The key role of hydrogen in the growth of SiC/SiO(2) nanocables.

SiC/SiO(2) nanocables, consisting of a crystalline SiC core surrounded by an amorphous silica shell, have been grown by thermal chemical vapour deposition (CVD) at 950 °C on Ni-covered silicon substrates. The addition of methane to a 375 Torr hydrogen atmosphere, after heating the substrate in argon, leads to the growth of the SiC/SiO(2) nanocables, by the carbothermal reduction of silicon oxide as the initial stage. The growth mechanism follows the model previously proposed by us for a reducing medium. From the results obtained, several effects of hydrogen on the deposition process have been established: (a) reduction of the nickel nucleation sites, thus favouring the formation of SiC from the initial stage; (b) oxygen removal in the medium hindering the oxidative effect over the SiO and C species, thus promoting the nanocable growth, and (c) increase of the SiO concentration in the neighbourhood of the active nucleation sites. In addition, it is important to mention that SiC/SiO(2) nanocables, following the already proposed model, are obtained uniquely in a narrow hydrogen pressure range. At high hydrogen pressure, the unexpected formation of silica nanowires together with the SiC/SiO(2) nanocables has been detected.