Silicon nanowires (NWs) and vertical nanowire-based Si/Ge heterostructures are expected to be building blocks for future devices. eg. field-effect transistors or thermoelectric elements In principle two approaches can be applied to synthesise these NWs the 'bottom-up' and the 'top-down' approach The most common method for the former is the vapour-liquid-solid (VLS) mechanism which can also be applied to grow NWs by molecular beam epitaxy (MBE) Although MBE allows a precise growth control under highly reproducible conditions, the general nature of the growth process via a eutectic droplet prevents the synthesis of heterostructures with sharp interfaces and high Ge concentrations We compare the VLS NW growth with two different top-down methods The first is a combination of colloidal lithography and metal-assisted wet chemical etching, which is an inexpensive and fast method and results in large arrays of homogenous Si NWs with adjustable diameters down to 50 nm. The second top-down method combines the growth of Si/Ge superlattices by MBE with electron beam lithography and reactive ion etching Again, large and homogeneous arrays of NWs were created, this time with a diameter of 40 nm and the Si/Ge superlattice inside. (c) 2009 Elsevier B.V. All rights reserved