Si based nanostructures for solar cells

The major drawback of the top-down method is that the surfaces of the structure are damaged during the process resulting in nanowires with a poor crystal quality. In addition the lithographic techniques may not be able to produce sufficiently small structures for further downscaling of devices. In order to produce small enough nanowires of high enough crystal quality, the bottom-up approach is thought to be a potential alternative. The idea is to build-up nanosized structures and devices by using nanoscale building blocks to initiate growth directly at desired positions and with designed dimensions and properties. In contrast to the lithographic and etching techniques used in the top-down methodology, the bottom-up approach involves the direct growth of one-dimensional nanostructures onto a substrate. The typical method to fabricate NWs by bottom-up approach is the catalytic growth with random metallic nanoparticles. A better understanding of the catalytic nanowire growth process is necessary to pin down the growth mechanism and to be able to rationally control their compositions, sizes, crystal structures, and growth directions. Gold was used as a catalyst for the nucleation of the Si nanophase. The choice to use gold relies on the advantages of this metal respect to others, like Ni, Sn, Al and Cu, such as its low eutectic temperature with Si, the opportunity to easily form alloys with the growth precursor and the fast inter-diffusion of Si through gold nanodot. Gold, however, presents some drawbacks, because it creates deep band gap defects and for this reason the data shown represent a proof of concept, and al