Improving ALD-Al2O3 Surface Passivation of Si Utilizing Pre-Existing SiOx

Abstract

Al 2 O 3 has rapidly become the surface passivation material of choice for p + layers of solar cells because of its high negative fixed charge, good long-term and thermal stability, and no parasitic absorption. In this article, the surface saturation current density, fixed charge, and interface state density are compared for Al 2 O 3 deposited on Si substrates where the pre-existing out-of-the-box SiO x layer was not removed, with substrates where the SiO x was removed by hydrofluoric acid. The depositions are performed by atomic layer deposition at temperatures in the 150–300 °C range, using trimethylaluminium, H 2 O, and O 3 as precursors. The samples where the native oxide was not removed achieve a higher level of surface passivation for every tested deposition temperature, with the sample deposited at 200 °C exhibiting a surface saturation current density of only 0.9 fA/cm 2 after annealing, a fixed charge of −4.2 × 10 12 cm −2 , and a density of interface states of 9.8 × 10 9 cm −2 eV −1 . Capacitance and conductance voltage characteristics reveal a strong correlation between the surface saturation current density and the density of interface states and fixed charges. It is also determined that the long-term stability of the surface passivation depends on the deposition temperature, with higher deposition temperatures resulting in improved long-term stability. The results indicate that H-terminated Si prior to Al 2 O 3 deposition may have a detrimental effect on the surface passivation.