Here is listed the current research activities in the LED materials and devices team.
SiC photonics and nanofabrication
SiC is the only compound in the group IV semiconductors and has been wide implemented in the power electronics thanks to it wide bandgap, high breakdown field and good thermal conductivity. With the development of SiC growth and the improvement of crystal quality, SiC is becoming active in optoelectronics. Right now we focus on optical characterization of fluorescent SiC for white LED light source, nanostructuring the surface for light extraction efficiency enhancement. This area is in close collaboration with Meijo University in Japan, Linköping university in Sweden and Erlangen university in Germany.
Recent publications (selected):
[1] Y. Ou, D. Corell, C. Dam-Hansen, P. Petersen, and H. Ou, “Antireflective sub-wavelength structures for improvement of the extraction efficiency and color rendering index of monolithic white light-emitting diode,” Optics Express 19(S2), A166-A172 (2011).
[2] Y. Ou, V. Jokubavicius, S. Kamiyama, C. Liu, R. Berg, M. K. Linnarsson, R. Yakimova, M. Syväjärvi, and H. Ou, “Donor-acceptor-pair emission characterization in N-B doped fluorescent SiC,” Optical Materials Express 1(8), 1439-1446 (2011).
[3] Y. Ou, V. Jokubavicius, M. K. Linnarsson, R. Yakimova, M. Syväjärvi, and H. Ou, “Characterization of donor-acceptor-pair emission in fluorescent 6H-SiC,” Physica Scripta T148, 014003 (2012).
[4] Y. Ou, V. Jokubavicius, P. Hens, M. Kaiser, P. Wellmann, R. Yakimova, M. Syväjärvi, and H. Ou, “Broadband and omnidirectional light harvesting enhancement of fluorescent SiC,” Optics Express 20(7), 7575-7579 (2012).
[5] Y. Ou, V. Jokubavicius, R. Yakimova, M. Syväjärvi, and H. Ou, “Omnidirectional luminescence enhancement of fluorescent SiC via pseudo-periodic antireflective sub-wavelength structures,” Optics Letters 37(18), 3816-3818 (2012).
[6] A. Argyraki, Y. Ou, and H. Ou, “Broadband antireflective silicon carbide surface produced by cost effective method,” Optical Materials Express 3(8), 1119-1126 (2013).
[7] Y. Ou, I. Aijaz, V. Jokubavicius, R. Yakimova, M. Syväjärvi, and H. Ou, “Broadband antireflection silicon carbide surface by self-assembled nanopatterned reactive-ion etching,” Optical Materials Express 3(1), 86-94 (2013).
[8] Y. Ou, X. Zhu, V. Jokubavicius, R. Yakimova, N. A. Mortensen, M. Syväjärvi, S. Xiao, and H. Ou, “Broadband antireflection and light extraction enhancement in fluorescent SiC with nanodome structures,” Scientific Reports 4, 4662 (2014).
[9] H. Ou, Y. Ou, A. Argyraki, S. Schimmel, M. Kaiser, P. Wellmann, M. K. Linnarsson, V. Jokubavicius, J. Sun, R. Liljedahl, and M. Syväjärvi, “Advances in wide bandgap SiC for optoelectronics,” The European Physical Journal B 87, 58 (2014).
[10] W. Lu, Y. Ou, P. M. Petersen, and H. Ou, "Fabrication and surface passivation of porous 6H-SiC by atomic layer deposited films," Optical Materials Express 6(6), 1956-1963 (2016).
[11] W. Lu, Y. Ou, V. Jokubavicius, A. Fadil, M. Syväjärvi, P. M. Petersen, and H. Ou, “Wavelength-conversion efficiency enhancement in nano-textured fluorescent 6H-SiC passivated by atomic layer deposited titanium oxide,” Physica Scripta 91, 074001 (2016).
[12] Y. Ou, A. Fadil, and H. Ou, "Antireflective SiC Surface Fabricated by Scalable Self-Assembled Nanopatterning," Micromachines, 7(9), 152, (2016).
[13] W. Lu, Y. Ou, E. M. Fiordaliso, Y. Iwasa, V. Jokubavicius, M. Syväjärvi, S. Kamiyama, P. M. Petersen, and H. Ou, "White light emission from fluorescent SiC with porous surface," Scientific Reports 7, 9798 (2017).
