Evaluation of the bulk lifetime of silicon wafers by immersion in hydrofluoric acid and illumination

We report on a procedure to temporarily attain a very high level of surface passivation for silicon wafers at room temperature. When applied during a photoconductance measurement, the procedure permits an accurate assessment of the bulk lifetime, even for high-lifetime samples (τ_bulk > 5 ms) that are otherwise difficult to measure. It is already established that the surfaces of a silicon wafer can be well passivated by immersion in hydrofluoric acid (HF). Here, we show that the HF passivation is greatly enhanced by illuminating the wafers just prior to measurement, and that the HF passivation depends critically on the surface preparation, where the best passivation is attained after etching the wafers in tetramethylammonium hydroxide. We assess the level of passivation for a range of HF concentrations and wafer resistivities, and we demonstrate that S < 5 cm/s can be attained on 0.8-1000 Ω-cm n- and p-type silicon wafers. We demonstrate the value of the method with two examples.