Mechanisms of Ni diffusion at silicon surface

A comparative study of the mechanisms of Ni transport along a clean Si(111) surface and along a surface with submonolayer coverages of Co or Fe has be en carried out by Auger electron spectroscopy and low energy electron diffr action. It has been shown that both at the clean and at adsorbate-covered s urfaces the Ni transport occurs by diffusion of Ni atoms through the bulk o f Si rather than by intrinsic surface diffusion. The distinction between th ese two cases is as follows. At a clean Si surface the concentration distri butions of transferred Ni appear on the surface after the completion of ann ealing and cooling the sample down as a result of Ni segregation to the sur face due to the decrease of Ni solubility at decreasing temperature. The Ni distributions at the clean Si surface can be detected only after annealing at temperatures > 750 degrees C. In the presence of the submonolayer quant ities of adsorbed Co or Fe, the temperature range where the Ni transport is observed on Si surfaces expands substantially towards low temperatures, sp ecifically to 500 degrees C on a surface with adsorbed Co and to 600 degree s C on a surface with adsorbed Fe. Unlike clean surfaces, on the surfaces w ith adsorbed metal atoms the Ni distributions are formed due to the adatom- induced trapping and accumulation of Ni atoms on the surface at high temper atures, during the annealing. This mechanism of Ni transport dominates at t he Si surfaces with adsorbed Co or Fe at temperatures below 750 degrees C.