In situ study of growth of NiSi2 nanoplates in Si(001) wafer

Full text: Composite thin films formed by NiSi2 silicide nanocrystals embedded in single-crystalline Si have attracted a renewed interest due to their potential applications in electronic devices. We describe here an in situ study of the formation and growth of NiSi2 nanoplates in Si(001) wafer near the surface using Grazing Incidence Small Angle X-ray Scattering (GISAXS). A new preparation method was used to obtain well separated NiSi2 nanoplates embedded in Si. The sample was prepared using two precursor wet solutions deposited on the Si wafers through spin-coating method. The first solution is composed of 0.150 g of Ni(NO3)2.6H2O salt diluted in isopropyl alcohol. The second solution is composed of a mixture of 0.050 g of TEOS (tetraethyl orthosilicate) and 0.30 of HNO₃ diluted in isopropyl alcohol. Both solutions were deposited on the Si(001) in the same order present here. The sample was dried in air and then heated at 300 ℃ in 5% H2+He flow to promote the reduction of Ni oxide and evaporation of the volatiles. As result we obtained a Ni-doped SiO₂ thin film (thickness ~100 nm) covering one of the surfaces of the Si wafer. This sample was placed in a specially designed high temperature furnace installed at the XRD2 beam line of the Brazilian Synchrotron Radiation Laboratory (LNLS). The GISAXS intensity produced by the thin film and by the thin Si layer close to its external surface was measured several times during annealing at 405 ℃ in He flow. 2D GISAXS intensity patterns were recorded each 60 seconds of the thermal treatment. Previous Scanning Transmission Electron Microscopy (STEM) results of a sample prepared using the same method but annealed at 500 ℃ during 50 min indicated the formation of NiSi2 nanoplates in Si is a consequence of the diffusion of the Ni atoms from the Ni-doped SiO₂ thin film into Si followed by the reaction with the Si atoms. This previous study showed the NiSi2 nanoplates have the shape of nearly regular hexagons with their larger surface parallel to the planes of the Si{111} crystalline form. In the here reported in situ study a lower temperature was preferred to avoid nanoplates growing too fast. From the analysis of the GISAXS intensity patterns we determined the average value of the largest diameter of the hexagonal surface, thickness, number and total volume of the nanoplates as a function of the treatment time. The results showed the nanoplates reach their final size after 10 minutes of annealing while the number of the nanoplates increases until 50 minutes. This behavior indicates the final size of the nanoplates is not limited by the concentration of Ni atoms in the thin film. The dependence on time of the total volume of the NiSi2 nanoplates, V(t), was compared to the function predicted by the Jhonson-Mehl-Avrami theory. The parameters determined from the best fit of this function to the experimental V(t) function are consistent with the growth of planar shaped nanoparticles formed by an heterogeneous nucleation process. (author)