Surface morphology of homoepitaxial c-plane GaN: Hillocks and ridges

We investigated the properties of a GaN epilayer grown by metalorganic vapour phase epitaxy on a c-plane bulk GaN substrate obtained by ammonothermal growth. X-ray diffraction measurements showed that the epilayer and substrate were fully relaxed, had a miscut angle of 0.3±0.05° towards m and had omega rocking curve width values of 20-30 arcsec, limited by the instrumental broadening. Scanning capacitance microscopy data of the sample in cross-section indicated that the substrate had n-type conductivity with a carrier concentration of at least 1019 cm−3. Combined optical Nomarski microscopy, atomic-force microscopy and scanning electron microscope-cathodoluminescence studies showed the presence of large hexagonal pyramids on the surface, each associated with one or two dislocations with a screw-component threading from the substrate. This observation leads us to calculate a lower limit of the threading dislocation density of 3×102 cm−2. We predict that the formation of such hexagonal hillocks during epitaxy can be avoided with a slightly larger miscut angle of 0.4° or 0.5°. Another type of defect observed were ridge-like surface structures with narrow arrays of edge-type threading defects with a local density of 109 cm−2. However, the absence of threading defects below the regrowth interface at a ridge suggested that this type of structure is linked to (polishing) damage to the substrate surface and is therefore rated as an avoidable problem.