Comparison of bulk and surface passivation properties of plasma nitrides on Si and SiGe solar cells

In this paper the effects of direct and remote plasma nitrides on the performance of Cz-Si and SiGe bulk cells are analysed. The surface passivation properties of both kinds of nitrides are compared to a thin thermal oxide, grown at high temperatures. Internal quantum efficiency measurements prove that the surface recombination velocities are lowest in case of the remote plasma nitride layer. The extracted values of the surface recombination velocity are as low as 1.5×10³ cm/s for the remote plasma nitride, whereas the value for the thermal oxide is twice as high, comparable to the value obtained for the direct plasma nitride. SiGe-cells show the same tendency, although the blue response is lower in absolute value. However, when using the appropriate SiGe-absorption coefficient for parameter extraction by PC-ID, the surface recombination velocity of the plasma nitrides on SiGe-emitters is comparable to what is found in case of Si. In addition, we also found that for both remote plasma and direct nitride layers, there is a significant enhancement of the red response of all types of cells, compared to the samples without nitride. A comparison was made of the behaviour of the cells, with starting lifetime of 10-30 μs, showing that the enhancement of the red response by the use of a plasma nitride is comparable to the beneficial effects of a remote plasma hydrogenation. However, for starting lifetimes lower than 5 μs, the separate hydrogenation step brings an additional improvement of the red response compared to the only-nitride case and the effect of the two treatments seems to be cumulative