The ‘coolest’ semiconductor nanowires
A new method permits cost-effective production of silicon nanowires
Semiconductor nanowires are essential materials in the development of cheaper and more efficient solar cells, as well as batteries with higher storage capacity. Moreover, they are important building blocks in nanoelectronics. However, manufacturing semiconductor nanowires on an industrial scale is very expensive. The main reason for this is the high temperatures at which they are produced (600−900 °Celsius), as well as the use of expensive catalysts, such as gold. Scientists at the Max Planck Institute for Intelligent Systems in Stuttgart, formerly the Max Planck Institute for Metals Research, have now been able to produce crystalline semiconductor nanowires at a much lower temperature (150 °Celsius) while using inexpensive catalysts, such as aluminium. In this way, nanostructured semiconductors can even be deposited directly on heat-sensitive plastic substrates.
Nanowires made of semiconductors such as silicon (Si) or germanium (Ge) will be indispensable for many technical applications in the future. Until now, they have been manufactured using a process that was first described in 1964. The so-called vapour-liquid-solid (VLS) mechanism utilises tiny particles of metal catalysts as seeds for the growth of the nanowires. The metal seeds are deposited on a solid substrate, melted and exposed to a gas atmosphere containing silicon or germanium. The metal droplets will then take up semiconductor atoms from the gas until they are supersaturated, and the excess semiconductor material precipitates at the boundary with the substrate: a nanowire grows. In most cases, gold is used as a catalyst, since it can dissolve a lot of silicon or germanium when molten. The use of this expensive catalyst and the high processing temperature of 600 to 900 ºCelsius lead, however, to high production costs.
Materials scientists from Eric Mittemeijer’s department at the Max Planck Institute for Intelligent Systems have now discovered a method to produce semiconductor nanowires at a strikingly lower temperature of only 150 °Celsius, while using cheap catalysts like aluminium. Together with colleagues from the Stuttgart Center for Electron Microscopy, a research facility at the same Institute, the scientists have managed to observe nanowire growth at an atomic scale in real time.
To this end, the scientists prepared a bilayer of crystalline aluminium and amorphous silicon. The layer was produced in vacuum and at room temperature using thermal evaporation. Whereas the atoms are disordered in the amorphous silicon phase, they are arranged in an ordered crystalline lattice in the aluminium layer. In fact, the Al layer is constituted of billions of tiny aluminium crystals, each of them of size as small as about 50 nanometres. The crystal grains are in tight contact with each other. Their boundaries thus form a two-dimensional grain-boundary network within the aluminium layer.