Amorphous Si1-xCx layers were grown using low-pressure chemical vapor deposition with SiH4 and C2H4 at 780 degrees C and 6.65 kPa on Si substrates. These layers, which exhibit a high optical bandgap and atomically sharp interface, were combined with P-doped polycrystalline layers to form a two-layer emitter structure. The addition of PH3 during growth drastically changes the electrical, compositional, and structural properties of the layers. The distribution of P and the stability on annealing of these materials have been studied using anneal treatments in N-2 at 800 to 900 degrees C. At these higher temperatures P diffuses in the amorphous Si1-xCx material and accumulates at the Si1-xCx interface. The presence of P also promotes the formation of nanocrystalline beta-SiC crystals. These crystals greatly enhance P diffusion in the Si1-xCx material. In practical emitter structures, processed using a lower thermal budget, P is found to segregate at or near the Si1-xCx-Si interface. It is very likely that this segregated P affects and possibly dominates the heterojunction bipolar transistor barrier mechanism explaining the enhanced current gains of these transistors.