Elucidation of efficiency droop mechanism in nitride semiconductorbased light emitting devices by scanning near field optical microscopy

为了阐明效率下降现象的主要因素，我们使用扫描近场光学显微镜进行了PL和时间分辨PL测量。对于蓝色样品，虽然在高载流子密度下载流子从局域中心溢出，但由于在非辐射复合中心（NRC）周围形成了势垒，因此几乎不发生对NRC的捕获。另一方面，对于绿色样品，载流子密度的增加增强了从强PL畴向对应于高In组分区域的弱PL畴的扩散，在该区域中大量的NRC与穿透位错相关联地分布。发现这种载流子复合动力学是绿色样品中效率下降的主要机制，这是由于载流子寿命和穿透位错数量的增加。

To clarify main factor of efficiency droop phenomena, we performed PL and time-resolved PL measurement using a scanning near field optical microscope. For the blue sample , although carriers overflow from localization centers under the high carrier density, the capture to nonradiative recombination centers (NRCs) hardly takes place because the potential barriers are formed around the NRCs. On the other hand, for the green sample, the increase of carrier density enhances the diffusion from the strong PL domains to the weak ones corresponding to high In composition area, where a large number of NRCs are distributed in association with threading dislocations. Such carrier recombination dynamics was found to be a major mechanism of the efficiency droop in the green sample due to the increase of both carrier lifetime and number of threading dislocations.