Triangular defects as source of doping inhomogeneity in germanium-doped bulk GaN

This study presents analysis of growth irregularities in highly-doped n-type gallium nitride crystals grown via Halide Vapor Phase Epitaxy. Triangular defects - regions of inhomogeneous dopant incorporation associated with hillock merging during bulk crystallization - were identified in germanium-doped gallium nitride crystals with two different doping levels. In this work, we demonstrate a direct correlation between the presence of triangular defects and impurity incorporation, resulting in free carrier concentration gradients. To identify these regions, optical microscopy with ultra-violet illumination and X-ray topography were employed. Time-of-flight secondary ion mass spectrometry was used to determine the impurity levels within the triangular defects compared to the crystal matrix. Furthermore, micro-Raman spectroscopy was utilized to track the free carrier concentration and local strain of the crystal lattice. The results indicate a significantly lower concentration of incorporated donors and free electrons within the triangular defect regions. These findings indicate that the formation of triangular defects should be always considered regardless of the growth technique and the doping level.