The Template-Assisted Selective Epitaxy (TASE) method, developed at IBM Research Europe – Zurich, permits to create a homogeneous integration route for various semiconductor materials which is compatible with the CMOS process. With this approach, a III-V nanowire can be grown horizontally inside a silicon oxide template by metal-organic chemical vapor deposition (MOCVD). The confined growth implies that the diffusion mechanisms of group III and group V components are different when compared with both planar epitaxy. This means that TASE allows us to grow complex III-V heterostructures. In this work we have used TASE to create an InGaAs-InP type II superlattice system with dimensions of approximately 200 nm x 200 nm in cross section and 700 nm in length with < 10-nm-thick layers in the superlattice stack. Our goal was to evaluate and optimise heterointerface quality. Based on our fabrication approach, we observed different, facet-dependent, growth rates for the InP and InGaAs layers. In particular, early experiments on <001> Silicon-on-Insulator (SoI) wafers resulted in a complex facet morphology with different behaviour of the group III and V components. Here we observe diffusion of the III component through the interface leading to alloying and loss of interface sharpness. However, the interfaces were well defined for the group V component. To improve the process even further we have introduced buffer times at the switching of the precursor flows. We used a group V precursor overpressure while switching the III precursor flows to avoid temperature-driven desorption of group V atoms from the first surface layers, adjusting the concurrent group V element switching process to minimize group V element substitution at the interface. In order to evaluate the InGaAs/InP type II superlattice heterointerfaces, we employed characterization techniques such as Scanning Transmission Electron Microscopy (STEM) and Energy Dispersive Xray Spectroscopy (EDS). We have investigated both crystalline quality, in terms of defects and facet selection, and composition intermixing for both III and V components. Our results highlight the challenges posed by the complex and varied diffusion dynamics in such complex InGaAs/InP type II superlattice heterostructure and give indications of how to solve those challenges, namely alloying, facet control, and process control. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 860095. The authors thank the Cleanroom Operations Team of the Binnig and Rohrer Nanotechnology Center (BRNC) for their help and support. References:  M. Borg, H. Schmid, K. E. Moselund, D. Cutaia, and H. Riel, “Mechanisms of template-assisted selective epitaxy of InAs nanowires on Si,” Journal of Applied Physics, vol. 117, no. 14, pp. 1–8, 2015, doi: 10.1063/1.4916984.