However, although RHEED has gained great importance for quantitative 2D layer growth studies, in case of NW growth, RHEED has mostly been restricted to qualitative conclusions. In situ RHEED equipment, in contrast to TEM and XRD, is usually already integrated into commercial MBE systems and therefore broadly available. However, special growth chambers equipped with X-ray windows are required as well as access to heavy-duty diffractometers at high-flux synchrotron light sources. 35 In both cases, NW growth close to standard growth conditions with epitaxial connection to the substrate can be monitored. By using microfocused beams, even the properties of individual NWs can be in situ examined. In situ XRD during growth 21,31–34 probes the evolution of representative structure properties averaged over a large statistical NW ensemble. the impact of diffusion processes on the substrate as well as material flux shadowing by the NW ensemble. 20,26–29 The investigations are typically performed with pre-grown NWs 26,27 or NWs without epitaxial connection to any substrate, 20,28,29 therefore excluding a number of growth effects under standard conditions, e.g. Within the available portfolio of in situ techniques, in situ transmission electron microscopy (TEM) during NW growth offers unrivalled spatial resolution down to the atomic scale, together with high temporal resolution, but it is restricted to special equipment not broadly available. Powerful techniques allowing in situ characterization during growth can serve as a key to understand and optimize the morphological properties of NWs. tapered NW shape, 21 or the fabrication of axial heterostructures formed of different polytypes along the NW growth axis 22,23 allowing exploitation of their band structure differences. Good control over the droplet itself enables the realization of dedicated NW morphologies, e.g. Their simultaneous occurrence is called polytypism. Self-catalysed GaAs NWs adopt mainly the cubic zinc blende (ZB), its rotational twin (TZB), and the hexagonal wurtzite (WZ) crystal structure. 14 The reason is inherent in the VLS growth mode, more precisely in the liquid catalyst particle, which is responsible for the axial growth of the NWs and directly determines the NW morphology, 15–20 such as the shape and the crystal structure. Moreover, an increase in the number density of NWs is accompanied by changing NW diameters 3,13 and/or crystal structure. 1,2,5,8 However, those studies have also shown that these properties cannot be optimized separately via growth parameters. For the growth of self-catalysed GaAs NWs, progress was achieved in control of NW yield, 2,4,6 shape 3,4,7 and density, 3,6,7 as well as in the crystal structure. 10 In contrast, the self-catalysed or Ga-assisted growth 11,12 in case of GaAs NWs ensures fabrication without any risk of this possible contamination. NWs grown in the vapour–liquid–solid (VLS) mode 9 by metalorganic vapor-phase epitaxy (MOVPE) or MBE should avoid foreign elements such as Au as catalyst particles, because of the possibility of incorporation in the growing NWs. 1–8 The integration of these dissimilar material systems is possible due to the small footprint of NWs, facilitating an epitaxial connection. Introduction In recent years improved control over the growth of self-catalysed III–V nanowires on Si has led to substantial progress, which is mainly driven by the promise of the integration of III–V semiconductors on the cost-effective Si platform. Furthermore, we demonstrate, how careful analysis of in situ RHEED if supported by ex situ XRD and scanning electron microscopy (SEM), all usually available at conventional MBE laboratories, can also provide highly quantitative feedback on polytypism during growth allowing validation of current vapour–liquid–solid (VLS) growth models. In particular, the combination of RHEED and XRD allows for translating quantitatively the time-resolved information into a height-resolved information on the crystalline structure without a priori assumptions on the growth model. Exploiting the complementarity by a correlative data analysis presently offers the most comprehensive experimental access to the growth dynamics of statistical NW ensembles under standard MBE growth conditions. Simultaneously recorded in situ RHEED and in situ XRD intensities show strongly differing temporal behaviour and provide evidence of the highly complementary information value of both diffraction techniques. We analyse and compare the methodical potential of reflection high-energy electron diffraction (RHEED) and X-ray diffraction reciprocal space imaging (XRD) for in situ growth characterization during molecular-beam epitaxy (MBE). Therefore, quantitative feedback over the structure evolution of the NW ensemble during growth is highly desirable. Design of novel nanowire (NW) based semiconductor devices requires deep understanding and technological control of NW growth.
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