Effect of air ambient on surface recombination and determination of diffusion length in silicon wafer using photocurrent generation method

An n+–p–p+ structure was formed by depositing a thin Al layer on one side and a semitransparent Pd layer on other side of a p–Si wafer, and the diffusion length L was determined from the slope of short circuit current density Jsc vs. illumination intensity Pin characteristics by illuminating the p+ side with a laser light of wavelength λ = 789 nm, following the photocurrent generation method. The slope was found to decrease after the specimen was exposed to air for a few hours before the measurement. The present investigation revealed that the degradation of slope was owing to increase in surface recombination velocity S significantly, e.g. to 2800 cm/s for air exposure of 8 h, above its initial value 10 cm/s. This resulted from adsorption of moisture on Pd surface which decreases work function of Pd and lowers the accumulation potential barrier. The adsorbed water molecules could be removed from Pd surface by vacuum pumping and then S again became small and the slope regained its original value. The true value of L, smaller or larger than the wafer thickness, can be determined using single monochromatic illumination if S is known or has a negligibly small value, as in our case where the specimen was stored in vacuum. In our specimen we determined L = 56 μm when S was negligibly small. However, using the experimental slopes of Jsc–Pin curves at two monochromatic illuminations obtained by Sharma et al. we determined both S and L and found their values 1390 cm/s and 94 μm, respectively.