Characterization of Si wafers by μ-PCD with surface electric field

Carrier recombination lifetime in Si wafers was measured by the microwave reflectance photoconductivity decay method with a voltage applied between the wafer and an electrode placed just above the wafer. From the dependence of the lifetime on the voltage, the following conclusions were drawn: (1) the surface Fermi level is close to the valence band for the wafers stored in air for several years; (2) positive charge is induced and the surface Fermi level is shifted to near the conduction band by the dilute HF treatment, and the positive charge remains on the surface under air exposure for more than 1 month; (3) positive charge is also induced by the NH4OH+H2O2+H2O (SC1) treatment, but its amount is significantly reduced by 1 week of air exposure.