High-quality surface passivation of crystalline Si is achieved using 10 nm thick Al2O3 films fabricated by thermal atomic layer deposition at 100 °C. After a 5 min post deposition annealing at 200 °C, the effective carrier lifetime is 1 ms, indicating a functional level of surface passivation. The interplay between the chemical and the field effect passivation is investigated monitoring the density of interface traps and the amount of fixed charges with conductance-voltage and capacitance-voltage techniques. The physical mechanisms underlying the surface passivation are described. The combination of low processing temperatures, thin layers, and good passivation properties facilitate a technology for low-temperature solar cells.