The enhanced diffusion of boron in silicon after high-dose implantation and during rapid thermal annealing

Abstract

In this paper we have studied the diffusion of boron in silicon after high-dose implantation (50 keV, 5 × 10 15 ions cm 2 ) and during rapid thermal annealing at 1100°C, under nitrogen gas. We confirm that some enhanced as well as “anomalous” diffusion takes place during the early stage of annealing and that this phenomenon must be related to the defects generated by ion implantation. Experiments were performed on companion samples by SIMS, XTEM and resistivity methods. “Damage” calculations were obtained by running the computer code LUPIN to generate the defect profile (displacements) due to the bombardment. For samples which were subjected to increasing annealing periods (1 s, 3 s, 5 s, etc.) the dopant profile can be simulated only when assuming a phenomenological depth-dependent diffusion coefficient which is always many times higher than according to the “classical” theory. The discussion is conducted by comparing the depth variation of the diffusion coefficient with the position and density of the extended defects seen by XTEM. We show that the formation of a dense band of dislocations and loops around the boron projected range (within 1 s at 1100° C) corresponds to the ejection and clustering of Si interstitials due to the activation of boron. For larger annealing times, boron diffusion is dependent on the motion of these interstitials emitted from the extended defects until they dissolve into the bulk. These experiments clearly evidence the role played by Si interstitials and lead us to reject the idea that the “collisional” damage is responsible for the enhanced diffusion of boron in silicon.