GaN/InGaN LED Sidewall Defects Analysis by Cathodoluminescence and Photosensitive Kelvin Probe Force Microscopy

This paper delves into the impact of sidewall defects on a GaN/InGaN epi-LED, utilizing advanced analytical techniques with nanometric resolution: cathodoluminescence (CL) and photosensitive Kelvin probe force microscopy (KPFM). CL investigations at varying excitation energies enable an in-depth study of the epi-LED by distinctly probing the top p-GaN layer and InGaN quantum wells within the mesa structure. These measurements permit the analysis of the impact of sidewall defects on the light-emitting diode (LED) light emission in different epilayers, shedding light on the phenomena affecting LED optoelectronic properties. The results reveal reduced intensities related to the GaN near-band-edge (NBE) and InGaN quantum well emission, along with an increased yellow luminescence band near the mesa edge, attributed to sidewall defects induced during the etching process. Conversely, the top p-GaN layer shows no NBE band and uniform ultraviolet luminescence line (UVL) related to MgGa acceptors, suggesting no effect from sidewall defects. Similarly, the surface photovoltage (SPV) measured on the top p-GaN surface by KPFM shows no significant changes near the mesa boundary. Moreover, KPFM measurements show a slow SPV decay after illumination, indicating charge trapping of holes in the surface of p-GaN, as evidenced by positive SPV and consistent with the simulations. We attribute the absence of the sidewall defects effect and the charge trapping in the p-GaN layer to high-density Mg-related defects, as supported by the uniform CL UVL emission. To ensure that sidewall defects were not masked by the high concentration of Mg impurities, we conducted photosensitive KPFM at low temperatures. At these temperatures, defects are "frozen", and different types of defects exhibit distinct detrapping time scales. The consistent behavior observed at both the center and border of the mesa structure across three different temperatures confirms that only one type of defect associated with Mg doping rather than sidewall defects is being observed. These findings suggest that the etching process used in LED fabrication does not significantly impact the role of the top p-GaN layer in LED miniaturization. Moreover, they provide crucial insights into defect-induced processes and their influence on device performance, particularly in the vicinity of LED edges. Understanding these effects is essential for fabricating high-performance microLEDs.