Sub-surface retention of Pb atoms in silicon after low-energy ion implantation and electron beam annealing

Low-energy Pb ion implantation into (100) Si and subsequent high-vacuum electron beam annealing was used to study the potential of sub-surface retention of Pb atoms after applying a high temperature annealing process. 7keV Pb+ ions were implanted into p-type (100) Si at room temperature with a fluence of 4×1015ionscm−2. The implantation results in a Pb depth distribution that has a calculated Pb peak concentration of 23.9at.% at a depth of 8.0nm. The Pb implanted Si substrates were annealed with a high-current 20keV electron beam at 200–700°C for 15s. The Pb loss by out-diffusion was measured with RBS. Key results are: (i) minimal Pb loss in samples annealed up to 400°C, (ii) emerging out-diffusion above 400°C, (iii) retention of Pb atoms in the near-surface region in samples annealed up to 700°C. Comparison of the RBS data with the calculated evaporation rate of Pb under similar conditions reveals two distinctive temperature ranges in which the measured Pb loss of the implanted samples disagrees with the calculated Pb loss: (1) Pb atoms diffused out of the samples at a higher rate in the temperature range up to 400°C and (2) the Pb atoms diffused out of the samples at a much slower rate above 450°C. Both phenomena are attributed to the ion implantation process.