Point defect-based modeling of diffusion and electrical activation of ion implanted boron in crystalline silicon

Abstract

The time evolution of the transient enhanced diffusion and of the electrical activation of boron in crystalline silicon during thermal annealing subsequent to boron ion implantation is modeled by a system of diffusion-reaction equations for the dopant species and the silicon point defects. The concept of point defect impurity pair diffusion under equilibrium conditions is used to describe the diffusion process. Outdiffusion of implantation-induced silicon self-interstitials and the kick-out reaction Bi■Bs+I are assumed to be the leading mechanisms for boron activation. In the case of low-dose boron ion implantation, we start from a defect distribution of Gaussian shape with one interstitial per implanted boron atom. For higher boron doses, the area density of this interstitial distribution is constant, but the depth position of its peak depends on boron dose. Local equilibrium for the reactions between the point defects and the boron species is postulated to be realized before the onset of diffusion. The computed boron depth profiles are compared to data from the literature. Implantation doses from 2×1014 cm−2 up to 5×1015 cm−2 are analyzed, annealing temperatures and times are considered over the ranges 800–1000 °C and 10 s–8 h, respectively. Although this approach is characterized by a number of simple assumptions, essential deficiencies are only found in certain cases of annealing subsequent to high-dose boron implantation. Trapping of free interstitials by extended defects seems to become important at low temperatures and for long annealing times. If the depth region with maximum boron concentration is in its as-implanted state close to amorphization, a boron overactivation which is beyond the present model can be found. For all other cases it is possible to achieve a reasonable modeling of transient enhanced diffusion and of electrical activation.