Field-effect passivation of the SiO2Si interface

Abstract

The field-effect passivation of the interface of thermal oxides on silicon is experimentally investigated by depositing corona charges on the oxide of solar cells and of lifetime test structures. The open circuit voltage of solar cells with interdigitated rear contacts can be increased by +12 mV or decreased by −34 mV, respectively, by depositing positive or negative corona charges on top of the front oxide. The resulting effective surface recombination velocity, Seff, is determined on carrier lifetime test structures for different injection levels and charge densities using microwave-detected photoconductance decay and a new expression for the Auger-limited bulk lifetime. Seff can be varied between 24 cm/s and 538 cm/s on a 1 Ω cm p-type wafer with a thermal oxide of 105 nm thickness. The measurements are compared with theoretical predictions of an analytical model for the calculation of the surface recombination. Measured values for the capture cross sections and interface trap densities are used for the calculation. The model predicts an optimum passivation for strong positive compared to strong negative charge densities. This is due to the asymmetry of the capture cross sections for electrons and holes. This prediction is in very good agreement with the measured Seff values. However, the predicted Seff values of well below 1 cm/s for 1 Ω cm p-type silicon cannot be achieved in the experiment. This discrepancy can be explained by an inhomogeneous charge distribution resulting in potential fluctuations and additional loss currents. With a new extended analytical model for the calculation of Seff the measured Seff values can be described quantitatively.