The Ga-Nitride/air Two-Dimensional Photonic Quasi-crystals Fabricated on GaN-based Light Emitters
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The Ga-Nitride/air Two-Dimensional Photonic Quasi-crystals Fabricated on GaN-based Light Emitters Bei Zhang*, ZhenSheng Zhang, Jun Xu, Qi Wang, ZhiJian Yang, WeiHua Chen, XiaoDong Hu, ZhiXin Qin, GuoYi Zhang, DaPeng Yu School of Physics, Peking University, and State Key Laboratory for Mesoscopic Physics, Beijing 100871, CHINA ABSTRACT The two-dimensional (2D) Ga-nitride/air photonic quasi-crystals (PQC) were successfully fabricated by the technique of focused Ga ion beam (FIB) milling on GaN based epitaxial wafers. The effects of the PQC on the current injected edge emitting GaN-based light emitters were investigated. The quasi-crystal structures studied in this work are based on square-triangular tiling with 8-fold or 12-fold symmetry. The air hole diameter in the different PQC patterns was varied from 95nm up to 1200nm, the filling factor of air hole was in the range of 10% to 50% and the depth of the hole was 90nm to 370nm, respectively. Among these, the nearest center-to-center distance of the holes and/or the lattice constant was reached to be 170nm. The photonic quasi-crystals on the GaN-based light emitters demonstrated a two to three factor of magnitude enhancement of surface extractive emission. The blocking of the propagation of planar-guided modes by the photonic quasi-crystals was observed. By comparison of symmetry effect among the triangular lattice PC and PQCs, it was found that the GaN-based photonic quasi-crystal (PQC) with 12-fold symmetry provided a much favorable enhancement of the extractive light emission over that of triangle lattice PC and 8-fold symmetric PQC as well. INTRODUCTION Photonic crystals (PCs) have potential applications in many research areas particularly in light emitting devices. By using PCs, the improvement of light extraction was obtained by out-coupling of remaining guided modes [1, 2]. Currently, the photonic crystal composed of quasi-crystal structure attracts much interest. Quasi-crystals are the structures with high-order rotational and line symmetries but without translation symmetries. It was suggested that photonic quasi-crystals with higher level of symmetry also possess photonic band-gaps. Moreover, photonic quasi-crystals exhibit unique light behaviors, such as isotropic of band-gap and light scattering, even with a lower air hole filling factor (f) and with relatively lower refractive index of materials than that of conventional PCs. Zoorob etc [3] demonstrated the complete photonic band-gap of the quasi-crystals on silicon nitride waveguide. Thus, it is expected that PCs especially quasi-crystals could be promising for application in a variety of optical devices. However, there is a big technical challenge in the fabrication of GaN based PCs with relatively smaller scale due to the shorter emitting wavelength. Recently, incorporation of PCs with triangular lattice patterns of air holes on GaN LEDs using electron beam lithography
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combined with dry etching has demonstrated improved extraction efficiency [4,5]. Unfortunately, to our best knowledge
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