Surface-plasmon-enhanced LEDs
GaN-based LEDs have shown a promising role in the search for energy efficient visible light sources. By incorporating indium (In), the bandgap of InGaN can be tuned from near UV to over the entire visible spectrum. LEDs based on III-arsenide and phosphide material systems already constitute efficient light sources from the red to yellow spectral region, thus III-nitrides could play a crucial role for LEDs from blue to green. The internal quantum efficiency (IQE) of blue InGaN/GaN LED grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrate, easily reaches above 80 %. However, in moving towards longer wavelengths by increasing the In composition, the IQE suffers a significant decrease, resulting in a poor performance of green LEDs. One approach to improve the IQE of LEDs, involves the fabrication of Ag nanoparticles (NPs) in the near field of the active light emitting region, where the coupling between excitons and localized surface plasmon (LSP) modes is expected to provide a faster radiative decay channel thereby improving the IQE. As opposed to random self-assembled Ag NPs, with a regular array the surface plasmon resonance can be tailored to improve IQE at desired wavelengths. A low-cost fabrication technique is investigated as an alternative to E-beam lithography, based on nanosphere lithography (NSL).
Recent publications (selected):
[1] A. Fadil, D. Iida, Y. Chen, J. Ma, Y. Ou, P. M. Petersen, and H. Ou, “Surface plasmon coupling dynamics in InGaN/GaN quantum-well structures and radiative efficiency improvement,” Scientific Reports 4, 6392 (2014).
[2] D. Iida, A. Fadil, Y. Chen, Y. Ou, O. Kopylov, M. Iwaya, T. Takeuchi, S. Kamiyama, I. Akasaki, and H. Ou, "Internal quantum efficiency enhancement of GaInN/GaN quantum-well structures using Ag nanoparticles, " AIP Advances 5, 097169 (2015).
[3] A. Fadil, D. Iida, Y. Chen, Y. Ou, S. Kamiyama, and H. Ou, “Influence of near-field coupling from Ag surface plasmons on InGaN/GaN quantum-well photoluminescence,” Journal of Luminescence 175, 213-216 (2016).
[4] A. Fadil, Y. Ou, D. Iida, S. Kamiyama, P. M. Petersen, and H. Ou, "Combining surface plasmonic and light extraction enhancement on InGaN quantum-well light-emitters," Nanoscale 8, 16340-16348 (2016).
Nanostructured LEDs
Nano-fabrication technologies have enabled the improvement of currently existing LEDs and can be used to overcome the issue of low quantum efficiency of green GaN-based LEDs. Different nano-fabrication techniques have been applied to fabricate nanopillars on InGaN∕GaN quantum-well LEDs. By etching through the active region, it is possible to improve both the light extraction efficiency and, in addition, the internal quantum efficiency through the effects of lattice strain relaxation.
Recent publications (selected):
[1] A. Fadil, Y. Ou, T. Zhan, K. Wu, D. Suyatin, W. Lu, P. M. Petersen, Z. Liu, and H. Ou, “Fabrication and improvement of nanopillar InGaN∕GaN light-emitting diodes using nanosphere lithography,”Journal of Nanophotonics 9, 093062 (2015).
[2] Y. Ou, D. Iida, A. Fadil, and H. Ou, "Enhanced Emission Efficiency of Size-Controlled InGaN/GaN Green Nanopillar Light-Emitting Diodes," International Journal of Optics and Photonic Engineering 1, 1 (2016).
[3] Y. Ou, D. Iida, J. Liu, K. Wu, K. Ohkawa, A. Boisen, P. M. Petersen, and H. Ou, "Efficiency enhancement of InGaN amber MQWs using nanopillar structures," Nanophotonics 7, 317-322 (2018).
Ge photonics
Germanium possesses unique optical properties rather than Si, meanwhile compatible with the CMOS processing. One of the unique properties is that Ge could be tuned into a direct bandgap semiconductor by strain engineering, therefore it could solve the most challenging problem, i.e. a Si-based light source. We focus on the methods to forming Ge nanocrystals, and characterizing them in terms of size, absorption, emission etc. by means of TEM, SEM, SIMS, Raman spectroscopy, PL etc.
Recent publications (selected):
[1] H. Ou, Y. Ou, C. Liu, R. Berg, and K. Rottwitt, “Formation and characterization of varied size germanium nanocrystals by electron microscopy, Raman spectroscopy, and photoluminescence,” Optical Materials Express 1(4), 643-651 (2011).
Si photonics
Silicon (Si) is the most abundant element in the earth crust and has been dominant for more than half a century in microelectronics. Benefiting from the well-developed CMOS processing, Si is also emerging as an essential material platform for photonics, which is expected to address the current challenges in Si microelectronics. Combining the inherent advantages of Si (high refractive index) with the state-of-the-art nanofabrication facilities at DTU Danchip, we focus on designing and fabricating Si optical devices for polarization and dispersion management, format conversion, mode-division multiplexing etc, and implementing them in the system experiments in collaboration with High Speed Optical Communications group.
Recent publications regarding Si photonics can be found from here.