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.
Using analytical transmission electron microscopy, the scientists were able to directly observe that silicon atoms begin to flow from the silicon layer into the aluminium catalyst at a temperature as low as 120 °Celsius. At such low temperatures, the aluminium catalyst is solid and cannot dissolve any silicon atoms. Microscopic investigations reveal that the silicon atoms are instead accommodated at the boundaries between the aluminium crystals. As more and more silicon atoms gather at the aluminium grain boundaries, they are restructured into tiny crystalline nanowires, as this reduces the total energy of the system. This produces a network of crystalline nanowires, the pattern of which is precisely determined by the aluminium grain-boundary network. Wires as thin as 15 nanometres can thus be produced.
Clearly the growth mechanism of nanowires discovered by the material scientists in Stuttgart is fundamentally different from the conventional VLS growth mechanism. Most strikingly, the new growth method does not require semiconductor solubility in the metal catalyst and can therefore be realized at low temperatures (150 °Celsius), while using cheap catalysts like aluminium.
The major benefits of the new method are therefore that it does not require high substrate temperatures or expensive catalysts. In addition, materials scientists can tailor the size of the aluminium grains and thereby the form of the aluminium grain-boundary network, to produce the desired pattern of silicon nanowires. The Al catalyst can easily be removed through selective etching. Since aluminium films have been used in microelectronics for decades, their production and processing are widely established. Other catalysts may also be suitable for the method. Another advantage is that nanostructured silicon devices can be grown directly on most plastic substrates, even if they are heat-sensitive.