Annealing behavior of Al2O3 thin films grown on crystalline silicon by atomic layer deposition

Abstract

Al2O3 thin films deposited by Atomic Layer Deposition provide an excellent passivation on both n-type and p-type silicon surfaces due to a field effect caused by negative fixed charges. To activate the passivation an annealing step at moderate temperatures (~ 425°C) for 30 min after deposition was reported to be essential. A drop in minority carrier lifetime was reported if an additional firing step was carried out. This work demonstrates that the order of the annealing and firing step determines the quality of the surface recombination velocity (SRV). Applying the firing step before the annealing step results in a lower SRV compared to the SRV of samples subjected to the reference process. The short firing step is carried out at a peak temperature of ~ 600°C as determined at the wafer surface. The maximum surface recombination velocity assuming an infinite bulk lifetime of the best samples after firing and subsequent anneal is determined to be 5.1 cm/s and 5.8 cm/s, whereas the lowest values for the reference process are 8.3 cm/s and 9.7 cm/s for n- and p-type Si wafers, respectively. Moreover, the minority carrier lifetime of the n-type samples after firing and subsequent annealing is comparable to the one of only annealed samples without any firing step.