based on patterned carbon nanotube and silver nanoparticle composite films", Proceedings of the National Academy of Sciences (PNAS), Vol. 111, pp. 1703-1707, 2014.
High performance electronic
Low power and high performance transistors are one of the most important technologies for future electronics. The power consumption of transistors has exponentially increased in recent years to satisfy human demand of electronics such as personal computers and cell phones. In order to decrease power consumption in transistors, there are two ways. One is to shrink down the size of transistors. Another is to explore new material systems that have higher mobility and saturation velocity than those used for current technology. The first way (smaller transistors) using conventional materials such as silicon, will become a problem in near future because the size of Si transistors is already reaching fundamental limit. Because of that, the latter way is a more promising approach to reduce power consumption. To explore new material systems, we have proposed an ultrathin epitaxial layer transfer technique with high mobility III-V semiconductors onto Si/SiO2 substrates. By using an elastomer polymer stamp (PDMS), we have successfully transferred ultrathin InAs for n-type and InGaSb with InAs cap layers for p-type field-effect transistors (FETs) onto Si substrates. As can be seen from the TEM image below, the transfer yields an impressively smooth interface between the layers and SiO2/Si substrates. The FET devices were characterized and effective mobilities >4000 cm2/Vs for n-type FETs and >800 cm2/Vs for p-type FETs on Si substrates were demonstrated. These mobilites are >5x higher than those of Si FETs. In addition, we benchmarked the FETs, finding a high peak transconductances ~0.6-1.7 mS/um and Ion/Ioff ~500-20,000 at VDD=0.5 V depending on the thickness of InAs layers (5-13 nm) although the channel length is still relatively large (~200 nm) compared to the conventional Si transistors. The performance differences between 5 nm and 13 nm thick InAs are mostly due to quantization of the channel and scattering mechanisms as well as quantum contact resistance. The next goal of this project is to reveal the performance limit of freestanding III-V semiconductor FETs.
References
1. H. Ko*, K. Takei*, R. Kapadia*, S. Chuang, H. Fang, P. W. Leu, K. Ganapathi, E. Plis, H. S. Kim, S.-Y. Chen, M. Madsen, A. C. Ford, Y.-L. Chueh, S. Krishna, S. Salahuddin, A. Javey, “Ultrathin compound semiconductor on insulator layers for high performance nanoscale transistors”, Nature, Vol. 468, pp. 286-289, 2010. *Authors equally contributed.
2. K. Takei, S. Chuang, H. Fang, R. Kapadia, C.-H. Liu, J. Nah, H. S. Kim, E. Plis, S. Krishna, Y.-L. Chueh, A. Javey, “Benchmarking the performance of ultrathin body InAs-on-insulator transistors as a function of body thickness”, Applied Physics Letters,Vol. 99 , p.103507, 2011.
3. K. Takei*, H. Fang*, S. B. Kumar*, R.Kapadia, Q.Gao, M.Madsen, E.Plis, S.-Y.Chen, S. Krishna, Y.-L. Chueh, J. Guo, A. Javey,“ Quantum confinement effects in nanoscale-thickness InAs membranes", Nano Letters, Vol. 11, pp.5008-5012, 2011.
4. K. Takei*, M. Madsen*, H. Fang, R. Kapadia,S. Chuang, H. S. Kim, C.-H. Liu, E. Plis, J. Nah, S. Krishna, Y.-L. Chueh, J. Guo, A. Javey, "Nanoscale InGaSb heterostructure membranes on Si substrates for high hole mobility transistors”, Nano Letters, Vol. 12, pp. 2060-2066 2012.