ON THE ORIGIN OF PHOTOCURRENT OSCILLATION AT SI ELECTRODES

Photocurrent oscillations at n-type Si(100) were investigated in ammonium fluoride solutions for a variety of electrolyte compositions and pH values. The data show a linear correlation between the etch rate for anodic oxides and the oscillation frequency at fixed applied voltage. A model based on minority carrier collection at the boundary between pores and the silicon substrate is suggested. Because of the large diffusion length of holes in silicon, the strong photocurrent modulation observed can be explained by changes in a relatively small number of charge-collecting pores while a substantial overall oxide thickness is maintained. A transition to noisy behaviour is also observed when the solution composition is changed to give a higher etch rate. The charge collection model used here to interpret the photocurrent oscillation is also the basic functioning principle of silicon point contact solar cells.