Boron Segregation and Out-diffusion in Single-Crystal Si 1-y C y
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Boron Segregation and Out-diffusion in Single-Crystal Si1-yCy E. J. Stewart and J.C. Sturm Center for Photonics and Optoelectronic Materials, Department of Electrical Engineering Princeton University, Princeton NJ ABSTRACT Boron segregation and its effect on carbon diffusion is studied in single-crystal Si1-yCy. We find that boron segregates from silicon to Si0.996C0.004 at a level m=[B]SiC/[B]Si = 1.7 during a 2 hour, 850°C anneal in N2. After this anneal, if most of the carbon is then removed from the Si1-yCy layer (via an oxidation-enhanced outdiffusion process), most of the boron segregation is removed as well. This argues against immobile B-C defects as the predominant mechanism driving the segregation. Boron is shown to increase carbon diffusion during the N2 anneal, but also appears to enhance carbon precipitation during a subsequent oxidation. INTRODUCTION Si1-x-yGexCy and Si1-yCy alloys are of great interest for controlling dopant profiles in scaled Si-based devices. Adding small amounts of substitutional carbon to Si or Si1xGex has been shown to dramatically decrease the diffusion of boron and phosphorus. Carbon suppresses the concentration of silicon interstitials in the lattice, on which boron and phosphorus depend to diffuse [1]. Reducing unwanted diffusion during thermal processing is critical for devices where sharp dopant profiles must be maintained. This has been applied most notably to the base of a heterojunction bipolar transistor [2], and has also been investigated for MOSFET channels and source/drains [3,4]. In addition to diffusivity reductions, carbon also can induce dopant segregation. Boron has been observed to strongly segregate to polycrystalline Si1-x-yGexCy layers from polysilicon during thermal anneals [5]. This provides additional control over dopant profiles, as boron will not only diffuse slower but also be drawn into carbon-containing regions. This has been exploited in the gates of p-channel MOSFETs, where polycrystalline Si1-x-yGexCy layers reduce boron penetration into the substrate [6]. The segregation has been shown to increase with carbon level and occurs with or without germanium, indicating that carbon by itself can drive the process. It has also been observed in single-crystal Si1-x-yGexCy, revealing that it is not entirely a polycrystalline effect associated with grain boundaries. In this work, we study boron segregation and related diffusion in single-crystal Si1-yCy structures. The segregation process is shown to be mostly reversible by removing the carbon from the Si1-yCy layer. Interstitial gradients are discussed as a driving mechanism for segregation. Finally, we discuss the effect of boron on carbon diffusion. EXPERIMENT Two structures (labeled "A" and "B") were grown by Rapid Thermal Chemical Vapor Depostion at 625°C and 750°C, using SiCl2H2 and Si2H6 as silicon sources and
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SiCH6 and B2H6 as carbon and boron sources, respectively. For the low carbon concentrations used in this work (0.4%), the carbon is primarily incorporated substitutionally [
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