In this Letter, we report that both thermal atomic layer deposition (ALD) with H2O, and plasma ALD with an O2 plasma, can be used to deposit Al2O3 for a high level of surface passivation of crystalline silicon (c‐Si). For 3.5 Ω cm n‐type c‐Si, plasma ALD Al2O3 resulted in ultralow surface recombination velocities of Seff < 0.8 cm/s. Thermal ALD Al2O3 also showed an excellent passivation level, with Seff < 2.5 cm/s. In contrast to plasma ALD Al2O3, thermal ALD Al2O3 provides some surface passivation in the as‐deposited state, although annealing is required to activate it to the full extent. For thermal ALD, the optimal temperature for this anneal was found to be slightly lower, ∼375 °C, than for plasma ALD Al2O3, ∼425 °C. The minimal Al2O3 thickness without compromising the passivation properties was 5 nm for plasma ALD Al2O3, whereas for thermal ALD, films >10 nm were required. Thermal stability against a high temperature firing step was demonstrated for ultrathin thermal and plasma ALD Al2O3 films of 5 nm by Seff < 9.2 and < 6.5 cm/s, respectively. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The thermal and ultraviolet (UV) stability of crystalline silicon (c-Si) surface passivation provided by atomic layer deposited Al2O3 was compared with results for thermal SiO2. For Al2O3 and Al2O3/a-SiNx:H stacks on 2 Ω cm n-type c-Si, ultralow surface recombination velocities of Seff&lt;3 cm/s were obtained and the passivation proved sufficiently stable (Seff&lt;14 cm/s) against a high temperature “firing” process (&gt;800 °C) used for screen printed c-Si solar cells. Effusion measurements revealed the loss of hydrogen and oxygen during firing through the detection of H2 and H2O. Al2O3 also demonstrated UV stability with the surface passivation improving during UV irradiation.

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Surface passivation schemes based on Al2O3 have enabled increased efficiencies for silicon solar cells. The key distinguishing factor of Al2O3 is the high fixed negative charge density (Qf = 10 12 -10 13 cm -2 ), which is especially beneficial for p-and p+ type c-Si, as it leads to a high level of field-effect passivation. Here we discuss the properties of Al2O3 surface passivation films synthesized with plasma atomic layer deposition (ALD), thermal ALD (using H2O as oxidant) and PECVD. We will show that with all three methods a high level of surface passivation can be obtained for Al2O3 deposited at substrate temperatures in the range of 150-250 o C. Furthermore, the role of chemical and field-effect passivation will be briefly addressed. It is concluded that the passivation performance of Al2O3 is relatively insensitive to variations in structural properties. Al2O3 is therefore a very robust solution for silicon surface passivation.