Silicon Interstitial Driven Loss of Substitutional Carbon from SiGeC Structures
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Silicon Interstitial Driven Loss of Substitutional Carbon from SiGeC Structures M. S. Carroll and J. C. Sturm, Dept. of Electrical Engineering, Princeton University, Princeton, NJ; E. Napolitani, D. De Salvador and M. Berti, INFM and Dept. of Physics, University of Padova, Padova, Italy; J. Stangl and G. Bauer, Institute for Semiconductor Physics, JohannesKepler University Linz, Linz, Austria; D. J. Tweet SHARP Laboratories of America Inc., Camas WA Abstract The effect of annealing silicon capped pseudomorphic Si0.7865Ge0.21C0.0035 or Si0.998C0.002 layers on silicon substrates in nitrogen or oxygen at 850°C was examined using x-ray diffraction (XRD) and secondary ion mass spectrometry (SIMS). Most substitutional carbon is lost from the alloy layers due to carbon out-diffusion rather than from precipitation. The carbon is found to diffuse more rapidly out of the SiGeC layer than the SiC layer after nitrogen and the carbon is found to leave the sample entirely, an effect that is enhanced by oxidation and thin cap layers. All substitutional carbon can be removed from the sample in some cases implying negligible formation of silicon-carbon complexes. Furthermore, it is found that each injected silicon interstitial atom due to oxidation causes the removal of one additional carbon atom for the SiGeC layer. Introduction Substitutional carbon incorporation in silicon and SiGe has drawn significant attention because of reduced boron diffusion in carbon’s presence, due to its ability to consume silicon interstitials, which mediate boron diffusion [1]. However, the potential that the interstitial-carbon product is a defect (i.e ß-SiC precipitation or carbon clusters [2,3]) may limit the usefulness of carbon for diffusion engineering. Previous studies of carbon thermal stability in SiGeC confirm that carbon can precipitate in SiGeC [2,4]. In this letter carbon out-diffusion from thin SiGeC layers is examined and carbon out-diffusion is found to be the dominant mechanism of carbon loss for samples close to the surface, even in the regime of carbon concentration far above solid solubility or in the presence of excess interstitials injected during oxidation. Experiment Two test structures with 25 nm thick Si0.7865Ge0.21C0.0035 layers capped by 50 or 280 nm of silicon and one 150 nm thick Si0.998C0.002 layer capped by a 45 nm silicon layer were grown on silicon substrates by rapid thermal chemical vapor deposition (RTCVD) at temperatures between 550ºC and 750ºC using dichlorosilane, disilane, germane, and methylsilane as the silicon, germanium, and carbon sources respectively [5]. Samples of the as-grown and annealed structures were examined using secondary ion mass spectrometry (SIMS), which were sputtered using 1-2 keV Cs+ (Fig. 1) or O+ (Fig. 2) ions. Depths were determined using standard profilometry of the sputtered craters leading to a 5% uncertainty in depths, a 20% (Fig. 1) or 10% (Fig. 2)
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uncertainty in carbon concentrations and approximately a 2% uncertainty in the absolute germanium fraction. 1021
850°C, N2
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