The Precipitation of Nickel and Copper at Grain Boundaries in Silicon
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THE PRECIPITATION OF NICKEL AND COPPER AT GRAIN BOUNDARIES IN SILICON H. J. MOLLER, U. JENDRICH, L. HUANG, AND A. FOITZIK Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106 ABSTRACT The precipitation of copper and nickel at grain boundaries in cast polycrystalline silicon is investigated. The metals are diffused into the specimens from a surface source between 800 1000 OC and the precipitation after cooling is studied by TEM. Copper precipitates in form of colonies containing hundreds of particles with a size between 5 - 60 nm. In the grain boundary they nucleate preferentially at dislocations and steps. The distribution and size of the precipitates depend on the cooling rate after the diffusion. Nickel forms only few large (micrometer size) plate-like or three dimensional precipitates at and near grain boundaries. The main features of the results and the differences between the two elements are explained under the assumption that the precipitation requires the transport of native point defects. INTRODUCTION Many of the electronic properties of polycrystalline semiconductors are related to grain boundary phenomena. Recent experimental investigations [1-4] have shown that the electronic properties of grain boundaries can be changed by the presence of impurities. The transition metals are of particular interest since they are very mobile even at low temperatures and introduce deep levels themselves. Although their properties are well known, there is little information available on the interaction with grain boundaries and the influence on the electrical activity. It is therefore the purpose of a comprehensive study in our group to investigate the segregation and precipitation behavior of the 3d transition elements quantitatively. Copper and nickel are the elements with the highest diffusion coefficients and solubilities in silicon among the 3d transition elements and it has been shown that even after quenching they cannot be kept in solution [5]. The precipitation behavior in dislocation free, single crystalline silicon is complicated and has been studied extensively [6-19]. It is generally believed now that intrinsic point defects such as vacancies or silicon-interstitials play an important role in the precipitation behavior of metal impurities, and the experimental results for a number of elements have lead to the development of models which can explain some of the observed features. From this perspective the investigation of the interaction with grain boundaries may also yield additional information about the precipitation behavior of these impurities in silicon. The major difference between the behavior of copper and nickel lies in the properties of the metal-silicides which precipitate. The usually observed i" - Cu 3 Si phase [20] has a large lattice mismatch with silicon, whereas the structure of the NiSi 2 - phase matches the silicon matrix almost perfectly (mismatch 0.4%), therefore one can expect that this difference also influences the precipitation behavio
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