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. We can fabricate the world-class LEDs and laser diodes in the yellow and red range.


Present research topics are as follows.

• Original MOVPE technology for high-quality InGaN


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. 

  JCG Fig.jpgvisble spectra.jpg

References

[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).

[2] K. Ohkawa, F. Ichinohe, T. Watanabe, K. Nakamura, D. Iida, "Metalorganic vapor-phase epitaxial growth simulation to realize high-quality and high-In-content InGaN alloys", Journal of Crystal Growth 512, pp.69-73 (2019). DOI:  j.jcrysgro.2019.02.018​

 


 Red InGaN LEDs for micro-LED displays​


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 commercial blue LEDs and green LEDs are made of nitride semiconductors, but the red LEDs are phosphide. We try to realize the alternative material for red LEDs by nitrides. Recently, we achieved the InGaN-based red LEDs using our MOVPE technique. In the near future, our red LEDs can contribute realization of RGB-LEDs by only nitrides, th​e monolithic devices for the micro-LED display are possible. It is a great benefit and breakthrough to fabricate the future display, like VR&AR. The display is energy-saving and environmentally friendly.     ​​


red LEDs_research.jpgCIE2.jpg

References

[1] D. Iida, Z. Zhuang, P. Kirilenko, M. Velazquez-Rizo, M. A. Najmi, and K. Ohkawa, “633-nm InGaN-based red LEDs grown on thick underlying GaN layers with reduced in-plane residual stress​”, Appl. Phys. Lett. 116, 162101 (2020). DOI: 10.1063/1.5142538​ ​

[2] Z. Zhuang, D. Iida, and K. Ohkawa, "Effects of size on the electrical and optical properties of InGaN-based red light-emitting diodes", Appl. Phys. Lett. 116, 173501 (2020). DOI10.1063/5.0006910




 Novel InGaN laser diodes in the yellow and red range


We will develop yell​ow 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.​​​
Yellow LD.pngOhkawaLD.jpg


• 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.

 

Strain Fig2.jpgYellow InGaN QWs2.jpg

Reference

[1] D. Iida, S. Lu, S. Hirahara, K. Niwa, S. Kamiyama, K. Ohkawa, “Enhanced light output power of InGaN-based amber LEDs by strain-compensating AlN/AlGaN barriers”, Journal of Crystal Growth 448, 105 (2016).

         

Heterostructure analysis


We are investigating the heterostructure of III-nitrides and oxide materials. The heterostructure analysis by TEM is significantly important for crystal growth. High-resolution TEM can perform the atomical scale observation, which can reveal the strain, composition, and defects in the crystal. Understanding of these informations, we can realize new structures and develop high-performance devices. 





References

[1] M. Velazquez-Rizo, D. Iida, and K. Ohkawa, "Photoelectrochemical and Crystalline Properties of a GaN Photoelectrode Loaded with α-Fe2O3 as Cocatalyst", Sci. Rep. 10, 12586 (2020). DOI: 10.1038/s41598-020-69419-8​​

[2] D. Iida, Z. Zhuang, P. Kirilenko, M. Velazquez-Rizo, and K. Ohkawa, "Demonstration of low forward voltage InGaN-based red LEDs", Applied Physics Express 13, 031001 (2020). DOI: 10.35848/1882-0786/ab7168​


• 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).​

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