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Philipp Schroth1 2 Julian Jakob2 Ludwig Feigl2 Seyed Mohammad Mostafavi Kashani1 Tilo Baumbach2 Ulrich Pietsch1

1, University of Siegen, Siegen, , Germany
2, Karlsruhe Institute of Technology, Karlsruhe, , Germany

The morphology of GaAs Nanowires (NW), grown by the self-catalyzed method, is strongly influenced by the liquid Ga-droplet at the NW tip.1 The size of the droplet correlates with the actual NW diameter.2 Its composition and shape at the liquid-solid interface and the involved facets determine the micro-structure of the growing NW3, and certain control of the NW crystal structure at later stages of growth has been achieved recently4.
At the onset of growth however, the wetting conditions may change dramatically. Whereas the liquid droplet is in contact with the oxide-covered Si substrate prior to GaAs nucleation, it begins to wet the NW side-walls once the GaAs NW starts to grow. It has been shown, that the wetting conditions between droplet and substrate are crucial for NW growth in terms of vertical yield.5 Further, the wetting conditions between droplet and the NW have impact on the evolution of polytypism during growth.1,3
Although there are reports about the impact of substrate treatment5-8, an investigation of it the effect on the crystalline phase of the NWs, especially during the nucleation phase, is pending.
Here, we report on the growth of self-catalysed GaAs nanowires onto Silicon (111) substrates using the portable molecular beam epitaxy setup of LAS/IPS at KIT9, specially designed for in-situ growth studies by X-ray diffraction (XRD). We vary the size and shape of the liquid Ga droplets by pre-growth substrate treatment, and investigate the deposited Ga-droplets, that serve as seed particles for further NW growth.
Furthermore, we probe the crystallographic properties of the GaAs NW grown from these droplets by means of time-resolved in-situ Reflection-High-Energy-Electron-Diffraction. After the growth, the NWs are investigated by ex-situ Scanning-Electron-Microscopy (SEM) and XRD. We reveal differences in the crystalline phase composition of the grown NW samples in terms of polytypism.10 Employing a statistical model for the atomic stacking sequences in the NWs11, we model the XRD data and find indications for polytypism beyond the 2H and 3C phases of self-catalyzed GaAs NWs.12

References
1P. Krogstrup et al., Nano Lett., 10, (2010)
2B. O’Dowd et al., J. Appl. Phys., 116, (2014)
3P. Krogstrup et al., Phys.Rev. Lett., 106, (2011)
4L. Balaghi et al., Nano Lett., 16 (7), (2016)
5T. Tauchnitz et al., Cryst. Growth Des., 17, (2017)
6T. V. Hakkarainen et al., Nanotechnology, 26, (2015)
7Y. Zi et al., Nano Lett., 17, (2017)
8H. Küpers et al., Journal of Crystal Growth, 459, (2017)
9T. Slobodskyy et al., Rev. Sci. Instrum., 83, (2012)
10P. Schroth et al., Phys. Rev. Lett., 114, (2015)
11M. Köhl et al., J Synchrotron Radiat., 23, (2016)
12 We are grateful for David Reuther, Jörg Strempfer and Sonia Francoual at P09, Hans Gräfe, Bärbel Krause and Annette Weißhardt at IPS, KIT. We thank Simone Dehm at INT, KIT and Gernot Buth at SCD beamline at ANKA. The project was supported by German BMBF (05ES7CK and 05K13PS3).

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