The negative fixed charge of atomic layer deposited aluminium oxide—a two-dimensional SiO₂/AlO_x interface effect

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. (paper)