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In all walks of life today, led a wide range of applications, regardless of the display, lighting, or communications field. But in the usual light-emitting devices due to total internal reflection transmission mode, the light efficiency greatly curtailed in recent years made a lot of methods to improve led torch extraction efficiency, such as surface roughness, and the cavity method, the use of these methods have been observed to varying degrees of light efficiency.
Similar semiconductor electronic band gap of the photonic band gap in the photonic crystal, it is expected to become the next generation of optical semiconductor to become the research hot topics. Due to the existence of photonic crystals, photonic band gap, making the photons of certain frequencies can not be, this feature can be used to implement a variety of optoelectronic devices with excellent characteristics, such as photonic crystal lasers, photonic crystal angle bent waveguide.
This paper studies the bandgap of the photonic crystal structure to improve the efficiency of the led torch. Use of finite difference time domain method is analyzed theoretically the change of the lattice constant the band gap position change and the corresponding impact on the efficiency of the led torch, the use of semiconductor process fabricated photonic crystal structure, and experimentally verified photonic crystals to improve the efficiency of led torch effect.
2 Theoretical analysis and experimental production
2.1 energy band structure analysis
FDTD method is mainly used to calculate the domain of electromagnetic wave propagation and scattering in photonic crystal inside through the analysis of electromagnetic field of the same time in different locations, distribution, and the electromagnetic field distribution at different times the same location, the analysis of photonic crystal internal electromagnetic field propagation purpose. FDTD method is not only able to analyze the energy band structure of photonic crystals, the same time to different locations or at different times the same position of the electromagnetic field distribution. We use two-dimensional FDTD method to obtain the energy band structure of photonic crystals, and also analyzed the time crystals to change the lattice constant changes with location. Figure 1 shows a = 600nm r / a = 013 two-dimensional infinite photonic crystal TE mode photonic crystal band structure of the normalized frequency range of 0121 ~ 0128 of the photonic band gap can be found from the figure, photon density of states is zero in this frequency range. The also found that the band gap does not appear in the energy band structure of the TM polarization mode.
2.2 photonic crystal micro-machining
There are many ways the production of photonic crystal structures in semiconductor materials have an advantage, but the process compatible with semiconductor technology to achieve optoelectronics integration. Due to the improvement of the micro-processing technology, the fine structure such as photonic crystal device fabrication is more accurate. There are a variety of methods of processing photonic crystal, we mainly used the method of electron beam lithography to generate graphics using reactive ion beam etching (RIE) to transfer the graphics.
First try only using the PMMA glue to do the mask, and graphics directly transferred from the plastic to the InP material photonic crystal structure. EBL defined photonic crystal graphics. To increase led flashlight of the mask in the etching process and materials, after developing chips for more than 2h, bake in a hot environment to make more rigid PMMA mask.
RIE system to etch the InP material directly. Etching in CH4 + H2 + Ar gas mixture, the etching after the completion of the graphics shown in Figure 2. The figure shows, due to the poor selectivity of the PMMA mask, etching after the graphics have been deformed. Is formed in the material in accordance with the theoretical design and graphics of the EBL-defined structure of air holes etched after the results of cone-shaped column of material, lateral erosion is more serious in the etching process, the adjacent hole connected to each other, forming tapered cylindrical structure. Even so, the uniformity of the process etching graphics remain good, which also shows that by optimizing the process to improve the selectivity of the mask can get the required graphics.