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GaAs nanowires (NWs) are grown on GaAs (311)B substrates by gold assisted molecular beam epitaxy technology. Combined scanning and transmission electron microscopy analyses, the crystallographic orientations of NWs are studied. It is found that crystallographic orientations of NWs are closely related to their crystal structures: NWs of zinc blende structure grow along 〈001〉 directions and NWs of wurtzite structure grow along 〈0001〉 directions. The influence of impinging Ga flux on morphology and crystal structure of the NWs is also discussed. It is observed that NWs prefer to grow along zinc blende 〈001〉 directions at lower Ga flux, while NWs tend to grow along the wurtzite 〈0001〉 directions with only a small portion along the zinc blende ?001? direction at a higher Ga flux. The control of crystal structure and orientation of NWs can be achieved effectively by changing the Ga flux. [ABSTRACT FROM AUTHOR]

We investigated the quantum dots-templated growth of a (0001) GaN film on a c-plane sapphire substrate. The growth was carried out in a radio-frequency molecular beam epitaxy system. The enlargement and coalescence of grains on the GaN quantum dots template was observed in the atom force microscopy images, as well as the more ideal surface morphology of the GaN epitaxial film on the quantum dots template compared with the one on the AlN buffer. The Ga polarity was confirmed by the reflected high energy electron diffraction patterns and the Raman spectra. The significant strain relaxation in the quantum dots-templated GaN film was calculated based on the Raman spectra and the X-ray rocking curves. Meanwhile, the threading dislocation density in the quantum dots-templated film was estimated to be 7.1 × 107 cm-2, which was significantly suppressed compared with that of the AlN-buffered GaN film. The roomtemperature Hall measurement showed an electron mobility of up to 1860 cm2/V . s in the two-dimensional electron gas at the interface of the Al0.25Ga0.75N/GaN heterojunction. [ABSTRACT FROM AUTHOR]