Strain relaxation in AlGaN multilayer structures by inclined dislocations

Abstract

To examine further the strain relaxation produced by inclined threading dislocations in AlGaN, a heterostructure with three AlGaN layers having successively increasing Ga contents and compressive strains was grown on an AlN template layer by metalorganic vapor-phase epitaxy. The strain state of the layers was determined by x-ray diffraction (XRD) and the dislocation microstructure was characterized with transmission electron microscopy (TEM). As the GaN mole fraction of the heterostructure increased from 0.15 to 0.48, the increased epitaxial strain produced inclined dislocations with successively greater bend angles. Using the observed bend angles, which ranged from 6.7° to 17.8°, the measured strain relaxation within each layer was modeled and found to be accounted for by threading-dislocation densities of 6–7×109/cm2, in reasonable agreement with densities determined by TEM and XRD. In addition to the influence of lattice-mismatch strain on the average bend angle, we found evidence that local strain inhomogeneities due to neighboring dislocations influence the specific bend angles of individual dislocations. This interaction with local strain fields may contribute to the large spread in the bend angles observed within each layer. A detailed TEM examination found that the initial bending of threading dislocations away from vertical often occurs at positions within <15 nm of the AlGaN/AlN heterointerface. Under the assumption that dislocation climb mediated by bulk-defect diffusion is effectively suppressed at the growth temperature, this result implies that inclination is established by processes occurring at the dynamic growth surface. We describe a mechanism where dislocation bending occurs by means of dislocation-line jogs created when surface steps overgrow vacancies that attach to threading-dislocation cores at their intersection with the growth surface.