Mechanism of nucleation and growth of catalyst-free self-organized GaN columns by MOVPE

The growth mechanism of catalyst-free self-organized GaN nuclei and three-dimensional columns on sapphire by metal organic vapour phase epitaxy (MOVPE) is investigated. Temperature-and time-dependent growth is performed. The growth behaviour can be characterized by two different kinetic regimes: mass-transport-limited growth and thermodynamically limited growth. The sum of activation energies for thermodynamic barrier of nucleation and for surface diffusion/mass-transport limitation, i.e. W-het + E-d, is 0.57 eV in the 'low'-temperature region and 2.43 eV in the 'high'-temperature region. GaN columns grown under the same conditions have very comparable height, which is not dependent on their diameter or the distance to other columns. Therefore, the growth rate is presumably limited by the incorporation rate on the top surface of columns. The height and diameter at the top of the GaN columns increase linearly with time and no height limit is observed. The GaN columns can reach more than 40 mu m in height. Moreover, the investigated GaN columns are Ga-polar.