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


The high-temperature InGaN growth is one of the key technologies to cover the whole visible spectrum with light-emitting devices. Our MOVPE technique increases the growth temperature by 60oC compared to typical MOVPE systems. We demonstrated the deep-red 740 nm InGaN-based LEDs for the first time.  
   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


Nitride photocatalyst was invented by K. Ohkawa in 2001. The nitride photocatalyst can split water into H2 and O2, and reduces CO2 into CH4, HCOOH, C2H5OH and so on. There are so-called “water splitting” and “artificial photosynthesis”, respectively. In water splitting, it is not only efficient but also very stable due to our original co-catalyst technology. 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.
  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


InGaN-based LEDs and LDs are developed by the micro-flow channel MOVPE. We are challenging to develop 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.
      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 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.

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).