Improvement of AlN Film Quality by Controlling the Coalescence of Nucleation Islands in Plasma-Assisted Molecular Beam Epitaxy

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