The origin of the commonly observed negative fixed charge density (Q fix) in atomic layer deposited (ALD-)aluminium oxide is still a matter of debate despite its widespread applications in (opto-)electronics, particularly in silicon photovoltaics. Q fix plays a crucial role for excellent Si surface passivation, which is mandatory for high efficiency solar cells. Often, Q fix is believed to originate from structural or compositional specifics of the first few nanometres of ALD-AlO x adjacent to the Si-interface. Here, we demonstrate that the negative Q fix is solely an interfacial effect of ALD-AlO x and the SiO2 ultra-thin film that grows inevitably during ALD on Si. Furthermore, it is proven that a second Q fix-layer exists at the upper AlO x /SiO2 interface of SiO2/AlO x /SiO2-stacks, which can carry up to a quarter of the total Q fix. We show that both SiO2/AlO x interfaces can be separated by a charge-lean material such as HfO2 (rather than AlO x ) without significant impact on the measured Q fix. This renders the location of Q fix exactly at the two-dimensional interface of SiO2 and AlO x, rather than in the near-interfacial AlO x volume. The origin of Q fix is discussed in detail. The possibility to obtain very high charge densities of around -5 × 1012 cm-2 by sub-nm thick ALD-AlO x enables advanced applications such as passivating hole-selective contacts for Si solar cells or nanoelectronic Si-doping strategies via Al-induced SiO2 modulation doping.