Observation of Anisotropic Growth and Compositional Discontinuity in AlGaN Electron-Blocking Layers on GaN Microrods

In GaN microrods, phase separation of the AlGaN electron-blocking layer is an enormous obstacle for achieving high-efficiency light-emitting diodes, as this phenomenon negatively affects the device efficiency by inducing unwanted band-energy modulations. Here, we found that the AlGaN electron-blocking layer on each <i>m</i>-plane of the GaN microrod appears to be phase separated, and each electron-blocking layer has a different thickness and length. Our careful analysis based on atom probe tomography reveals that the Al distribution in AlGaN is not uniform and that Al-rich and Al-deficient regions are clearly present. In addition, the longer surface diffusion length of Ga adatoms, as compared to Al adatoms, and the different initial strain state of each <i>m</i>-plane in the GaN rods are deeply associated with the different growth rates and inhomogeneous Al composition of AlGaN, resulting in phase separation of the AlGaN electron-blocking layer. These atomic-scale observations in the structural and chemical composition of AlGaN grown on GaN microrods could provide expanded opportunities for building a wide range of high-quality AlGaN electron-blocking layers