NiSi 2 on Si(111) : II. Effects of substrate temperature and defect structure

Abstract

Using LEED and AES, the growth of NiSi 2 has been investigated on stepped Si(111) surfaces misoriented by 4° toward the [1 1 0] direction. The reaction of Ni with Si(111) was followed as a function of the amount of Ni deposited and the deposition temperature on surfaces with two types of step arrangements. The first is the clean surface which undergoes a structural phase separation at T ≲ 820° C resulting in large flat regions separated by regions of high step density. The second surface contains the Ni-induced ( 19 × 19 ) R23.4° reconstruction which can be pinned in a single rotational domain with corresponding changes in the step structure: the step edges rotate to follow the 19 unit cell, the step-step separation increases to match the width of the 19 unit cell, and the 19 reconstruction is correlated across the steps. On both types of stepped surfaces, even at the extremely slow Ni-deposition rate of ∼ 0.4 ML/min, the previously reported reversal in silicide orientation with increasing Ni-coverage occurs. This change from B-type to A-type occurs at a slightly lower coverage on the clean, stepped surface than on the clean planar surface. On the 19 -reconstructed stepped surface the quality of the B-type silicide is improved and the formation of A-type is suppressed, although no such changes occur on the planar 19 -reconstructed surface. Deposition of Ni at increasing substrate temperatures results in increased diffusion of Ni into the substrate. This is accompanied by broadening of the LEED beams indicating structural damage to the surface. Growth following deposition at 250° C gives poor quality B-type silicide, but no change in the quality of A-type. At a substrate temperature of 450° C, Ni deposition results in no surface Ni observable by AES. The stages of the Ni-Si reaction were monitored during step-wise annealing. When the Ni-Si reaction is mostly complete as determined by AES, the I– V curves first become characteristic of B-type silicide. At low coverages, further annealing improves the quality of the B-type. At high coverages, further annealing causes the I– V curves to change to those characteristic of A-type. Finally, at even higher annealing temperatures, the thin silicide films aggregate to form NiSi 2 islands. The temperature of aggregation is much lower on the stepped surfaces than on planar surfaces.