Disilicide Phase Research Articles

Comparison of the Three Classes (Rare Earth, Refractory and Near-Noble) of Silicide Contacts

It is well established that near-noble metals and refractory metals form two distinct classes of silicide contacts with silicon. Rare earth metals have been studied in the same manner and found to form a new class that is very distinct from the other two in terms of properties and characteristics. Some of these characteristics are the formation of a disilicide phase, as an apparently first and last phase, at a surprisingly low temperature (250–400°C). Marker motion study using implanted krypton and argon showed silicon to be the dominant diffusing species for ErSi2. The Schottky barrier height to n-Si is 0.40 ± 0.04 eV and to p-Si is 0.70 ± 0.04 eV for all six of the metals studied. The surface morphology after reaction indicates the formation of a tensile stress by the silicide reaction. Oxidation of the rare earth metals is a severe problem although a variety of passivation schemes have been shown to work.

Epitaxial growth of the nickel disilicide phase

The nickel disilicide (NiSi 2) phase which forms epitaxially on silicon single-crystal substrates in many ways exhibits an interesting behavior. A regular network of the silicon-rich phase was observed to originate at or near the Si-silicide interface using the Berg-Barrett X-ray topographic technique. The regularity, the thickness and the density of the network increase as the epitaxial layer increases in thickness. For (100)-oriented silicon substrates the network runs along two perpendicular 〈110〉 directions; for (111)-oriented silicon substrates the network runs along 〈112〉 directions. Large voids on the surface of the epitaxial layer and fracturing of the layer along the edges of the network for thick epitaxial layers were also observed. The results are explained in terms of the corrugated morphology of the NiSi 2-Si structure.

Investigation of the Cr-Si system in the disilicide region

1. It has been established that the chromium disilicide phase crystallizing at the composition CrSi1.95 expands when the temperature drops, shifts in the direction of silicon, and at 1250°C corresponds to the limiting composition CrSi1.98−1.99to CrSi2.02−2.03. 2. From the data of investigating the electrophysical characteristics of single-phase alloys we established that the stoichiometric composition corresponds to a minimum microhardness, maximum thermal conductivity, minimum value of the hole concentration, and, assuming invariability of the scattering mechanism, a minimum effective density of the carrier states. 3. The stoichiometric composition corresponds to a maximal value of the width of the forbidden band (∼0.7 eV) which falls near the single-phase boundaries to 0.4–0.5 eV. 4. From a comparison of the data on the electrophysical characteristics and from the measured and theoretical density of the alloys in the region CrSi2, we have proposed that the high concentration of holes (∼5·1020 per cm3) in the stoichiometric composition is related with intracrystalline defects. The defects decrease when there is a deviation from stoichiometry toward chromium and with a composition of CrSi1.95 of the pulled single crystals there are almost no defects, whereas with deviation toward an excess of silicon atoms defects remains approximately constant. One molecular defect gives from 0.5 to 1 carrier in the valence band.