Intrinsic and doped amorphous silicon carbide films for the surface passivation of silicon solar cells

Abstract

The amorphous silicon carbide films investigated throughout this work were produced on the basis of plasma enhanced chemical vapor deposition (PECVD). The two different PECVD reactors used for the deposition of the films were presented. The principle development of the material was conducted at the AK400M batch-type reactor from Roth&Rau and the processes were then transferred to an industrial type SINA in-line reactor from the same company. For standard passivating Si-rich a-Si1-xCx layers deposited in the AK400M reactor working with RF power only, SIMS measurements revealed rather low C-atom densities (< 10 at.%) in the matrix. Doping atom densities of 1×1021 cm-3 for phosphorous and of 5×1020 cm-3 for boron were determined for a typical gas flow of 100 sccm (B2H6 or PH3 highly diluted in H2). The a-Si1-xCx films fabricated in the SINA reactor exhibit a similar composition as well as similar electrical properties as their batch-type counterparts.A parallel analysis of a-Si1-xCx/c-Si and a-Si1-xCx/c-Ge systems was performed. From literature it is known that the main differences in terms of surface passivation of crystalline silicon and germanium substrates are the instability of (thermally grown) GexOy and the limited practicability of hydrogen for the passivation of Ge dangling bonds. From a compositional viewpoint, the performed transmission electron microscopy (TEM) studies reveal no difference in the film structure depending on the substrate type. With increasing C-content, the Si-rich a-Si1-xCx films become less dense, a fact which is attributed to an increasing microvoid density. The c-Si surface passivation quality is found to be directly correlated with the Si-H bond density in the film. The onset of Si-H bond rupture at around 300°C therefore coincides with the onset of electrical degradation independently of the carbon content and the doping density in the film. The surface passivation of germanium by a-Si1-xCx demonstrates a fairly different behavior. First of all, the thermal stability of intrinsic films is significantly increased. The electrical degradation starts at temperatures as high as 450°C and therefore indicates a “decoupling” of hydrogen present in the film and passivation quality.