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Using quantum dots and nanograting to improve emission efficiency of III-nitride based light-emitting diodes (LEDs) PIs: 程育人, 施閔雄, 張亞中,(RCAS), 王興宗,(NCTU), 綦振瀛(NCU & RCAS) This research proposal will focus on two areas: 1) Studies of cavity exciton-polaritons in III-nitride with and without quantum dots and the dependence of emission efficiency on exciton-photon coupling. 2) Effects of nano-scale grating and geometry on the emission efficiency. The goal is to understand scientific issues of the exciton-photon (light-matter) interaction dynamics in III-nitride quantum well/dot LEDs and find ways to improve the emission efficiency via nanotechnology. The III-nitride based optoelectronic devices covers applications from solid state lighting, high density optical data storage (blue/UV lasers) to high speed/power electronics. The III-nitride based LED is considered the most promising device for energy efficient lighting application. This material system also finds important applications in cavity polariton research due to its high exciton binding energy and large oscillator strength. Research in III-nitride material and devices are crucial for the next generation optoelectronic and energy devices. Many scientific and technological issues related to III-nitride devices remain unsolved. For example, the photoluminescence of III-nitride material was found to be much less sensitive to crystal defect than other well developed III-V alloys, such as GaAs. III-nitride based microcavities can have very large and fast optical nonlinearity. III-nitride growth technology still has a lot of room to improve. The strain built up from lattice-mismatch during growth usually leads to the formation of crystal defects and/or self-assembled nano clusters. Aside from lattice-mismatch, the InGaN/InN compound can also spontaneously form nano dots or clusters due to its phase segregation property. These nano-scale embedded dots together with crystal dislocation defects all affect its photoluminescent characteristics. We plan to investigate the optoelectronic properties and the potential applications to energy efficient solid state lighting device. Furthermore, when excitons in the excited semiconductor material interact with cavity photons in the strong interaction regime, they can form a quasi bosonic particle state at room temperature, the so called cavity polaritons. The condensation of polaritons can lead to spontaneous coherent radiation and have applications in polariton lasers, nonlinear optics, and quantum information. In the past, the research has been carried out in materials that can operate only under cryogenic temperature. Recently, this scientific research has been moved to III-nitride material as it becomes more available and promises a room temperature operation due to its large exciton binding energy. We plan to study the exciton-photon interaction dynamics in III-nitride material and the polariton laser realization. We will also fabricate properly designed nanoscale photonic crystal cavity and couple it with the III-nitride light-emitting device. We have internationally competitive expertise in laser cavity, nanostructure electronic states and electromagnetic wave modeling, as well as photonic-crystal cavity fabrication. We will explore the effect of exciton-photon coupling inside the cavity and find the best design to optimize the extraction efficiency of light from the III-nitride LEDs. Research approaches: 1. Develop fabrication processes that can produce controlled nano structures and quantum dots in shape, size, and composition [1]. The formation of quantum dots often relies on the self-assembled process, which is often induced by strain built up and/or subsequent annealing. It always carries certain degrees of randomness. It is crucial to develop process parameters with low degree of randomness. 2. Characterize the optoelectronic properties, e.g. PL spectrum, temperature dependence, etc, by various analysis tools (e.g. micro/nano PL/EL/Raman, NSOM, SEM, TEM, etc.) and compare with theoretical simulation based on realistic models. The focus is to gain knowledge about the dependence of quantum well/dot energy level and luminescent properties on its physical and material parameters. 3. Design photonic crystal cavity to optimize the light emission by using an efficient plane-wave based Green’s function method developed by Y. C. Chang [2]. Once the best design is found, the actual fabrication will be made by solid state fabrication processes [3]. The fabrication procedure includes electron beam lithography, and plasma-assisted dry etching etc. 4. Aside from the above practical application, cavity polariton dynamics is an important scientific research subject. The device structure for realization can be a monolithically grown quantum well(s)/dot(s) in vertical surface emitting cavity [4]. The cavity polariton dynamics will be investigated by photoluminescent spectral measurement. References: [1]. C.-K. Chao, H.-S. Chang, T. M. Hsu, C.-N. Hsiao, C.-C. Kei, S.-Y. Kuo, and J.-I. Chyi , Nanotechnology, 17, 15, 3930, Aug. 2006; J.-I. Chyi et al., Phys. Rev. Lett. 96. 117401, (2006) [2]. Y.-C. Chang, G. Li, H. Chu, J. Opsal, J. Opt. Soc. A 23, 638 (2006);Y. C. Chang, H. Chu, J. Opsal; US patents 6867866B1 and 7038850B2. [3]. M. H. Shih, W. Kuang, M. Bagheri, A. Mock, S. J. Choi, J. D. O’Brien and P. D. Dapkus, Appl. Phys. Lett. 90, 121116 (2007); M. H. Shih et al., IEEE Photon. Tech. Lett. 18, 535 (2006). [4]. Y.-J. Cheng, J.-T. Chu, H.-C. Kuo, T.-C. Lu, and S.-C. Wang, Frontiers in Optics, postdeadline paper PDP-B8, (2007); Y.-J. Cheng, PRL 97, 093601, 2006.