Impact of substrate microstructure on self-induced nucleation and properties of GaN nanowires grown by plasma-assisted MBE

Marta Sobanska ,  Kamil Klosek ,  Aleksandra Wierzbicka ,  Jolanta Borysiuk ,  Slawomir Kret ,  Giorgi Tchutchulashvili ,  Sylwia Gierałtowska ,  Elżbieta Lusakowska ,  Piotr Nowakowski ,  Zbigniew R. Zytkiewicz 

Institute of Physics, Polish Academy of Sciences, Warsaw 02-668, Poland

Abstract
Recent in-situ studies of growth of GaN nanowires (NWs) by plasma-assisted molecular beam epitaxy on silicon by Hestroffer et al. [1] show that a massive self-induced nucleation of GaN NWs starts only after partial amorphization of the silicon nitride film that inevitably forms on silicon substrate under nitrogen flux. This result indicates existence of a barrier for GaN nucleation on monocrystalline substrate, so creation of nucleation sites (e.g. by partial amorphization of the substrate) seems to be needed to enhance nucleation rate of GaN NWs.
To explore this effect in a more details we have compared efficiency of self-induced nucleation of GaN NWs on three different substrates, namely on bare c-oriented sapphire, on Si(111) with a native silicon nitride layer and on Si(111) coated with a thin amorphous Al2O3 buffer deposited at low temperature by atomic layer deposition. On all substrates catalyst-free growth of NWs proceeded under the same, highly nitrogen-rich conditions at ~750 oC [2].

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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Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 3, by Marta Sobanska
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-04-15 15:44
Revised:   2013-07-23 17:34