Silicide formation at the Ti/Si(111) interface: Diffusion parameters and behavior at elevated temperatures

We have used high-resolution x-ray photoelectron spectroscopy to investigate the kinetics of formation of the Ti/Si(111)-7×7 interface as a function of temperature. At room temperature, we observe chemically shifted Si 2p core levels at binding energies of -0.6 and -1.1 eV relative to the substrate, corresponding to the formation of TiSi and Si in solution in Ti. When the temperature is increased (T≲340 °C), the rate of Ti diffusion through the TiSi layer to the Si substrate increases, more Si is released from the substrate, and Si is better able to out-diffuse into the Ti overlayer. There is then heterogeneous TiSi growth, together with diffusion of Si along grain boundaries to the surface. Measurements of the Si and Ti core-level emission as a function of time and temperature for 100-Å-thick Ti overlayers on Si(111) make it possible to determine lower limits for the effective diffusion coefficients and an upper limit for the effective activation energy for this process. By using Fick’s second law with simplified boundary conditions, we found values of (7.5±2.5)×${10}^{\mathrm{-}17}$ and (7.5±2.5)×${10}^{\mathrm{-}16}$ ${\mathrm{cm}}^2$/sec for the diffusion coefficient at 275 °C and 340 °C, respectively, and an activation energy of 24±9 kcal/mol (1.0±0.4 eV/atom).