Effect of C and Ge Concentration On The Thermal Stability of RTCVD Grown Si 1-x-y Ge x C y Alloys
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ABSTRACT Si / Sil-xyGexCy / Si heterostructures containing up to 17 at.% Ge and 1.9 at.% C were grown on (001) silicon by low pressure Rapid Thermal Chemical Vapor Deposition, using a mixture of silane, germane and methylsilane, diluted in hydrogen. The samples were then annealed in a Rapid Thermal Processing furnace, under an atmospheric pressure of nitrogen, at temperatures ranging from 900 to 1130 °C. The samples were characterized using infrared spectroscopy and x-ray diffraction. SIMS profiling and TEM observation were performed on some of the samples. Substitutional C gradually disappeared, either precipitating out to form cubic silicon carbide (B-SiC), or simply vanishing into interstitial positions. In any case, the in-plane lattice constant remained constant after annealing, indicating that there was no mechanical strain relaxation by formation of misfit dislocations. The perpendicular lattice constant increased due to the decrease in substitutional C concentration, as well as it decreased due to the germanium out-diffusion. This variation of the strain during annealing was modeled, and allowed the determination of the kinetics of the substitutional carbon disappearance. The same behavior was observed for all samples. Indeed, the Cs disappearance rate was always increased for samples with higher initial Ge and C concentrations. The kinetics of this precipitation was found in very good agreement with previous published results. INTRODUCTION
Group IV based heterostructures epitaxially grown on silicon are of great interest for Si microelectronics technology. Most efforts up to now have been centered on the Si-Ge alloys (see for example the review ref. [1]). These binary alloys, however, have the drawback to be highly metastable and to relax via plastic flow (i.e. nucleation of misfit dislocations). Moreover, the possibilities of band gap engineering are limited, as composition and strain are variing simultaneously. The incorporation of C onto substitutional sites (Cs) in SiGe films allows to tailor the Ge concentration and strain independently (see for example the review paper of ref. [2]). It provides additional flexibility for band-gap engineering. Furthermore, the thermal stability of nearly lattice-matched films should be much improved. Because Ge is totally soluble in Si, the stability of strained SiGe layers during subsequent processing (i.e. ion implantation, thermal oxidation or annealing) is not a chemical but only a mechanical problem. The case of SiC or SiGeC films is more complicated. Their metastable composition favors B-SiC precipitation or at least substitutional-interstitial migration of C, i.e. chemical relaxation. A detailed study of the annealing behavior of pseudomorphic SiGeC layers is a prerequisite for their technological integration. Previous studies of the thermal stability of epitaxial SiGeC layers [3-5] mostly dealt with conventional furnace anneals. It has been demonstrated that ternary epitaxial films are more stable than binary ones, i.e. they do not relax their strain by plast
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