Origin of pyramidal hillocks on GaN thin films grown on free-standing m -plane GaN substrates

The relationship between pyramidal hillocks and dislocations was studied for GaN thin films grown by metalorganic chemical vapor deposition on low-dislocation-density free-standing m -plane GaN substrates. Four-sided pyramidal hillocks were observed on GaN thin films grown on nominally on-axis m -plane GaN substrates. Cathodoluminescence measurements revealed the presence of a dislocation at the apex of each pyramidal hillock. High-resolution atomic force microscopy images showed a pinned step at the apex of each pyramidal hillock and a spiral ramp around the termination of the step, indicating that the pyramidal hillocks arise from spiral growth around screw-component dislocations intersecting the surface of the crystal.