An arrangement of self-assembled GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on a Si(111) substrate is studied as a function of the temperature at which the substrate is nitridized before GaN growth. We show that the NWs grow with the c-axis perpendicular to the substrate surface independently of nitridation temperature with only a slight improvement in tilt coherency for high nitridation temperatures. A much larger influence of the substrate nitridation process on the in-plane arrangement of NWs is found. For high (850 °C) and medium (450 °C) nitridation temperatures angular twist distributions are relatively narrow and NWs are epitaxially aligned to the substrate in the same way as commonly observed in GaN on Si(111) planar layers with an AlN buffer. However, if the substrate is nitridized at low temperature (~150 °C) the epitaxial relationship with the substrate is lost and an almost random in-plane orientation of GaN NWs is observed. These results are correlated with a microstructure of silicon nitride film created on the substrate as the result of the nitridation procedure.

A high-resolution Laplace-transform deep-level-transient spectroscopy has been used to study electon emission from the DX centers related to group-IV (silicon) and group-VI (tellurium) donor elements in ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As (0.25<x<0.76). This provides experimental evidence that substitutional-interstitial atom motion is responsible for DX behavior and for the associated metastability effects. The atom which is subjected to this transition is, for DX(Si), silicon itself, and so only one group of peaks is observed in the spectra; while for DX(Te), it can be either gallium or aluminum, producing two groups of peaks.

We report on plasma-assisted molecular beam epitaxial growth of GaN nanowires (NWs) on Si(111) substrates with a thin amorphous Al₂O₃ buffer layer deposited by atomic layer deposition. Comparison of nucleation kinetics shows that presence of amorphous Al₂O₃ buffer significantly enhances spontaneous nucleation of GaN NWs. Slower nucleation was observed on partially amorphous silicon nitride films. No growth of NWs was found on sapphire substrate under the same growth conditions which we explain by a low density of defects on monocrystalline substrate surface where NWs may nucleate. Our finding shows that tuning of substrate microstructure is an efficient tool to control rate of self-induced nucleation of GaN NWs.

The growth mode and structural and optical properties of novel type of inclined GaN nanowires (NWs) grown by plasma-assisted MBE on Si(001) substrate were investigated. We show that due to a specific nucleation mechanism the NWs grow epitaxially on the Si substrate without any SixNy interlayer, first in the form of zinc-blende islands and then as double wurtzite GaN nanorods with Ga-polarity. X-ray measurements show that orientation of these nanowires is epitaxially linked to the symmetry of the substrate so that [0001] axis of w-GaN nanowire is directed along the [111](Si) axis. This is different from commonly observed behavior of self-induced GaN NWs that are N-polar and grow perpendicularly to the surface of nitridized silicon substrate independently on its orientation. The inclined NWs exhibit bright luminescence of bulk donor-bound excitons ((DX)-X-0) at 3.472 eV and exciton-related peak at 3.46 eV having a long lifetime (0.7 ns at 4 K) and observable up to 50 K.