Lattice strain and composition of Boron-Interstitial Clusters in Crystalline Silicon
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Lattice strain and composition of Boron-Interstitial Clusters in Crystalline Silicon D. De Salvador, G. Bisognin, E. Napolitani, L. Aldegheri, A.V. Drigo, A. Carnera. MATIS-INFM and Dipartimento di Fisica, Università di Padova, Via Marzolo 8, 35131 Padova, Italy S. Mirabella, E. Bruno, G. Impellizzeri, F. Priolo MATIS-INFM and Dipartimento di Fisica e Astronomia, Università di Catania, Via S. Sofia 64, I-95123 Catania, Italy ABSTRACT In this work we have investigated the average composition of Boron Interstitials Clusters (BICs) and the strain induced in the Si crystal by BICs. We have formed BICs by Si implantation and subsequent annealing of two Si samples, grown by molecular beam epitaxy, containing thin buried layers doped with different B concentrations (1019 and 1020 at/cm3). By B chemical profiles diffusion analysis, we have extracted the doses of Si self-interstitials (I) and Boron atoms trapped at the BICs. The B/I stoichiometric ratio is about 1 for the low B concentration and about 3.5 for the high B concentration sample. High-resolution x-ray diffraction analyses provided an estimate of strain profile. While in the low B concentration sample no appreciable strain was detected after BIC formation, at the higher B concentration we found that the tensile strain present in the as grown B doped layer changes to a strong compressive strain as a consequence of BICs formation. For this kind of clusters, the mean volume expansion with respect to the Si matrix is of (29 ± 6) Å3 for each B atom trapped at the BICs. INTRODUCTION Ion implantation of boron in crystalline silicon is a widely used process to realize shallow ptype doped regions in microelectronic devices. However, during the annealing step required to remove the implant damage and to electrically activate boron, the interaction of the ion beam generated Si-self interstitials (I) with boron promotes the well known transient enhanced diffusion (TED) of B [1] and the formation of boron-interstitial clusters (BICs), even far below the B solubility limit [1-4]. In most cases, BICs are small, stable, and electrically inactive agglomerates which severely limit the achievement of shallow junctions with low sheet resistance as required for the shrinkage of future microelectronic devices. Despite the intense effort spent so far both from an experimental [1-4] and a theoretical [1,5,6] point of view, no clear and homogeneous picture is still available on the phenomenology of the BIC formation and dissolution, nor on their microstructure. Important information about BIC structure concerns the stoichiometry of BIC (B/I ratio) and the strain induced in the Si matrix. Theoretical studies predict that a B/I ratio near to 1 is preferred in order to minimize the strain energy [6] . This indication is confirmed by some experimental papers [3,4] where BICs are formed at a B concentration of about 1019 at/cm3. In this paper we report a quantitative investigation of the B/I stoichiometric ratio and of the lattice strain relative to BICs formed by Si ion implantation i
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