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 x 10(12) 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.

Automated summary

This study demonstrates that the negative fixed charge density in ALD-aluminium oxide is an interfacial effect between ALD-AlO (x) and the SiO2 ultra-thin film grown on silicon during the ALD process. Additionally, a second fixed charge layer is shown to exist at the upper AlO (x) /SiO2 interface in SiO2/AlO (x) /SiO2-stacks. The presence of high charge densities through sub-nm thick ALD-AlO (x) enables advanced applications such as passivating hole-selective contacts for Si solar cells or nanoelectronic Si-doping strategies.