Surface preparation of substrates from bulk GaN crystals

Large gallium nitride (GaN) crystals were grown using a hydride vapor phase epitaxy (HVPE) technique and were processed into substrates for device applications. Polishing procedures were developed for GaN substrates to produce surfaces prepared for epitaxial growth. Surface preparation of (0 0 0 1) and (0 0 0 1¯) substrates was examined, along with preparation of (1 1 2¯ 0) and (1 1¯ 0 0) non-polar surfaces. For all surfaces, chemical mechanical polishing (CMP) resulted in an average root mean square (RMS) surface roughness on a 5 μm×5 μm scanning probe microscope (SPM) image of <0.2 nm. Characterization of the surfaces of polished substrates by cross-sectional transmission electron microscope (TEM) showed no sub-surface damage and no epitaxial defects generated at the substrate/epi interface during homoepitaxial growth. Cathodoluminescence (CL) imaging was used to verify the defect density and that no defects related to polishing were present. The average dislocation density of the substrates was <5×106 cm−2.