LEED investigations on the interaction of Pd and Ni with different Si(111) surfaces

Abstract

The structural properties of Pd and Ni on cleaved Si(111)-2 × 1 and − 7 × 7 surfaces have been studies under ultrahigh vacuum conditions by LEED to evaluate the microstructure in the early stages of suicide-silicon interface formation. In contrast to the indifferent behavior of Ni, the reaction of Pd reveals strong evidence for the dependence of the nucleation and growth of epitaxial Pd 2Si on the initial surface reconstruction. A 5 × 1 superstructure emerges on the 2 × 1 surface after deposition of submonolayer Pd at room temperature, suggesting a rather well ordered lateral arrangement of Pd 2Si(1000)-1 × 1 nuclei, whereas epitaxial Pd 2Si on 7 × 7 is only established at much higher coverage ( θ ⩾ 3). An unexpected 3 × 3 reconstruction is formed after annealing at ∼ 200 ° C for θ ⩽ 3 monolayers of Ni, independent of the initial reconstruction and well below the formation temperature of epitaxial NiSi 2. The coverage dependence of the 3 structure implies the presence of an unknown epitaxial film. This finding represents the first evidence of a well defined intermediate state of reaction between unreacted Ni and NiSi 2, which has not been anticipated from other UHV experiments. The qualitative difference in the interaction of Pd and Ni with the Si lattice is demonstrated further by the reappearance of the 7 × 7 reconstruction even after several monolayers of Ni, which has not been achieved with Pd under any process condition, and the pronounced faceting of stepped surfaces under Pd.