Effects of ultrathin AlN prelayers on the spontaneous growth of GaN nanowires by plasma assisted molecular beam epitaxy

The effects of ultrathin AlN prelayers, with nominal thicknesses between 0 and 1.5 nm, on the spontaneous growth of GaN nanowires (NWs) on Si (1 1 1) substrates were investigated. The morphological and structural characteristics of GaN NWs were analyzed by electron microscopy and X-ray diffraction techniques. The results quantify how the AlN prelayer thickness affects GaN NWs. The increase of AlN thickness gradually limits nitridation of the substrate surface and accelerates 3D GaN nucleation. The formation of amorphous SixNy by Si nitridation is completely avoided for 1.5 nm of AlN that fully covers the Si surface. The dependence of the height, diameter and density of GaN NWs on the AlN thickness was also determined. The 1.5 nm AlN provided the optimum condition for GaN NW nucleation and growth; the NWs exhibited a large homogeneous height with almost no parasitic GaN formation between them. High resolution transmission electron microscopy showed the full relaxation of misfit strain of AlN on Si (1 1 1) and of GaN NWs on AlN. In-situ reflection high energy electron diffraction during AlN nucleation revealed the immediate relaxation of the AlN prelayer before GaN NW nucleation. Formation of beta-Si3N4 before AlN nucleation was also observed.