Comparison of nucleation kinetics by RHEED showed that no NWs nucleation occurred  on sapphire. This agrees with observation by  Geelhaar et al. [3] who have reported that on the crystalline sapphire NWs do not form under any PAMBE growth conditions. Slow GaN nucleation was observed on partially amorphous silicon nitride film naturally created on the surface of Si(111) due to a high affinity of nitrogen to silicon. Finally, significantly enhanced nucleation of GaN NWs was observed on amorphous Al₂O₃ buffer. Post growth analysis by scanning and transmission electron microscopy show separated NWs on a bare silicon whereas high density of GaN nuclei are found between NWs grown on Al₂O₃ buffer (Fig. 1).

Fig. 1 Cross-section SEM views of GaN NWs (a) on Si(111) substrates with 2 nm thick amorphous Al₂O₃ buffer and (b) on bare Si(111).

Energy electron loss spectroscopy was used to check if creation of any AlN islands that might make GaN nucleation on amorphous Al₂O₃ easier as compared to crystalline sapphire took place in our case. However, precise analysis of oxygen and nitrogen distributions across the GaN/Al₂O₃ interface eliminated such explanation. Instead, we consider defects in Al₂O₃ buffer as possible GaN nucleation sites. Localization of Ga adatoms at these defects should speed up creation of supercritical nuclei and lead to shorter NWs incubation times.
Finally, arrangement on the substrate as well as structural and optical properties of GaN NWs are studied and correlated with microstructure of substrates on which they are grown.

This work was partly supported by the European Union within European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08). MS thanks for support from European Social Fund through Human Capital Program and local authorities (Samorząd Województwa Mazowieckiego - „Potencjał naukowy wsparciem dla gospodarki Mazowsza – stypendia dla doktorantów”).

[1] K. Hestroffer et al., Appl. Phys. Lett. 100, 212107 (2012).
[2] A. Wierzbicka et al., Nanotechnology 24, 035703 (2013).
[3] L. Geelhaar et al., Appl. Phys. Lett. 91, 093113 (2007).

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