Silicon surface passivation by anodic oxidation annealed at 400°C

We investigate the surface passivation attained by anodic oxidation of silicon wafers. A low surface recombination velocity S of 20-35 cm/s is achieved by submersing silicon wafers in nitric acid (HNO3) and applying a constant voltage, followed by annealing at 400° C in oxygen and then forming gas. We examine the SiO2 thickness as a function of HNO3 concentration and conclude that water is the primary source of oxygen for the anodic oxidation process. It is demonstrated that the HNO3 purity has a substantial impact on the magnitude and repeatability of S. When the HNO3 impurity concentration is in the parts per million range, S = 10-1000 cm/s, whereas when the impurity concentration is reduced to parts per billion, S = 20-35 cm/s. We find that the anodic SiO2 films degrade when exposed to the ambient air, primarily due to the ingression of water vapor. This low-temperature anodic oxidation provides a level of passivation required by high efficiency solar cells.