With the advent of the IoT era, there is a strong demand for various high-performance electronic devices. In our laboratory, extensive research is being conducted for this era.
The main research themes are as follows.
▶Materials and devices for brain-like information processing （AI related）
▶High-performance information storage materials / devices （IoT related）
▶Flexible biometric information sensors, etc. （IoT related）
▶Next-generation power generation devices (solar cells, etc.) （SDGs related）
▶Related core technologies (nano fabrication, advanced materials, thin films, etc.)
In addition, our laboratory conducts joint research with prominent researchers overseas to promote cutting-edge research.
First, I will introduce an example of research on artificial intelligence materials and devices for brain-like information processing.
As the brain processes information, it changes the strength of synaptic connections.
The change often happens, so highly reliable functional materials are required.
However, for conventional materials, a change in film thickness of 5.5 to 9% has been observed due to the phase change.
Therefore, a large internal stress occurs in the device, and voids gradually appear in the device.
This will result in poor contact and high resistance failure.
To solve this problem, we added other element to conventional materials to develop a new material that does not change in film thickness due to phase change.
In the conventional material GeTe, heating causes internal stress associated with crystallization, resulting in many cracks.
However, the addition of N to this material resulted in the elimination of cracks.
Also, due to the addition of nitrogen, the film thickness change rate slightly changed from 7.5% of the conventional material at first, then approached almost zero, and finally increased by about 6%.
If this material with almost zero change in film thickness is applied to artificial intelligence devices, it is expected that the reliability will be greatly improved.
Next, I will introduce an example of research on materials and devices for high-performance information storage.
This is the development of large-capacity information storage ultra-multilevel technology.
In the IoT era, large amounts of information will be collected, stored and processed.
The amount of information is increasing exponentially.
Therefore, there is a need for large-capacity information storage technology that can store a large amount of information at low cost.
However, it is difficult to obtain moderate resistance with conventional technology, and since it can only take two states of "0": low resistance and "1": high resistance, it becomes a normal binary information storage device.
Therefore, in this research, we developed a multi-level storage technology that can precisely control the phase change process and region by taking advantage of the characteristics of phase change materials.
We proposed a device with a 50 nm thick phase-change control layer added on top of the phase-change layer.
In the proposed structure, intermediate resistances can be easily obtained, and 16-level ultra-multilevel storage has been demonstrated.
I believe that ultra-multi-level storage technology will play an important role in the IoT era, where it is necessary to store huge amounts of information.
Next, I would like to introduce research on flexible biological information sensors.
In the IoT era, various kinds of information are collected. Therefore, many kinds of sensors are required.
Among them, the market for flexible sensors is expected to grow to $7.6 billion by 2027.
The conventional printing method used for sensor fabrication requires large-scale equipment and is not suitable for new development in laboratory.
Therefore, in this research, we used the sputtering method, which is relatively easy for new development, to develop a nanoparticle resistance-type flexible sensor.
Depending on how the flexible substrate is bent, the distance between the nanoparticles changes and the tunneling current increases or decreases, resulting in a resistance change.
This is a schematic diagram of the structure of the sensor.
Also, a measurement system was constructed using LabVIEW.
We confirmed that the sensor resistance changes depending on how the flexible substrate is bent.
In the future, we will try to acquire biometric information, and I think that we can contribute to health and welfare in the future.
Next, I would like to introduce research on next-generation power generation devices.
The world demand for electricity is increasing year by year.
Many fossil fuels are used for power generation, and problems such as global warming and depletion of fossil fuels are becoming serious.
Therefore, the development of power generation technology using natural energy such as sunlight has become an urgent task.
Si solar cells have become popular in recent years, but they have the problem of low conversion efficiency.
Therefore, in this research, we developed an optical rectenna that is expected to greatly exceed the conversion efficiency of this Si solar cell.
An optical rectenna consists of an antenna part and a rectifier part.
Light is received by the antenna part, and the received light, which is an electromagnetic wave, is converted into direct current through the rectification part.
In this study, the antenna part was examined by simulation, and it was found that the directivity of the patch antenna is good, so it is suitable for the optical rectenna.
We also fabricated a MIM element using resist as an insulator and evaluated its electrical characteristics.
We were able to obtain high asymmetry.
In the future, we can expect to contribute to the SDGs by using inexhaustible solar energy to supply us with electricity.
Finally, I will introduce related fundamental technologies.
Nanofabrication is a fundamental technology in the field of nanoelectronic devices.
Advances in nanofabrication technology will enable the creation of innovative electronic devices.
For the fabrication of high-performance devices such as quantum dot solar cells, formation of aligned nanostructures is required.
The bottom-up self-assembly method using block copolymers can form large-area nanostructures, but the nanostructures are not aligned and become random.
On the other hand, top-down electron beam lithography is characterized by its ability to easily form aligned structures, but it is inefficient because it is difficult to process large areas.
Therefore, in this research, we developed a nanofabrication technology that can form aligned nanostructures over a large area.
First, electron beam lithography is used to fabricate posts and guide lines, and then nanostructures are fabricated by self-assembly.
We have successfully formed aligned nanostructures using this method.
If this technology is applied to the fabrication of cutting-edge nanodevices, significant performance improvements can be expected.
International joint research
Our laboratory actively promotes international joint research.
We have conducted joint research with researchers from well-known overseas research institutes, and obtained significant research results, which have been published in famous journals (ACS Nano, Nat. Commun., etc.).
1. Through joint research with the Massachusetts Institute of Technology MIT in the United States and the Korea Advanced Institute of Science and Technology KAIST, we were able to reduce the power consumption of phase change information storage devices to about 1/20.
Papers： ACS Nano, 7, 2651-2658 (2013)、ACS Nano, 9, 4120-4128 (2015)、Chemistry of Materials, 27, 2673-2677 (2015).
2. In collaboration with Zhejiang University ZU in China, we analyzed 2D materials using first principles.
Papers： International Journal of Modern Physics C, 28, 1750131 1-11 (2017)、Journal of the Korean Physical Society, 66, 1031-1034 (2015).
3. We conducted joint research with Nanyang Technological University NTU in Singapore and University of Electronic Science and Technology UESTC in China to develop artificial intelligence devices and construct neural networks. Using it, we succeeded in recognizing handwritten digits.
Papers： Scientific Reports, 8, 12546 1-7 (2018)、Nature Communications., 6, 7522 1-8 (2015).
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