Interlayer methods for reducing the dislocation density in gallium nitride

The ability of various interlayers to lower the density of threading dislocations (TDs) has been studied for the growth of c-plane (0 0 0 1) GaN epilayers on sapphire by metalorganic vapor-phase epitaxy (MOVPE). The TD density in the films may be reduced to 9×107 cm−2 using in-situ SiNx interlayers formed from silane and ammonia. This TD reduction method relies on the formation of facetted islands on the SiNx-treated GaN surface and the formation of dislocation half loops between bent-over TDs during the lateral overgrowth, and not by physically blocking the TDs. Thus, the TD 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 have also proved effective for defect reduction; dislocation densities of 3×107 cm−2 have been reached by depositing thin (5–15 nm) Sc metal layers on 500 nm GaN-on-sapphire seed layers, then annealing in NH3 to form ScN, followed by overgrowth of GaN. The low dislocation densities are 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.