The influence of nucleation coalescence upon the crystalline quality of AlN films grown on sapphire by plasma-assisted molecular beam epitaxy was investigated. The coalescence speed was controlled by the V/III ratio chosen for the growth after nucleation. A slightly Al-rich condition, corresponding to slow coalescence, could significantly reduce the density of edge threading dislocations, which was found to be dominant in AlN epilayers. The cross-sectional TEM image of the AlN epilayer grown under this condition clearly revealed an automatically formed boundary where an abrupt decrease of edge threading dislocation density occurred.

Improvement of AlN Film Quality by Controlling the Coalescence of Nucleation Islands in Plasma-Assisted Molecular Beam Epitaxy. Z.Chen, Z.B.Hao, F.Ren, J.N.Hu, L.Yi: Chinese Physics Letters, 2010, 27[5], 058101