Physical mechanism underlying the enhancement effect of carbon in heavily phosphorus-doped Czochralski silicon substrate on phosphorus out-diffusion within n/n+ epitaxial wafer

Abstract

The effects of carbon in heavily phosphorus-doped Czochralski (HP-Cz) silicon (Si) substrates, with the concentrations across an order of magnitude from 1016 to 1017 cm−3, on the out-diffusion of phosphorus impurities within n/n+ epitaxial Si (Epi-Si) wafers have been investigated. It is found that the increase in the carbon concentration ([C]) from 1.0 × 1016 to 1.0 × 1017 cm−3 leads to the enhanced phosphorus out-diffusion within the n/n+ Epi-Si wafer when subjected to anneal at 1100 °C in an N2 or an O2 ambient or to anneal at 1150 °C in an N2 ambient, but hardly affects the phosphorus out-diffusion within the n/n+ Epi-Si wafer when subjected to anneal at 1150 °C in an O2 ambient. Based on the density functional theory calculations, it is derived that the increase in the [C] from 1.0 × 1016 to 1.0 × 1017 cm−3 in the HP-Cz Si substrate results in a significantly increased thermal equilibrium concentration of self-interstitial silicon (SiI) atoms at 1100 or 1150 °C, which, in turn, leads to the increased concentration of the SiI atoms that out-diffuse from the substrate to the epitaxial layer because the SiI atoms diffuse extremely fast in Si. Such a carbon-induced increase in the concentration of SiI atoms is believed to be responsible for the aforementioned enhanced phosphorus out-diffusion, which is predominantly dictated by the interstitialcy mechanism. In the case of anneal in the O2 ambient at sufficiently high temperatures such as 1150 °C, a large number of excessive SiI atoms injected into the Epi-Si wafer substantially mask the carbon enhancement effect on the phosphorus out-diffusion. Of technological significance, it is deduced that the [C] should be not higher than 1.0 × 1016 cm−3 in the HP-Cz Si wafers as the substrates of n/n+ Epi-Si wafers used for power devices.