Electrical properties of silicon with divacancies

Abstract

The conductivity was measured at room temperature of silicon samples with a small content of oxygen, carbon, and phosphor, after irradiation with electrons of the energy of ≈1 MeV and doses of the order of 1018 electrons cm−2. It is found that the hole conductivity after a dose of ≈1018 electrons cm−2 becomes only slightly dependent on the dose when the latter increases in 2 to 3 times; this conductivity can be described by the expression p = A exp (-a/kT) where A approximately coincides with the density of states in the valence band Q = 2(2πmeffkT)3/2h−3 and a ≈0.4 eV. The value of a coincides with the value 1/2(ϵ1 + ϵ2) where ϵ1 and ϵ2 are the positions of the donor and the acceptor levels, respectively of the divacancy in relation to the edge of the valence band (0.27 and 0.51 eV correspondingly). This experimental result shows that in the irradiated samples the ultimate position of the Fermi level is realized, when the equilibrium conductivity is determined by divacancies, but is independent of the concentration of the divacancies ND. The increase of ND with increasing dose leads only to the redistribution of the electrons among the donor and the acceptor levels of the divacancies, not affecting the concentration of the conductivity holes. In silicon samples with higher concentrations of impurities (O, C, P etc.) the ultimate position of the Fermi level could not be obtained at the doses noted. The theoretical analysis shows that the relatively small concentration of electrically active centres created during the irradiation (2 to 3 orders of magnitude less than the divacancy concentration) prevents the realization of the ultimate position of the Fermi level. [Russian Text Ignored]