Research

 

ECO Devices Laboratory is developing energy-saving devices and energy-creating devices based on material science. Our tool is MOVPE of Nitrides and Oxides. The leading-edge MOVPE technique is unique in the world. Developing energy-saving devices is a great contribution to the future of society and environment. We are currently focused on the growth of high-quality InGaN and its heterostructures to study carrier and strain behaviors. Their applications are LEDs, laser diodes, water splitting, and artificial photosynthesis.


Present research topics are as follows.

• High-quality InGaN growth by the original micro-flow channel MOVPE


High-temperature InGaN growth is one of the key technologies for covering the entire visible spectrum with light-emitting devices. Our unique MOVPE technique increases the growth temperature by 60oC compared to a typical MOVPE system. We demonstrated the deep-red 740 nm InGaN-based LEDs for the first time. 

 

Do you know a future display “micro-LED display”? The new display has clear colors, and low power consumption as RGB LEDs are present on the surface without a polarizer. The current blue LEDs and green LEDs are made of nitride semiconductors, but the red LEDs are phosphide. When RGB-LEDs are realized from nitrides, the monolithic devices for the micro-LED display are possible. It is the great benefit and breakthrough to fabricate the future display. The display is energy saving. It is environmentally friendly.​  
   JCG Fig.jpgLED spectra.jpg

Reference

[1] K. Ohkawa, T. Watanabe, A. Hirako, et al., “740-nm emission from InGaN-based LEDs on c-plane sapphire substrates by MOVPE”, Journal of Crystal Growth 343, pp.13-16 (2012).

 

Hydrogen generation by using GaN and InGaN photocatalyst


We have realized “the artificial photosynthesis” which can remake a huge amount of CO2 from industries and vehicles into sustainable fuel. We can convert CO2 into not only HCOOH but also C2H5OH, CH4, C2H4, CH3CHO, C3H6OH and so on. Such technology to produce sustainable energy will be a key to suppress global warming.

 

In photocatalysis phenomena, it is not only efficient but also very stable due to our original co-catalyst technology. The discovery of good co-catalyst is the crucial point to realize high-efficiency and long-durability in water splitting and artificial photosynthesis. We are the first group to discover the co-catalyst for Nitride photocatalyst. We have improved the energy conversion efficiency by one order.

 

The figure below shows the durability test for the last 490-500 hours. The nitride photocatalyst is stable without any photo-corrosion. We are studying band structure and strain control of InGaN photocatalyst to realize the highest efficiency in energy conversion.

 

Let us make innovative ideas together to change the world in the views of “clean energy” and “sustainable society”.​

  PC.jpgSchematic view2.jpghydrgen experiment result.jpg

Reference

[1] K. Ohkawa, W. Ohara, D. Uchida, et al., “Highly stable GaN photocatalyst for producing H2 gas from water”, Japanese Journal of Applied Physics 52, 08JH04 pp.1-3 (2013).


Highly efficient light-emitting diodes and laser diodes


The micro-flow channel MOVPE develops InGaN-based LEDs and LDs. We are challenging to create more efficient InGaN-based LEDs and LDs in the yellow-to-red region to replace phosphides which are explosive materials rather than nitrides. The monolithic nitride solid-state lighting should be safe, efficient and comfortable for our life.

 

We will develop yellow InGaN lasers. The yellow one is the current missing part of the laser diodes. The yellow region is essential for the medical, communication, and display fields. We can contribute to our great future.​

    

orange2.jpg  Yellow LD.png

    ​OhkawaLD.jpg

Reference

[1] D. Iida, K. Niwa, S. Kamiyama, K. Ohkawa, “Demonstration of InGaN-based orange LEDs with hybrid multiple-quantum-wells structure”, Applied Physics Express 9, 111003 (2016).


• A novel strain compensating technique


The crystal quality of InGaN is very important for optical devices. We have realized that InGaN quantum wells improve the crystalline quality by strain-compensating technique.

 

InGaN layers are heavily compressively strained. It is important to reduce the strain for suppressing the defect density. This strain-compensating technique is effective in high-In-content InGaN-based devices such as LEDs and LDs. To realize this idea to photocatalysts and solar cells, our further material development is in progress. Further development is necessary to obtain higher-performance devices such as more efficient and/or more powerful.​

Strain Fig2.jpgYellow InGaN QWs2.jpg

Reference

[1] D. Iida, S. Lu, S. Hirahara, K. Niwa, S. Kamiyama, K. Ohkawa, “Investigation of amber light-emitting diodes based on InGaN/AlN/AlGaN quantum wells”, Japanese Journal of Applied Physics 52, 08JB13pp.1-3(2013).