Dopant Diffusion in SiGeC Alloys

Alloying germanium and/or carbon to the Si lattice can change diffusion coefficients of B, P, As, and Sb by more than one order of magnitude. This effect has a major impact on design and fabrication of Si-based heterojunction devices such as high-speed npn and pnp SiGe HBTs and strained channel FETs. The most prominent example for the exploitation of suppressed dopant diffusion is the npn SiGe HBT. Reduced diffusion of boron in SiGe has facilitated the fabrication of extremely sharp base doping profiles resulting in excellent RF performance. Carbon doping of the SiGe layer has been identified as a means for further suppression of the diffusion of boron and consequently became an indispensable feature of state-of-the-art high-performance SiGe HBTs. In this paper, we discuss the impact of germanium and carbon on the diffusion of common dopants in Si-based alloys. We review results of various diffusion experiments and discuss the basic physical mechanisms of the observed changes of diffusion coefficients as a function of alloy composition. Results of new marker layer diffusion experiments are presented for binary Si1-xGex and Si1-yCy and ternary Si1-x-yGexCy layers. It is shown that boron and phosphorus marker layers in ternary alloys exhibit a superposition of the reduction/enhancement of their diffusion coefficients due to germanium and due to carbon.