Spatially Resolved Spectroscopy of Blue and Green InGaN Quantum Wells by Scanning Near‐Field Optical Microscopy

We investigated nanoscopic spectroscopy of blue and green InGaN multiple quantum wells (MQWs) by scanning near-field optical microscopy (SNOM). The photoluminescence (PL) spectra showed emission peaks related to GaN and a main InGaN peak both in the blue and green MQWs. In addition, local emission peaks appeared on the higher-energy side of the main InGaN peak (high-E emission) in only the green InGaN MQW. The PL intensity map acquired from the high-E emission in the green MQW was compared with the GaN emission intensity map, and there was no clear correlation between the high-E emission regions and the dark spots in the GaN emission intensity map at room temperature (RT). The high-E emissions are not due to potential barriers, but to other defects in the quantum wells. This may be because there is no high-E emission related to the potential barrier at RT owing to the small V-pit size, which reduces the suppression of carrier diffusion into dislocations especially at RT. Thus, the SNOM-PL is measured for the green MQW at low temperature. In contrast to RT, there was a clear correlation between the high-E emission regions and the dark spots in the GaN emission intensity map at 40K, indicating that potential barriers formed around threading dislocations. The potential barrier height estimated from the SNOM-PL spectra of the green MQW was sufficient to suppress carrier diffusion into threading dislocations.