Most publications on (opto)electronic devices based on 2D materials rely on single monolayers embedded in classical 3D semiconductors, dielectrics and metals. However, heterostructures of different 2D materials can be employed to tailor the performance of the 2D components by reduced defect densities, carrier or exciton transfer processes and improved stability. This translates to additional and unique degrees of freedom for novel device design. The nearly infinite number of potential combinations of 2D layers allows for many fascinating applications. Unlike mechanical stacking, metal-organic vapour phase epitaxy (MOVPE) can potentially provide large-scale highly homogeneous 2D layer stacks with clean and sharp interfaces. Here, we demonstrate the direct successive MOVPE of MoS2/WS2 and WS2/MoS2 heterostructures on 2” sapphire (0001) substrates. Furthermore, the first deposition of large-scale MoS2/graphene and WS2/graphene heterostructures using only MOVPE is presented and the influence of growth time on nucleation of WS2 on graphene is analysed.

Semi-polar (1122) GaN films were grown on (1100) m-plane sapphire substrates. Growth demonstrated surface striations aligned perpendicular to the in-plane GaN m-axis. SiNx interlayers were incorporated into the as-grown films with the purpose of decreasing the density of defects in the material. Inclusion of interlayers increased the characteristic length of surface striations and feature size. X-ray rocking curves widths are shown to be correlated to specific threading dislocation geometry. Skew-symmetric omega scan peak broadening suggests a decrease in the proportion of screw-type dislocations to edge-type dislocations with increasing number of interlayers.

A series of Al0.47Ga0.53N/GaN heterostructures with different AlN interlayer thicknesses ranging from 1nm to 50nm has been examined. It was found that when the interlayer thickness is greater than ∼5nm, it becomes possible to grow 250nm of Al0.47Ga0.53N without cracking. The interlayers are then believed to be sufficiently relaxed to place the AlGaN under compressive strain. The mechanisms for this relaxation have been studied using high angle annular dark field (HAADF) imaging, conventional transmission electron microscopy (TEM), energy-filtered TEM (EFTEM) and electron energy loss spectroscopy (EELS). It is found that relaxation takes place through both the small-scale cracking of the interlayer and the generation of misfit dislocations at the GaN/AlN interface. EELS and EFTEM have been used to probe the Al and Ga content of both the material filling the interlayer cracks, and the interlayer itself. This chemical analysis suggests Ga-rich AlGaN areas inside the interlayer cracks and also significant compositional variations in defect-free interlayer regions. It is observed that relaxation by the generation of misfit dislocations results in an increase in the threading dislocation density of the AlGaN layer, in part due to the bending up of misfit dislocations at crack walls.