Direct Observation of High Current Density Area by Microscopic Electroluminescence Mapping in Vertical GaN p–n Junction Diodes

This paper presents electroluminescence intensity mapping on a p–n junction plane of vertical GaN diodes under forward‐biased conditions for the first time. By this mapping, it has been discovered that current crowding existed, corresponding to the naturally formed surface stripes on epitaxial layers grown on freestanding GaN substrates. Detailed analyses by AFM and TOF‐SIMS clarified that the concentration of doped Mg acceptors on one slope of the stripe was higher than that on the other slope. The higher Mg‐concentration region should have lower electric resistance, which would cause the current crowding. By improving the surface flatness, the current crowding was suppressed and a low specific on‐resistance was obtained. Characteristic stripe‐shaped surface morphology often appears on MOVPE‐grown layers at a certain range of c‐axis off‐angle of GaN substrates. The p–n junction diode fabricated on such surface shows a non‐uniform electroluminescence intensity pattern (reflecting its current‐density distribution) under the anode electrode matching with the morphology. This method is a strong tool for evaluating the quality of GaN epitaxial layers.