Decoupling high surface recombination velocity and epitaxial growth for silicon passivation layers on crystalline silicon

We have critically evaluated the deposition parameter space of very high frequency plasma-enhanced chemical vapour deposition discharges near the amorphous to crystalline transition for intrinsic a-Si:H passivation layers on Si (1 1 1) wafers. Using a low silane concentration in the SiH4-H2 feedstock gas mixture that created amorphous material just before the transition, we have obtained samples with excellent surface passivation. Also, an a-Si:H matrix was grown with embedded local epitaxial growth of crystalline cones on a Si (1 1 1) substrate, as was revealed with a combined scanning electron and high-resolution transmission electron microscopy study. This local epitaxial growth was introduced by a decrease of the silane concentration in the feedstock gas or an increase in discharge power at low silane concentration. Together with the samples on Si (1 1 1) substrates, layers were co-deposited on Si (1 0 0) substrates. This resulted in void-rich, mono-crystalline epitaxial layers on Si (1 0 0). The epitaxial growth on Si (1 0 0) was compared to the local epitaxial growth on Si (1 1 1). The sparse surface coverage of cones seeded on the Si (1 1 1) substrate is most probably enabled by a combination of nucleation at steps and kinks in the (1 1 1) surface and intense ion bombardment at low silane concentration. The effective carrier lifetime of this sample is low and does not increase upon post-deposition annealing. Thus, sparse local epitaxial growth on Si (1 1 1) is enough to obstruct crystalline silicon surface passivation by amorphous silicon.