Passivation of phosporus diffused silicon surfaces with Al2O3: Influence of surface doping concentration and thermal activation treatments

Thin layers of Al 2O3 are well known for the excellent passivation of p-type c-Si surfaces including highly doped p + emitters, due to a high density of fixed negative charges. Recent results indicate that Al 2O3 can also provide a good passivation of certain phosphorus-diffused n + c-Si surfaces. In this work, we studied the recombination at Al 2O3 passivated n + surfaces theoretically with device simulations and experimentally for Al 2O3 deposited with atomic layer deposition. The simulation results indicate that there is a certain surface doping concentration, where the recombination is maximal due to depletion or weak inversion of the charge carriers at the c-Si/Al2O3 interface. This pronounced maximum was also observed experimentally for n + surfaces passivated either with Al 2O3 single layers or stacks of Al 2O3 capped by SiNx, when activated with a low temperature anneal (425 °C). In contrast, for Al 2O3/SiNx stacks activated with a short high-temperature firing process (800 °C) a significant lower surface recombination was observed for most n + diffusion profiles without such a pronounced maximum. Based on experimentally determined interface properties and simulation results, we attribute this superior passivation quality after firing to a better chemical surface passivation, quantified by a lower interface defect density, in combination with a lower density of negative fixed charges. These experimental results reveal that Al 2O3/SiNx stacks can provide not only excellent passivation on p + surfaces but also on n + surfaces for a wide range of surface doping concentrations when activated with short high-temperature treatments.