Measuring near field distribution of photonic crystal GaN-based blue Light Emitting Diodes with NSOM

초록

Recently, Photonic Crystal (PhC)-patterned Light Emitting Diodes (LED) becomes main research subject, because the incorporated PhC structure diffracts waveguide modes out of LED and thus can greatly improve its light extraction efficiency. Accordingly, it is very important to find the optimal PhC structure for realizing high efficient blue LEDs. In our study, the near-field distribution on the chip surface of PhC GaN-based blue LED was measured with Near-field Scanning Optical Microscopy (NSOM). Through these near-field measurements, both the validness of the design procedure and the accuracy of the fabrication procedure were verified. The measured PhC GaN-based blue LED has the layer structure of Sapphire substrate - n-GaN - Multi Quantum Well (MQW) - p-GaN - ITO - encapsulated epoxy resin, and the PhC is incorporated onto either the p-GaN layer or ITO layer. For the measurement the epoxy resin was removed in advance. When the current was applied to the MQW, the light was emitted out of LED and the near-field distribution at the surface of LED chip was picked up by the fiber probe of NSOM system. The system was made by ourselves, and the distance between the probe and the surface was controlled by shear force feedback control method using tuning fork, where Lock-in amplifier was used for noise reduction and for dithering the probe. By using NSOM, the three-dimensional profile of PhC surface was measured as well as the near field distribution.