Diffusion Engineering by Carbon in Silicon

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Diffusion Engineering by Carbon in Silicon Ulrich Goesele, Pierre Laveant, Rene Scholz, Norbert Engler, and Peter Werner Max Plack Institute of Microstructure Physics Weinberg 2, D-06120 Halle, Germany ABSTRACT The possibility to suppress undesirable diffusion of the base dopant boron in siliconbased bipolar transistor structures by the incorporation of a high concentration of carbon has lead to renewed interest in the behavior of carbon in crystalline silicon. The present paper will review essential features of carbon in silicon including solubility, diffusion mechanisms and precipitation behavior. Based on this information the possibilities to use carbon to influence diffusion of dopants in silicon by the introduction of non-equilibrium concentrations of intrinsic point defects will be discussed as well as the reason for the relatively high resilience against carbon precipitation. Interactions between carbon and oxygen will be mentioned, especially in the context of an as yet unexplained fast out-diffusion of carbon close to the surface. INTRODUCTION Besides oxygen carbon is probably the best investigated electrically inactive impurity in crystalline silicon, mostly because carbon is present in appreciable concentrations in silicon after crystal growth. Initially it was of major interest just to characterize its overall behavior in silicon, such as its solubility, diffusion coefficient and precipitation behavior. It was shown that carbon was involved in certain agglomerates of self-interstitials formed during crystal growth of float-zone materials and in certain types of polycrystalline silicon ribbons used for solar cells. It was also noticed that carbon facilitated oxygen precipitation, which had become important for intrinsic gettering purposes. Overall carbon was well investigated and fairly well understood since the middle of the 1980s [1-3]. It was concluded that carbon was a fairly benign impurity which was not really damaging for device processing but did not offer any specific advantages either. Later on it was shown that implanted carbon, in contrast to boron, did not lead to interstitial-type dislocation loops for comparable doses [4]. In addition, implanted carbon showed good gettering capabilities [5] and, at sufficiently high doses, could be used as a very effective etch-stop [6]. Since carbon contracts the silicon lattice considerably it was then investigated carefully as a potential remedy to compensate the lattice expansion associated with germanium in silicon used for heterobipolar transistors [7,8]. For these investigations carbon was incorporated into the silicon lattice during epitaxial crystal growth. Surprisingly, it turned out that carbon can be incorporated in concentrations up to a few percent which is many orders of magnitude above its thermodynamic solubility of carbon in silicon. Most recently carbon incorporation during crystal growth of silicon-germanium alloys was used to induce Si-Ge quantum dot structures [9]. The observation that implanted carbon could reduce the so-called Transi

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