Studies of photoconductance decay method for characterization of near-surface electrical properties of semiconductors

The Photoconductance Decay (PCD) method in a standard version allows determination of the bulk properties of semiconductors but is not suitable for evaluation of the surface and near-surface region of the wafer during processing. This experiment is concerned with PCD measurements devised specifically for the purpose of electrical probing using temporary contacts of the near-surface (ns-PCD) region of semiconductor substrates, including thin films. The method is tested through the measurements of the single-crystal, indirect bandgap elemental semiconductor wafers (Si and Ge), as well as selected direct bandgap compound semiconductor wafers (GaAs and InP). In the former case, surface properties of the material tested are altered in a controlled fashion and the PCD response to the surface alteration, in terms of the minority carrier lifetime, is measured. In the case of all substrates studied, a direct correlation between carrier lifetime measured using PCD and the condition of the wafer surface was observed. Furthermore, the ns-PCD method was proven very effective in measuring minority carrier lifetime at the semiconductor-dielectric interface without having to form MOS test structures. The results obtained suggest that the modified PCD method employing temporary contacts to the measured surface can be used to monitor the condition of semiconductor surfaces during device processing.