Dr. Sang-Youp Lim's reserach team succeeded in observing and analyzing microscopic areas for low efficiency of green LED

□ Dr. Sang-Youp Lim's research team at the Advanced Photonics Research Institute at the Gwangju Institute of Science and Technology (GIST, President Seung Hyeon Moon) has observed that the low efficiency factor of green LED by using micro fluorescence imaging technique, and the micro luminescence centers intensify the non-luminescence loss due to the Auger coupling process.

□ The fact that no LED material with high efficiency is found in the green wavelength region is called "green gap." * In particular, much research has been conducted on the decrease in the efficiency of the green LED obtained by increasing the indium composition of the indium gallium nitride (InGaN) compound. However, previous studies have proposed various possibilities for the low efficiency of green LEDs, mostly from a macroscopic point of view, but the debate over key factors is ongoing in the absence of clear research results.

* It refers to a region (wavelength of 530nm ~ 570nm) in which quantum efficiency is drastically reduced in general semiconductor light emitting devices and high efficiency is difficult to obtain.

∘ In addition, the InGaN compound is formed with a micro-luminescence center that generates bright light in the local region due to crystal growth characteristic. It has been continuously suggested to use this as a way to overcome low optical efficiency. However, there is also no study that shows its efficiency.

□ The researchers used the InGaN semiconductor quantum well structure and applied micro fluorescence imaging technique to measure and analyze optical properties in the micro region. This technique can acquire spectral information together with spatial information of an imaging area by using a fluorescence spectral acquisition device with a microscope. By extracting and reconstructing the intensity and wavelength information of the fluorescence spectrum, it is possible to closely observe how the charge carriers generated by the excitation light source * behave in the micro-region. In particular, the basic data on the light efficiency can be obtained in the micro area.

* Light source for exciting the medium (optical pumping). The charge carriers excited in the medium consume energy while emitting light or heat.

∘ As a result, the emission center of the sub-micrometer size generates very bright light at the low excitation light intensity, but the light efficiency decreases sharply as the excitation light intensity increases. It was confirmed that the light efficiency of the light emitting region of the size of several micrometers was increased.

□ The researchers have shown directly that the non-luminescent Auger coupling phenomenon is extensively expressed in the sub-micrometer luminescent center due to the limited high-density charge carriers in the narrow region. In addition, it has been confirmed that the formation of such a sub-micrometer emission center reduces the total volume of the effective light emitting region of the LED.

□ Dr. Sang-Youp Lim said, "The results of this research are significant for the first observation and analysis of the low efficiency of green LEDs in the micro-region, and we hope this will be used as a cornerstone for the successful development of micro-structured devices, such as micro LEDs."

□ This research was supported by the Korea Research Foundation Foundation and the Nanotechnology and Bioengineering Research Project of the GIST Advanced Photonics Research Institute. The results of this research are presented in ACS Photonics, an international scientific journal in the field of optics. It was posted online on January 8, 2018.