Fluorine in preamorphized Si: Point defect engineering and control of dopant diffusion

While it is known that F modifies dopant diffusion in crystalline Si, the physical mechanisms behind this process are still unclear. In this work we report experimental studies about the F control of the point defect density in preamorphized Si layers. These studies put the basis for the understanding of the F behavior and for the realization of ultra-shallow junctions. We first investigated the F incorporation process during the solid phase epitaxy (SPE) of amorphous Si layers. We elucidated the role of the SPE temperature on the F incorporation and suggested a new route towards a F profile engineering. Moreover, we explained the role of F in modifying the point defect population (self-interstitials, Is, and vacancies, Vs), employing B and Sb spike layers as markers for Is and Vs, respectively. We clearly showed that F decreases the B diffusion while enhances the Sb one, pointing out the capacity to induce an Is undersaturation or a Vs supersaturation. These data rule out the hypothesis of a chemical bonding between F and the dopants. Such F ability in modifying the Is/Vs density resulted to be a transient effect, because strictly correlated with the presence of F in the Si samples, which decreases with the annealing time. In addition, we evidenced that even if F is spatially separated from B, i.e., localized between shallow-implanted B and the end-of-range (EOR) region, it still suppresses the enhancement of B diffusivity, due to the EOR defects dissolution. These studies, besides improving the current understanding of the physical mechanisms by which F influences the dopant diffusion in Si, could be helpful for the realization of ultra-shallow junctions for the future metal-oxide-semiconductor devices.