We have studied the growth conditions for epitaxial overgrowth over metallic features using low-pressure metalorganic vapour phase epitaxy. The aim of this study is to optimise the growth conditions and to establish a processing sequence for the realisation of 3D metal-semiconductor quantum devices. First. we investigate the effect of the metal pattern (orientation and shape) on the resulting growth behaviour under fixed epitaxial conditions. Then we use the same pattern and vary the growth temperature in order to optimise the lateral growth above the metal. An excellent crystalline quality of the overgrown material is thereafter demonstrated using transmission electron microscopy. Finally, it is demonstrated how these structures may be used in two types of quantum devices, namely as storage elements in a semi-insulating layer and as an active part in a resonant tunnelling transistor.