Semiconductor nanowires produced at record low temperatures
1 min read
In what is being described as a significant breakthrough towards cheaper and more efficient solar cells, scientists have been able to produce crystalline semiconductor nanowires at a significantly low temperature and using inexpensive catalysts such as lead instead of gold.
The materials scientists from Eric Mittemeijer's department at the Max Planck Institute for Intelligent Systems in Stuttgart, in collaboration with colleagues from the Stuttgart Center for Electron Microscopy, first prepared a bilayer of crystalline aluminium and amorphous silicon in vacuum and at room temperature using thermal evaporation. The atoms were then arranged in an ordered crystalline lattice in the aluminium layer which consisted of billions of tiny aluminium crystals approximately 50nm in size.
Using analytical transmission electron microscopy, the researchers directly observed the tightly packed crystal grains forming a 2d grain boundary network within the aluminium layer. The silicon atoms then began to flow from the silicon layer into the aluminium catalyst at temperatures as low as 120°C compared to traditional methods that require between 600 and 900°C.
At such low temperatures, the aluminium catalyst was solid and could not dissolve any silicon atoms. Microscopic investigations revealed that the silicon atoms were instead accommodated at the boundaries between the aluminium crystals. As more and more silicon atoms gathered at the aluminium grain boundaries, they were restructured into tiny crystalline nanowires, which produced a network of crystalline nanowires. This pattern was precisely determined by the aluminium grain boundary network and meant that wires as thin as 15nm could be produced.
While the new growth mechanism discovered is fundamentally different from the conventional vapour-liquid-solid mechanism, which utilises tiny particles of metal catalysts as seeds for the growth of the nanowires, the scientists believe it could offer significant advantages in the development of cheaper solar cells.
Most notably, it does not require semiconductor solubility in the metal catalyst and can therefore be realised at low temperatures while using cheap catalysts like aluminium. 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.
