Arsenic uphill diffusion during shallow junction formation

The behavior during annealing of low-energy As-implanted Si have been investigated by comparing secondary ion mass spectrometry (SIMS) and simulated profiles. Z-contrast scanning transmission electron microscopy (STEM) imaging has also been used to determine the As local distribution in proximity of the sample surface. The implants have been performed with energies between 1 and 10 keV both through a thermally grown 11 nm thick oxide and without any oxide mask. SIMS and STEM profiles show, after short annealing at 800-1000 degrees C, an As pileup in the first nanometers of the Si matrix in proximity of the SiO2/Si interface. We demonstrate that this phenomenon can be explained with a "Fickian" standard diffusion by assuming the presence of unspecified "dopant traps" near the SiO2/Si interface that cause a drastic reduction of the dopant able to diffuse inside the bulk. We have also verified that removing before annealing the superficial 4 nm of Si does not eliminate the As pileup. Different mechanisms proposed in literature to explain the uphill diffusion are discussed. Furthermore, the availability of a suitable simulation model allows us to evaluate the dopant diffusivity during the annealing and investigate the transient enhanced diffusion (TED) phenomena.