Characterization of bulk gallium nitride by defect-selective wet etching
Authors:Dano Pagenkopf, Paul Von Dollen
Mentor:Siddha Pimputkar, Postdoctoral Researcher, University of California Santa Barbara
Gallium nitride (GaN) enables high intensity white, blue and green light emitting diodes (LEDs) and blue-violet lasers (e.g., Blu-ray ™). Hetero-epitaxial grown devices have proven successful despite high densities of extended structural defects caused by the lattice mismatch to the sapphire substrates. The next generation of GaN semiconductors will be grown homo-epitaxially on bulk GaN substrates. Simple, reliable, low cost characterization and preparation methods need to be developed for bulk GaN substrate qualification. Bulk GaN samples were prepared using a diamond wire saw for wafer separation from bulk GaN boules, planarized and polished using mechanical and chemical mechanical polishing (CMP) techniques to produce surfaces ready for epitaxial growth and etching. Methods utilized to characterize bulk GaN crystals included X-ray Diffraction (XRD) for crystallographic orientation, and crystal quality determination as well as a defect-selective wet etchant process for bulk GaN crystals. The resulting etch pit densities were counted using scanning electron microscopy (SEM), and differential interference contrast microscopy (DICM), and can be correlated to material quality and dislocation densities. For bulk GaN layers, the etch pit density on the order of 10^8 cm^-2 suggests a dislocation density of approximately 10^8 cm^-2. These results can be compared to hetero-epitaxial GaN on sapphire which has a dislocation density of 10^9 cm^-2.