Electrical transport properties in polysilicon emitters investigated by variation of poly-Si thickness

Shallow poly-Si emitters with low values of emitter resistance fabricated by rapid optical annealing are discussed. The polysilicon thickness was varied from 100 nm to virtually zero without changing doping profiles. As long as a continuous grain boundary exists at the interface, the experimental results indicate the presence of an effective hole barrier at the poly/mono-Si interface. The residual and discontinuous interface oxide layer cannot explain the independence of base currents from the poly-Si thickness. The low values of interface resistance as well as the ability of the interface to block minority carrier transport can be explained by recent arsenic segregation models. However, it is possible to exclude tunneling or thermionic emission as the dominant mechanisms determining effective surface recombination velocity. The experimental findings suggest that virtually all holes recombine in the interfacial region. The blocking action of the interface and the resulting low values of emitter transit time make the poly-Si emitter a serious rival to heterojunction emitters for scaled-down devices