Schottky Barrier and Electronic States at Silicide-Silicon Interfaces
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P.E. SCHMID*, M. LIEHR**, F.K. LEGOUES** and P.S. HO** * Ecole Polytechnique de Lausanne, CH-1015, Lausanne, Switzerland ** IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598
ABSTRACT This paper reviews the recent studies on Schottky barrier and interface states at silicidesilicon interfaces, with emphasis placed on the results obtained from the epitaxial Ni silicides. A model based on interfacial defect states has been proposed to account for the overall chemical correlation between the barrier height and the metal electronegativity. Measurements on the barrier heights of type A, B and C epitaxial Ni silicides show that these three interfaces can be formed with high degrees of perfection to yield a barrier of 0.78 eV. Similar interfaces formed under less ideal conditions or with impurity incorporation decrease the barrier to 0.66 eV. The density and distribution of the interface states measured by a capacitance spectroscopy method correlate well with the structural perfection of the single and mixed-phase interfaces. A consistent picture seems to have emerged suggesting that the barrier height at silicide-Si interfaces is formed as a result of Fermi level pinning by interfacial defect states which are controlled primarily by the degree of perfection of the interface instead of the specific epitaxy.
INTRODUCTION During the past several years there has been considerable interest in studying Schottky barrier formation at silicide-silicon interfaces. This is largely stimulated by the use of silicides to form device contacts in very large-scale integrated circuits. This problem is of basic interest as well since this class of interfaces is highly reactive, and the reaction leading to silicide formation provides a class of interfaces with properties which can be controllably varied for studying the formation of the Schottky barrier. The reaction alters the interfacial characteristics, such as the chemical environment, stoichiometry and structure, all of which can influence the electrical properties. For such reactive interfaces, one fundamental question concerns the effects of chemistry and microstructure on the barrier height. Results from studies addressing this question were reviewed in 1983 [1]. It was concluded that the barrier height definitely depends on the metal species but is not influenced by the variation in the material characteristics of the bulk silicide phase, such as the silicide stoichiometry and structure. The effect of silicide formation was found mainly in changing the nature of the interface from one loaded with extrinsic imperfections, e.g. contaminations and process-induced defects, to one with intrinsic metal-silicon bonds. Hence as reaction proceeds, the imperfections at the original interface are left behind and an intrinsic barrier is established. These observations, plus the synchrotron results on band bending [2] showing Fermi level pinning with a few monolayers of metal coverage, led to the conclusion that the barrier of the silicide-Si interface is a true interface propert
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