Nanoscale characterization of two semi-vertical GaN stacks by scanning probe microscopy

Energy-efficient power devices benefit from GaN’s superior intrinsic properties, such as wide band gap, high electron-saturation velocity, and high thermal conductivity. However, the efficiency of GaN-based devices is hindered by the presence of dislocations within GaN stacks. In this work, structural and electrical properties of threading dislocations in metal-organic vapor-phase epitaxy-grown Si-doped GaN (0001) were determined by combining multiple scanning probe microscopy approaches. Specifically, two semi-vertical GaN stacks on QST® substrates, characterized by different buffer layers, were investigated. The dislocation density was quantified using Atomic Force and Scanning Electron Microscopy, aiding in the identification of the most suitable buffer layer. The space charge region at the dislocation sites was examined through differential capacitance measurements (dC/dV) via Scanning Capacitance Microscopy. Additionally, Conductive Atomic Force Microscopy revealed leakage spots on the surface, constituting approximately 1% of the dislocation density, compatible with the hypothesis that pure screw dislocations are electrically active ones. To comprehend the electrical transport mechanism, I-V curves were acquired at leakage spots and different models were fitted. Furthermore, distinct morphological features on one sample’s surface were investigated and attributed to a gallium oxide adlayer. The impact of KOH etching on the GaN surface was also explored, linking the formation of anisotropic steps to nitrogen-dangling bonds.