Speaker: Akihiko Yoshikawa, Center for SMART Green Innovation Research, Chiba University, Japan

Title: Proposal of “1 ML” InN/GaN Matrix Coherent-Structure QW System and Its Application for Novel Structure Photonic Devices

Abstract:
We have proposed a novel 1 ML-thick InN/GaN matrix-based coherent structure QW system and have already successfully developed unique ALE-like InN deposition processes to fabricate those QWs on +c-GaN by using a custom-made MBE system equipped with a spectroscopic ellipsometry (SE) (J. A. Woollam, M-2000). The attached SE system is quite useful and effective as an in-situ monitoring, deep understanding, and precise controlling tool for the growth processes. The novel InN QWs were fabricated under self-ordering and self-limiting processes achieved at remarkably higher growth temperatures (600-700 C) than the higher side critical temperature (~500 C) for continuous InN film growth under +c-polarity growth regime by MBE. The fundamental structure of the novel QWs consisted of 1 ML-thick InN well coherently embedded in GaN matrix, and the QW could be controlled in fractional ML-thick and also 2 ML-thick InN wells in the GaN matrix depending on the growth conditions [1-4]. Then, these InN QWs are generically expressed here as “1 ML” InN QWs.

On the basis of above mentioned development on coherent structure “1 ML” InN/GaN matrix QWs, we have also been investigating to extend them to (InN)m/(GaN)n short-period superlattices (SPSs), where we simply call them as SMART structure and/or SMART process. Here SMART stands for “Superstructure Magic Alloys fabricated at Raised Temperature”. We expect that the InN/GaN SPS-based SMART structures can play a role as high quality artificial or digital InGaN ternary alloys. Of course, the GaN barrier layer thickness must be also ultimately reduced in this case down to a few monolayers or several monolayers level so that localized wave functions at the InN wells can be overlapped with each other. Furthermore, it should be noted that the SMART process has been successfully developed first in MBE but it has been extended to MOVPE now. It was found that the structural quality of the SMART structure devices and/or corresponding InN/GaN SPSs is getting worse in the case of MOVPE than that of MBE, but MOVPE-grown devices often show better device performances than those for MBE, probably due to the effects of remarkably higher growth temperatures.

In this lecture, I first summarize what is the novel “1 ML”-InN/GaN matrix QW system and then also discuss how and where those “1 ML”-InN QWs are fabricated on or inside the GaN matrix by MBE. Next, the basic idea for their potential application to InN/GaN SPS-based artificial or digital InGaN ternary alloys, i.e. SMART structure, will be reported. Furthermore, our idea and recent efforts for applying the “1 ML” InN QW/GaN matrix QW system and the SMART structure to develop blue-green light emitters and high efficiency III-N tandem solar cells [5-8], respectively, are also discussed, and their initial stage results will be reported.

1. A. Yoshikawa, S. B. Che, W. Yamaguchi, H. Saito, X. Q. Wang, and E. S. Hwang, Appl. Phys. Lett. vol. 90, 073101 (2007).

2. E. S. Hwang, S. B. Che, H. Saito, X. Wang, Y. Ishitani, A. Yoshikawa, Journal of Electronic Materials vol. 37, 597 (2008).

3. A. Yoshikawa, S. B. Che, N. Hashimoto, H. Saito, and X. Q. Wang, J. Vac. Sci. Technol. B vol. 26, 1551 (2008).

4. Akihiko Yoshikawa, Song Bek Che, Yoshihiro Ishitani, Xinqiang Wang, J. Crystal Growth vol. 311, 2073 (2009).

5. S. B. Che, A. Yuki, H. Watanabe, and A. Yoshikawa, Appl. Phys. Express vol. 2, 021001 (2009).

6. Akihiko Yuki, Hiroshi Watanabe, Song-Bek Che, Yoshihiro Ishitani, and Akihiko Yoshikawa, Phys. Status Solidi C vol.6, S417 (2009).

7. A. Yoshikawa, Y. Ishitani, and K. Kusakabe, U.S. patent 0,174,894 A1 (21 March 2013); First filled as PCT/JP2011/063, 911 (17 June 2011).

8. Kazuhide Kusakabe and Akihiko Yoshikawa, Proc. SPIE 8986, 89861B 1 (2014).

Speaker Bio:
Akihiko Yoshikawa was born in Japan on January 23, 1947. He received his B.S. and M.S. degrees in Electronics Engineering from Chiba University in 1969 and 1971, respectively, and the Ph.D. degree in Physical Electronics from Tokyo Institute of Technology in 1977.

He is a Professor of Semiconductor Physics at Chiba University and also a Director of “Center for SMART Green Innovation Research”, Chiba University. His major interest is in studying/understanding the material- and epitaxy-control of wide bandgap semiconductors, such as III-nitrides including GaN and InN, by MBE and MOVPE. He is also interested in nano-structured super functional photonic and electronic devices utilizing the supra material properties of wide bandgap semiconductors.

He is now a project leader for developing SMART III-N solar cells as one of ALCA (Advanced Low Carbon) projects promoted by JST (Japan Science and Technology Agency).

He has been a Chairman for about 10 years of The 162nd Committee on Wide-bandgap Semiconductor Photonic and Electronic Devices, Japan Society for the Promotion of Sciences (JSPS). The 162nd Committee is playing an important role in Japan for promoting industry-academia-government collaboration in the area of widegap semiconductors including III-Ns, SiC, C, and oxides etc. Almost of all active Japanese professors working in the area of widegap semiconductors and also about 40 related industries are members of this committee.

He is a Fellow Member of both ”The Japan Society of Applied Physics” (JSAP) and ”The Institute of Electronics, Information and Communication Engineers” (IEICE).