The ability of various interlayers to lower the density of threading dislocations  was studied for the growth of c-plane (00•1) GaN epilayers on sapphire by metalorganic vapour-phase epitaxy. The threading dislocation density in the films may be reduced to 9 x 107/cm2 using in situ SiNx interlayers formed from silane and ammonia. This threading dislocation reduction method relied upon the formation of facetted islands on the SiNx-treated GaN surface and the formation of dislocation half loops between bent-over threading dislocations during the lateral overgrowth, and not by physically blocking the threading dislocations. Thus, the threading dislocation density was reduced at the expense of greater film thickness by increasing the SiNx coverage and delaying intentionally the coalescence of the GaN islands. Scandium nitride interlayers had also proved effective in defect reduction; dislocation densities of 3 x 107/cm2 were reached by depositing thin (5–15nm) Sc metal layers on 500nm GaN-on-sapphire seed layers, then annealing in NH3 to form ScN, followed by overgrowth of GaN. The low dislocation densities were attributed to a dislocation-blocking effect induced by the different (but compatible) crystal structures of GaN and ScN, whereby the dislocations threading from the seed layer cannot propagate through the ScN layer. The remaining dislocations appear to form on coalescence of the dislocation-free GaN islands which nucleate on the ScN interlayer.

Interlayer Methods for Reducing the Dislocation Density in Gallium Nitride. M.J.Kappers, M.A.Moram, Y.Zhang, M.E.Vickers, Z.H.Barber, C.J.Humphreys: Physica B, 2007, 401-402, 296-301