Experimental Study on the Mechanism of Carbon Diffusion in Silicon

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N.E.B. COWERN* , B. COLOMBEAU*, F. ROOZEBOOM , M. HOPSTAKEN , H. SNIJDERS†, P. MEUNIER-BEILLARD‡ and W. LERCH§ *School of Electronics, Computing and Mathematics, University of Surrey, Guildford, Surrey, GU2 7XH, United Kingdom † Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands ‡ Philips Research, IMEC, Kapeldreef 75, B-3001 Leuven, Belgium § Mattson Thermal Products GmbH, Daimlerstrasse 10, D-89160 Dornstadt, Germany

ABSTRACT CVD-grown lightly C-doped superlattices with peak C concentrations of 2.1018/cm2 and 2.1019/cm2 were annealed in NH3, N2/H2, N2, and O2 ambient gases to investigate the influence of a range of point-defect conditions on C diffusion at the nanometer scale. C profiles were measured by secondary-ion mass spectroscopy. The profiles exhibit exponential-like diffusion consistent with a ‘long hop’ diffusion process with a characteristic migration length λ (=19 ± 3 nm at 850 °C). Within experimental errors the value of λ is the same for all the ambient gases used, whereas the migration frequency g increases by two orders of magnitude as the ambient gas is changed from NH3 ambient (interstitial undersaturation) to O2 ambient (interstitial supersaturation), and decreases as a function of C concentration in the as-grown superlattice. The results confirm that C diffuses predominantly by a kick out mechanism under nearequilibrium diffusion conditions. Initial results support the chemical-pump model for suppression of diffusion in C-doped silicon.

INTRODUCTION Carbon plays a potentially important role as a suppressor of interstitial-mediated dopant diffusion and clustering in advanced silicon-based devices. There are two ways in which C is believed to influence dopant diffusion. First, since C is a fast interstitial-mediated diffuser it acts as a ‘chemical pump’ expelling a flux of interstitial atoms from the C-doped region, thus creating a localised undersaturation of interstitials [1]. Second, the clustering of C atoms at high C-concentrations can consume excess interstitials [2]. Both effects inhibit the motion of dopants that rely on interstitial-mediated diffusion mechanisms. In order to get a proper understanding of the relative importance of these two phenomena in different device applications, quantitative data on the fundamental mechanism and parameters of C-diffusion are required. This paper presents a fundamental study of the C-diffusion mechanism using an approach first developed in Ref. [3]. Essentially all substitutional impurities in silicon diffuse by converting for brief intervals into a fast-diffusing point-defect complex (of vacancy or interstitial type), a process which has been described as ‘intermittent’ or 1

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Figure 1: Illustration of a single dopant migration step. A native point defect (dashed line) reacts with a substitutional dopant atom to form a short-lived fastmigrating species. After traveling some distance the migrating