Investigation of crystalline silicon surface passivation by positively charged PO_x/Al₂O₃ stacks

We investigate the passivation of crystalline Si (c-Si) surfaces by phosphorus oxide (PO_x) thin films deposited in an atomic layer deposition (ALD) reactor and capped in-situ by ALD Al₂O₃. Passivation is demonstrated on both n- and p-type (100) Si surfaces, and for PO_x/Al₂O₃ stacks deposited at both 25 °C and 100 °C. In contrast to Al₂O₃ alone, PO_x/Al₂O₃ passivation is activated already by annealing at temperatures as low as 250 °C in N₂ in all cases. Best results were obtained after annealing at 350 °C and 450 °C for films deposited at 25 °C and 100 °C respectively, with similar implied open-circuit voltages of 723 and 724 mV on n-type (100) Si. In the latter case an outstandingly low surface recombination velocity of 1.7 cm/s and saturation current density of 3.3 fA/cm² were obtained on 1.35 Ω cm material. Passivation of p-type Si appeared somewhat poorer, with surface recombination velocity of 13 cm/s on 2.54 Ω cm substrates. Passivation was found to be independent of PO_x film thickness for films of 4 nm and above, and was observed to be stable during prolonged annealing up to 500 °C. This excellent passivation performance on n-type Si is attributed partly to an unusually large positive fixed charge in the range of 3–5 × 10¹² cm⁻² (determined from capacitance–voltage measurements) for stacks deposited at both temperatures, which is significantly larger than that exhibited by existing positively charged passivation materials such as SiN_x. Indeed, passivation performance on n-type silicon is shown to compare favourably to state-of-the-art results reported for PECVD SiN_x. PO_x/Al₂O₃ stacks thus represent a highly effective positively charged passivation scheme for c-Si, with potential for n-type surface passivation and selective doping applications.