Recently, it was shown that Al₂O₃ thin films synthesized by (plasma-assisted) atomic layer deposition (ALD) provide excellent surface passivation of n, p and p⁺ type c-Si as highly relevant for c-Si photovoltaics. It was found that a large negative fixed charge density (up to 10¹³ cm^-2) in the Al₂O₃ film plays a key role in the passivation mechanism of Al₂O₃ [1, 2]. The surface passivation quality of Al₂O₃ strongly increases with film thickness before reaching saturation around 10 nm as determined by carrier lifetime spectroscopy. In this contribution a study into the thickness effect will be presented in order to distinguish between the influence of field-effect passivation, i.e. electrostatic shielding of charge carriers by the fixed negative charge, and chemical passivation, i.e. by a reduction of the interface defect density. To this goal the nonlinear optical technique of second-harmonic generation (SHG) has been utilized. SHG is highly surface and interface specific and allows for the contactless determination of internal electric fields (≥ 10⁵ V/cm^-1). Spectroscopic SHG, carried out with a femtosecond Ti:sapphire laser tunable in the 1.33-1.75 eV photon energy range, has revealed a thickness independent electric field for Al₂O₃ films with thicknesses ranging from 2 to 20 nm. This implies that the field-effect passivation is not affected by the film thickness and that the thickness dependence in passivation quality can be attributed to a changing level of chemical passivation. Moreover, this result confirms that the fixed negative charges are located at the Al₂O₃ interface as also indicated by conventional C-V measurements. In addition, SHG shows clear differences between measurements performed on Al₂O₃ films grown by thermal and plasma-assisted ALD. These are likely related to the properties of the interfacial SiO_x induced by either growth process. The presence of this oxide is suggested to be responsible for the chemical passivation quality. Furthermore, the differences indicate a smaller contribution of field-effect passivation for the Al₂O₃ grown with thermal ALD compared to the film from the plasma-assisted process. These results have led to a deeper understanding of the c-Si surface passivation by Al₂O₃ as will be discussed.

[1] B. Hoex et al. J. Appl. Phys. 104, 044903 (2008)

[2] J.J.H. Gielis et al. J. Appl. Phys. 104, 073701 (2008)