AB-INITIO MODELING OF C-B INTERACTIONS IN SI Chun-Li Liu, Wolfgang Windl, Len Borucki, Shifeng Lu*, Xiang-Yang Liu** Advanced Process Development and External Research Lab., Motorola *Process and Materials Characterization Laboratory, Motorola **Physical Sciences Research Laboratory, Motorola ABSTRACT We present the results of ab-initio calculations for the structure and energetics of small boron-carbon (BCI) as well as carbon-carbon (C 2 I) clusters in Si , a continuum model for the nucleation, growth, and dissolution of the clusters, and experimental investigation by SIMS. The modeling results suggest that these clusters may play a role in controlling B diffusion in Si and SiGe systems and the experimental results seem to support the modeling findings. I. INTRODUCTION SiGeC has several beneficial materials properties over SiGe which include the compensation of film strains by adjustment of the Ge/C ratio, enhanced thermal stability, increased critical film thickness, suppressed transient enhanced diffusion (TED) of boron [1], and preservation of the narrowed band gap of strained SiGe. One particular device application of SiGeC films is the construction of heterojunction bipolar transistors (HBT) using Si/SiGeC/Si heterostructures. SiGeC HBTs have increased performance (higher frequency) due to the smaller band gap [2] and stability of the base profile. The currently prevalent explanation of suppression of diffusion of boron by carbon is that carbon reduces the free silicon interstitial (I) concentration by forming a C- Si complex, resulting in fewer B-Si complexes, which are believed to be responsible for B TED in Si [1,3]. Reference [1] further showed that the diffusion of carbon incorporated in silicon well above its solid solubility causes an undersaturation of silicon self-interstitials, which further retards boron diffusion. In this work, we further explore the atomic mechanisms for the effect of carbon on B diffusion through ab-initio investigation of C-B interactions by focusing on C-B split interstitial pairs (CBI). For the first time the results of our ab initio calculations indicate that carbon and boron can interact directly by forming CBI pairs. For the modeled case, CBI is the most important cluster containing B, while the predominant cluster capturing and storing Si interstitials is the C2I cluster. Our modeling predicts that the CBI concentration may not be sufficient to surpass the effect of carbon in reduction of Si interstitials by forming C2I clusters, and thus that the direct interactions between C and B by forming C-B pairs may play a secondary role in suppressing B diffusion. However, a comparison to experiment indicates that BCI complexes might play an even more central role than C2I clusters (our ab initio calculations have error bars of ~0.3 eV). II. COMPUTATIONAL METHODS AND ASSUMPTIONS VASP (the Vienna Ab-initio Simulation Package) [4] was used for the calculations in this work. The ultra-soft pseudopotentials with a plane wave basis supplied with VASP and the generalized gradient approximation (GGA) were used. A supercell of 64 Si atoms and a cutoff energy of 21 Ry corresponding to the cutoff energy of the C pseudopotential were chosen for all calculations. Diffusion barriers were determined using the nudged elastic band method (NEBM) [5] implemented in VASP, Mat. Res. Soc. Symp. Proc. Vol. 669 © 2001 Materials Research Society J4.6.1