Reverse saturation current density imaging of highly doped regions in silicon: A photoluminescence approach

We present a camera-based technique for the local determination of reverse saturation current densities J ₀ of highly doped regions in silicon wafers utilizing photoconductance calibrated photoluminescence imaging (PC-PLI). We apply this approach to 12.5×12.5 cm ² float zone silicon samples with textured surfaces and a homogeneous phosphorous diffusion with sheet resistances between 24 and 230 Ω/□. We find enhanced photoluminescence emission at metallized regions of a sample due to reflection of long-wavelength light at the rear side of the sample. Our measurement setup comprises an optical short pass filter in front of the camera effectively blocking wavelengths above 970 nm and therefore ensuring a correct calibration of the PL signal in terms of excess charge carrier density Δn. We analyze two sets of samples comprising metal contacts to highly doped regions prepared by Laser Transfer Doping (LTD) as well as standard tube furnace phosphorus diffusion. We find a considerably smaller J ₀ value of 370 fA/cm ² for the LTD approach compared to a standard diffusion process resulting in J ₀=570 fA/cm ². On the basis of these results we demonstrate that J ₀ imaging is a powerful analysis technique for process optimization.