supporting information by C. Blumberg et al. . 1 Mask-less MOVPE of arrayed n-GaN nanowires on site- and polarity-controlled AlN/Si templates” C. Blumberg, F. Wefers, F.-J. Tegude, N. Weimann, W. Prost Lotharstr. 55 (ZHO), 47057 Duisburg, Germany E-mail: christian.blumberg@uni-due.de Supporting information Reflected interferometry on SAE islands In this publication reflected interferometry data has been used to analyze and interpret the growth of GaN islands on an AlN/Si surface. Here, we explain possible causes for signal oscillation and signal decreasing during the GaN island and NW growth on AlN/Si 2D-layers. To simplify the model, we do not consider the Si pillars in the following analyzation but chose a flat AlN- and Si-layer. Furthermore, absorption effects are neglected. The measured reflected interferometer intensity is a superposition of reflected light from the GaN islands and from light reflected at the AlN/Si surface, where no GaN should grow. Thus, we have calculated the light path and the intensity change during its propagation through the crystals at different positions. Figure 1 schematically illustrates how the light propagates, if it hits the r-facet of a GaN island (a) and how it is reflected without (b) and with increasing parasitic growth (c and d) on the flat AlN/Si-template. First, the incident light on a GaN island is discussed (Figure 1 a). We have calculated the incident ( ), the exit and the reflected angle based on the fixed angular dependence of the  facets and the Fresnel equations. E.g. the angle between the m-direction ([11 0]) and the ̅ 2 r-direction [(11 ]) is 61.5°, and thus, the incident angle at the air/r-facet interface is also ̅ 21  61.5°. Using the refractive indexes of air (1), GaN (2.387), AlN (2.17) and Si (3.87) at the interferometer wavelength of 633 nm, the exit angles at each interface and the total reflection (R) and transmission (T = 1 - R) can be calculated. The results are summarized in Table 1. Electronic Supplementary Material (ESI) for CrystEngComm. This journal is © The Royal Society of Chemistry 2019