The Chemical Structure of Metal/Semiconductor Interfaces as Determined by X-Ray Photoelectron Spectroscopy

Citation

Grunthaner, Paula Jean (1980) The Chemical Structure of Metal/Semiconductor Interfaces as Determined by X-Ray Photoelectron Spectroscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0458-3943. https://resolver.caltech.edu/CaltechTHESIS:07232025-170156584

Abstract

The chemical nature of transition metal/silicon interfaces has been investigated using x-ray photoelectron (XPS) and Rutherford backscattering (RBS) spectroscopy, In particular, the Ni/Ni₂Si, Si/Ni₂Si, and Ni/Si transitional regions have been examined in detail. The effect of oxygen impurities on the Ni/NiSi interface has been studied using ion implantation and ¹⁶O(d,α)¹⁴N nuclear reactions in conjunction with XPS and RBS. Preliminary results on the Pd/Pd₂Si and Pt/Pt₂Si interfaces are presented and contrasted to the Ni system.

A technique for studying silicide/silicon and silicide/metal interfaces has been developed in the course of this work. The approach exploits the exponential attenuation of photoemission intensities to dynamically monitor the advancing planar silicide growth front during the in situ growth of the silicide. The technique allows the examination of a realistic interface bounded on either side by an extended solid without the chemical and structural perturbations caused by conventional depth profiling methods. The local chemical environment of both the silicon and transition metal atoms has been established through analysis of the observed binding energy shifts in the photoemission spectra ⁴He⁺ backscattering has been used to follow the progression of the thin film reaction and to provide quantitative information on atomic composition.

The Ni/Ni₂Si interface was examined using both amorphous and crystalline Si as a substrate. In each case, it was demonstrated that the Si atoms in the transition region are in a substantially more Ni-rich environment than that found for Si in Ni₂Si. These regions are again of graded composition. The first Si 2p signal observed is consistent with a metallic-like Si substituted in the fee lattice of Ni metal. The essential difference between the Ni/Ni₂Si interface using a crystalline substrate as compared to an amorphous substrate is that the former is 2 - 3 times narrower (0.6λ) than the latter (1.5λ).

The as-deposited Ni/Si interface was investigated by monitoring the evolution of the Ni and Si core levels as multiple monolayers of Ni were deposited on Si (100). The data indicate the presence of a chemically graded transition region which ranges in stoichiometry from Ni atoms bound in the Si interstitial voids on the (100) Si side of the interface to Si atoms substituted in the Ni metal lattice on the Ni° side of the interface.

The chemical nature of the Ni/Si interface as a function of the substrate temperature was also examined. It was shown that substantial chemical interaction occurs between the Ni and Si substrate at temperatures as low as 100° K. Ni metal was deposited on an inert substrate to demonstrate that aggregation effects were not responsible for the observed chemical shifts.

The dependence of the Ni 2p binding energy on the substrate temperature also demonstrated that the initial Ni atom deposited on the Si surface must drop into the interstitial Si voids rather than being bound on the surface. The behavior of the Ni 2p binding energy as a function of Ni coverage at elevated substrate temperatures suggests the presence of several distinct Ni environments.

The effect of oxygen impurities on the Ni/Ni₂Si interface was investigated via ion implantation using XPS, RBS, and ¹⁶O(d,α)¹⁴N nuclear reactions. It was shown that 2.2 x 10¹⁶ O/cm² are sufficient to block the diffusion of the Ni metal and thereby inhibit the silicide reaction. The data demonstrate that as the advancing Ni/Ni₂Si interface encounters oxygen in the Ni film, silicon suboxides are formed. As more oxygen is encountered, SiO₂ is formed. When a sufficient layer of SiO₂ has formed, the Ni metal is no longer able to diffuse through to the Si/Ni₂Si interface to continue the solid phase reaction.

Preliminary results for the Pd/Pd₂Si and Pt/Pt₂Si interfaces are presented and contrasted to those found for the Ni/Ni₂Si system. The data suggest that, as the Ni case, the limiting environment for Si at the Pd/Pd₂Si interface is metallic-like with a high ligancy. We propose this is a Si atom substituted in the fee lattice of Pd metal. The Si atoms at the Pt/Pt₂Si interface, on the other hand, rest in a site of substantially lower coordination number in the fee Pt lattice. This is suggestive of an interstitial defect site.

Both the Pd/Pd₂Si and Pt/Pt₂Si interfaces are found to be graded in composition. Estimations of these widths indicate that the metal/ silicon interface widths decrease in the order Ni > Pt > Pd.

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