  Self assembled AlN nanowires (NWs) are grown by plasma assisted molecular
beam epitaxy (PAMBE) on SiO2 / Si (111) substrates. Using a combination of
in-situ reflective high energy electron diffraction and ex situ X ray
diffraction (XRD), we show that the NWs grow nearly strain free, preferentially
perpendicular to the amorphous SiO2 interlayer and without epitaxial
relationship to Si(111) substrate, as expected. Scanning electron microscopy
investigation reveals significant NWs coalescence, which results in their
progressively increasing diameter and formation of columnar structures with non
hexagonal cross section. Making use of scanning transmission electron
microscopy (STEM), the NWs initial diameters are found in the 20 to 30 nm
range. In addition, the formation of a thin (30 nm) polycrystalline AlN layer
is observed on the substrate surface. Regarding the structural quality of the
AlN NWs, STEM measurements reveal the formation of extended columnar regions,
which grow with a virtually perfect metal-polarity wurtzite arrangement and
with extended defects only sporadically observed. Combination of STEM and
electron energy loss spectroscopy (EELS) reveals the formation of continuous
aluminum oxide (1 to 2 nm) on the NW surface. Low temperature photoluminescence
measurements reveal a single near band edge (NBE) emission peak, positioned at
6.03 eV (at 2 K), a value consistent with nearly zero NW strain evidenced by
XRD and in agreement with the values obtained on AlN bulk layers synthesized by
other growth techniques. The significant full width at half maximum of NBE
emission, found at 20 meV (at 2 K), suggests that free and bound excitons are
mixed together within this single emission band.
