Dislocation bending in GaN/step-graded (Al,Ga)N/AlN buffer layers on Si(111) investigated by STM and STEM

The distribution and bending of dislocations in GaN/step-graded (Al,Ga)N/AlN buffer layers grown on Si(111) are investigated by cross-sectional scanning tunnelling microscopy (STM) and scanning transmission electron microscopy (STEM). We observe dislocations with -type Burgers vector intersecting the m-plane cleavage surface and having line directions bent off the [0001] growth direction toward non-polar directions. The spatial distribution of dislocations intersecting the m-plane cleavage surface indicates consecutive bending of dislocations due to strain at interfaces between subsequent lattice mismatched buffer layers and at doping junctions, reducing the density of threading dislocations at the (0001) growth front. No interface misfit dislocations, v-shaped defects, or loss of crystalline quality are observed, demonstrating the high performance of the step-graded (Al,Ga)N/AlN buffer layers on Si for relaxing the lattice constant without creating large defect concentrations