Credit: © 2009 IBM

Most applications of semiconductor nanowires require the composition of the nanowire to change along its length so as to build in features such as quantum dots and field-effect transistors. Moreover, the junctions between the different regions need to be abrupt and free from defects. This is routinely done for pairs of III–V semiconductors such as indium arsenide and indium phosphide, but until now it has not been possible to grow nanowires with abrupt junctions between different group IV semiconductors, such as silicon and germanium. Now Frances Ross and co-workers at IBM, Purdue University and University of California, Los Angeles have grown such nanowires1.

Si and Ge nanowires are usually grown from gaseous silane (SiH4) or germane (GeH4) with the help of a liquid-metal catalyst. The gas is dissociated by the catalyst and the Si or Ge dissolves in the liquid metal, before precipitating out in the form of a nanowire when its concentration reaches a certain value. Changing the gas from silane to germane will, therefore, result in a Si–Ge junction. However, the interface between the two regions will be diffuse because large amounts of Si, for example, can be dissolved in the catalyst, and will continue to be included in the growing nanowire after the silane has been replaced with germane. Ross and co-workers overcome this problem by using solid aluminium–gold alloy nanoparticles as catalysts. The solubility of Si and Ge is very low in this alloy, leading to abrupt junctions in the nanowires.