Surface Passivation Mechanism of Atomic Layer Deposited Al2O3 Films on c-Si Studied by Optical Second-Harmonic Generation

Recently, it was shown that Al2O3 thin films synthesized by (plasmaassisted) 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 fieldeffect 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 fieldeffect 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 plasmaassisted ALD. These are likely related to the properties of the interfacial SiOx 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.