Electron diffraction on GaAs nanowhiskers grown on Si(100) and Si(111) substrates by molecular-beam epitaxy

The crystal structure of GaAs nanowhiskers grown by molecular-beam epitaxy on Si(111) and Si(100) substrates is investigated using reflection high-energy electron diffraction (RHEED). It is revealed that, in both cases, the electron diffraction images contain a combination (superposition) of systems of reflections characteristic of the hexagonal (wurtzite and/or 4H polytype) and cubic (sphalerite) phases of the GaAs compound. The growth on the Si(111) substrates leads to the formation of nanowhiskers with hexagonal (wurtzite and/or 4H polytype) and cubic (sphalerite) structures with one and two orientations, respectively. In the case of the Si(100) substrates, the grown array contains GaAs nanowhiskers that have a cubic structure with five different orientations and a hexagonal structure with eight orientations in the (110) planes of the substrate. The formation of the two-phase crystal structure in nanowhiskers is explained by the wurtzite-sphalerite phase transitions and/or twinning of crystallites.