We demonstrate that thin AlN buffer layers improve the orientation of GaN nanowire grown on Si(111). The deposited GaN initially forms into islands which act as a seed for the wires. By raising substrate temperature, the actual amount of grown material decreases but wire density increases and well-separated wires are achieved. Fast wire length growth rate at high growth temperature is assigned to an enhancement of adatom diffusion. The upper limit of length growth rate is determined by the supply rate of active nitrogen.

We have performed a real-time in situ x-ray scattering study of the nucleation of GaN nanowires grown by plasma-assisted molecular beam epitaxy on AlN(0001)/Si(111). The intensity variation of the GaN diffraction peak as a function of time was found to exhibit three different regimes: (i) the deposition of a wetting layer, which is followed by (ii) a supralinear regime assigned to nucleation of almost fully relaxed GaN nanowires, eventually leading to (iii) a steady-state growth regime. Based on scanning electron microscopy and electron microscopy analysis, it is proposed that the granular character of the thin AlN buffer layer may account for the easy plastic relaxation of GaN, establishing that three-dimensional islanding and plastic strain relaxation of GaN are two necessary conditions for nanowire growth.