Oct 21, 2009
Study explores gold migration in silicon nanowires
For silicon nanowires (SiNWs) grown by the vapour-liquid-solid (VLS) mechanism, catalyzed by gold nanodroplets, it is important to know in view of planned applications in all sorts of electronics where and how much gold resides in the SiNW itself and at its surface, as well as where the gold goes upon annealing/oxidation under different annealing conditions.
The enormous impact that even the smallest concentrations of gold in silicon have, makes it essential to understand the extent to which gold atoms diffuse in SiNWs and remain incorporated there and on the SiNW surface. This is especially true after it has been shown that gold atoms from the catalyst nanodroplets are very mobile and diffuse all over the SiNW surfaces and between SiNWs at temperatures as low as 600 °C thus leading to Ostwald ripening of gold nanoparticles at the SiNW sidewalls.
To provide some answers, researchers in Germany have studied the oxidation behaviour of SiNWs grown in an electron beam evaporation (EBE) reactor by the gold-catalyzed VLS growth process. The oxide layer thickness as well as oxide and SiNW morphology depend strongly on annealing conditions (time and temperature) as shown by varied oxidation processing and subsequent cross-sectional transmission electron microscopy (TEM) including high-resolution studies as well as scanning electron microscopy (SEM) studies.
Results strongly suggest that the SiNWs can be fully oxidized to form silica nanowires that can either keep their initial shape or, under certain annealing conditions do not keep their initial wire shape but assume a bamboo-like shape that forms most likely as a result of locally high stresses, which are related to the formation of nanocracks. These nanocracks form in the growing oxide layer mediated by the presence of gold nanoparticles that yield gold-enhanced SiNW oxidation and thus a faster oxidation rate, locally.
SiNWs that may be fully or partially oxidized are considered as potential active device materials for future nanoelectronics. Uses include light absorbers and emitters or wave guides or light absorbers, for example in solar cells, photodetectors or LEDs and as a large surface area to be functionalized for sensing applications. In all of these cases, the interfacial properties between the SiNW core and the SiO2 (silica) shell depend strongly on the gold presence there and are thus of key importance in terms of device functionality.
The researchers presented their work in the journal Nanotechnology.
About the author
Essential parts of the work were carried out at the Max Planck Institute of Microstructure Physics (MPI/Halle, Halle/Saale Germany), supported by work carried out at the Institute of Photonic Technology (IPHT, Jena, Germany) and the Martin Luther University Halle-Wittenberg (MLU, Halle/Saale, Germany). Financial support was given by the German Science Foundation (DFG, contract number CH-159/1) and by the Max-Planck Society in the framework of the "Nanostress" project. Dr Vladimir A Sivakov is a postdoctoral researcher at the MPI/Halle in the group of Dr Silke H Christiansen in the department of Prof. Dr Ulrich Gösele, one of the directors of the MPI/Halle. Dr Roland Scholz is a specialist in transmission electron microscopy at the MPI. Dipl.-Phys. Frank Syrowatka is a specialist in scanning electron microscopy at MLU. Dr Fritz Falk is leading the Photovoltaics Systems department at IPHT where Dr Silke H Christiansen is leading the Semiconductor Nanostructures department in addition to her appointment at MPI.