The electronic structures and X-ray photoelectron spectra of silicon models with octahedral B 6 , icosahedral B 12 , or cubooctahedral B 12 clusters are investigated using first-principles calculations. It is found that the B 6 and B 12 clusters act as double acceptors in silicon and that the simulated chemical shift of the B 1s orbital signals of the B 6 and cubo-octahedral B 12 clusters in X-ray photoelectron spectra coincides exactly with the chemical shift of B 1s experimentally observed in as-implanted silicon at an extremely high dose of boron. These results reveal that the B 6 and cubo-octahedral B 12 clusters are the origin of hole carriers in silicon. We propose a mechanism for hole generation and a physical model for boron cluster formation at implantation-induced divacancy sites and multi vacancy sites.
We have investigated the formation energies, ionization energies, and chemical natures of substitutional group-II, III, V, and VI impurity atoms in 4H-SiC. It is shown that the impurity atoms have lower formation energies on a carbon site than on a silicon site for nitrogen, oxygen, and sulfur regardless of the crystal growth conditions, whereas the favorable sites for boron and selenium depend on the composition. With the exception of the above elements, impurity atoms always substitute on a silicon site. The cluster calculations suggest that antimony introduces a much shallower donor level than those of conventional n-type dopants such as nitrogen and phosphorus. However, its high energy of formation will make it difficult to dope SiC with a high impurity concentration. These results suggest that antimony is a good candidate for an extremely shallow n-type dopant.
Ab initio calculations of the atomic and electronic structure of crystalline silicon (c-Si) with X@B 6 and X@B 12 (X ¼ H{Br) clusters have been performed to investigate carrier generation by doping atoms inside the cage of the boron clusters. We confirmed that octahedral B 6 , cubo-octahedral B 12 (B 12 -CO) and icosahedral B 12 (B 12 -ICO) can exist stably in c-Si and should act as double acceptors. We also found that H atoms can be settled in B 12 -CO clusters and the H@B 12 -CO cluster can introduce a very shallow single acceptor level whose activation energy is lower than those of B 6 , B 12 (-CO, -ICO) and substitutional boron atom (B s ). It is found on the basis of the formation energies that B@B 6 and B@B 12 will inevitably be formed and may degrade the efficiency of carrier generation. The H@B 12 -CO cluster is one of the most promising candidates as the cluster dopant for the improvement of the efficiency of boron implantation and the formation of a high-performance extremely shallow junction.
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