We investigate the passivation of crystalline Si (c-Si) surfaces by phosphorus oxide (POx) 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 POx/Al2O3 stacks deposited at both 25 degrees C and 100 degrees C. In contrast to Al2O3 alone, POx/Al2O3 passivation is activated already by annealing at temperatures as low as 250 degrees C in N-2 in all cases. Best results were obtained after annealing at 350 degrees C and 450 degrees C for films deposited at 25 degrees C and 100 degrees 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/cm(2) were obtained on 1.35 Omega cm material. Passivation of p-type Si appeared somewhat poorer, with surface recombination velocity of 13 cm/s on 2.54 cm substrates. Passivation was found to be independent of POx film thickness for films of 4 nm and above, and was observed to be stable during prolonged annealing up to 500 degrees 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 x 10(12) 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 Sibix. Indeed, passivation performance on n-type silicon is shown to compare favourably to state-of-the-art results reported for PECVD SiNx. POx/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.