Effect of the thickness of amorphous silicon carbide interlayer on the passivation of c-Ge surface by aluminium oxide films

Surface passivation is of paramount importance in photovoltaic devices based on high-quality crystalline absorbers such as crystalline germanium (c-Ge). In this work, we report on the surface passivation of p-type c-Ge by an amorphous silicon carbide/aluminium oxide film stack (a-SiCx/Al2O3). In particular, we focus on the impact of the thin (0, 1, 2, and 4 nm) a-SiCx interlayer which is used to provide chemical passivation while the Al2O3 is responsible for the field-effect passivation due to its high negative fixed charge density. The measured effective surface recombination velocity (Seff) values show that the introduction of the a-SiCx interlayer improves surface passivation, but the thinner the a-SiCx film the better. High frequency capacitance-voltage characterization leads to the determination of the fixed charge density (Qf) which indicates that the a-SiCx is shielding the negative charge located at the first nanometers of the Al2O3 film, as suggested by chemical characterization of the interface. As a consequence, the best result is Seff = 18 cm/s for the case of 1 nm of a-SiCx.