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.

Characterization of nanometer scale compositionally inhomogeneous AlGaN active regions on bulk AlN substrates

“…Therefore, the highly strained NCI-AlGaN film grown on bulk AlN is expected to contain a significantly lower dislocation density than similar fully relaxed films grown on c-plane sapphire or templates consisting of thin III-Nitride layers grown on c-plane sapphire. It should be noted that other groups have reported on strain relaxation in high AlN mole fraction films (>60%) grown on sapphire or SiC substrates through the formation of dislocations [11,12] in contrast to what we observe for growth of these films on bulk AlN. Understanding this result in terms of the differences in the strain relaxation mechanisms between the AlGaN/ bulk AlN system and the more commonly investigated AlGaN/AlN/c-plane sapphire system requires further investigation.…”

Curvature and bow of bulk GaN substrates

“…23,32,33 TEM studies done by Cantu et al reported a TDD of $3 Â 10 10 cm À2 , where the TD was inclined by 10 -25 in the 1 100 direction and had a misfit component, thus relieving misfit strain. Observations of Follstaedt et al for compressively strained AlGaN indicated that the TD inclination occurred prior to the introduction of Si, where the average inclination angle was $19 , and the TDD is $6.9 Â 10 9 cm À2 .…”

Analysis of reaction between c+a and -c+a dislocations in GaN layer grown on 4-inch Si(111) substrate with AlGaN/AlN strained layer superlattice by transmission electron microscopy

“…[14][15][16][17][18][19] As mentioned in Ref. 18, Burgers vector analysis is required to prove this.…”