Nucleation and growth of GaN/AlN quantum dots

We study the nucleation of GaN islands grown by plasma-assisted molecular-beam epitaxy on AlN(0001) in a Stranski-Krastanov mode. In particular, we assess the variation of their height and density as a function of GaN coverage. We show that the GaN growth passes four stages: (i) initially, the growth is layer by layer, (ii) subsequently, two-dimensional precursor islands form, which (iii) transform into genuine three-dimensional islands. Island height and density increase with GaN coverage until the density saturates. (iv) During further GaN growth, the density remains constant and a bimodal height distribution appears. A fit of an equilibrium model for Stranski-Krastanov growth to the variation of island distributions as a function of coverage is discussed.